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Southern California Geology

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

Post-batholithic Geologic Structures

Post-batholithic geologic structures in and around the San Bernardino Mountains fall into three categories:

Late Miocene uplift structures. --These are associated with uplift of the ancestral San Bernardino Mountains in late Miocene time (about 12 to about 5 million years ago) (Meisling and Weldon, 1982, 1989). These include the Squaw Peak thrust fault in the Cajon Valley region and east-trending north-dipping reverse faults and left-lateral faults in the western and central part of the mountains, including the Santa Ana Fault. The Santa Ana Fault is an east-striking reverse fault located in the interior of the range. Displacement on this fault has placed crystalline rocks against the Santa Ana Sandstone. The fault is obscured by landslides and colluvial debris along much of its length. Near the fault, the bedding of the Santa Ana Sandstone steepens and locally is overturned (Sadler, 1993).

San Andreas Fault zone .--As discussed above, several strands of the San Andreas Fault zone traverse the southeastern San Bernardino Mountains and flank the southwestern base of the range (Matti and others, 1992a; Matti and Morton, 1993). Older strands of the zone include the Wilson Creek, Mission Creek, and Mill Creek faults; the modern trace of the fault in this region is represented by the San Bernardino strand. The older strands have generated considerable right-lateral displacements that over the last few million years have juxtaposed far-traveled crystalline basement rocks against the main mass of the San Bernardino Mountains. The modern San Bernardino strand is capable of generating large earthquakes, although the strand apparently did not rupture during the 1857 earthquake that occurred along the Mojave Desert segment to the northwest. Locally, as in the San Gorgonio Pass region and in the Yucaipa area, complexities in the San Andreas Fault have created associated reverse and thrust-fault zones and normal dip-slip fault zones (Matti and others, 1992a,b).

Quaternary uplift structures .--Structures associated with Quaternary uplift of the range include the north-frontal fault zone (Meisling, 1984; Miller, 1987; Sadler, 1982a) and faults along the south part of the range that facilitated uplift (the San Gorgonio Pass Fault zone of Matti and others, 1992a). Meisling and Weldon (1989) indicate that uplift was accomplished in early Quaternary time by north-directed upward movements of the San Bernardino Mountains block along south-dipping low-angle structures that underlie the range. This uplift created the impressive topographic relief along the north face of the San Bernardino Mountains. Although largely complete by middle Quaternary time (Meisling and Weldon, 1989; Spotila and others, 1998), tectonism presumably associated with uplift of the range has continued into the late quaternary, giving rise to strike-slip and thrust-fault scarps that locally break late Quaternary alluvial deposits adjacent to the northern range front (Miller, 1987; Miller and others, 1998; Powell and Matti, 2000).

Continue to Geology of San Gorgonio Pass Region

Selected References

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