Geology of the Eastern Tehachapi Mountains and Late Cretaceous-Early Cenozoic tectonics of the southern Sierra Nevada Region, Kern County, California

摘要

Many geologic studies have inferred that the California continental margin in the vicinity of the western Mojave Desert was tectonically disrupted after emplacement of the Cretaceous Cordilleran batholith and prior to Neogene displacements on the San Andreas fault system. The causes of this regional deformation, however, are poorly understood. Located along the northern margin of this disrupted region at the southern end of the comparatively little deformed Sierra Nevada batholith, the eastern Tehachapi Mountains are ideally situated to study the possible mechanisms of this disruption. In view of this, the geology and structure of the eastern Tehachapi Mountains were investigated using geologic field mapping at scales of 1:6,000 through 1:24,000, detailed petrographic studies, and structural and kinematic analysis of deformation fabrics and structures in the field and in the lab. The study area is divided by a generally N trending shallowly SE dipping ductile-cataclastic fault zone called the Blackburn Canyon fault into the eastern Tehachapi gneiss complex in the footwall and the Oak Creek Pass complex in the hangingwall.ududThe eastern Tehachapi gneiss complex is composed of two different sequences of metasedimentary rocks that have been intruded by three generations of plutonic rocks. The Brite Valley group metasedimentary rocks consist largely of pelites and graphitic quartzite with subordinate marble. The Antelope Canyon group metasedimentary rocks consist of a lower section composed mostly of thinly laminated dirty quartzite overlain by an upper section of marble. The earliest intrusive rocks in the area (group I orthogneisses) are lithologically diverse and include granite augen gneiss, garnetiferous hornblende diorite gneiss, and hornblende biotite quartz diorite gneiss. Both groups of paragneiss and the group I orthogneisses are intruded by group II plutons of the Tehachapi Intrusive Complex. The Tehachapi Intrusive Complex is composed of comagmatic gabbro, quartz diorite, and tonalite and it is inferred to be continuous with the large ~100 Ma Bear Valley Springs tonalite pluton exposed to the west. The group III intrusives are small bodies and thin sheets of leucocratic biotite granite which intrude all of the other lithologies.ududThe rocks in the gneiss complex have had a complex deformational history. The metasedimentary rocks are folded into map-scale N to NW trending SW vergent isoclinal F1 folds. Later (?) intrusion of the group I orthogneisses was accompanied (?) and followed by amphibolite facies metamorphism and the localized formation of NE trending shallow plunging open to tight F2 folds. During (?) and after intrusion of the ~100 (?) Ma Tehachapi Intrusive Complex the gneiss complex was metamorphosed at amphibolite facies and deformed by map-scale open to tight NW trending SW vergent F3 folds. After much of the F3 folding the basement rocks in the Tehachapi Valley area appear to have been folded into a regional dextral-sense convex-west F4 oroclinal fold. In the later stages of F4 folding part of the southwest limb of the Tehachapi Valley orocline is inferred to have been transposed into a NW trending shallow NE dipping noncoaxial ductile shear zone called the eastern Tehachapi shear zone. The shear zone has a structural thickness of ~1 km, top to the S-SW shear sense, and most shearing appears to have occurred during greenschist facies retrograde metamorphism. The shear zone appears to continue to the north across Tehachapi Valley where it is inferred to merge with the steeply E dipping dextral-slip proto-Kern Canyon fault. Motion on the shear zone is inferred to have ended at about the time when the Late Cretaceous (?) group III leucogranites intruded. Following shear zone activity rocks in the gneiss complex locally were folded in gentle NE trending subhorizontal F5 folds. Late top to the NE shearing in the upper structural levels of the gneiss complex suggests that a normal fault may be concealed beneath the alluvium of Tehachapi Valley.ududThe lithologies and deformation history of the Oak Creek Pass complex are very different from the eastern Tehachapi gneiss complex. The Oak Creek Pass complex is composed mostly of granodioritic plutonic rocks (group IV intrusives) which commonly are cataclastically deformed and metamorphosed at greenschist and lower grade. Arkosic sandstones and conglomerates of the Late Cretaceous (?)-Eocene (?) Witnet Formation locally are unconformable above the granodiorite. Emplacement of the Oak Creek Pass complex above the eastern Tehachapi gneiss complex along the Blackburn Canyon fault took place after most of the activity along the eastern Tehachapi shear zone. Shear sense along the Blackburn Canyon fault is top to the S or SE. The Oak Creek Pass complex is divided into a number of structural plates by low-angle (?) ductile-cataclastic fault zones one of which is the NE trending Mendiburu Canyon fault. Synclinal F6 folding of the Witnet Formation and NW vergent overthrusting of the Witnet Formation by granitic rocks along the Mendiburu Canyon fault are interpreted to postdate motion along the Blackburn Canyon fault. Deformation of the Witnet Formation is inferred to be pre-Miocene in age based on correlation with a similar deformation across Tehachapi Valley.ududThe Brite Valley group metasedimentary rocks are suggested to correlate with the western facies of the Triassic-Jurassic age Kings sequence and the Antelope Canyon group rocks may correlate with the eastern facies of the Kings sequence or possibly with Late Proterozoic-Cambrian age rocks of the miogeocline. Juxtaposition of the two groups of metasedimentary rocks may have been along a cryptic structure that was active prior to intrusion of some of the group I plutons which are inferred to be mid-Cretaceous in age. Formation of the NE trending F2 folds between ~117 Ma and ~100 Ma is suggested to have resulted from the local reorientation of the regional stress field in the vicinity of a weak strike-slip (?) fault such that the direction of maximum compressive stress during the deformation was oriented subparralel to the trend of the Sierra Nevada batholith. The F3 folds, F4 folds, and the eastern Tehachapi shear zone are interpreted to have formed more or less sequentially during a protracted period of contractional deformation in the middle to lower crust of the southern Sierra Nevada batholith from ~100 Ma to ~80 Ma. Top to the S-SW motion along the shear zone may reflect the and underthrusting of Rand schist beneath the batholith at lower structural levels during low-angle Laramide subduction.ududThe Blackburn Canyon fault and a number of other previously identified low-angle faults in the southern Sierra Nevada region are suggested to be extensional faults along which part of the southern Sierra Nevada batholith was unroofed. The source region for the out of place Oak Creek Pass complex and other inferred allochthonous rocks is suggested to be the area in the Sierra Nevada east of the proto-Kern Canyon fault and south of South Fork Valley. Exposures of Witnet Formation may be the remnants of a synextensional sedimentary deposit that accumulated in a supradetachment basin. This inferred extensional exhumation of the southeastern Sierra Nevada may have begun as early as ~85-90 Ma and ended at ~80 Ma or later based on data from previous studies in the region. Thus, contractional deformation in the middle crust of the southern Sierra Nevada region may have been coeval with upper crustal extensional deformation in Late Cretaceous time.ududCorrelation of the Cretaceous structural histories of the eastern Tehachapi gneiss complex and the northern Salinian block in the Coast Ranges of central California supports previous suggestions that the two areas may have evolved in close proximity to one another. The relative westward offset of the Salinian block from the Sierra Nevada prior to the Neogene may in part be the result of Late Cretaceous-early Cenozoic (?) westward extrusion of wedges of middle to lower crust bounded by thrust faults below and E dipping extensional faults above in a manner analogous to recent models for deformation in the Himalayas. The upper plate rocks of the Blackburn Canyon fault appear to be rotated about 90° clockwise relative to their inferred source region and the F4 folds in the Tehachapi area appear to have dextral vergence. The vergence of the folding and the sense of rotation both are consistent with Late Cretaceous dextral-oblique convergence indicated by plate motion models and with the presence of Late Cretaceous synbatholithic dextral transpressional and strike-slip shear zones in the Sierra Nevada to the north.