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2003 Fall Meeting

Cite abstracts as: Eos. Trans. AGU, 84(46), Fall Meet. Suppl., Abstract #####-##, 2003.

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els. The approach consists of two steps. First the large-scale mantle flow field is calculated in a global model,in the whole mantle. This global and robust flow fieldserves as a background flow in the regional model, inwhich the interaction of the mid-Atlantic ridge and theIceland plume is calculated. A time-dependent model ofthe large-scale mantle flow field was obtained by usingpaleogeometries of the Atlantic and Eurasian plates re-constructed from magnetic anomalies and by advectingdensity anomalies backward in time. The motion of theplume source on the bottom of the model box is calcu-lated according to the distortion of an initally verticalplume conduit in the large-scale mantle flow field. Inthe regional model the changing large-scale flow fieldand the moving plume source are introduced as timedependent boundary conditions. In this way, the in-teraction of the Iceland plume and the mid-Atlanticridge is investigated in a 3D model containing detailedridge geomtery. Results with time independent bound-ary conditions show that the large-scale mantle flowfield controls the development of the plume. In theregional model the plume is tilted to the north in theupper mantle, which is also shown by seismology. How-ever, the northward channeling of the plume materialin the model does not explain the geochemical anoma-lies, which show an increased plume influence south ofIceland. Simulations with time dependent boundaryconditions (time dependent ridge geometry and plumesource position) modify this channelling of plume ma-terial and give a more precise view of plume-ridge in-teraction.

T41C-0243 0830h POSTER

The Dynamics of Slab detachment:Process Initiated by Melting ofSubducted Crust

Taras Gerya1,2 (+49-234-3223518;[email protected])

David Yuen3 ([email protected])1Institut of Geology, Mineralogy and Geophysics, SFB

526, Ruhr-Universiy Bochum, Universitaetstrasse150, Bochum D-44780, Germany

2Institute of Experimental Mineralogy RussianAcademy of Sciences (at present Alexander vonHumboldt Foundation Fellow), Chernogolovka,Moscow 142432, Russian Federation

3University of Minnesota Supercomputing Instituteand Department of Geology and Geophysics Insti-tute of Experimental Mineralogy Russian Academyof Sciences, University of Minnesota, Minneapolis,MN 55455-0219, United States

It is well recognized that slab detachment orbreakoff is a realistic geological process, as shown by re-cent tomographic imaging [1]. Using 2-D upper-mantlemodel with an area of 660 km deep and 2000 km widewe have investigated with a finite-difference and markernumerical technique the multi-resolutional character ofthermomechanical phenomena related to this complexgeological process. We have used up to 50 million mark-ers on a shared-memory computer for delineating thecomplex multiscale structures in the composition, vis-cosity, accumulated strain, shear heating, and otherfield variables. Our experiments show that this pro-cess can be initiated by slab weakening due to thethermal relaxation of the slab and related melting ofthe subducted oceanic crust. The melting propagateswithin the subducted oceanic crust at the top of theslab occurring at the restricted depth interval of 100to 200 km defined by the non-linear dependence of wetsolidus temperature of the oceanic crust with pressure.The detachment process is self-accelerating due to thestrain and thermal erosion focussing and strong ther-mal feedback from the shear heating. Slab detachmentdevelops around 10% faster with viscous dissipation in-cluded, thus showing the importance of shear heatingin this process. Detached slab rapidly fall down hav-ing a tendency of coherent rotation. This may producenear horizontal relatively cold slab fragments laying ondenser mantle at 660 km discontinuity. Influence of atemperature- and pressure-dependent thermal conduc-tivity for the process of thermal relaxation of the slabis significant. Overall 20% increase in thermal con-ductivity of mantle produce 20% decrease in timescaleof detachment. This support the idea that breakoffprocess is mainly driven by focussed thermal erosionwith timescale linearly dependent on heat conductiv-ity. Rapid changes in topography and significant vol-canic activity due to the massive melting of subductedoceanic crust during the slab detachment process areplausible consequences of this vigorous geodynamic sce-nario. [1] Levin, V., Shapiro, N., Park, J. and M. Ritz-woller, Seismic evidence for catastrophic slab loss be-neath Kamchatka, Nature, 418, 763-767, 2002.


Slab Dynamics and Non-NewtonianRheology in the Upper Mantle

Magali I Billen1 ([email protected])

Greg Hirth2 ([email protected])

Peter B Kelemen2 ([email protected])1U. C. Davis, Geology Department, Geology/Physics

Building, One Shields Avenue, Davis, CA 95616,United States

2Woods Hole Oceanographic Institution, Geology andGeophysics, 360 Woods Hole Rd, Woods Hole, MA02543, United States

One of the fundamental aspects of plate tectonicsand mantle convection in the Earth is one-sided sub-duction of plates at trenches with apparent slab dipsranging from 30◦ to 90◦ . While this aspect of platetectonics is not often reproduced in large scale mantleconvection simulations, the importance of slab dip tothe thermal structure of the slab-wedge system has leadto numerical models of slab thermal structure, whichfix the slab dip at a specified value and impose one-sided subduction.Understanding the balance of forcesthat lead to the range of observed slab dips can provideconstraints on the viscosity structure of the shallowmantle and the importance of phase changes in modify-ing the buoyancy forces driving subduction. We present2-D, time-dependent, finite element models of thermalconvection exploring the dependence of slab dynamicson the viscosity structure and phase changes in the up-per mantle. The viscosity structure evolves in timeand is defined by a composite rheology which is tem-perature, pressure and strain-rate dependent, includ-ing both diffusion (Newtonian) and dislocation (non-Newtonian) flow laws and a yield criterion at low tem-peratures. The plate boundary within the lithosphereis included as a narrow shear zone with low viscosity.We find that slab dynamics are strongly dependent onthe assumed boundary conditions and proximity of thesubduction zone to the side boundaries.Flat slab sub-duction and/or two-sided subduction often occurs formodels in which the viscosity is not strain-rate depen-dent. The strain-rate dependence of viscosity can leadto weak regions that cut through the full thickness ofthe subducting lithosphere within the subduction zone,producing steeply dipping slabs.


On the Curvature of Oceanic Arcs

Gabriele Morra1 (++41-1-6332720;[email protected])

Klaus Regenauer-Lieb1

([email protected])

Domenico Giardini1

([email protected])1ETH Zurich - Geophysics Institute, ETH Hoengger-

berg, Zurich 8093, Switzerland

The key feature of plate tectonics is the subduc-tion of cold oceanic plates into a hot convective man-tle. These subducting plates, as seen from the sur-face, mostly portray a distinct concave arc shape atthe trench with respect to the leading edge of subduc-tion. The origin of arc curvature is not yet understood.A common belief is that it is probably an effect of theEarth’s sphericity. However, the spherical effect of theEarth creates convex, long-wavelength arc shapes. Wethus investigate whether concave arc curvature can beexplained by: (1) Exogenic feedback between the mi-grating lithosphere and the secondary induced mantleflow, (2) Endogenic heterogeneities within the litho-sphere itself, e.g. owing to differences in cooling ages ofthe plate at the trench. Although both mechanisms cre-ate concave arcs, for isolate subduction systems, onlythe endogenic effects are sufficient to explain the mag-nitude of observed arc curvature. We compare our re-sults to the Aleutian and Sandwich arcs. Our methodis based on a novel 3-D numerical tool. We model thesubduction process as a solid (lithosphere) - fluid (man-tle) interaction. Two different numerical methods areused to solve for the constituents: Implicit Finite El-ement (FEM) for the lithosphere and Implicit Bound-ary Elements (BEM) for the mantle. The methods arechosen on the basis of a critical isotherm allowing afluid mechanical approximation of the full continuum-mechanical problem above 1200 K. Thus the calculus ofan approximate average drag effect is feasible throughsemi-analytical methods. This approach extends the 2-D setup of Funiciello et al., (JGR, 2003) into 3-D byadding the BEM solution. The BEM method builds onthe stokeslet theory as a semi-analytical solution forthe mantle drag. It shows that the drag mainly dependsby the integration of the singularities at the lateral ex-tremities of the slab.

URL: http://www.sg.geophys.ethz.ch/geodynamics/gabriele/


Blob Tracing Models for the CentralMexican Volcanic Belt

Vlad Manea1 ([email protected])

Marina Manea1 ([email protected])

Vladimir Kostoglodov1

([email protected])

Granville Sewell2 ([email protected])1Dept. Seismology/Institude Geophysics/UNAM,

Ciudad universitaria, Circuito de la Inv. Cient.,Mexico, df 04510, Mexico

2University of TExas, El Paso, P.O.Box 12141, ElPaso, tex 799137, United StatesThe numerical model of steady state temperature

and velocity fields in the mantle wedge of the Cen-tral Mexican Volcanic Belt (CMVB) is used to com-pute a dynamic model of buoyant blob tracing in thenon-newtonian mantle wedge velocity field. Consider-ing that the main component of the volcanic material isgenerated by the melting processes on the subductingplate surface, a dynamic model simulating the motionof detached blobs in a viscous mantle wedge flow wasdeveloped. The blob’s motion is determined by the ac-tion of drag, mass, and buoyancy forces in the mantlewedge velocity field. The blobs of the realistic diame-ter of 0.2 - 2.0 km show very different trajectories onlyat very low wrapping viscosity ( 1015 Pas). The blobrise time which is necessary to reach the bottom of thecontinental crust is from 0.04 up to 12.5 million yearsdepending on the plume diameter and surrounding vis-cosity.

T41D MCC: Level 1 Thursday0830h

Deformation Mechanisms: From theLab to the Lithosphere I Posters (jointwith V, MR)

Presiding: I Katayama, YaleUniversity; D L Goldsby, BrownUniversity

T41D-0247 0830h POSTER

Dislocation Creep in Magnesium Calcite

Lili Xu1 (617-252-1974; [email protected])

Xiaohui Xiao1 (617-253-3319; [email protected])

Brian J Evans1 (617-253-2856; [email protected])1Dept. Earth, Atmospheric and Planetary Sciences,

Massachusetts Institute of Technology, Cambridge,MA 02139, United StatesTo investigate the effect of dissolved Mg on plas-

tic deformation of calcite, we performed triaxial defor-mation experiments on synthetic calcite with varyingamount of Mg content. Mixtures of powders of cal-cite and dolomite were isostatically hot pressed (HIP)at 850◦C and 300 MPa confining pressure for differ-ent intervals (2 to 20hrs) resulting in homogeneous ag-gregates of high-magnesium calcite; Mg content var-ied from 0.07 to 0.17 mol%. Creep tests were per-formed at differential stresses from 20 to 160 MPa at700 to 800◦C. Grain sizes before and after deforma-tion were determined from the images obtained fromscanning electron microscope (SEM) and optical micro-scope. Grain sizes are in the range of 5 to 20 micronsdepending on the HIP time, and decrease with increas-ing magnesium content. Both BSE images and chemicalanalysis suggest that all dolomite are dissolved and theMg distribution is homogeneous through the sample,after 2 hrs HIP.At stresses below 40 MPa, the samples deformed in dif-fusion region (Coble creep), as described previously byHerwegh. The strength decreases with increasing mag-nesium content, owing to the difference of grain size.At stresses above 80 MPa, the stress exponent is greaterthan 3, indicating an increased contribution of disloca-tion creep. The transition between diffusion to dislo-cation creep occurs at higher stresses for the sampleswith higher magnesium content and smaller grain size.Preliminary data suggests a slight increase in strengthwith increasing magnesium content, but more tests areneeded to verify this effect. In a few samples, somestrain weakening may have been evident. The acti-vation energy in the transition region (at 80 MPa) is∼ 200 KJ/mol with no dependence on magnesium con-tent, agreeing with previous measurements of diffusioncreep in natural and synthetic marbles.

URL: http://www.agu.orglilixu


The Effect of Humidity and ParticleCharacteristics on Friction andStick-slip Instability in Granular FaultGouge

Jennifer L. Anthony1 (814-360-3269;[email protected])

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2003 Fall Meeting

Chris J. Marone1 (814-865-7964; [email protected])1Department of Geosciences, The Pennsylvania State

University, 522 Deike Building, University Park, PA16802, United States

Previous studies have shown that particle charac-teristics such as shape, dimension, and roughness af-fect friction in granular shear zones. Other work showsthat humidity plays a key role in frictional healing andrate/state dependence within granular gouge. In orderto improve our understanding of grain-scale deforma-tion mechanisms within fault gouge, we performed lab-oratory experiments using a double-direct-shear test-ing apparatus. This assembly includes three rigid forc-ing blocks with two gouge layers sandwiched betweenrough or smooth surfaces. Roughened surfaces weretriangular grooves 0.8 mm deep and 1 mm wavelength.These promote distributed shear throughout the layerundergoing cataclastic deformation. Smooth surfaceswere mirror-finished hardened steel and were used topromote and isolate grain boundary sliding. The cen-ter block is forced at controlled displacement rate be-tween the two side blocks to create frictional shear.We studied gouge layers 3-7 mm thick, consisting of ei-ther quartz rods sheared in 1-D and 2-D configurationsand smooth glass beads mixed with varying amountsof rough sand particles. We report on particle diame-ters that range from 0.050-0.210 mm, and quartz rods1 mm in diameter and 100 mm long. The experimentsare run at room temperature, controlled relative hu-midity ranging from 5 to 100%, and shear displace-ment rates from 0.1 to 300 microns per second. Exper-iments are carried out under a normal stress of 5 MPa,a non-fracture loading regime where sliding friction forsmooth spherical particles is measurably lower thanfor rough angular particles. We compare results fromshear between smooth boundaries, where we hypothe-size that grain boundary sliding is the mechanism influ-encing granular friction, to rough sample experimentswhere shear undergoes a transition from distributed,pervasive shear to progressively localized as a functionof increasing strain. For shear within rough surfaces,stick-slip instability occurs in gouge that consists ofless than 30We expand on previous work done by Fryeand Marone 2002 (JGR) to study the effect of humidityon 1-D, 2-D, and 3-D gouge layer configurations. Ourdata show that humidity has a significant effect on fric-tional strength and stability and that this effect is ob-served for both smooth surfaces, where grain boundarysliding is the dominant deformation mechanisms, andfor shear within rough surfaces where gouge deforma-tion occurs by rolling, dilation, compaction, and grainboundary sliding.


Influence of Intermediate Stress onYielding of Berea Sandstone

Serguei Jourine1 ([email protected])

Stephen L Karner2 ([email protected])

Andreas K Kronenberg2 (979-845-0132;[email protected])

Frederick M Chester2 ([email protected])1Texas A&M University, Department of Petroleum

Engineering, College Station, TX 77843, UnitedStates

2Texas A&M University, Center for TectonophysicsGeology and Geophysics Department, College Sta-tion, TX 77843, United States

The onset of brittle failure of Berea sandstone hasbeen investigated under varying stress states by sub-jecting solid and hollow cylindrical samples to confin-ing pressure Pc (to 120 MPa) and axial stress σz (to260 MPa) in a conventional triaxial deformation appa-ratus. Inelastic yielding of solid and hollow samples ismarked by cascading acoustic emissions and axial dis-placements that depart from the initial linear elasticresponse. For hollow samples, uniaxial (σz > σΘ =σr = 0) and biaxial (σz > σΘ > σr = 0 or σΘ >σz > σr = 0) stress states are obtained with devia-tors that (1) are maximum at the inner wall, (2) decayin a predictable manner with increasing radius prior toyielding, and (3) are insensitive to frictional boundaryconditions at piston-sample contacts throughout muchof the sample. X-ray CT scans of deformed samplesreveal different modes of failure associated with thestress states imposed. Zones of anomalous X-ray den-sity within hollow samples that mark deformed sand-stone are localized near the inner wall, nearly midwaybetween sample ends, and inner walls have lost theircylindrical geometry. Axial cavities of hollow samplesbecome elliptical when axial stress is the intermedi-ate principal stress (σz = σ2), with geometries resem-bling wellbore break-outs used to infer in-situ horizon-tal stresses in scientific drilling studies, while toroidalspalls are developed when the tangential stress is theintermediate stress (σΘ = σ2). Our results for solidspecimens under conventional triaxial conditions (σz> σr = σΘ > 0) are in agreement with results re-ported for Berea sandstone. However, the results forhollow specimens require a yield criterion that includesintermediate stress σ2. Yield criteria that fit our com-bined data may be expressed in terms of first and sec-ond invariants of stress I1 and J2; for example, using

the modified Wiebols and Cook (1968) criterion, J21/2

= A + B I1 + C I12, data for Berea sandstone are well

matched by parameters A = 16.5 MPa, B = 1.05, andC = -0.0013 MPa−1.


Experimental Study of Hybrid Fracturesand the Transition From Joints toFaults

Jonathan M. Ramsey1,2 (1-832-636-1000;jake [email protected])

Frederick M. Chester1 (1-979-845-3296;[email protected])

1Center for Tectonophysics, Department of Geologyand Geophysics, Texas A & M University, CollegeStation, TX 77843, United States

2Anadarko Petroleum Corp., 1201 Lake RobbinsDrive, The Woodlands, TX 77380, United States

Joints and faults are end members of a contin-uous spectrum of brittle fractures including the hy-brid fractures, hypothesized to form under mixed com-pressive and tensile stress. However, unequivocal ev-idence for the existence of hybrid fractures has notbeen presented. To investigate this transition, we haveconducted triaxial extension experiments on dog-boneshaped cylindrical samples of Carrara marble at roomtemperature, an axial extension rate of 2x10−2 mms−1, and confining pressures between 7.5 and 170 MPa.Two parallel suites of experiments were completed, oneusing very weak, latex jacketing to obtain accuratefailure strength, and another using copper foil jacket-ing to preserve fracture surfaces. The combined dataset provides strong evidence for the existence of hy-brid fractures on the basis of the progressive changein failure strength, fracture orientation, and fracturesurface morphology from joints to faults. At the low-est confining pressures (7.5 to 60 MPa), fractures areoriented approximately parallel to the maximum prin-cipal compressive stress, form at a tensile axial stressof approximately -7.75 MPa (i.e. the uniaxial tensilestrength), and display fracture surfaces characterizedby many reflective grain-scale cleavage faces, consistentwith jointing. At the highest confining pressures (130to 170 MPa), fractures are oriented from 13.4 to 21.6degrees to the maximum principal compressive stress,form under completely compressive stress states wherethe axial stress is between 0 and 4.3 MPa, and are char-acterized by short slip lineations and powdery, finelycomminuted grains consistent with faulting. At inter-mediate confining pressures (70 to 120 MPa), fracturesare oriented from 3.7 to 12.4 degrees to the maximumprincipal compressive stress, form under mixed stressconditions with the axial stress ranging from -10.6 to -3.0 MPa, and display both reflective cleavage faces andshort slip lineations with comminuted grains, consis-tent with hybrid fracturing.


Acoustic Emission Analysis of Stick SlipBehavior on Rough and SmoothFractures in Westerly Granite

Ben D Thompson1 (1-416-978-1276;[email protected])

R Paul Young1,2 ([email protected])

David A Lockner3 ([email protected])1Dept. Earth Science, University of Liverpool, 4

Brownlow Street, Liverpool L69 3GP, United King-dom

2Lassonde Institute, University of Toronto, Rm 119,170 College Street, Toronto, ON M5S 3E3, Canada

3US Geological Survey, 345 Middlefield Rd, Menlo Pk,CA 94025, United States

We present results from stick slip experiments on arough and a smooth fault, and use Acoustic Emission(AE) data to make a comparison between the natureof slip on these two surfaces. Two Westerly Granitecores were pre-fractured to represent rough and smoothend member models of fracture surface geometry. Onecore featured a rough, natural fracture which was prop-agated quasi-statically by triaxial loading (under AE-feedback control). The second core contained a saw-cut fracture, the surfaces of which were hand lappedto produce a smooth finish. The pre-fractured coreswere triaxially loaded to induce stick slip, at a con-fining stress of 150 MPa. In the rough fractured sam-ple, one stick slip event was recorded at an axial stressof 625 MPa. During a five minute period about thisslip, over 4000 AE events were triggered. A markedcontrast is seen for slip on the smooth fractured sam-ple, where a total of three discrete slip events occurred,at axial stresses of 380, 400 and 460 MPa. These slipevents are characterized by a seismic quiescence; only90 AE events were triggered during the five minute pe-riod about the final slip event. This extremely large

difference in the amount of AE activity observed be-tween slip on a rough and smooth fracture can be ex-plained, as expected, by the presence of interlockingasperities on the rough fracture surface. In order tofurther analyze the characteristics of stick slip, we lo-cate AE hypocenters to identify nucleation sites, cal-culate b-values, and resolve AE focal mechanisms toinvestigate the fundamental micromechanical processesoperating on smooth and rough fracture surfaces.


Photogrammetrical fracture analysis ofrock bodies

Yukiyasu Fujii1 (81-3-3944-8010; [email protected])

Shinzaburo Hori2 (81-3-3537-6633;[email protected])

1Fukada Geological Institute, 2-13-12 Hon-KomagomeBunkyo-ku, Tokyo 113-0021, Japan

2DPT Corporation, 1-1-4 Kyodo-building 2F Hatty-oubori Chuo-ku, Tokyo 104-0032, Japan

We tried to introduce a new way to determine the 3-D distribution of a fracture system. The fractured rockbodies are stereo-photographed by a calibrated analogcamera, and the photos were processed using a stereo-comparator to get raw data and the computer softwareto get the digital terrain model of the rock bodies andthe 3-D distribution of the fracture systems. A targetfor this trial was chosen from the Cretaceous sandstonelayers exposed in the Pacific Coast near Nakaminato,Ibaraki, which have been suffered different fracturingsuccessively since its deposition. THe 3-D distributionof the fractures suggests the following facts. (1) Foursets of small scale faults are distinguished, making ho-mogeneous domains; (2) Joints are not displaced byfaults; (3) Joints are not continuous across the bound-aries of the fault domains; (4) Each joint system de-veloped within a fault-bounded is different in orienta-tion,from that of neighboring. From these observationswe can safely conclude that, in this particular case,faults are formed at first, making fault bounded blocksin the Cretaceous sandstone. Then joint systems wereformed under subsequent stress condition, but accord-ing to the preexisting fault systems, resulted stress fieldwere in different orientation according to the block.This suggests that the boundaries of the blocks gavenew boundary condition when joints were formed. Jointsystems were accordingly oriented to slightly differentdirections in neighboring blocks. This is only an exam-ple, but shows that this new method is useful enoughfor 3-D analysis of the fractures.


Development of Discrete CompactionBands in Two Porous Sandstones

Sheryl Tembe1 (631-631-8302;[email protected])

Patrick Baud2 ([email protected])

Teng-fong Wong1

([email protected])1Dept of Geosciences, SUNY, Stony Brook, NY 11794-

2100, United States2Universite de Louis Pasteur, IPG, 5 rue Rene

Descartes, Strasbourg 67084, France

Compaction band formation has been documentedby recent field and laboratory studies as a localizedfailure mode occurring in porous sandstones. The cou-pling of compaction and localization may significantlyalter the stress field and strain partitioning, and actas barriers within reservoirs. Two end-members of thisfailure mode that develop subperpendicular to the max-imum principal stress have been identified: numerousdiscrete compaction bands with a thickness of onlyseveral grains, or a few diffuse bands that are signif-icantly thicker. Much of what is known about dis-crete compaction bands derives from laboratory exper-iments performed on the relatively homogeneous Ben-theim sandstone with 23% porosity. In this study weobserve similar compaction localization behavior in theDiemelstadt sandstone, that has an initial porosity of24.4% and a modal composition of 68% quartz, 26%feldspar, 4% oxides, and 2% micas. CT scans of theDiemelstadt sandstone indicate bedding correspondingto low porosity laminae. Saturated samples cored per-pendicular to bedding were deformed at room tempera-ture under drained conditions at a constant pore pres-sure of 10 MPa and a confining pressure range of 20-175 MPa. Acoustic emission activity and pore volumechange were recorded continuously. Samples were de-formed to axial strains of 1-4% and recovered from thetriaxial cell for microstructural analysis. The mechan-ical data map the transition in failure mode from brit-tle faulting to compactive cataclastic flow. The brittleregime occurred at effective pressures up to 40 MPa, as-sociated with failure by conjugate shear bands. At aneffective pressure range of 60-175 MPa strain harden-ing and shear-enhanced compaction were accompaniedby the development of discrete compaction bands, that

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2003 Fall Meeting


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was manifested by episodic surges of acoustic emission.Preliminary microstructural observations of the failedsamples suggest that bedding influenced the band ori-entations which varies between 75-90◦relative to themaximum principle stress. Our study demonstratesthat despite their different mineralogy, failure modesand development of the compaction localization aresimilar in the Diemelstadt and Benthiem sandstones.


Formation of Compaction Bands inSandstone as a Phase DecompositionAssociated With a Non-convex StressPotential

William A Olsson1 (505 844 7344;[email protected])

David J Holcomb1 (505 844 2157;[email protected])

1Sandia National Laboratories, MS 0751 POB 5800,Albuquerque, NM 87185-0751, United States

Permanent compaction of pore volume into local-ized regions, compaction bands, in sandstone has beenobserved in the outcrop and the laboratory. The cur-rent theoretical framework treats the onset of com-paction banding as a bifurcation of the deformationfield at a particular set of values of constitutive prop-erties. A shortcoming of the theory as it exists is thatit says nothing about the evolution of the specimen af-ter the onset of banding. Stress-shortening curves forspecimens undergoing compaction band formation andspread are characterized by a plateau of nearly constantstress during shortening. During this plateau the com-paction bands form and spread throughout the speci-men, in effect, changing the specimen from the origi-nal porosity (density) to a new, lower porosity (higherdensity). This type of curve is characteristic of manymaterials that show band formation and spread (inter-face propagation). Such a stress-deformation relationarises from the global minimization of an associatednon-convex, multi-well stress potential curve that ischaracterized by three numbers λ1, λ2 and pM (note:three numbers only if there are just two wells in the en-ergy curve) that may be construed as constitutive pa-rameters of the original porous rock. The deformationstate at the onset of banding is characterized by λ1,deformation at the completion of banding by λ2 andthe plateau stress is the Maxwell stress pM . Duringa triaxial test on a specimen of porous sandstone, thestress difference first increases from zero to peak stresswhere the deformation is λ1. At this point a new phaseof deformation characterized by λ2 appears. Through-out the plateau, there is a continuous rearrangement ofphases, λ1 being replaced by λ2. The volume fractionsof the two λ’s are related to the overall shortening λ bythe standard mixture rule. Nothing is predicted aboutthe distribution of phases, only the relative amounts.Thus deformation could proceed as one thickening bandor a series of intercalated bands of compacted and un-compacted material. Furthermore, the bands could befew in number and thick, or multitudinous and thin.All that is required is that the mixture rule be obeyed.Currently, we are examining a Hertzian fracture mecha-nism as the possible origin of the energy non-convexity.Identification of the appropriate micro-mechanism maylead to better understanding of the effects of such vari-ables as grain size and distribution of sizes on the typeof compaction observed—thick bands, thin bands, orhomogeneous deformation.


Frictional Properties of Mylonite UnderHigh Pressure and High Temperature

Koji Masuda1 (81-29-861-3994;[email protected])

Takashi Arai2

Koichiro Fujimoto2

Norio Shigematsu2

1Geoinformation Division, National Institute of Ad-vanced Industrial Science and Technology, AISTTsukuba Central 7, Tsukuba 305-8567, Japan

2Institute of Geoscience, National Institute of Ad-vanced Industrial Science and Technology, AISTTsukuba Central 7, Tsukuba 305-8567, Japan

In order to understand the earthquake generationprocess, we need to understand the frictional and rhe-ological properties of fault zone materials under high-pressure and high-temperature conditions. Laboratorydata on frictional properties of fault surfaces of faultzone rocks are useful for that purpose. We carriedout a series of conventional triaxial compression testsof mylonite at constant displacement rate. The strainrate of deformation was 5.5 x 10−6s−1, the temper-ature was raised at a rate of 10C/min for all experi-ments. We analyzed the stress-strain relation and thefrictional behavior of the fault surface formed in the

tests. Mylonite samples taken from an exposed brittle-ductile transition zone, the Hatagawa fault zone, north-east Japan, are tested under the confining pressure upto 200 MPa and temperatures up to 800C both in thedry and wet conditions. In the wet conditions, porewater pressure was applied up to 70 MPa. The sampleshape was a cylinder of 16.0 mm diameter and 40.0 mmlength. The sample axis of the mylonite samples was30 degrees from the orientation of the foliation struc-ture of the mylonite block. Under the dry conditionof 200 MPa confining pressure at 800C, samples showthe ductile behavior. The yield stress of mylonite sam-ple is much smaller than that of granite sample un-der the same condition. The internal structure of faultrocks such as foliation structure may significantly af-fect the deformation process under the high-pressureand high-temperature regime. Even under the same ef-fective confining pressure, presence of pore water dra-matically reduces the peak shear stress at the temper-ature regime higher than 600C. Frictional properties offault surface formed during the deformation tests areinvestigated. In the dry conditions, stick-slip behav-iors were observed at the room temperature and 200C.For the temperature range up to 600C, frictional forcesare almost same level. In the wet condition, we didn’tobserve stick-slip behavior for all temperature ranges.The frictional force decreased as the temperature in-creased. Fluid such as water in the deep crust mayplay an important role in deformation process.

URL: http://staff.aist.go.jp/koji.masuda/


Conditions for Localized Deformation inPorous Granular Materials UnderAxisymmetric Loading Using a TwoYield Surface Model

Vennela Challa1 (315-268-4400;[email protected])

Kathleen A. Issen1 (315-268-3880;[email protected])

1Clarkson University, Mechanical and Aeronauti-cal Engineering, Potsdam, NY 13699-5727, UnitedStates

Strain localization in porous granular rock occursin field and laboratory settings. Compaction bandsand dilation bands are of particular interest since lo-calized deformation may increase (or decrease) poros-ity/permeability, possibly affecting fluid flow withingeological formations and impacting drilling and ex-traction applications. Mollema and Antonellini (1996)first identified compaction bands as “thin planar zonesof pure compressional deformation,” oriented perpen-dicular to maximum compression. Besuelle (2001), andDu Bernard, Eichhubl and Aydin (2002) recently re-ported dilation bands (oriented perpendicular to mini-mum compression) in laboratory and field settings, re-spectively. Rudnicki and Rice (1975) modeled strain lo-calization as a bifurcation from homogeneous deforma-tion using a single yield surface model to describe shearlocalization in low porosity rock. However, recent reex-aminations of this model reveal that predicted band ori-entations do not agree with experimental observationsof compaction bands in high porosity sandstone. Mi-crostructural observations by Menendez, Zhu and Wongsuggest multiple active damage processes, promptingdevelopment of a two yield surface model by Issen todescribe strain localization in high porosity sandstone.The first yield surface corresponds to a dilatant, fric-tional mechanism, while the cap corresponds to a com-pactant mechanism. This model successfully predictsthe experimentally observed compaction bands underaxisymmetric compression (ASC) when the slope of ef-fective mean stress-inelastic volume strain curve is zeroor slightly positive, corresponding to the stress plateaucharacteristic of compaction band formation. Deter-mining conditions for dilation band formation underaxisymmetric extension (ASE) using the two yield sur-face model is facilitated by certain mathematical sym-metries with compaction band conditions for ASC. Theconditions for dilation band formation though complex,depend largely on the dilation coefficient and slope ofthe shear yield surface with dilation bands being pre-dicted for a wide range of probable material parame-ter values. These conditions are less restrictive thanthe analogous conditions for compaction band forma-tion under ASC, suggesting that dilation band forma-tion could be a common deformation mode for highporosity sandstone. Furthermore, since multiple stresspaths are possible for a single stress state (e.g., ASCor ASE), the chosen stress path defines the region ofthe yield surface activated (and therefore the appropri-ate constitutive model), thus influencing whether thelocalization condition can be satisfied. This may haveimportant implications for band formation in geologicalsettings.


Effect of Confining Pressure onCompaction Localization in NotchedSamples of Bentheim Sandstone:Experimental Observations and FiniteElement Modeling

Veronika Vajdova1 ((631)632-8302;[email protected])

Teng-fong Wong1 ((631)632-8212;[email protected])

Vennela Challa2 ([email protected])

Kathleen A Issen2 ([email protected])1SUNY Stony Brook, Department of Geosciences,

SUNY SB, Stony Brook, NY 11794-2100, UnitedStates

2Clarkson University, Mechanical and Aeronauti-cal Engineering, Potsdam, NY 13699-5725, UnitedStatesIn tectonic settings the coupled development of

compaction and strain localization may significantlyimpact the stress field, strain partitioning and fluidflow, and therefore it is desirable to have a better un-derstanding of how such localization develops at var-ious burial depths. Field studies indicate that com-paction localization may develop due to structural andstress heterogeneity. In a previous laboratory study toinvestigate these phenomena a stress concentration wasintroduced by a V-shaped circumferential notch in acylindrical sample of Bentheim sandstone and conven-tional triaxial experiments were conducted at the con-fining pressure of 300 MPa. Our acoustic emission andmicrostructure data indicated that discrete compactionbands initiated from the notch tips and propagated bysequential increments as ”anti-cracks”. The transversepropagation of a compaction band was inferred to befaster than the axial displacement rate by 2 orders ofmagnitude. Energy dissipated for compaction band for-mation was estimated to be comparable to the shearfracture energy for shear band propagation. Guidedby experimental observations, a finite element analy-sis was conducted to simulate the initiation and evolu-tion of compaction localization. The ABAQUS modelwas developed using a Drucker-Prager with cap con-stitutive model, and the numerical simulations confirmthat a stress concentration exists at the notch causinga stress state favoring an axially compacted zone to ex-tend perpendicular to the maximum compressive stress.To clarify the pressure effect we conducted additionalexperiments at confining pressures of 250 and 350 MPa.Our mechanical data show that the critical stress forthe initiation of a compaction band from a notch tipdecreased with increasing confining pressure, similar tothe yield stress for an unnotched sample that maps outa cap with negative slope in the stress space. Differen-tial stress vs. axial strain plots from numerical simu-lations support the experimental observation that theyield stress shows negative pressure dependence. Pre-liminary microstructural observations indicate a similarfailure mode for notched samples at the three differentconfining pressures. Synthesis of the experimental andnumerical results can provide useful constraints on thestress singularity at the notch tip and how it influencesdevelopment of compaction localization.


Anisotropic schist foliation orientationdetermined using time domainelectromagnetics

Jamie L Collins1 ([email protected])

Mark E Everett1Texas A&M University, Dept. of Geology and Geo-

physics Texas A&M University, College Station, TX77843-3115, United States

Remote detection of metamorphic rock formationsbeneath sedimentary cover using geophysical methodswould greatly assist efforts of geologists to study re-gional tectonics. Metamorphic rocks are often charac-terized by foliations oriented in a preferred direction.The foliations may generate anisotropic physical prop-erties. Electromagnetic methods can be used to detectanisotropy in electrical conductivity at depth withinthe earth. Around 1.1 billion years ago the Llano upliftexposed Precambrian igneous and metamorphic rocksin central Texas. In centrally located Mason, Texasthe uncovered Packsaddle Schist maintains the charac-teristic preferred foliation orientation of metamorphicrocks, providing an ideal survey region. Time domainelectromagnetic surveys with a 20 - 40 meter offset loopconfiguration were conducted azimuthally in the Pack-saddle Schist. A transmitter loop with a five meterradius was centrally located. Voltage induced within areceiver loop was recorded over time and plotted on 360degree polar azimuthal graphs. The graphs consistentlyshow elliptical voltage responses over time with largervoltage readings trending northwest-southeast at earlytime. Larger voltage readings within the elliptical re-sponse correspond to slower decay of the induced volt-

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2003 Fall Meeting

age and lower apparent conductivity. Further interpre-tation of electromagnetic responses in anisotropic me-dia involves consideration of the paradox of anisotropy.A paradox of anisotropy occurs when apparent conduc-tivity is larger across strike than along strike due tolocal induced electric current flow control of the re-sponse. Accordingly, lower apparent conductivity val-ues correlate with the along strike direction. There-fore, the graphs generated by electromagnetics showthe orientation of the Packsaddle Schist foliation trendsnorthwest-southeast. Additional DC resistivity surveysof the same region show higher apparent resistivityoriented more subtly northwest-southeast. Under theparadox of anisotropy, higher apparent resistivity, orlower apparent conductivity, correlates with the alongstrike direction of the foliation. DC resistivity supportsthe same preferred foliation orientation found based ontime domain electromagnetics. Utilizing time domainelectromagnetics with an understanding of the para-dox of anisotropy provides an effective and noninvasivemeans to detect the foliations characteristic of meta-morphic rocks and determine the preferred orientationof the foliations.


Impact of tectonic stresses oncompaction in the toe thrust region ofa Tertiary Delta

Tina G Fitts1 (713-431-4534;[email protected])

Joann Welton1 (713-431-6940;[email protected])

James M DeGraff2 (281-654-5116;james.mdegraff@exxonmobil)

1ExxonMobil Upstream Research Company, 3120 Buf-falo Speedway, Houston, TX 77252-3189, UnitedStates

2ExxonMobil Exploration Company, P.O.Box 4778,Houston, TX 77210-4778, United States

Increased mean and shear stresses during toe-thrusting can accelerate compaction within thrustsheets, even away from bounding faults. Tectoniccompaction in the toe-thrust regime is inferred whenthrusted sediments appear to be anomalously consol-idated compared to the same materials sampled fromundeformed sections. Elevated mean stresses withinthrust sheets can be estimated from the degree of over-consolidation of thrusted sediments relative to thosecompacted in near-uniaxial and neutral tectonic set-tings. Core data (sands) and well logs (shales) alonga set of toe-thrusts of a Tertiary Delta have been usedto describe the mechanism for compaction and esti-mate the magnitude of deviation from the normal trendin undeformed areas. Compaction mechanisms withinthrust sheets appear to be dominated by grain rear-rangement rather than cataclasis, which appears to oc-cur only locally in thrust fault zones based on coredata. Estimates of horizontal/vertical stress ratios(<1.5) compare favorably with estimates based on thelow thrust angle and mechanical rock property rangesassuming relatively weak rocks in this shale-prone sec-tion. Results based on physical properties provide anarrower stress range than would be derived from thepotential range of mechanical properties and structureinputs alone.


Fundamentally Different FailureMechanisms Around Boreholes in twoHigh Porosity Sandstones

Bezalel Haimson1 (608-262-2563;[email protected])

HiKweon Lee1 (608-265-3021; [email protected])1Dept. of Materials Science and Engneering and Geo-

logical Engineering Program, University of Wiscon-sin, 1509 University Avenue, Madison, WI 53706-1595, United States

We compare the shape and mechanism of failurearound vertical boreholes drilled in blocks of two high-porosity sandstones subjected to unequal far-field prin-cipal stresses. Tablerock sandstone has a porosityof 28%, and is composed of 55% quartz and 37%weaker feldspar grains. Grain cementation is substan-tial through microcrystalline quartz. Critical far-fieldstresses induce failure around boreholes in the form ofV-shaped (dog-eared) breakouts, the result of dilatantintra-and trans-granular microcracking subparallel toboth the maximum horizontal far-field stress and tothe borehole wall. No localized deformation ahead ofthe breakout tip is observed. On the other hand, bore-holes in Mansfield sandstone, which has similar poros-ity (26%), but contains mainly quartz grains (90%)held together primarily by spot-sutured contacts, failby developing fracture-like breakouts. These are longand very narrow (several grain diameters) tabular fail-ure zones perpendicular to the maximum stress. Evi-dence provided mainly by SEM observations suggests a

failure process initiated by localized grain-bond loos-ening along the least horizontal far-field stress spring-line, the packing of these grains into a lower porositycompaction band resembling those discovered in Navajoand Aztec sandstones, and the emptying of the loosenedgrains by the circulating drilling fluid starting from theborehole wall. Although the immediate several grainlayers at the breakout tip often contain some cracked oreven crushed grains, the failure mechanism enabled bythe formation of the compaction band is largely non-dilatant, a major departure from the dilatant mecha-nism considered typical for rocks. The experimentalresults suggest that unlike our previous assertion, thetype of grain bonding and mineral composition, and notthe porosity, are major factors in the formation of com-paction bands and the ensuing fracture-like breakouts.


Potential field Modeling of the 3-DGeologic Structure of the SanAndreas Fault Observatory at Depth(SAFOD) at Parkfield, California

Darcy K McPhee (650-329-4173; [email protected])

U.S. Geological Survey, MS 989, 345 Middlefield Rd.,Menlo Park, CA 94025, United States

Gravity and magnetic data, along with other geo-physical and geological constraints, are used to develop2-D models that we use to characterize the 3-D geolog-ical structure of the San Andreas fault (SAF) zone inthe vicinity of SAFOD near Parkfield, CA. The grav-ity data, reduced to isostatic anomalies, comprise acompilation of three different data sets with a maxi-mum of 1.6 km grid spacing for the scattered data andclosely spaced (∼40 m) stations along one SW-NE pro-file crossing the SAFOD pilot hole. Aeromagnetic datawere flown at a nominal 300 m above the terrain alongSW-NE flight lines perpendicular to the San AndreasFault. Data were recorded at ∼50 m spacing alongflight lines approximately 800 m apart. Ground mag-netic data recorded every 5 m along lines ∼300 m apartcover a 3 x 5 km area surrounding the SAFOD pilothole. Previous modeling showed that magnetic graniticbasement rocks southwest of the SAF are divided by aninferred steep fault sub-parallel to the SAF. We com-pute 2-D crustal models along 5 km-long southwest-northeast profiles, one of which extends through theSAFOD pilot hole near and along the high-resolutionseismic refraction/reflection survey completed in 1998(Catchings et al., 2002). Our models are constrained bypilot hole measurements, where we see a boundary be-tween sediment and granitic basement at ∼770 m andan order of magnitude increase in magnetic susceptibil-ity at ∼1400 m, possibly the same depth at which theSW dipping Buzzard Canyon Fault intersects the pilothole. Regional gravity, magnetic and geologic data in-dicate two very distinct basement blocks separated bya steeply dipping SAF. The shallowly dipping sedimen-tary section SW of the SAF coincides with the low ve-locity zone observed with seismic measurements. Shal-low slivers of magnetic sandstone on the NE side ofthe SAF explain higher frequency features in the mag-netic data. In addition, we show a flat lying, tabu-lar body of serpentinite sandwiched between 2 blocksof Franciscan rock on the NE side of and truncatingat the SAF. The Salinian granitic rocks to the SW ofthe SAF contain a magnetic body of unknown originfurther to the SW and overlay a high density graniticroot, possibly a deeper, denser phase of granitic rock.Furthermore, a shallow magnetic body SE of the SAF,possibly a sliver of granitic rock or serpentinite, givesrise to a significant NW trending high on the groundmagnetic map parallel to the SAF. We use 2-D modelsboth NW and SE of the profile that extends throughthe SAFOD pilot hole to explore the 3-D nature of theunknown magnetic bodies SW of the SAF and their sig-nificance to future drilling at SAFOD as well as to otherfaults sub parallel to the SAF. Our models are the basisfor a 3-D digital model of the upper crust surroundingSAFOD that will act as a tool for directly comparingquantitative subsurface interpretations based on vari-ous methods including seismic refraction and reflection,seismicity, magnetotelluric, gravity, magnetic, and geo-logic techniques throughout the lifetime of the SAFODproject.


”Intelligent design” of a 3D reflectionsurvey for the SAFOD drill-hole site

Gabriel Alvarez1 (650 724 0461;[email protected])

John A Hole2 ([email protected])

Simon L Klemperer1 ([email protected])

Biondo Biondi1 ([email protected])

Matthias Imhof2 ([email protected])1Department of Geophysics, Stanford University,

Stanford, CA 94305-2215, United States

2Virginia Polytechnic Institute, 4044 Derring Hall,Blacksburg, VA 24061-0420, United States

SAFOD seeks to better understand the earthquakeprocess by drilling though the San Andreas fault (SAF)to sample an earthquake in situ. To capitalize fullyon the opportunities presented by the 1D drill-holeinto a complex fault zone we must characterize thesurrounding 3D geology at a scale commensurate withthe drilling observations, to provide the structural con-text to extrapolate 1D drilling results along the faultplane and into the surrounding 3D volume. Excellentactive-2D and passive-3D seismic observations com-pleted and underway lack the detailed 3D resolution re-quired. Only an industry-quality 3D reflection surveycan provide c. 25 m subsurface sample-spacing hor-izontally and vertically. A 3D reflection survey willprovide subsurface structural and stratigraphic con-trol at the 100-m level, mapping major geologic units,structural boundaries, and subsurface relationships be-tween the many faults that make up the SAF faultsystem. A principal objective should be a reflection-image (horizon-slice through the 3D volume) of thenear-vertical fault plane(s) to show variations in phys-ical properties around the drill-hole. Without a 3D re-flection image of the fault zone, we risk interpretingdrilled anomalies as ubiquitous properties of the fault,or risk missing important anomalies altogether. Sucha survey cannot be properly costed or technically de-signed without major planning. ”Intelligent survey de-sign” can minimize source and receiver effort withoutcompromising data-quality at the fault target. Suchoptimization can in principal reduce the cost of a 3Dseismic survey by a factor of two or three, utilizingthe known surface logistic constraints, partially-knownsub-surface velocity field, and the suite of scientific tar-gets at SAFOD. Our methodology poses the selection ofthe survey parameters as an optimization process thatallows the parameters to vary spatially in response tochanges in the subsurface. The acquisition geometryis locally optimized for uniformity of subsurface illumi-nation by a micro-genetic algorithm. We start by ac-curately establishing the correspondence between thesubsurface area of the target reflector (in this case, thesteeply-dipping SAF) and the part of the surface areawhose sources and receivers contribute to its image us-ing 3D ray-tracing. We then use dense acquisition pa-rameters in that part of the survey area and use stan-dard parameters in the rest of the survey area. This isthe key idea that allows us to get optimum image qual-ity with the least acquisition effort. The optimizationalso requires constraints from structural geologists andfrom the community who will interpret the results. Themost critical parameters to our optimization processare the structural model of the target(s) (depth and ge-ological dips) and the velocity model in the subsurface.We seek community input, and have formed a scientificadvisory committee of academic and industry leaders,to help evaluate trade-offs for the community betweencost, resolution and volume of the resultant data-set,and to ensure that an appropriate range of piggy-backexperiments is developed to utilize the seismic sourcesavailable during the 3D experiment. The scientific out-put of our project will be a community-vetted designfor a 3D reflection survey over SAFOD that is techni-cally feasible, cost-effective, and most likely to yieldthe image and seismic parameter measurements thatwill best constrain the physical properties of the faultzone and their spatial variation.


Comparison of SAFOD Pilot Holephyllosilicate mineral assemblages tothe Punchbowl fault: Recognizingpost-faulting alteration in exhumedfault zones

John G Solum1 (734-647-2157; [email protected])

Ben A van der Pluijm1 (734-763-0373;[email protected])

1University of Michigan, Department of GeologicalSciences 425 E University Ave. 2534 C.C. LittleBldg, Ann Arbor, MI 48109-1063

The chlorite assemblages in cuttings from two sam-pled intervals of the SAFOD Pilot Hole (488-914m and1585-2012 m) can be separated into two populationsbased on X-ray diffraction characteristics. The first ischaracterized by peaks with a width of ∼0.1-0.3 ◦2θand a ratio of the area of the 002 chlorite peak to the001 chlorite peak of ∼0.3-4.4, while the second exhibitspeak widths of ∼0.2-1.0 ◦2θ and a peak area ratio of∼0.1-0.6. The first population occurs in the deeper in-terval, and rarely in the shallower, while the secondpopulation occurs only in the shallower interval. Thedifference in peak area ratio indicates a difference inchemistry (most likely in octahedral iron and magne-sium), and the difference in peak width indicates thatthe deeper samples have larger crystallites and fewerexpandable interlayers than the shallower population.The X-ray characteristics for the deeper populationmatch those for samples from protolith and cataclasitefor the exhumed Punchbowl fault, while samples fromthe intensely-deformed ultracataclasite are dissimilarto that population. This supports previously publishedfindings, based on scanning and transmission electron

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microscopy as well as X-ray techniques, that the min-eral assemblages in the ultracataclasite formed afterthe cessation of motion along the fault, and that fault-related mineral assemblages have been overprinted bypost-faulting alteration. This illustrates how detailedcharacterizations of the phyllosilicate mineral assem-blages in the SAFOD Pilot Hole can be used to con-strain interpretations of exhumed fault zones by estab-lishing a baseline characterization of phyllosilicate min-eral assemblages at depth. This allows for the recog-nition of fault-related and non-fault-related phyllosil-icates in exhumed fault zones, aiding interpretationsof fault strength and permeability structure, both ofwhich can be strongly influenced by phyllosilicates.


Predicting Macroscale PhysicalProperties Using Microscale ImageData

J T Fredrich (505-844-2096; [email protected])

Sandia National Laboratories, MS 0750, Albuquerque,NM 87185-0750, United States

Geologic materials, including tight crystalline rocks,shales, and weakly consolidated sandstones and lime-stones, exhibit geometrically complex microscale struc-tures that control physical and mechanical propertiesat the macroscale. The past decade has seen remark-able development of several new techniques that en-able high-resolution three-dimensional imaging of thepore structure of complex geomaterials. This, coupledwith advances in numerical simulation methods, com-puter hardware, and development of fast computer ar-chitectures, provides unprecedented opportunities forthe prediction of bulk physical and/or mechanical prop-erties directly from microscale image data. We presentdata obtained using the two highest fidelity methodsfor 3D imaging, synchrotron computed microtomogra-phy and laser scanning microscopy, and discuss the ad-vantages and disadvantages that each method presentsin the specific context of microscale imaging and sub-sequent use of 3D image data in numerical simulations.We also contrast the application of these modern tech-niques with conventional serial sectioning techniques.We directly apply the image data in massively par-allel numerical simulations of single phase fluid flow.Using data obtained for several natural and syntheticsandstones at a range of resolutions and encompass-ing different solid volumes, we explore fundamental is-sues related to representative volumes and length scalesnecessary to characterize geometrically complex porousmedia and enable accurate prediction of physical prop-erties at the macroscale. This work was performed atSandia National Laboratories funded by the US DOEunder Contract DE-AC04-94AL85000. Sandia is a mul-tiprogam laboratory operated by Sandia Corporation,a Lockheed Martin Company, for the United States De-partment of Energy.

T41E MCC: 2002-2004 Thursday1020h

At the Seismogenic Front: DynamicProcesses at Convergent Margins II(joint with G, S)

Presiding: S Bilek, New MexicoInstitute of Technology; Y Liu,Harvard University

T41E-01 1020h INVITED

Friction Mechanics at the Updip Limitof Seismogenic Faulting AlongSubduction Megathrusts.

Chris Marone1 (814-865-7964; [email protected])

Demian M. Saffer2 (307-766-2981; [email protected])1Dept. of Geosciences, Penn. State Univ. 536 Deike

Bld., University Park, PA 16803, United States

2Dept. of Geology and Geophysics, Univ. ofWyoming, Laramie, WY 82071, United States

The mechanical behavior of plate boundary faultzones can be divided into three main zones: a deepaseismic zone, the seismogenic zone, and an updipaseismic zone. Identifying and understanding the sta-bility transitions from seismic to aseismic faulting arekey goals in understanding subduction zone megath-rusts. We focus on the mechanics and frictional prop-erties of the upper stability transition from stable tounstable faulting. Two hypotheses for the updip limitof subduction seismicity have been proposed. Theclay mineral hypothesis posits that a thermally- driven

transition from dominantly smectite to dominantly il-lite clay produces a transition from aseismic to seis-mic behavior. The consolidation/lithification hypothe-sis posits that the stability transition is the result of achange from distributed granular shear, in which aseis-mic behavior is related to grain crushing, consolidation,and strain- rate dependent dilatancy, to localized shearwithin highly consolidated material, for which unsta-ble friction behavior results from properties of adhesivecontact junctions. We summarize laboratory frictiondata and constitutive laws in the context of require-ments for unstable faulting. We report on laboratoryexperiments designed to investigate the frictional be-havior of smectite-illite clays and clay-quartz mixtures,with emphasis on processes that control frictional sta-bility. Double-direct shear friction experiments werecarried out on powders (2-500 5 µm) at normal stressesfrom 5-150 MPa, sliding velocities from 0.1-200 µm/s,and shear strains up to 20 at room temperature. Wefind that the coefficient of friction (µ) is 0.42-0.68 forillite shale, consistent with previous work. Over the fullrange of conditions studied, illite shale exhibits onlyvelocity-strengthening behavior, opposite to the widelyexpected, potentially unstable velocityweakening be-havior assumed in the clay mineral hypothesis. Smec-tite sheared under identical conditions exhibits low fric-tion (µ = 0.15-0.32) and a transition from velocityweakening at low normal stress to velocity strength-ening at higher normal stress (>35 MPa). Our datasuggest that the transformation of smectite to illite re-sults in an increase in friction, but do not support thehypothesis that the smectite-illite transition is respon-sible for the seismic-aseismic transition in subductionzones. We show that mixtures of smectite and quartzundergo a transition from adhesive frictional behavior,in which contact junctions exhibit time-dependent be-havior and friction exhibits rate and state properties,to viscous behavior in which shear strength is purelyrate dependent. We suggest that processes, such as ce-mentation, consolidation, and slip localization, play animportant role in determining the updip limit of theseismogenic zone in subduction zones, and that theseprocesses, in addition to clay mineralogy, should be thefocus of future investigations.

T41E-02 1035h

The Upper Aseismic to SeismicTransition: A Silica MobilityThreshold

Christie D Rowe1 (831-459-2762; [email protected])

J. Casey Moore1 (831-459-2574; [email protected])1Dpt. Earth Sciences, UC Santa Cruz 1156 High St,

Santa Cruz, CA 95064, United States

The up-dip portions of accretionary subductionzone decollements slide stably and are therefore aseis-mic, but become seismogenetic at a depth of 5-15km. Thermal models of modern subduction zones andaccretionary wedges predict that the aseismic-seismictransition occurs at 100-150◦C. This correlation be-tween temperature and the onset of seismogenesis sug-gests that fault behavioral properties are modified bya diagenetic-metamorphic reaction affecting the faultzone mineralogy. The Kodiak Accretionary Complex,Alaska, is a well-exposed sediment wedge associatedwith Mesozoic through recent Aleutian subduction. Wecompare two tectonic units that were subducted, oneto just above the seismogenic transition, and one towithin the seismogenic zone. The Eocene rocks weresubducted to approximately 2.4-3.9 km and experi-enced temperatures of 100-125◦C before accreting intothe wedge. The Paleocene rocks subducted to 10-14 km (280-320 MPa) and reached 215-290◦C. Bothformations host disrupted zones interpreted as paleothrust faults by previous authors, which were proba-bly associated with paleodecollement systems. Quartzcementation is rare in the paleo-thrust faults of theEocene rocks but ubiquitous and extensive in the paleo-thrust faults of the Paleocene rocks, in fault-paralleland fault-crossing geometries. We suggest that the for-mation of a quartz network along and across fault zonesmay cause the onset of seismogenesis. The frictional be-havior of sheet silicates is generally velocity strength-ening, resulting in stable sliding behavior, while quartzexhibits velocity weakening, or stick-slip frictional be-havior. Thus, the aseismic-seismic transition may becontrolled by quartz mobility and the appearance ofvolumetrically significant quartz ± calcite precipitatesfilling, coating, and cementing fault surfaces, creating“deadbolts” across slip surfaces, establishing frictionalcontrol over surfaces whose properties were previouslycontrolled by sheet silicates.

T41E-03 1050h

Seismic activity in the Japan Trenchforearc from network observation inthe seafloor

Eiichiro Araki1 ([email protected]); SelwynSacks1; Alan Linde1; Toshihiko Kanazawa2;Masanao Shinohara2; Hiromi Fujimoto3; RyotaHino3; Hitoshi Mikada4; Hiroyuki Matsumoto4;Takeshi Sato4; Kiyoshi Suyehiro4

1DTM, Carnegie Institution of Washington, 5241Broad Branch Rd, NW Washington, DC 20015

2ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo 1130032,Japan

3Tohoku Univ., Tohoku Univ, Aobaku 9808578, Japan

4JAMSTEC, 2-15 Natsushimacho, Yokosuka 2370061,Japan

In the forearc region of off-Sanriku area in theNortheast Japan Arc, subduction of the Pacific platebeneath the arc gives rise to seismogenesis of smallestto M7-class earthquakes. Seismicity in the area showsnon-uniform distribution of earthquakes and its rela-tion to the structural heterogeneity at the plate bound-ary has been suggested. Such a structural heterogene-ity may relate to horst and graven structure of the sub-ducting plate that is prominent in the oceanic crust ofthe Pacific plate before subduction in the area. Thesubducting plate motion is accommodated by deforma-tions and displacements near the plate boundary. Con-dition of the plate boundary is subject to change undersubduction. This, in turn, control the manner of seis-mogenesis and are observed as non-uniform seismicity,transition of earthquake mechanisms, or possible aseis-mic slip in the plate boundary. In order to understandsuch processes, observation of seismic activity to defineprecise location and mechanisms of earthquakes in re-lation to the subducting plate geometry is important.As the occurrence of large earthquakes account for onlyabout 30 % of the plate motion of about 10 cm/y acrossthe Japan Trench, we also need to search for possibleslow slip in the area. Existing land observatories, whichare apart from the area by more than 100km, are in-appropriate for these targets. Therefore, we deployednetwork of seafloor instruments consisted of boreholeseismo-geodetic observatories, seafloor pressure gauges,and arrays of ocean bottom seismometers (OBS) inthe forearc region of off-Sanriku area. Two boreholeobservatories (JT1 and JT2) were installed in 1999.We installed 2 broadband seismographs (CMG1T andPMD2123), tiltmeter (AG510), and a strainmeter. Sen-sors are buried near the hole bottom approx. 1.1km be-low seafloor. With these state-of-art borehole sensorssituated only 10km above the plate boundary, we ex-pect earthquakes and slow crust deformation observedat the highest precision. We have conducted mainte-nance cruises to the borehole observatories, and recov-ered total of a half-year data from JT1 and two sets of3-month data from JT2. The long period performancesof the borehole observatories are good enough for tidaltilting of the boreholes to be observed. The seafloorpressure gauges (Paroscientific 8B7000) deployed ad-jacent to the JT1 and JT2 sites are useful to detectvertical crustal movement as small as a few millime-ters. In June 2003, we recovered the first 8-month datafrom near JT1 site. We started 1-year pressure obser-vation at the two sites from June 2003. Series of OBSarray deployments in 2001, 2002 and 2003 of each sev-eral months enabled us to get detailed image of earth-quakes with plate subduction in this area. In 2002, wedeployed 17 OBS at 7-15km spacing around the JT2site. During the deployment, we also shot airgun along5 seismic lines on the OBS network. In 2003, we focusedon the JT1 site deploying 5 OBS in 5 n.m. separationsfrom the JT1 borehole, and we made simultaneous ob-servation with the JT1 and JT2 borehole. From theyear 2002 OBS dataset, clusters of microearthquakesare clearly imaged near the depth of plate boundary,and a small number of earthquakes are identified be-low the oceanic crust of the subducting plate. The air-gun shot data along with the seismic structure obtainedfrom previous airgun-OBS surveys enables us to corre-late the seismicity with the crustal structure under thenetwork.

T41E-04 1105h

Seismicity along the Nankai troughseismogenic zone: results frommicro-seismicity observations usingocean bottom seismographs

Koichiro Obana1 (+81-45-778-5436;[email protected]); Shuichi Kodaira1

([email protected]); Kimihiro Mochizuki2

([email protected]); Masanao Shinohara2

([email protected]); Kiyoshi Suyehiro3

([email protected]); Yoshiyuki Kaneda1

([email protected])

fm03-T41D

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