Geology

doi: 10.1130/0091-7613(2002)030<0183:PRTMFT>2.0.CO;2 2002;30;183-186Geology

 An Yin Schist in southern California, United StatesPassive-roof thrust model for the emplacement of the Pelona-Orocopia  

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q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; February 2002; v. 30; no. 2; p. 183–186; 4 figures. 183

Passive-roof thrust model for the emplacement of thePelona-Orocopia Schist in southern California,

United StatesAn Yin*

Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California,Los Angeles, California 90095-1567, USA

ABSTRACTThe early Tertiary Orocopia-Chocolate fault in southern California was originally in-

terpreted as a southwest-directed thrust assisting tectonic underplating of the OrocopiaSchist beneath North America during subduction. However, the predicted thrusting di-rection is opposite to that observed for the fault. Although this problem appeared to beresolved by interpreting the Orocopia-Chocolate fault as a normal fault, this alternativeis inconsistent with the fact that neither an early Tertiary breakaway nor early Tertiaryupper plate extension has been observed. I suggest that the fault was a northeast-directedpassive-roof thrust that linked with a southwest-directed basal decollement (the Vincentthrust) during early Tertiary flat subduction of the Farallon plate beneath North America.This model may also explain a similar structural relationship between the Main Centralthrust and the South Tibet detachment fault in the Himalaya.

Keywords: passive roof thrust, fault-bend fold, Pelona, Orocopia, exhumation, erosion.

Figure 1. Tectonic map of southern California, after Law et al. (2001), Tosdal (1990), andSpencer and Reynolds (1990). VF—Vincent fault, OCF—Orocopia-Chocolate fault.

INTRODUCTIONThe origin of the Vincent and Orocopia-

Chocolate faults in southern California hasbeen long debated. These faults juxtapose thePelona-Orocopia Schist below and the NorthAmerican cratonal rocks above (Fig. 1). ThePelona-Orocopia Schist is thought to have de-rived either from a melange complex or partof a forearc basin (Barth and Schneiderman,1996; Jacobson et al., 1996). Although theVincent fault is clearly a ductile thrust (Ehlig,1981; Jacobson, 1997), the origin of the Or-ocopia-Chocolate fault remains unclear. Ki-nematic studies on this fault show a top-to-the-northeast sense of shear (Simpson, 1990;Jacobson and Dawson, 1995), opposite to thatpredicted by the northeastward subductionmodel of Burchfiel and Davis (1981). This ledto the suggestions that the Orocopia-Choco-late fault was either a suture (Haxel and Dil-lon, 1978) or a northeast-directed thrust(Crowell, 1981), but both possibilities wereruled out by later studies (Tosdal, 1990; Ja-cobson and Dawson, 1995). The apparent dis-crepancy between the northeastward-subduc-tion model and the observed fault kinematicsmay be resolved if the Orocopia-Chocolatefault is considered to be an east-directed de-tachment fault (Jacobson et al., 1996). How-ever, as shown herein, this alternative modelis also inconsistent with the regional geology.

The 40Ar/39Ar thermochronology indicates

*E-mail: yin@ess.ucla.edu.

that the Vincent thrust was active at 60–55 Maand its motion stopped after 50 Ma (Groveand Lovera, 1996; Jacobson, 1990). The Or-ocopia-Chocolate fault in the Orocopia Moun-tains was probably active between 41 and 52Ma, as indicated by cooling ages of musco-vites from the footwall (Jacobson, 1990). Inthe Gavilan Hills, 40Ar/39Ar muscovite and bi-otite ages show that the hanging wall and foot-wall of the early Tertiary Orocopia-Chocolatefault ceased its motion ca. 48 Ma (Jacobsonet al., 2001). Rapid cooling from ;350 8C to150 8C at 30–24 Ma in the hanging wall ofthe Orocopia-Chocolate fault in the Gavilan

Hills was attributed to later normal faulting(Jacobson et al., 2001), but could also becaused by middle Tertiary igneous activitiesin the region (Sherrod and Tosdal, 1991).

Jacobson et al.’s (1996) detachment-faultmodel can be tested by examining the originalgeometry of the Vincent and Orocopia-Choc-olate faults, because it predicts the existenceof an early Tertiary breakaway and significanthanging-wall extension in the early Tertiary(Fig. 2A). Palinspastic reconstruction and ob-servations made in the region show no evi-dence for either the presence of an early Ter-tiary breakaway or early Tertiary upper plate

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184 GEOLOGY, February 2002

Figure 2. Models for origin of Vincent and Orocopia-Chocolate faults. A: Detachment-fault model. B: Passive-roof thrust model.P-Osch—Pelona-Orocopia Schist, VF—Vincent fault, OCF—Orocopia-Chocolate fault.

Figure 3. Early Tertiarypaleogeologic map ofsouthern California andwestern Arizona afterremoving middle andlate Tertiary extensionand strike-slip faulting(Richard, 1993). Mapcombines reconstruc-tions of Ehlig (1981)and Hall (1991) withconsideration of Pow-ell’s (1993) estimates ofrotation for San Gabrieland Orocopia-Choco-late. Note that Pelona-Orocopia Schist wasnot exposed at this time(Ehlig, 1981).

extension in the region (Ehlig, 1981; Richard,1993) (Fig. 3).

PASSIVE-ROOF THRUST MODELThe Pelona-Orocopia Schist is exposed

along the axial zone of the .30-km-wide and200-km-long Chocolate Mountains anticlinor-ium (Figs. 1 and 3) (Richard, 1993). It trendseast and is defined by the Pelona-OrocopiaSchist in the core and Precambrian gneiss, Ju-rassic metasedimentary rocks, and Jurassic toLate Cretaceous plutonic rocks along its twolimbs (Haxel et al., 1985). The involvement ofLate Cretaceous plutons suggests that the an-ticlinorium formed in the Cenozoic. The pre-Cenozoic crystalline and metasedimentaryrocks in the hanging wall of the Orocopia-Chocolate fault along the crest of the anticli-norium are depositionally overlain by Oligo-cene-Miocene volcanic and clastic sedimentaryrocks (Haxel et al., 1985; Richard, 1993). Thelack of early Tertiary sedimentation on top ofthe anticlinorium implies, but does not re-quire, that the region was a structural high,possibly resulted from the formation of the an-ticlinorium in the early Tertiary. The anticli-norium was cut by mid-Tertiary normal faultsin the Chocolate Mountains (Sherrod and Tos-dal, 1991), suggesting that it formed in theearly Tertiary. The broad Chocolate Moun-

tains antiform was clearly superposed by lateTertiary folding, as indicated by short wave-length folds involving Miocene strata in theeastern Orocopia Mountains (Crowell, 1975;Law et al., 2001).

The development of the anticlinorium mayhave been associated with marine sedimenta-tion of the Paleocene (ca. 66–57 Ma) SanFrancisquito Formation in the San GabrielMountains (Ehlig, 1981; Grove, 1993) and the

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GEOLOGY, February 2002 185

Figure 4. Emplacement history of Pelona-Orocopia Schist. A: Between 85 and70 Ma, Big Maria fold belt (MFB) and Mule Mountains thrust (MMT) developed,causing sediments to be shed westward from arc. B: Between 70 and 65 Ma,forearc-basin sediments and melange complex were underthrust beneath NorthAmerica during flat subduction of Farallon plate. C: Between 65 and 50 Ma, fault-bend fold was generated above new east-dipping thrust ramp and was extrudedwestward. Extruded thrust sheet is bounded at top by Orocopia-Chocolate fault(OCF) acting as a passive roof thrust.

lower to middle Eocene (ca. 57–43 Ma) Man-iobra Formation in the eastern OrocopiaMountains (Fig. 2; Crowell, 1975). The twoformations could have been originally part ofa forearc basin along the western edge ofNorth America that was locally partitioned bythe anticlinorium into the western and easternsubbasins (Fig. 3). The eastern subbasin waseventually filled up above sea level betweenlate Eocene and early Oligocene time, as rep-resented by the sedimentation of the Oligo-cene-Miocene Diligencia Formation above theManiobra Formation (Crowell, 1975).

If development of the anticlinorium was re-lated to early Tertiary deep-seated faulting, theassociated fault must have the same age,length, and trend. The only structure with acomparable size and age is the Vincent-Orocopia-Chocolate fault system. Its spatialrelationship to the antiform (Fig. 3) suggeststhat the two could have been kinematically re-lated in a fault-bend fold system (Fig. 2B).That is, the Orocopia-Chocolate fault is a pas-sive roof thrust, which began as a southwest-directed thrust and was reactivated later as anortheast-directed roof thrust. The Vincentthrust could be both a part of the basal thrustand the northeast-directed passive-roof thruston the forelimb of the fault-bend fold. In thiscase, the passive roof thrust (Orocopia-Choc-olate fault) was connected with a southwest-directed decollement (Vincent thrust) alongwhich the forearc melange complex and pos-sibly part of the forearc basin were subducted(Fig. 4).

The structural model proposed here requiresexhumation of the Pelona-Orocopia Schist byerosion during contraction (Fig. 2B). Apatitefission-track and (U-Th)/He analyses of rocksin the hanging wall of the Vincent thrust showthat the most dominant long-wavelength (.40km) signature is a cooling event that occurredbetween 60 and 40 Ma (Blythe et al., 2000).The similarity of apatite fission-track and (U-Th)/He ages to muscovite and K-feldspar agesin the hanging wall of the Vincent thrust (Ja-cobson, 1990; Grove and Lovera, 1996; Bly-the et al., 2000) suggests that cooling from;350 8C to ;100–60 8C was relatively rapidbetween ca. 60 and 50 Ma. This result impliesthat a .10-km-thick crustal section was re-moved during early Tertiary thrusting alongthe Vincent thrust. Erosional denudation hasbeen shown to be an efficient way of exhum-ing rocks in contractional orogenic belts(Schneider et al., 1999; Yin and Nie, 1993).

DISCUSSIONPassive roof thrusts are common in foreland

thrust duplexes (Banks and Warburton, 1986)or fault-bend folds (e.g., Yin and Oertel,1993). Clearly, a rigorous application of the

flexural-slip folding mechanism to quantifythe geometry of the inferred fault-bend foldmay not be appropriate, because crystal-plas-tic deformation was involved when the Vin-cent and Orocopia-Chocolate faults developed(Simpson, 1990). Nevertheless, the modelshown in Figures 3B and 4 requires only thatdeformation occurred along discrete shearzones with a flat-ramp geometry. Both the var-iation of schist composition from graywacketo mafic-schist layers (e.g., Ehlig, 1981; Ja-cobson and Dawson, 1995) and the presenceof foliation surfaces could have provided themechanical anisotropy needed for the devel-opment of the inferred fault-bend fold system.Thrusts in plastically deformed crystalline ter-ranes, such as the Main Central thrust systemin the Himalaya, have been shown to followthe geometry of a fault-bend fold (e.g., John-son et al., 2001).

Westward extrusion of the thrust sheet be-low the Orocopia-Chocolate fault (Figs. 3Band 4) could have been caused by westwardspreading of the ductile lower crust from analready thickened region beneath the Big Ma-ria fold belt (;60 km; Spencer and Reynolds,

1990) (Fig. 4). It is also possible that the water-rich schist was significantly lighter and rosealong the subduction zone to initiate motionon the proposed roof thrust (e.g., Chemendaet al., 2000).

Retrograde metamorphism in the lowerplate schist has been cited as major evidencefor the interpretation that the Orocopia-Choc-olate fault is a detachment fault (Jacobson andDawson, 1995). However, retrograde meta-morphism could have occurred during west-ward extrusion of the fault-bend fold if ero-sion rates kept pace with rock uplift. Thefluids derived from prograde metamorphismof the schist could have been transported up-ward along the roof fault, causing retrogrademetamorphism. This explanation requires ret-rograde metamorphism to be synkinematicwith respect to motion on the Orocopia-Choc-olate fault. In this regard, there is no differ-ence in terms of the timing of retrograde meta-morphism between the passive-roof–faultmodel and the detachment-fault model. Alter-natively, the retrograde metamorphism in theupper and lower plates of the Orocopia-Choc-olate fault could have been induced by youn-

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186 GEOLOGY, February 2002

ger normal faults (Fig. 2B). This scenario pre-dicts that retrograde metamorphism postdatesmotion on the Orocopia-Chocolate fault.

Although the roof thrust in my proposedmodel appears to be locally normal slip (i.e.,on the back limb of the fault-bend fold) (Fig.4C), its role is to accommodate horizontalshortening and vertical thickening rather thancrustal thinning as predicted by the detach-ment-fault model. The South Tibet detach-ment fault of Burchfiel et al. (1992) may haveplayed a similar role to the Orocopia-Choco-late fault, assisting crustal thickening of theHimalaya. That is, the High Himalayan crys-talline terrane was once a fault-bend-foldthrust sheet moving along a gentle basal thrust(Main Central thrust). The thrust sheet origi-nated in the lower crust due to southward pushby spreading of the thickened lower Tibetancrust, causing the initiation of its roof fault,the South Tibet detachment system. This pos-sibility may be tested in the future near theHimalayan syntaxes where the connection be-tween the Main Central thrust and the SouthTibet detachment may still be preserved. It isinteresting to note that the tectonic settings forthe Himalayan collisional belt and the Pelona-Orocopia Schist are remarkably similar, bothinvolving flat subduction.

ACKNOWLEDGMENTSCarl Jacobson first introduced me to the problems

of the Pelona-Orocopia Schist and has generouslyshared his thoughts with me in the past two decades.Discussion with Gary Axen, Marty Grove, CraigManning, and Ray Ingersoll has sharpened myideas. I thank Ann Blythe for a helpful review. Thiswork was partially supported by the National Sci-ence Foundation.

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Manuscript received June 15, 2001Revised manuscript received October 10, 2001Manuscript accepted October 25, 2001

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