Interaction Between Early San Andreas Strike-Slip Faulting and Extensional Tectonism in the Chocolate Mountains: A Prologue to Growth of the Salton Trough Along the Plate Boundary in Southern California
Abstract
The Chocolate Mountains (CM) along the NE margin of the southern Salton Trough (ST) lie NE of the post-5- Ma San Andreas fault (SAF) and SW of the early and middle Miocene Clemens Well-Fenner-San Francisquito strand (CW-F-SF) of the SAF system. The CM are a highly extended terrain that evolved during the late Oligocene-middle Miocene and is bounded by the CW fault. Constrained by reconstruction of a compelling array of paleogeologic patterns, the approximately 300 km displacement on the SAF NW of the Garlock fault is distributed to the SE on the SAF (ca 160 km, 0 to 5 Ma), San Gabriel fault (ca 40 km, 0-5 to 12 Ma), and CW-F-SF fault (ca 100 km, 13 to 17-22 Ma). The youngest rocks yet shown to be offset 300 km in southern CA are basalts in the Diligencia and Plush Ranch formations, as young as 22 Ma. Lack of evidence for a large-displacement dextral fault in AZ on-trend with the CW fault requires the existence of a tectonic mechanism for absorbing its dextral displacement to the SE. Structure in the CM manifests late Oligocene-middle Miocene extensional tectonism that culminated in exhumation of Orocopia Schist by tectonic denudation. In its early stages, tectonism was accompanied by sedimentation and by voluminous magma-generation producing a batholithic-to-volcanic edifice. The principal structural feature is a complexly faulted, NW-trending array of en echelon antiforms that runs the length of the range and continues SE into AZ and NW into the Orocopia Mts. In the anticlinorium core, Orocopia Schist is intruded by a late Oligocene composite batholith of mafic to felsic plutons. A succession of tectonic plates separated by detachment faults overlies the schist and plutons. The structurally lowest fault is ductile and juxtaposes mylonite against the schist. Three higher faults, all brittle, vertically stack plates of (1) Mesozoic orthogneiss, (2) little deformed Triassic and Jurassic plutonic rocks, Proterozoic gneiss and anorthosite, and dacitic to rhyolitic late Oligocene hypabyssal intrusive rocks, and (3) moderately to steeply tilted supracrustal sedimentary and volcanic rocks. Field relations and age data allow us to bracket sequential stages in the late Oligocene to middle Miocene (ca 28 and 13 Ma) magmatic-tectonic evolution of the CM. Our 40Ar-39Ar and K-Ar dates and published U- Pb indicate plutonism at 24 Ma, dacitic to rhyodacitic volcanism at 24 to 22 Ma. 20 Ma, and 17 Ma. At least the highest detachment is post-17 Ma and pre-13 Ma, the oldest flow age (ca 13 to 9 Ma) from untilted sections of basalt and conglomerate. This interval is coeval with displacement on the CW-F-SF fault. The basalt and conglomerate sections are fully offset on the modern SAF, pre-date growth of the ST, and span much of the interval between 13-Ma cessation of the SF-F-CW fault and 5-Ma start of the SAF. Spatial and temporal linkage between dextral displacement on the CW fault and extension in CM is compatible with a transfer mechanism whereby right-slip on the CW fault is accommodated to the SE by hyper-extension in the Orocopia-Chocolate Mts block. This strain pattern prefigures later development of the West Salton detachment that was associated with growth of the ST and that began perhaps as early as ca 10 Ma (Matti and Langenheim, this session). Unlike this later strain pattern, the extensional accommodation proposed here was not linked to opening of the Gulf of California, but rather occurred in an extensional zone between the CW fault and a reconstructed zone of sinistral shear along the southern boundary of the Transverse Ranges and SW margin of the CM.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T11A1839P
- Keywords:
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- 8038 Regional crustal structure;
- 8106 Continental margins: transform;
- 8108 Continental tectonics: compressional;
- 8111 Continental tectonics: strike-slip and transform;
- 8178 Tectonics and magmatism