We propose that the Los Angeles basin-inner California borderland (LAB-IB) and the southern borderland off Baja California formed by large-magnitude crustal extension beginning in latest Oligocene-earliest Miocene time, and that these regions represent major late Cenozoic rifts. The southern borderland is analogous to the Gulf of California rift, but the onset of rifting in this region may have preceded the opening of the Gulf by as much as 15 to 20 m.y. The LAB-IB rift is more complex than the southern borderland because it formed in the wake of the rotating western Transverse Ranges (WTR), a large crustal block that has undergone more than 90° of Neogene clockwise rotation. Total extension in both rifted regions is estimated to be in excess of 200 km, or comparable to the magnitude of late Cenozoic crustal extension now estimated for the Basin and Range province. Also, like the Basin and Range, the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults, voluminous Miocene volcanic rocks, and hanging-wall fragments of pre-rift strata and basement rocks stranded atop detachments during extension.

In support of this model, we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data (seismic reflection profiles). These profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging-wall rocks. We also suggest that a similar, but slightly earlier onset of crustal extension affected the Santa Maria basin and adjacent regions in central California, and that the southern margin of this "Santa Maria basin rift" was overridden during Neogene clockwise rotation of the WTR.

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