Nature of the southwestern boundary of the central Mojave Tertiary province, Rodman Mountains, California

Abstract

Tertiary volcanic and sedimentary rocks in the central Mojave Desert are largely confined to the Barstow-Bristol trough, an elongate tectonic depression filled with lower Miocene strata. The southwestern boundary of the trough is sharply defined and separates thick (∼2 km) Tertiary sequences from an area to the southwest that is virtually devoid of Tertiary rocks. Segments of this boundary have been interpreted as a buttress unconformity, as a right-slip sidewall fault that formed the lateral boundary of an early Miocene extensional terrane, and as a dip-slip fault caused by flexure of the North American plate above the subducted Mendocino fracture zone. The Silver Bell fault in the Rodman Mountains, central Mojave Desert, is the best exposed segment of the boundary, and relations exposed along it can be used to test these hypotheses. The Tertiary sequence north of the Silver Bell fault is at least 2.3 km thick and comprises at least 1.9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault. Stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed Tertiary section. Tuff layers intercalated in the lower part of the clastic deposits are 23–24 m.y. old, indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central Mojave metamorphic core complex. Structural and stratigraphic data indicate that the Silver Bell fault is primarily a dip-slip fault. The Tertiary section is folded into a west-trending, upright to overturned syncline located near the southern edge of exposed Tertiary rocks. The syncline records late Cenozoic north-south shortening along the northwest-striking Calico fault, which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip. Southwestward-dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension. However, bedding dip does not decrease upward in the Tertiary section, paleocurrent directions trend obliquely updip, and northeast-dipping normal faults are rare. Stratal tilting postdated the accumulation of the exposed section, which therefore was not deposited during rotational normal faulting. The new data contradict interpretation of the Silver Bell fault as a major right-slip fault bounding the central Mojave extensional terrane, although they permit small-magnitude extension across the fault. The amount of early Miocene crustal extension in this area therefore is apparently small in spite of stratal tilting, and the lateral limit of large-magnitude crustal extension in the central Mojave block is farther northwest. The data are consistent with the fault having formed by failure during plate flexure above the subducted Mendocino fracture zone.