CHARACTER AND ORIGIN OF CATACLASITE DEVELOPED ALONG THE LOW-ANGLE WHIPPLE DETACHMENT FAULT, WHIPPLE MOUNTAINS, CALIFORNIA Jan-Claire Phillips Mobil Oil Corporation P. O. Box 5444 Denver, CO 80217 ABSTRACT ment fault in the Whipple Mountains of southeastern Cal iforni a. The \'/hipple detachment fault in the \'Jhipple Mountains, southeastern California, is characterized Previ ous \~ork by a thin layer of cataclasite, a green or bro\'m aphanitic rock containing randomly oriented myloni­ Two studies dealing with lithified random­ tic porphyroclasts derived from lower plate rocks. fabric fault rocks (cataclasites and microbreccias) At least two generations of cataclasites are are those of Brock and Engelder (1977) and Anderson present. The cataclasites are composed of randomly and Osborne (1980). Brock and Engelder determined oriented quartz and feldspar porphyroclasts in a that cataclastic flow was the mechanism responsible random-fabric matrix. Color banding can be seen in for the deformation they observed beneath the Muddy the matrices of some green and most brown samples, Mountain Thrust in the Muddy Mountains, Nevada. but does not appear to be associ ated with oriented Anderson and Osborne did not address the problem of fabrics, shear zones, or other indications of flow. what mechanism was responsible for the cataclasites Porphyroclast size distributions for both green and deve loped along the San Gabri e1 fault in Southern brown cataclasites are of logarithmic-extreme value California, but they did provide an extensive type. petrologic and textural characterization of those rocks. The \'lOrk of Brock and Engelder and of The cataclasite matrix is composed of very Anderson and Osborne, along with the experimental small, equant, interlocking grains with an average studies of Engelder (1974) and Sammis and others size of 0.004-0.006 mm, sho\'/ing little evidence of (1980), i ndi cate that both the random-fabri c faul t deformation. Predominant matrix minerals are quartz rocks studied to date and experimentally formed and alkali feldspar. Much of the cataclasite fault gouge yield logarithmic-normal particle size mineralogy is probably secondary, although both distributions. Engelder (1974) and Brock and upper and lower plate components are present. Field Engelder (1977) have suggested that this type of relationships indicate that each cataclasite matrix particle size distribution is characteristic of was highly mobile at the time of its formation. cataclasis and cataclastic flow. The presence of uniformly sized interlocking Heidrick and Wilkins (1980) have presented matrix grains favors a creep mechanism for cata­ bulk chemistry data for cataclasites collected in clasite formation rather than one involving only the \·Ihipple and Buckskin Mountains, including the high fluid pressures or cataclastic flow. Although area of this study. They concluded that the the temperature of cataclasite formation was lower cataclasites formed primarily from lower plate than that expected for structural superplastic flow, rocks, although they were slightly enriched in the matrix textures suggest structural superplasti­ potassium and depleted in sodium relative to the city as a possible deformation mechanism for the lower plate rocks analyzed. cataclasite matrices. Superplasticity at these temperatures may be possible if diffusional Geologic Setting processes are enhanced by the presence of water. The 1ack of rotated porphyrocl asts and of matri x The ~Ihipple Mountains are located in eastern fabrics and the porphyroclast size distributions San Bernardino County, California, adjacent to the suggest that cataclastic flow was not the dominant Colorado River near Parker, Arizona (F igure 1). deforma t ion proces s . It issugges ted that cata­ This range is part of a large ( 3000 km 2) area of clastic flow initiated the deformation process and extensional faulting in the Colorado River trough was responsible for most grain size reduction, but of south;astern Cali forni a, western Ari zona, and that superplasticity may have become dominant after southernmost Nevada. The detached terrane a very small grain size was ach i eved. Both mecha­ coincides in part with the zone of metamorphic core nisms may have been aided by the presence of water complexes that extends throughout the southern and by higher than normal fluid pressures. Cordillera. An Oligocene to mid-Miocene low-angle fault surface, called the Whipple detachment fault I NTRODUCTI ON in the Whipple Mountains area, separates autoch­ thonous lower-pl ate igneous and metamorphic rocks Interest in earthquake prediction has contri­ of Precambrian to Tertiary (?) age from alloch­ buted to a vari ety of studi es of faul t rocks and thonous upper-plate igneous, metamorphi c, and fault mechanisms in recent years. However, most sedi mentary rocks rangi ng in age from Precambri an studies of cataclastic rocks have dealt exclusively to mi ddl e ~~i ocene. Thi s fault underl i es much of with clay gouge (e.g., \~u and others, 1975), non­ the Colorado River trough area and is exposed about lithified gouge or breccia (e.g., Anderson and the margins of adjacent ranges, including the others, 1980), or mylonitic rocks. Few studies have Chemehuevi , Sacramento, Bucks ki n, and Rawhi de focused on lithified, random-fabric fault rocks such Mountains. Rocks now seen in the lower plate and as those found along the lO\'i-angle Whipple detach- in one upper-plate location have been subjected to 109 MIO.-PlIO. OSBORNE WASH FM. OLlO.-MIO. OENE CANYON & COPPER BASIN FMS N UPPER - PLATE CRYSTALLINE 1 ROCKS LOWER - PLATE MYLONITIC ROCKS o , 2 '-----"---' LOWER - PL ATE NON - MYLONITIC km ROCKS CALIFORNIA Figure 1. Generalized geologic map of the vlhipple tliountains after Davis and others (1980). Boxes enclose areas sampled for this study. an earlier (mid- Cretaceous to early Tertiary (7)) locally absent in the western part of the range mylonitization that preceded detachment faulting. I'lhere the 10l'ier-plate rocks are not mylonitized. The region has also been subjected to a I'larping The cataclasite thus appears to be preferen­ event that formed a series of asymmetric anti­ tially developed in the eastern part of the range, forms and synforms that control the exposure of where myl on iti crocks underl i e the detachment the detachment fault around the margins of the surface. ranges. The chlorite breccia zone is described in The Colorado River trough area, and the Davis and others (1979; 1980) and in Krass (1980). Whipple Mountains in particular, have been the Occasionally there is a sharp contact between the subject of ongoing study at the University of cataclasite and chlorite breccia zone, but more Southern California for the past several years. often the latter becomes progressively more Da vi s and others (1979; 1980) provi de the mos t shattered and brecciated as the fault surface is recent overvi ews of the structure, petrology, and approached, finally grading into the microbreccia geologic history of the Whipple, Buckskin, and or cataclasite. Brecciation of the upper-plate Rawhide Mountains. rocks is much less pervasive and is limited to the first few meters above the fault plane. Extensive \~hipple Detachment Fault alteration, including iron and occasionally copper mineralization, is common in the cataclasite and The Whipple detachment fault apparently adjacent upper-plate rocks (Heidrick and \~ilkins, developed within crystall ine rocks, but in some 1980) . places now separates upper-plate Tertiary sedimentary and volcanic rocks from lower-plate CATACLASITES cryste.lline rocks. It is along such contacts that the cataclasite appears to be best developed. The Sample Locations detachment faul tis characteri zed by a typi cally smooth, planar fault surface which is underlain by a Sampling for the present study was confined to thin layer of cataclasite and microbreccia that the eastern and southern \~hipple Mountains, in the 1 tends to form a distinct ledge (Figure 2). The \~hipple \~ash and \~hipple Mountains SV! 7 2 minute cataclasite is usually 2 to 25 cm thick, but in quadrangles. Most samples were collected in areas places its thickness can be as great as 1.5 to 2 m. where Tertiary volcanic and sedimentary rocks are The cataclasite layer is usually underlain by a the predominant upper-plate units. The areas where thick (up to 250-300 m) zone of sheared and sampl es were coll ected have been mapped by brecciated lower-plate rocks that are chloritized G. A. Davis, K. V. Evans, E. G. Frost, V. A. Krass, and appear pale to dark green in color. This zone, S. H. Lingrey, J. A. Lopez, and L. C. Thurn; no termed the chlorite brecci a zone, appears to be further mapping was done for this study. preferentially developed where the lower plate consists of mylonitic gneisses and is thinner and Field Descriptions 110 Figure 2. The Whipple detachment fault exposed in the south-central Whipple Mountains (area B in Figure 1). The prominent ledge is formed of cataclasite developed beneath the fault. The cataclasite is an aphanitic, well-indurated of very low birefringence and appears to be a brown or green rock, often containing randomly fine-grained mass of quartz and feldspar, a conclu­ oriented floating porphyroclasts of 1 to 10 mm in sion verified by X-ray analysis. In places, the size with occas iona 1 severely fractured porphyro­ matrix is severely stained or opaque, particularly clasts up to 10 em in size. Porphyroclasts found in ~Iithin the brown cataclasite samples. Occasional the catacl asite above mylonitic lower-pl ate rocks porphyroclasts of alkali feldspar and epidote are are of three types: monomineralic quartz or feld­ also present. Identifiable plagioclase feldspar is spar, mylonitic rocks derived from the lower plate, rarely seen. Both euhedral (square and hexagonal) and fragments of older cataclasites. At least tlvO and irregularly shaped opaque minerals are common; generations of cataclasite are present. The younger the shapes of the euhedral opaques suggest that brown layer lies structurally above the older green they are pyrite and magnetite.