Annual Meeting - SCEC.org

Group 1 – LAD | Poster Abstracts
uncertainty of most of the dip observations is about 5-10 degrees. To examine the geometry of the
fault surface, we have developed a three-dimensional model of a dipping SAF, extending from
Parkfield in central California to the SAF’s southern termination at the Salton Sea. Knowledge
about the dip of the SAF is important for estimating shaking potential of scenario major
earthquakes and for calculating geodetic deformation.
In sections across the SAF, P-wave tomographic images of the mantle beneath southern California
(Kohler et al., 2003) suggests that the plate boundary extends into the mantle and is continuous
with the SAF in the crust. The dip of the plate boundary appears to steepen in the mantle.
Seismicity sections across the locked part of the SAF, from Indio towards the northwest, reveal
different seismicity regimes on either side of our model SAF surface, but do not reveal the fault
itself. These differences include changes in the maximum depth of the seismogenic zone and in the
abundance of seismicity over the past ~20 years. The different seismicity regimes may reflect
changes in physical properties and/or stress state of the crust on either side of the SAF at
seismogenic depths. Mantle velocities southwest of this projected plate boundary, within the
Pacific Plate, are relatively high and constitute the well documented upper-mantle high-velocity
body of the Transverse Ranges. This relationship is similar, in some ways, to that between the
Alpine fault of New Zealand and its underlying mantle, and suggests that in both California and
New Zealand, Pacific lithospheric mantle is downwelling along the plate boundary (Fuis et al.,
2007).
1-134
GEOLOGY AND TECTONICS OF THE CHOCOLATE MOUNTAINS Powell RE, and
Fleck RJ
The Chocolate Mountains (CM), situated along the NE margin of the southern Salton Trough, lie
NE of the post-5-Ma San Andreas (SA) fault and SW of the southeastward projection of the early
and middle Miocene Clemens Well (CW)-Fenner-San Francisquito-SA strand of the SA system. The
CM are the western part of a highly extended terrain in SE CA and SW AZ that evolved during the
late Oligocene-middle Miocene and is bounded to the NE by the CW-SA fault.
The principal structural feature in the CM is a complexly faulted, NW-trending array of en echelon
antiforms that runs the length of the range and continues into AZ. Orocopia Schist in the core of the
antiforms is structurally overlain by Proterozoic rocks (augen/pelitic/layered gneiss; anorthositesyenite)
and Mz mylonite, orthogneiss, and plutonic rocks (TR Mt Lowe, J mafic and intermediate
rocks) at the ductile CM thrust.
All these units are intruded by a late Oligocene composite batholith comprising plutons of gabbrodiorite,
granodiorite, and grante. Dacitic to rhyolitic hypabyssal, volcanic, and pyroclastic rocks cap
the batholith, and are coeval with it. Younger Miocene volcanic rocks also are present.
Structure in the CM manifests late Oligocene-middle Miocene extensional tectonism that
culminated in exhumation of Orocopia Schist by tectonic denudation. Tectonism was accompanied
by sedimentation and by magma-generation forming the batholith and its volcanic cap. Brittle
extensional faulting has largely reactivated and cut out the ductile CM thrust and has created a
stack of fault plates in the basement rocks above the thrust and in a superjacent steeply west-tilted
section of syntectonic Oligocene-Miocene redbeds, megabreccia, and volcanic and volcaniclastic
rocks. Along the NE flank of the CM, the extensional fault stack was dropped to the NE along a
normal fault representing final phase of extension.
2008 SCEC Annual Meeting | 139