## LSU Historical Dissertations and Theses

1994

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Civil and Environmental Engineering

Roy K. Dokka

#### Abstract

The southwest Cady Mountains of San Bernardino County, California, contain key relationships which allow separation of the timing of early Miocene detachment-style extension and vertical-axis clockwise rotation in the Mojave Desert. The rock succession consists of a pre-Tertiary granitic basement overlain by three successive volcanic and volcaniclastic units of early to middle Miocene age referred to informally as the formation of Poe, the formation of Troy Peak and the "Barstow Formation". Deposition of the volcanic upper Oligocene/lower Miocene formation of Poe was accompanied by extension and tilting, facilitated by faults which do not cut the unconformably overlying formation of Troy Peak basalts. The rocks then experienced a clockwise rotation of $63\sp\circ \pm 20\sp\circ$ prior to initiation of deposition of the "Barstow Fm." Following deposition of the mid-Miocene "Barstow Fm." ($<$14 Ma), this area was folded into an antiform and rotated an additional $67\sp\circ \pm 15\sp\circ$ clockwise. Upon removal of both clockwise rotations, the direction of early Miocene extension in the southwest Cady Mountains, indicated by geologic relationships, becomes aligned $\sim$N-S. The southwest Cady Mountains comprise a portion of the Daggett terrane of the Mojave extensional belt where all areas display similar orientation of extensional structures, except the southwest Cady Mountains. The removal of the post-14 Ma clockwise rotation from the southwest Cady Mountains brings the local direction of the early Miocene extension of the southwest Cady Mountains and the surrounding Daggett terrane into alignment. Other areas in and around the Mojave Desert block have also experienced early Miocene clockwise rotation. Upon removal of late Miocene to recent right-lateral strike-slip faulting, areas which have been affected by early Miocene clockwise rotation move into an approximate E-W alignment. This rotation can be explained by an early Miocene E-W directed right-lateral shear active prior to $\sim$18.5 Ma. Removal of this right-lateral shear straightens structural bends in traces of pre-Tertiary thrust systems, aligns a possible offset of the Independence dike swarm and removes clockwise rotations in and around the Mojave Desert block.

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