KINEMATICS DEFORMATION MECHANISMS - Confex
1
Abstract Much remains unknown about how continental margin architecture, built from a prior tectonic rifting event, can affect past and ongoing collisional orogenisis. West-central Taiwan can help us to understand the fundamentals of this problem because it is thought to be the site of a relic passive margin frac- ture zone that is controlling the contemporary uplift patterns of the Hsuehshan Range. We focus on microstructures within an oriented sample to identify the primary deformation mechanisms and kinematics. We found an abundance of clasts showing cataclasis while observing the deformation at grain scale. We found that our observations of the asymmetric microstructures are also consistent with the kinematics of the system observed in the field. The focal point of our study is a single fault within a suite of recently recognized northwest-striking faults cutting Pliocene-Miocene sandstone sequences just southwest of the Hsuehshan Range. The sampled fault material consists of fault rock and cohesive fault rock taken from a meter-scale thick fault zone east of the Anmashan Anticline near Kukuan Taiwan. Thin sections were made normal to the fault plane and both parallel and perpendicular to the slip direction. The bulk of the faults are north-northeast dipping oblique thrust faults and strain inversions suggests the maximum shortening direction to be south- west and northeast. These post-cleavage faults are north-northwest trending nearly normal to a northeast trending regional magnetic high that is believed to mark the edge of full-thickness continental crust northwest of the high. This offset nearly conforms with the topographic break that separates the higher Hsuehshan Range to the northeast from the lowlands of the Puli Basin to the southwest. We infer it to be the northeast facing margin of what appears to be a promontory in the lower plate pointing to the east. This promontory of continental crust in the footwall is now acting as a deformation guide as the trench- fill sediments making up the orogen move northwest in response to collision with the Luzon arc. Angelier et al. 2001: Science Academia Sinica, Taiwan Wang et al. 2002 Petrographic Characterization of Deformation Mechanisms and Kinematics in Post-Cleavage Faults Accommodating Differential Uplift of the Hsuehsha n Range: Taiwan Mark R. Smith, Jon C. Lewis, Timothy Byrne, Jean Crespi, David Mirakian, Chung Huang, Ellen A. Lamont (1) Indiana University of Pennsylvania (2) University of Connecticut KINEMATICS DEFORMATION MECHANISMS 0 0 2.4 cm 2.4 cm 4.3 cm 4.3 cm 0 2.4 cm 4.3 cm TT10.2.13.01 Billet Surface (left) and Cross-Polarized Light (right) TT10.2 TAKEN FROM HANGING WALL WALL Strike 334° Bottom Top 0cm 19.5 cm R R R R R Perpendicular to Slickenlines (Looking Southwest) Perpendicular to Slickenlines (Looking Southwest) Perpendicular to Slickenlines (Looking Southwest) Parallel to Slickenlines (Looking Southeast) Y P Taiw an G IS High :3941 Low :0 0 20 40 60 80 10 M iles – Taiw an G IS High :3941 Low :0 0 2 4 6 8 1 M iles fl Bottom of Hanging Wall Top of Hanging Wall A B C D E F A B C D E F 1B 1A 3A 5A 2A 4A 6A 2B 3B 4B 5B 6B Bottom of Hanging Wall Top of Hanging Wall Bottom of Hanging Wall Top of Hanging Wall Bottom of Hanging Wall Top of Hanging Wall 1km 2km 3km 0km 4km 120°58’ 24°13’ 24°13’ 120°58’ 24°02’ 121°00’ 121°00’ 24°02’ Tahengpingshan Syncline Kuohsing Ancline Kuohsing Fault Shuichanglin Fault Hsiasoan Ancline Lileng Syncline-Peikang Syncline Tapingng Ancline Meiyuan Fault Anmashan Ancline Hengliuhsi Fault Shuichanglin Fault Chungkeng Ancline Paimaoshan Fault Hoping Fault Tapingchiao Fault Maanliao Fault Miocene to Pliocene- Sandstones, shales, mudstones and siltstones Oligocene- Sandstones, shales and slates Holocene Alluvium- Gravel, sand and clay Pleistocene Terrace Deposits- Gravel, sand and clay Pleistocene Toukoshan Formaon- Conglomerate with thin to thick sandstone Pleistocene Toukoshan Formaon- Sandstone intercalated alternaons of sandstone and shale and thin bedded conglomerate River Stream Syncline Ancline Fault Shuantung Fault 0 2.4 cm 4.3 cm R R P R R’ R P Y Y Y S DEFORMATION MECHANISMS KEY (Evidence for near-surface cohesive brittle fault rocks with cohesive cataclasite and pseudotachylyte) A TT10.2.10.01 B TT10.2.13.01 1A) Interpreted to be pseudotachylyte in which the generation sur- face has been reactivated because of the abundance of inclusions. (Deformation mecahanism: cataclasis) 2A) Interpreted to be pseudotachylyte along generation surface con- taining some very small inclusions from the surrounding fault rock. The contact is sharp and separates the shear zone from the fault rock. (Deformation mechanism: ultra-cataclasis) 3A) Strong undulose extiction within quartz crystal parallel to the de- veloping slip plane located between 5A and 3A. (Deformation mechanism: dislocation creep) 4A) Chalcedony crystal showing an interpreted cataclastic fabric close to the generation surface. (Deformation mechanism: cataclasis) 5A) Dynamically recrystallized quartz grain in a less recent active region of the shear zone with polygonal aggregates. (Deformation mechanism: dynamic recrystallization) 6A) Dynamically recrystallized quartz grain in a less recent active region of the shear zone with interlobate aggregates. (Deformation mechanism: dynamic recrystallization) 1B) Cataclastic quartz fabric near reactivated zone with calcite and smaller subgrains infilling the fractures across the entire section and interpreted to have been fractured in-situ. (Deformation mechanism: cataclasis) 2B) Injection of pseudotachylyte with calcite filling in a large area of pore space within the reactivated generation zone. (Deformation mechanism: ultra-cataclasis) 3B) Injection of pseudotachylyte with calcite filling in the boundaries around grains with less pore space. (Deformation mechanism: ultra- cataclasis) 4B) Dynamically recrystallized quartz grain near generation zone that has not been reactivated, further indicating that the zone of interest has re- mained inactive for a prolongued period of time. (Deformation mecha- nism: dynamic recrystallization) 5B) Interpreted to be the youngest generation zone cutting a quartz grain in half leaving a young cataclastic fabric. (Deformation mechanism: cata- clasis) 6B) Tail end of the youngest generation zone revealing pseudotachylyte infiltrating available pore space. (Deformation mechanism: cataclasis) KINEMATICS KEY A) A cataclastic quartz fabric interpreted to be cut and broken by pseu- dotachylyte. (sense of shear: top to the right) B) Abundance of calcite indicating the opening direction of veins inter- preted to be text book stacking. (sense of shear: top to the right) C) A cataclastic quartz fabric interpreted to be cut and broken by pseu- dotachylyte. (sense of shear: top to the right) D) Abundance of calcite indicating the opening direction of veins inter- preted to be text book stacking. (sense of shear: top to the right) E) Quartz grain interpreted to be conjugate with R-shearing and P- shearing revealing the ideal slip directions. The distinct line to the right of the grain is interpreted to be a R’ shear. (sense of shear: top to the right) F) A cataclastic quartz fabric interpreted to be cut and broken by pseu- dotachylyte. Appears to be an injection vein after fracturing occurred. (sense of shear: top to the right) 1B 1A 3A 4A 6A 2B 3B 4B 5B 6B 2A 5A Photographs taken by Jon Lewis GIS Images by Ellen Lamont Crespi et al. (2010) Passchier, C., Trouw, R., 1996 RESULTS AND DISCUSSION The big picture of this research is designated to building a stable case to reveal the existence of a promontory at depth that is a key factor in driving collisional orogenisis in Taiwan. The promontory can easily be identified or interpreted to exist from topographic relief maps and maps showing mag- netic anomaly data. It is enough data to say that something exists at depth, but to say that it is controlling contemporary uplift in the Hseushan range, it is necessary to look at all the evidence with as many different perspectives as possible. This being said, it is my job to examine cohesive brittle fault rock taken from the eastern edge of the promontory to see if; one, faults do exist with the correct orientation (northewest striking) and two, that the faults along this promontory are late-stage and near the surface (brittle). We want to identify the faults as late-stage because it identifies the promontory as being a contributing crustal feature existing today in affecting collisional orogenisis. While examining the micro-structures we find that the kinematics correlate with the kinematics of the regional zone of interest. They all show evidence of reactivated oblique thrusting to the northwest along faults striking to the northwest. We wanted to get a closer look at the micro-structural deformation mechanisms and identify them as brittle. If the grain mass in thin section revealed signs of cataclasis or ultra-cataclasis in-situ, we can interpret the fault rock at grain scale to reflect what the faults are doing on a regional scale. If the faults are indeed brittle, they have responded to low temperature, high pressure situations interpreted as late-stage. While examin- ing the fault rock at grain scale we see what we interpret to be ultra-cataclasis as seen in the deformation mechanisms section. We see cataclastic fabric in quartz grains that are cross-cut regularly throughout the grain mass suggesting that each grain responded to stress in-situ. We interpret the faults to be late stage responding directly to the promontory observed at depth believed to be the edge of full-thickness continental crust from a prior rifting event. REFERENCES AND ACKNOWLEDGEMENTS Academia Sinica, Taiwan, Data management center, Institute of earth sciences. http://dmc.earth.sinica.edu.tw/geophysics/twn_topo.html Angelier, J., Lee, J.C., Chu, H.T., Hu, J.C., Lu, C.Y., Chan, Y.C., Lin, T.J., Font, Y., Deffontaines B. et Tsai, Y.B. (2001). - Le séisme de Chichi (1999) et sa place dans l'orogène de Taiwan. C. R. Acad. Sci. Paris, série IIa, Earth Planet. Sci., 333, 1, p. 5-21, 7 fig. Crespi, J.M., Underwood, H.R., and Chan, Y.-C. (2010). Orogenic curvature in the northern taconic allochthon and its relation to footwall geometry, Geological Society of America, p. 1-13, p. 11 fig. Passchier, C.W., Trouw, R.A.J., (1996) Micro-Tectonics, Springer publishing 1996. Wang, C., Huang, C.-P., Ke, L.-Y., Chien, W.-J., Hsu, S.-K., Shyu, C.-T., Cheng, W.-B., Lee, C.S., and Teng, L.S., 2002, Formation of the Taiwan Island as a solitary wave along the Eurasian continental plate margin: Mag netic and seismological evidence: Terrestrial, Atmospheric and Ocean Sciences, v. 13, p. 339-354. Wu, F.T., Chang, L.S., and Wu, Y.M., 2004, Precisely relocated hypocenters, focal mechanism and active orogeny in central Taiwan, in Malpas, J., Fletcher, C.J., Ali, J.R., and Aitchison, J.C., eds., Aspects of the Tec tonic Evolution of China, Volume Special Publications 226: London, Geological Society of London, p. 333-354. I would like to thank the National Science Foundation for award #0738953 as well as the principle investigator and my advisor, Jon C. Lewis from Indiana University of Pennsylvania for the opportunity and assistance. My research would not be made possible without the help of the Geoscience Department and faculty at IUP. I would like to thank Y-C Chan and J-C Lee of Academia Sinica for their help and for providing a vehicle for our field work. Thank you to Rick Allmendinger for providing the shareware StereoNet. I would also like to thank Tony Gutierrez and Graham Snodgrass from the U.S. Army institue of public health for the high resolution images of TT10.2.07.01 and TT10.2.10.01 made available by use of the Macropod. View of fault zone margin looking along strike toward the southeast View of fault zone margin looking perpendicular to the fault plane toward the southeast Looking at the underside of the hanging wall to the southwest NW SE SE NW SE
Transcript of KINEMATICS DEFORMATION MECHANISMS - Confex
Abstract Much remains unknown about how continental margin architecture, built from a prior tectonic rifting event, can a�ect past and ongoing collisional orogenisis. West-central Taiwan can help us to understand the fundamentals of this problem because it is thought to be the site of a relic passive margin frac-ture zone that is controlling the contemporary uplift patterns of the Hsuehshan Range. We focus on microstructures within an oriented sample to identify the primary deformation mechanisms and kinematics. We found an abundance of clasts showing cataclasis while observing the deformation at grain scale. We found that our observations of the asymmetric microstructures are also consistent with the kinematics of the system observed in the �eld. The focal point of our study is a single fault within a suite of recently recognized northwest-striking faults cutting Pliocene-Miocene sandstone sequences just southwest of the Hsuehshan Range. The sampled fault material consists of fault rock and cohesive fault rock taken from a meter-scale thick fault zone east of the Anmashan Anticline near Kukuan Taiwan. Thin sections were made normal to the fault plane and both parallel and perpendicular to the slip direction. The bulk of the faults are north-northeast dipping oblique thrust faults and strain inversions suggests the maximum shortening direction to be south-west and northeast. These post-cleavage faults are north-northwest trending nearly normal to a northeast trending regional magnetic high that is believed to mark the edge of full-thickness continental crust northwest of the high. This o�set nearly conforms with the topographic break that separates the higher Hsuehshan Range to the northeast from the lowlands of the Puli Basin to the southwest. We infer it to be the northeast facing margin of what appears to be a promontory in the lower plate pointing to the east. This promontory of continental crust in the footwall is now acting as a deformation guide as the trench-�ll sediments making up the orogen move northwest in response to collision with the Luzon arc.
Angelier et al. 2001: Science Academia Sinica, TaiwanWang et al. 2002
Petrographic Characterization of Deformation Mechanisms and Kinematics in Post-Cleavage Faults Accommodating Di�erential Uplift of the Hsuehshan Range: Taiwan
Mark R. Smith, Jon C. Lewis, Timothy Byrne, Jean Crespi, David Mirakian, Chung Huang, Ellen A. Lamont (1) Indiana University of Pennsylvania (2) University of Connecticut
KINEMATICS DEFORMATION MECHANISMS0 0
2.4 cm 2.4 cm
4.3 cm 4.3 cm
0
2.4 cm
4.3 cm
TT10.2.13.01 Billet Surface (left) and Cross-Polarized Light (right)
TT10.2 TAKEN FROM HANGING WALL WALL
Strike334°
Bottom Top
0cm 19.5 cm
R
R
R R R
Perpendicular to Slickenlines (Looking Southwest)
Perpendicular to Slickenlines (Looking Southwest)Perpendicular to Slickenlines (Looking Southwest)
Parallel to Slickenlines (Looking Southeast)
Y
P
Taiw an G IS
High : 3941
Low : 0 0 20 40 60 8010M iles
–
Taiw an G IS
High : 3941
Low : 0
0 2 4 6 81M iles
fl
Bottom of Hanging Wall
Top of Hanging Wall
A
BC D
EF
A
B
C
D
E
F
1B
1A 3A 5A
2A 4A 6A
2B
3B
4B
5B
6B
Bottom of Hanging Wall
Top of Hanging Wall
Bottom of Hanging Wall
Top of Hanging Wall
Bottom of Hanging Wall
Top of Hanging Wall
1km 2km 3km0km 4km
120°58’
24°13’ 24°13’
120°58’
24°02’
121°00’
121°00’
24°02’
Tahe
ngpi
ngsh
an S
yncl
ine
Kuoh
sing
Anti
clin
e
Kuoh
sing
Fau
lt
Shui
chan
glin
Fau
lt H
sias
oan
Antic
line
Lileng Synclin
e-Peikang Syncline
Tapi
ngtin
g Antic
line
Mei
yuan
Faul
t
Anmas
han
Anticli
ne
Hen
gliu
hsi F
ault
Shui
chan
glin
Fau
lt
Chun
gken
g An
ticlin
e
Paim
aosh
an F
ault
Hop
ing
Faul
t
Tapi
ngch
iao
Faul
t
Maa
nlia
o Fa
ult
Miocene to Pliocene- Sandstones, shales, mudstones and siltstones
Oligocene- Sandstones, shales and slates
Holocene Alluvium- Gravel, sand and clay
Pleistocene Terrace Deposits- Gravel, sand and clay
Pleistocene Toukoshan Formation- Conglomerate with thin to thick sandstone
Pleistocene Toukoshan Formation- Sandstone intercalated alternations
of sandstone and shale and thin bedded conglomerate
River
Stream
Syncline
Anticline
Fault
Shua
ntun
g Fau
lt
0
2.4 cm
4.3 cm
R
R
P
R
R’
R
P
Y
Y
Y
S
DEFORMATION MECHANISMS KEY (Evidence for near-surface cohesive brittle fault rocks with cohesive cataclasite and pseudotachylyte) A TT10.2.10.01 B TT10.2.13.01
1A) Interpreted to be pseudotachylyte in which the generation sur-face has been reactivated because of the abundance of inclusions. (Deformation mecahanism: cataclasis)2A) Interpreted to be pseudotachylyte along generation surface con-taining some very small inclusions from the surrounding fault rock. The contact is sharp and separates the shear zone from the fault rock. (Deformation mechanism: ultra-cataclasis)3A) Strong undulose extiction within quartz crystal parallel to the de-veloping slip plane located between 5A and 3A. (Deformation mechanism: dislocation creep)4A) Chalcedony crystal showing an interpreted cataclastic fabric close to the generation surface. (Deformation mechanism: cataclasis)5A) Dynamically recrystallized quartz grain in a less recent active region of the shear zone with polygonal aggregates. (Deformation mechanism: dynamic recrystallization)6A) Dynamically recrystallized quartz grain in a less recent active region of the shear zone with interlobate aggregates. (Deformation mechanism: dynamic recrystallization)
1B) Cataclastic quartz fabric near reactivated zone with calcite and smaller subgrains in�lling the fractures across the entire section and interpreted to have been fractured in-situ. (Deformation mechanism: cataclasis)2B) Injection of pseudotachylyte with calcite �lling in a large area of pore space within the reactivated generation zone. (Deformation mechanism: ultra-cataclasis)3B) Injection of pseudotachylyte with calcite �lling in the boundaries around grains with less pore space. (Deformation mechanism: ultra-cataclasis)4B) Dynamically recrystallized quartz grain near generation zone that has not been reactivated, further indicating that the zone of interest has re-mained inactive for a prolongued period of time. (Deformation mecha-nism: dynamic recrystallization)5B) Interpreted to be the youngest generation zone cutting a quartz grain in half leaving a young cataclastic fabric. (Deformation mechanism: cata-clasis)6B) Tail end of the youngest generation zone revealing pseudotachylyte in�ltrating available pore space. (Deformation mechanism: cataclasis)
KINEMATICS KEYA) A cataclastic quartz fabric interpreted to be cut and broken by pseu-dotachylyte. (sense of shear: top to the right)B) Abundance of calcite indicating the opening direction of veins inter-preted to be text book stacking. (sense of shear: top to the right)C) A cataclastic quartz fabric interpreted to be cut and broken by pseu-dotachylyte. (sense of shear: top to the right)D) Abundance of calcite indicating the opening direction of veins inter-preted to be text book stacking. (sense of shear: top to the right)E) Quartz grain interpreted to be conjugate with R-shearing and P-shearing revealing the ideal slip directions. The distinct line to the right of the grain is interpreted to be a R’ shear. (sense of shear: top to the right)F) A cataclastic quartz fabric interpreted to be cut and broken by pseu-dotachylyte. Appears to be an injection vein after fracturing occurred. (sense of shear: top to the right)
1B1A
3A
4A
6A
2B 3B4B
5B 6B
2A
5A
Photographs taken by Jon Lewis
GIS Images by Ellen Lamont
Crespi et al. (2010)
Passchier, C., Trouw, R., 1996
RESULTS AND DISCUSSION
The big picture of this research is designated to building a stable case to reveal the existence of a promontory at depth that is a key factor in driving collisional orogenisis in Taiwan. The promontory can easily be identi�ed or interpreted to exist from topographic relief maps and maps showing mag-netic anomaly data. It is enough data to say that something exists at depth, but to say that it is controlling contemporary uplift in the Hseushan range, it is necessary to look at all the evidence with as many di�erent perspectives as possible. This being said, it is my job to examine cohesive brittle fault rock taken from the eastern edge of the promontory to see if; one, faults do exist with the correct orientation (northewest striking) and two, that the faults along this promontory are late-stage and near the surface (brittle). We want to identify the faults as late-stage because it identi�es the promontory as being a contributing crustal feature existing today in a�ecting collisional orogenisis. While examining the micro-structures we �nd that the kinematics correlate with the kinematics of the regional zone of interest. They all show evidence of reactivated oblique thrusting to the northwest along faults striking to the northwest. We wanted to get a closer look at the micro-structural deformation mechanisms and identify them as brittle. If the grain mass in thin section revealed signs of cataclasis or ultra-cataclasis in-situ, we can interpret the fault rock at grain scale to re�ect what the faults are doing on a regional scale. If the faults are indeed brittle, they have responded to low temperature, high pressure situations interpreted as late-stage. While examin-ing the fault rock at grain scale we see what we interpret to be ultra-cataclasis as seen in the deformation mechanisms section. We see cataclastic fabric in quartz grains that are cross-cut regularly throughout the grain mass suggesting that each grain responded to stress in-situ. We interpret the faults to be late stage responding directly to the promontory observed at depth believed to be the edge of full-thickness continental crust from a prior rifting event.
REFERENCES AND ACKNOWLEDGEMENTSAcademia Sinica, Taiwan, Data management center, Institute of earth sciences. http://dmc.earth.sinica.edu.tw/geophysics/twn_topo.htmlAngelier, J., Lee, J.C., Chu, H.T., Hu, J.C., Lu, C.Y., Chan, Y.C., Lin, T.J., Font, Y., De�ontaines B. et Tsai, Y.B. (2001). - Le séisme de Chichi (1999) et sa place dans l'orogène de Taiwan. C. R. Acad. Sci. Paris, série IIa, Earth Planet. Sci., 333, 1, p. 5-21, 7 �g.Crespi, J.M., Underwood, H.R., and Chan, Y.-C. (2010). Orogenic curvature in the northern taconic allochthon and its relation to footwall geometry, Geological Society of America, p. 1-13, p. 11 �g.Passchier, C.W., Trouw, R.A.J., (1996) Micro-Tectonics, Springer publishing 1996. Wang, C., Huang, C.-P., Ke, L.-Y., Chien, W.-J., Hsu, S.-K., Shyu, C.-T., Cheng, W.-B., Lee, C.S., and Teng, L.S., 2002, Formation of the Taiwan Island as a solitary wave along the Eurasian continental plate margin: Mag netic and seismological evidence: Terrestrial, Atmospheric and Ocean Sciences, v. 13, p. 339-354. Wu, F.T., Chang, L.S., and Wu, Y.M., 2004, Precisely relocated hypocenters, focal mechanism and active orogeny in central Taiwan, in Malpas, J., Fletcher, C.J., Ali, J.R., and Aitchison, J.C., eds., Aspects of the Tec tonic Evolution of China, Volume Special Publications 226: London, Geological Society of London, p. 333-354.
I would like to thank the National Science Foundation for award #0738953 as well as the principle investigator and my advisor, Jon C. Lewis from Indiana University of Pennsylvania for the opportunity and assistance. My research would not be made possible without the help of the Geoscience Department and faculty at IUP. I would like to thank Y-C Chan and J-C Lee of Academia Sinica for their help and for providing a vehicle for our �eld work. Thank you to Rick Allmendinger for providing the shareware StereoNet. I would also like to thank Tony Gutierrez and Graham Snodgrass from the U.S. Army institue of public health for the high resolution images of TT10.2.07.01 and TT10.2.10.01 made available by use of the Macropod.
View of fault zone margin looking along strike toward the southeast
View of fault zone margin looking perpendicular to the fault plane
toward the southeast
Looking at the underside of the hanging wall to the southwest
NW
SE
SE
NW
SE
