ORTHOGONAL (N-S) EXTENSION AT THE LEADING EDGE OF A PROPAGATING CONTINENTAL RIFT: A REASSESSMENT OF HOMER MOUNTAIN AND ENVIRONS, LOWER COLORADO RIVER EXTENSIONAL CORRIDOR (CREC)

Homer Mountain lies within the CREC in SE California and exposes a crystalline core of PC gneiss and Cretaceous granite flanked by low-angle detachment faults and tilted Miocene hanging wall strata (Spencer & Turner, 1985). The footwall is intruded by Miocene E-W trending dikes - an orientation at odds with regional ENE-WNW extension, and interpreted by Spencer (1985) to reflect re-orientation of σ3 due to flexure of the footwall during extensional unroofing. New mapping and U-Pb geochronology clarify structural-magmatic relations and suggest an alternative tectonic interpretation.

The dense swarm of E-W trending rhyolite to diorite dikes at Homer Mountain document ~10% (800 m) of N-S extension and yield LA-ICPMS U-Pb ages ranging from 18.95 ± 0.25 to 17.55 ± 0.11 Ma. Following this ~1.5 Ma period of N-S magmatic accretion, the extension direction abruptly switched to E-W at ~ 17.0 Ma, and the area was successively intruded by N-S-trending 16.55 ± 0.15 Ma dikes (Homer Mt Rhyolite) and cut by a system of east-dipping normal faults. Evidence for large-scale(~60°) westward tilting (and high-angle initiation) of the “detachment” fault segments includes the gentle (~20-25°) E-dips of Homer Mt Rhyolite dikes and steep (~60°) W-dips of PC diabase sills in the footwall. Nearly flat-lying 14.5 Ma basalt lavas post-date most local extension.

The early (19-17 Ma) N-S extension at Homer Mountain was coeval with large magnitude NE-SW directed tectonic extension occurring in core complexes to the south (Whipples/Chemehuevis), consistent with northward propagation of magmatism and extension within the CREC. We suggest that the least principal stress at upper crustal levels was locally controlled by along-strike differences in crustal thickness such that high-standing (unextended) thick crust in the north spread gravitationally southward toward already stretched crust. This progression from early (minor) N-S extension, to major E-W extension is evident elsewhere in the CREC and Basin and Range, suggesting such complexities in the 3-D strain field are common. Transient N-S extension was likely accommodated by a component of N-S shortening (constrictional strain) in adjacent domains and may explain slip-parallel footwall corrugations and constrictional fabrics in MCCs and conjugate strike-slip faults elsewhere.