Evidence for Late Cretaceous Crustal Thinning in the Santa Rosa Mylonite Zone, Southern California

Evidence for Late Cretaceous crustal thinning in the Santa Rosa mylonite zone, southern California

Bradley G. Erskine, H.-R. Wenk Department of Geology and Geophysics, University of California, Berkeley, California 94720

ABSTRACT in general cut it at about 25°-35° (Figs. 1, 2). The Santa Rosa mylonite zone is part of a 100-km-long belt of deformation along A well-developed chloritic breccia zone is the eastern margin of the northern Peninsular Ranges batholith, southern California. commonly present beneath the fault surfaces. In the northern Santa Rosa Mountains, complexly deformed mylonitic plutonic and The upper-plate units above the Asbestos metasedimentary tectonites are cut by a stacked sequence of five low-angle faults. Evidence Mountain fault are composed primarily of from field relationships and the analysis of minor structures, microstructures, and textures moderately deformed granodiorites. Although (preferred orientation) suggest that the coaxial component of deformation ("pure shear") the granodiorites bear a strong lithologic re- dominated over a noncoaxial ("simple shear") component. It is proposed that mylonitic semblance to rocks found in the basement ter- deformation and low-angle faulting, which appear to be kinematically associated, are the rain, several characteristics are anomalous to result of Late Cretaceous extension and thinning of the continental margin. the area. The <5isO values are low compared to the surrounding plutons (see Silver et al., 1979, INTRODUCTION tures indicative of crustal extension that may be p. 99). Further, the granodiorites are magnetite- The Santa Rosa mylonite zone (SRM2') is explained by thinning of the continental margin. bearing, which is not characteristic of the east- part of the eastern Peninsular Ranges mylonite We report here some observations on textures, ern NPRB (Erskine and Marshall, 1982). zone (EPRMZ), a 100-km-long belt of defor- microstructures, and field data that constrain Magnetite-series granitoids that have low 6lsO mation along the eastern margin of the north- the mode of deformation in the SRMZ. values are associated with the western NPRB, ern Peninsular Ranges batholith (NPRB). De- which may represent a source terrain. tailed mapping of its northern part reveals that GEOLOGY The rocks of the SRMZ below the Asbestos the SRMZ is composed of complexly deformed On the basis of degree and style of deforma- Mountain fault are composed of two groups: mylonitic plutonic and metasedimentary tecton- tion, rocks in the area may be divided into (1) an upper metasedimentary sequence called ites cut by a series of low-angle faults. Previous three major groups: "upper-plate" units, the Palm Canyon group that is in fault contact work in the EPRMZ has called for deep-seated "lower-plate" rocks of the SRMZ, and NPRB with (2) a lower group of strongly deformed thrusting to account for the mylonitic deforma- basement. Associated spatially and geometri- plutonic roclc>. The characteristic feature of this tion (Sharp, 1979; Simpson, 1984). Our work cally with the SRMZ is a sequence of five im- unit is the penetrative mylonitic foliation strik- in the San Jacinto and northern Santa Rosa bricate low-angle faults that have been folded ing north-northwest and dipping from horizon- Mountains, however, has shown that the into a complex basin and domal geometry. The tal to about 60°E, and a strong downdip SRMZ and low-angle faults possess many fea- faults are locally subparallel to the foliation but lineation trending about N55°E. Structures within the SRMZ include tight isoclinal folds that are axial planar to the foliation with fold axes parallel to the lineation. Minimum width of the SRMZ is about 7 km. The lower boundary of the SRMZ is grada- tional and occurs primarily in plutonic rocks. No compositional difference can be discerned between mylonitic and moderately foliated granodiorites below the contact, and fabrics within the plutonic basement appear to be un- related to the mylonitic fabric in the SRMZ. Figure 1. Generalized Farther below the SRMZ is a screen of pre- geologic map of Santa batholithic metasedimentary rocks named the Rosa mylonite zone. Desert Divide group (Brown, 1981), which be- Basement terrain: ddg = long to a group of rocks that represent rem- Desert Divide group; prg = Peninsular Ranges gra- nants of the Paleozoic continental shelf (Miller nitics. Rocks of SRMZ: and Dockum, 1983). In contrast to the SRMZ, pprg = porphyroclastic the Desert Divide group dips steeply to the east Peninsular Ranges granit- and contains a southeasterly trending lineation ics; pcms = porphyroclas- that is probably related to an earlier deforma- tic Palm Canyon metasedimentary group; tion event. pg = porphyroclastic The low-angle faults may not be westward- gneiss. Upper-plate directed thrusts as proposed by Sharp (1979). granodiorite of Asbestos Mountain = gd. Barbs on One line of evidence is the possible western low-angle faults are on source terrain for the upper-plate granodiorites upper plate. discussed above. Another is the geometry of the

274 GEOLOGY, v. 13, p. 274-277, April 1985

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/13/4/274/3508080/i0091-7613-13-4-274.pdf?casa_token=94w0YNUEHp0AAAAA:rG1QUIPQdKG_xa7W3jotm7G3prfreDbLmXrdxDyJ4n0TBnZDtBLmIYymB7daf6RVanj8 by University of California Berkeley Library user on 06 May 2020 low-angle faults themselves. The faults are not The age of low-angle faulting is poorly con- v< observed to cut the lower SRMZ contact, nor strained. Truncation of the mylonitic foliation are they found within the NPRB basement by these faults show that at least some move- below. They are, however, found within the ment occurred after the formation of the mylo- upper-plate granodiorites. This geometry is sug- nitic foliation, but it is not clear when faulting gestive of a kinematic relationship between the started. The timing of faulting is given indi- "brittle" faulting and the ductile mylonitic fab- rectly by fission-track dates in the northern ric and is supported by a subparallel trend of Santa Rosa Mountains, where apatite, sphene, linear slip-related structures on the fault sur- and zircon ages from the upper-plate granodior- faces with the extensional lineation in the ites and the SRMZ below are concordant at SRMZ. In addition, faulting appears to be about 62 Ma (Dokka, 1984). Given the magni- northeastward-directed rather than southwest- tude of displacement (20 km minimum) and ward as indicated by "drag"-related structures direction (across the K-Ar cooling isochrons) of along some of the fault surfaces. the Asbestos Mountain fault, the concordant cooling dates suggest that the faulting which CONSTRAINTS ON TIMING OF juxtaposed the two units occurred before about MYLONITIC DEFORMATION 62 Ma. AND LOW-ANGLE FAULTING Silver et al. (1979) have shown that the age MICROSTRUCTURAL AND of the NPRB systematically decreases eastward, TEXTURAL ANALYSIS OF the SRMZ occupying the youngest part of the MYLONITIC ROCKS FROM batholith. U-Pb dates from the San Jacinto SANTA ROSA MYLONITE ZONE Mountains suggest that plutonic rocks were Carbonate rocks are widespread in the Palm emplaced at about 97 Ma (Hill, 1981). Because Canyon Group and possess microstructures that ' I ft',:' these rocks are overprinted by the SRMZ, this reflect deformation conditions. Optical micros- date provides an upper limit for the age of de- copy reveals that the transition from a coarse .r/f'fi ' ' formation. K-Ar cooling ages of hornblende marble to a fine-grained mylonite or ultramyl- impose a lower limit on the age of deformation. onite is the result of a complex interaction of Vf/t"' / -. They decrease eastward across the NPRB, many mechanisms. In carbonates, mechanical y,'f/ • / - the northern SRMZ occupying the 70-75 Ma twinning and kinking is followed by recrystalli- isochrons (Krummenacher et al., 1975). Thus, zation along grain boundaries and kink planes currently available geochronologic data suggest (Fig. 3). Transmission electron microscopy re- that deformation within the SRMZ occurred veals that in the coarse- as well as the fine- in the Late Cretaceous between 97 and grained recrystallized samples, mechanical 70-75 Ma. e = {0lT8} twinning and pile-ups of slip-

I'/'f/y, fe/i < 'i

Figure 3. Photomicro- graph of deformed marble from SRMZ. Note complex mechanical twinning and kinking, recrystallization at grain boundaries, and preferred orientation shown by parallelism of twins.

GEOLOGY, April 1985 275

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/13/4/274/3508080/i0091-7613-13-4-274.pdf?casa_token=94w0YNUEHp0AAAAA:rG1QUIPQdKG_xa7W3jotm7G3prfreDbLmXrdxDyJ4n0TBnZDtBLmIYymB7daf6RVanj8 by University of California Berkeley Library user on 06 May 2020 dislocations are common and that there is little 1982). Figure 4B is a simulated pole figure ob- clastic feldspar and sphene along the lineation. evidence of recovery. Even though recrystalliza- tained with the full-constraint Taylor theory for A noncoaxial component is suggested by the tion and perhaps grain boundary sliding may low-temperature mechanisms illustrating good presence of "S-C" bands and related structures. be active, slip and twinning continue to domi- agreement. For comparison, Figure 4C gives a Using the criteria outlined by Simpson and nate the deformation process. simulated pole figure for simple shear and iden- Schmid (1983) the S-C structures indicate an The accommodation of strain by intracrystal- tical mechanisms as in Figure 4B. The pole fig- east over west sense of shear consistent with line slip and mechanical twinning produces pre- ure is monoclinic and displays a 1120 girdle the southern EPRMZ (Simpson, 1984). This ferred orientation that is an indicator of the inclined 40° to the shear plane, which is clearly interpretation, however, is tentative until the strain path during the last major phase of de- different from the calcite mylonite fabrics ob- geometry and origin of the basement fabric formation. Pole figures of calcite in mylonitic served in the SRMZ. Dietrich and Song (1984) overprinted by the SRMZ is better understood. marbles measured by X-ray diffraction in re- have used the angle between the macroscopic The relative importance of simple and pure flection geometry show unusually strong tex- cleavage plane and the 1120 girdle to estimate shear cannot be derived from these structures, tures. The 1120 (a-axis) girdles in marbles the magnitude of the simple shear component but textures again provide important informa- generally lie in the foliation plane and corre- in naturally deformed limestones. Using this tion. Pole figures from quartz bands within the spond to c-axes maxima normal to the folia- method (Fig. 7 of their study), the calcite mylonitic plutonic rocks display strong ortho- tion. Some, such as that shown in Figure 4A, pole figures suggest that the noncoaxial rhombic tex tures with the {1120) = a aligned are inclined up to 10° from the foliation, but component did not exceed about 20% of the on a girdle with maxima disposed symmetri- without a consistent sense. The pole figures are total deformation. cally within a plane that is normal to the folia- thus more or less orthorhombic and have sym- The lack of significant monoclinic distortion tion and containing the lineation (Fig. 5A). metry axes corresponding to the mesoscopic in the bulk of calcite pole figures suggests that Only occasionally do maxima differ in intensity, fabric coordinates. deformation which produced the carbonate which may be due to the component of shear These natural textures are similar to experi- mylonites was primarily coaxial, flattening oc- (Fig. 5B; see also Lister and Hobbs, 1980; mentally produced and theoretically predicted curring normal to the foliaton and extension Fig. lib of Simpson and Schmid, 1983). Pole textures formed under conditions of plane along the lineation. Minor structures, micro- figures of quartz in homogeneous parts of plu- strain, pure shear at low temperatures where e structures, and quartz fabrics indicate that the tonic mylonites are always orthorhombic = {0118} twinning is the dominating deforma- same is probably true for the mylonitic plutonic (Fig. 5C) and provide the strongest evidence tion mechanism, accompanied by minor slip rocks as well. A coaxial component of defor- for the overall coaxial deformation of the my- on r = {10T4} and/= ¡0221} (Wagner et al., mation is suggested by extension of porphyro- lonite belt.

STRUCTURAL EVOLUTION OF SANTA ROSA MYLONITE ZONE Although some structures found in the SRMZ may be attributed to shearing, we think that the dominating mode of deformation in the mylonitic rocks was coaxial extension rather than westward-directed thrusting. The main lines of evidence discussed above are (1) the geometric and possible kinematic relationship of the low-angle faults with the formation of the mylonites, and the probable northeast direction of upper-plate transport; (2) mesoscopic and microscopic structures indicating exten- sions parallel to the well-developed lineation at Figure 4. Calcite 1120 pole figures of carbonate rocks. Upper-hemisphere equal-area projection. Contour interval 0.25 mrd (multiples of a random distribution) with >1.0 mrd shaded. A: all strain levels; and (3) the orthorhombic Mylonitic marble from Palm Canyon group of SRMZ; S, pole to foliation; L, lineation. B: Theo- symmetry of quartz and calcite fabrics and their retically simulated pole figure according to full constraint Taylor theory for low-temperature geometric concordance with fabric coordinates. conditions, pure shear, e = 30%. C: Same as B, but for simple shear, yy = 1.0. Mylonitic deformation as well as the low- angle faults may be explained in part with a model developed by Bott (1971) for deforma- Figure 5. Textures of quartz from mylonitic plutonic group of SRMZ. S = foliation; L = lineation. Projection as in Figure 4; contour interval 2.0 mrd with >1.0 mrd shaded. A: 1120 pole figure of quartzite band within granodiorite mylonite. B: Same as A; different sample. C: 1011 pole figure of quartz in grajiodiorite mylonite. 1011 = r+ zis strong X-ray reflection that provides reliable pole figure in this polymineralic rock.

276 GEOLOGY, April 1985

tion along passive continental margins. If all core complexes exposed nearby. Structures in tion and its application to quartzite: The influ- stresses due to plate interaction are effectively these rocks are similar to those within the ence of deformational history: Journal of reduced to zero, stresses in the lithosphere result SRMZ, and their origin is controversial (e.g., Structural Geology, v. 2, p. 355-370. Miller, R.S., and Dockum, M.S., 1983, Ordovician primarily from nonisotropic gravitational the coaxial extension model of Rehrig and conodonts from metamorphosed carbonates of forces, and deformation would occur from the Reynolds, 1980, versus the simple shear model the Salton Trough, California: Geology, v. 11, natural tendency of a continental margin to of Davis, 1983). A study of preferred orienta- p. 410-412. flow toward the oceanic basins. A finite ele- tion within the core complexes may reveal the Rehrig, W.A., and Reynolds, S.J., 1980, Geologic and geochronologic reconnaissance of a ment analysis of this system modeled for a pas- strain history and place constraints on their northwest-trending zone of metamorphic core sive margin in isostatic equilibrium shows that structural evolution. complexes in southern and western Arizona, in differential stresses in the crust rise to a maxi- Crittenden, M.D., Jr., Coney, P.J„ and Davis, mum of about 80 MPa at a depth of 10 km REFERENCES CITED G.H., eds., Cordilleran metamorphic core com- before decreasing to zero at the Mohoroviêic Bott, M.H.P., 1971, Evolution of young continental plexes: Geological Society of America Memoir margins and formation of shelf basins: Tectono- 153, p. 131-158. discontinuity (Bott and Dean, 1972; Fig. 6A). physics, v. 11, p. 319-327. 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A minor com- southeastern San Jacinto Mountains, in Brown, Some petrological, geochemical, and geochrono- ponent of simple shear could develop locally A.R., and Ruff, R.W., eds., Geology of the San logical observations of the Peninsular Ranges during flow toward the continental margin, Jacinto Mountains: South Coast Geological So- batholith near the international border of the particularly in the border zone of ductile ciety Annual Fieldtrip No. 9, p. 100-138. U.S.A. and Mexico, in Abbott, P.L., and Todd, Christie, J.M., and Ord, A., 1980, Flow stresses from V.R., eds., Mesozoic crystalline rocks: Peninsu- deformation. microstructures of mylonites: Example and cur- lar Ranges batholith and pegmatites; Point Sal Lateral flow within mid-crustal levels is ac- rent assessment: Journal of Geophysical Re- ophiolite: San Diego, California, San Diego companied by normal faults rooted into a series search, v. 85, p. 6253-6262. 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De- Letters, v. 11, p. 46-49. orientation in plane strain deformed limestone: Erskine, B.G., and Marshall, M., 1982, Magnetite- formation in the middle and upper levels of the Experiment and theory: Contributions to Miner- and ilmenite-series sub-belts in the northern alogy and Petrology, v. 80, p. 132-139. crust may be aided by synkinematic intrusion Peninsular Ranges batholith, southern Califor- of NPRB magmas at lower levels. nia: Their association with magnetic stability and implications on source regions: EOS (American ACKNOWLEDGMENTS Geophysical Union Transactions), v. 62, p. 849. Supported by National Science Foundation Grant CONCLUSIONS Hill, R.I., 1981, Field, penological and isotopic stud- EAR 82-07727, a Geological Society of America Field relations, minor structures, and micro- ies of the intrusive complex of San Jacinto H. T. Stearns Fellowship, and a grant by the Sigma structures in the SRMZ are ambiguous and can Mountain, in Brown, A.R., and Ruff, R.W., Xi Foundation. We appreciate thoughtful comments on the manuscript by S. J. Reynolds and C. Simpson. be interpreted as caused by either shear or eds., Geology of the San Jacinto Mountains: South Coast Geological Society, Annual Field- Discussions in the field with Brett Cox, John Dennis, coaxial extension. We think that textural analy- trip No. 9, Guidebook, p. 76-89. Eric Frost, Gordon Gastil, Gordon Haxel, Keith sis is a significant method for the determination Krummenacher, D., Gastil, R.G., Bushee, J., and Howard, Jon Matti, Robert Sharp, Sterling Shaw, of the importance of each component. Al- Doupont, J., 1975, K-Ar apparent ages, Penin- Carol Simpson, Victoria Todd, and Lionel Weiss were valuable for the development and clarification though our work is concerned with the SRMZ, sular Ranges batholith, southern California and of ideas presented in this paper. similar mechanisms may apply to other mylo- Baja California: Geological Society of America Bulletin, v. 86, p. 760-768. Manuscript received August 30, 1984 nite zones as well. One candidate is the group Lister, G.S., and Hobbs, B.E., 1980, The simulation Revised manuscript received November 26, 1984 of rocks collectively called the metamorphic of fabric development during plastic deforma- Manuscript accepted November 30, 1984

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