Deformation of Mylonites in Palm Canyon, California, Based on Xenolith Geometry

Journal of Structural Geology, Vol. 20, No. 5, pp. 559 to 571, 1998 # 1998 Elsevier Science Ltd PII: S0191±8141(97)00114±4 All rights reserved. Printed in Great Britain 0191-8141/98 $19.00 + 0.00 Deformation of mylonites in Palm Canyon, California, based on xenolith geometry HANS-RUDOLF WENK Department of Geology and Geophysics, University of California, Berkeley, CA 94720, U.S.A. ([email protected]) (Received 19 December 1996; accepted in revised form 26 November 1997) AbstractÐAn analysis of xenoliths in granodiorite and mylonites of the Peninsular Ranges batholith in southern California documents large deformation with Von Mises equivalent strains ranging from 1 to 4. From the geometry of xenoliths in mylonites it is concluded that much of the deformation occurred by ¯attening. If deformation had been homogeneous, the 800 m thick mylonite sequence would have extended originally over more than 10 km. Contrary to xenoliths associated with ballooning diapirs, these in Santa Rosa mylonites show much larger strains and are elongated in the lineation direction. It is proposed that the overall deformation pattern is consistent with a top to the west shear displacement, but most of the strain in the Santa Rosa mylonites was produced in ¯attening, during emplacement of the partially solidi®ed batholith. The large deformation may have produced folds that have coalesced with the foliation to accommodate the strain. # 1998 Elsevier Science Ltd. All rights reserved INTRODUCTION apply to a larger portion of the mylonite zone, as well as to many mylonitic rocks which display similar Mylonites have received a lot of attention in recent microstructures. years, in part sparked by a Penrose Conference (Tullis The aim is to evaluate strain from a statistical inves- et al., 1982) with a ®eld trip to mylonites in the tigation of the shape of xenoliths (also known as Peninsular Ranges of southern California. Since then enclaves, e.g. Didier and Barbarin, 1991) in highly the Santa Rosa mylonite zone south of Palm Springs deformed granitic rocks. A three-dimensional study is has become a classical site to study highly deformed possible due to excellent outcrops in the deeply eroded rocks, both in the ®eld and in the laboratory. canyons. The data were collected over several years Simpson and Schmid (1983) and Simpson (1984) during ®eld trips by the author and students, and described excellent examples of S±C microstructures should represent an unbiased collection. with consistent asymmetric augen structures and press- ure shadows in this region that are clear signs of shear deformation. This supported earlier conclusions based GEOLOGICAL BACKGROUND on ®eld studies (Sharp, 1967, 1979), introducing mylonites as part of a ``Zone of Shear'' as it is known on Figure 1 displays a simpli®ed geological map, and the California geological map (Rogers, 1965). These Fig. 2 a cross section of the area of interest. On the highly deformed granitic rocks and marbles have pro- eastern edge of the Peninsular Ranges batholith, highly vided much suitable material to investigate microscopic deformed rocks occur in a more or less continuous deformation mechanisms (e.g. Goodwin and Wenk, zone between Palm Springs (where the zone is trun- 1995) and the development of crystallographic pre- cated by the San Andreas fault), and Clark Valley ferred orientation (e.g. O'Brien et al., 1987; Wenk and (northeast of Borrego Springs) (where it is truncated Pannetier, 1990). A lot of information was accumu- by the San Jacinto fault) and may, after oset, con- lated about ductile deformation at conditions of tinue as the Borrego Springs mylonite zone (Simpson, amphibolite facies metamorphism of these rocks. 1984). On Fig. 1 a few topographic reference points While attention focused on the extreme degree of de- are indicated for orientation, as are sampling localities. formation and the large regional extent of highly Some local topographic names, which are not indi- deformed rocks, the question of total strain was never cated on the map, are used in the text to help those explored. Furthermore it has not been established how readers who may wish to visit some of the localities. much simple shear has contributed to the overall de- These names are readily available on U.S. Geological formation of these granitic mylonites. These two topics Survey topographic 7.5-minute quandrangles Palm are the subject of the present study: What is the ®nite View Peak, Rancho Mirage, Butter¯y Peak, Toro Peak strain in this large mylonite zone, over 1 km wide and and Martinez Mtn. extending over more than 100 km? The detailed obser- Starting in the west and at the geologically lowest vations refer to rocks in the Palm Canyon region level (Fig. 2), the relatively undeformed eastern south of Palm Springs, but conclusions are likely to Peninsular Ranges batholith consists of several distinct 559 560 H.-R. WENK Fig. 1. Generalized geological map of Palm Canyon and adjacent areas in southern California (modi®ed from Erskine and Wenk, 1985). Some lineations and foliations are indicated. Also shown are sampling localities (compare with Table 1). intrusions, all of granodioritic composition (e.g. Ague mineralogical composition as the protolith (O'Brien et and Brimhall, 1988) and Cretaceous age (97 Ma U±Pb al., 1987). The relatively sharp transition from proto- age of zircons for the San Jacinto Mtn. Pluton, Hill lith to mylonite can be mapped in the ®eld and is and Silver, 1980). These batholitic rocks are in contact shown on the map (Fig. 1). Equally, ultramylonites with roof pendants in the western part. To the east and phyllonites form well-de®ned zones, ranging from (western slope of Palm Canyon) the batholith is over- 2 cm in thickness to 20 m. They are most common in lain by a 700±900 m thick sequence of mylonites of upper Palm Canyon (near highway 74, between lo- similar composition to the protolith and a similar calities S2 and S3) and the southern extension where igneous age (97 Ma U±Pb, B. Nelson and L. the mylonite zone is narrowest (between localities S4 Ratschbacher, unpublished). These strongly foliated and S5). rocks are distinct in the ®eld (Fig. 3). The granodioritic mylonites are overlain by meta- During mylonitization a very regular E-plunging morphic rocks of the Palm Canyon Formation. These lineation and an E-dipping foliation developed, micro- are in ordinary igneous contact with the batholitic scopically expressed in preferred orientation of biotite rocks in the northern part (near locality N1 at and quartz (Wenk and Pannetier, 1990) and formed Hermit's Bench at the bottom of Palm Canyon, south during dislocation creep and dynamic recrystallization, of Palm Springs). In most places the contact is not with grainsize reduction of quartz and mica (Goodwin exposed, in some places it appears as a fault with sub- and Wenk, 1995). The degree of deformation is vari- stantial hydrothermal alteration, and in some areas the able: an undeformed protolith with moderate defor- Palm Canyon formation. is separated from the grano- mation features, such as undulatory extinction in dioritic mylonites by an intruded porphyritic tonalite quartz, transforms rather abruptly into mylonites, and (Fig. 2). The metamorphic rocks have various litholo- locally into ultramylonites and phyllonites. The latter gies: pelitic and leucocratic gneisses (with granulitic two are extremely ®ne grained rocks, resembling ¯int texture), migmatites, amphibolites (characteristically and slate, but with the same overall chemical and with oxyhornblende), pyroxenites, calcsilicates (with Deformation of mylonites in Palm Canyon, California, based on xenolith geometry 561 Fig. 2. E±W cross section through the Santa Rosa mylonite zone in central Palm Canyon. No vertical exaggeration. forsterite, diopside and scapolite) and marbles. The mation and its southern extension (towards Sheep and metamorphic grade is upper amphibolite facies as Martinez Mountains) are often mylonitic. A U±Pb age established by index minerals such as sillimanite and of zircons in migmatites of 110 Ma (Nelson and cordierite in pelitic schists, forsterite in marbles, and Ratschbacher, unpublished) indicates that the meta- orthopyroxene and spinel in amphibolites. morphism is of a similar age as the batholitic intru- Deformation of the Palm Canyon formation is very sions (inheriting some old components), rather than an heterogeneous. For example carbonate rocks may exist older event. Occasionally folding is observed and the as coarse marble or as ®ne-grained ultramylonites. fold axes are always parallel to the lineation. In the These calcite ultramylonites could be mistaken in the upper (and south eastern) part of the Palm Canyon ®eld for unmetamorphosed limestones, were it not for formation deformation of migmatites is more brittle, calcsilicate clasts of forsterite and diopside, document- with an extensive zone of pseudotachylites (Wenk and ing a metamorphic origin. As in the underlying mylo- Pearson, in preparation). The cataclastic rocks are best nites, shear indicators such as asymmetric tails around exposed in Deep Canyon and southwest of Palm porphyroclasts on a microscopic scale, and shear Desert. bands on a macroscopic scale, are consistent with a The Palm Canyon formation and associated rocks top-to-the-west movement. Lineations in the Palm are overlain by the Asbestos Mountain granodiorite of Canyon formation are parallel to those in the under- tonalitic composition (U±Pb age of 87 Ma, Nelson lying mylonites, suggesting that they were deformed at and Ratschbacher, unpublished). The structure of the same time. Migmatites in the Palm Canyon for- Asbestos Mountain tonalite is reminiscent of a gently Fig. 3. View from the western slope of Palm Canyon towards Santa Rosa Mountain in the background. In the fore- ground are strongly foliated mylonites, on the eastern slope of Palm Canyon (left) are metamorphic rocks of the Palm Canyon Formation. 562 H.-R. WENK Fig.

Load more