Early Pleistocene emergence of newdextral faults SW of the southern San Andreas fault, Salton Trough

Document Type

Presentation

Journal/Book Title/Conference

Abstractsfor the Annual Meeting of the Southern California Earthquake Center (SCEC)

Publication Date

2005

First Page

41

Abstract

Results from several years of field studies and magnetostratigraphy at 6 study sites in the SW Salton trough suggest a major plate reorganization in early Pleistocene time slightly before, during and after the 1.07 to 0.99 Ma Jaramillo subchron (Brown et al, 1991; Kirby, 2005; Lutz, 2005; Steely, 2005; Janecke et al 2004, 2005). From late Miocene to late Pliocene time the top-to-the-east West Salton detachment fault controlled sedimentation in subbasins of the SW Salton Trough. The 4 km thick syndetachment section is conformable above local unconformities at the base. Starting as early as 1.4 Ma and as late as 0.8 Ma, a new set of cross-cutting dextral and dextral oblique-slip faults offset, folded, and deactivated most of the West Salton detachment. The first sign of the new structural regime is seen in growth of the basement-cored San Felipe anticline before (?) and during deposition of the 1.1 to 0.5 Ma Ocotillo Formation. An angular unconformity across the crest of this >15 km long, E-W trending anticline changes north, south and east into a subregional disconformity. North and south of the anticline the contact between the Borrego and Ocotillo formations is conformable. Magnetostratigraphic dating of this progressive unconformity at a disconformable, conformable, and nearly conformable contact shows that its age is identical (~1.1 Ma) across the entire San Felipe-Borrego subbasin. Shortly after emergence of the transpressive San Felipe anticline, the dextral-normal Fish Creek Mountains fault uplifted older basin-fill deposits and basement in the Fish Creek and Vallecitos Mountains and provided source material for the Ocotillo and Brawley formations. The Fish Creek Mountains fault is part of the San Felipe fault zone, which currently stretches from the Elsinore fault in the NW to the San Jacinto fault in the SE. Southwestward coarsening and thickening of the Ocotillo and Brawley formations toward boulder-bearing fault-scarp breccias along the Fish Creek Mountains fault zone, NEdirected paleocurrents, and the presence of recycled sandstone clasts from newly uplifted older basin-fill deposits show that this fault uplifted the Fish Creek and Vallecito mountains for the first time in early Pleistocene time (~1.1 Ma). This uplift separated the formerly contiguous Fish Creek-Vallecitos subbasin from the San Felipe-Borrego subbasin, and drove progradation of a 600 m thick sheet of Ocotillo-Brawley gravel and sand 25 km NE into the former perennial lake basin of the Borrego Formation. By ~1.0 Ma the Clark lake and Santa Rosa segments of the Clark strand of the San Jacinto fault zone were shedding pebble to cobble gravel SW into the northern San Felipe-Borrego subbasin. By 0.9 to 0.8 Ma, the NW end of the Coyote Creek strand along Coyote Canyon had developed NW-flowing streams that deposited sandstone and conglomerate within the fault zone. At about 0.9 Ma the Fish Creek-Vallecito subbasin ended a 7 m.y. period of steady subsidence and began to be exhumed. Uplift SW of the Fish Creek Mtns fault is more likely to explain this fundamental change in the Fish Creek-Vallecito basin than initiation of the SE Elsinore fault (e.g. Johnson et al., 1983) because the NE strand of the Elsinore in the Tierra Blanca Mountains has a significant NE-down slip component.Structural and basin analysis in the San Felipe Hills, Borrego Badlands, and Yaqui Ridge area show that the fully modern San Jacinto fault zone appeared after 0.6 to 0.5 Ma. On the NE side of the Salton Trough an angular unconformity across the crest of a large NW-trending anticline in the Mecca Hills, parallel to the San Andreas fault, may date from the same time but probably records more missing time because the entire Jaramillo event was omitted (Boley et al 1994). Angular unconformities in the Indio Hills also appear to date from the Jaramillo subchron (Boley et al., 1994). If future work confirms the regional nature and synchroneity of this major unconformity we predict that it reflects a region-wide and abrupt inception of dextral strike-slip faults SW of the southern San Andreas fault. This major change may have been incited by changes and barriers to slip along the Big Bend of the San Andreas fault zone and coincides with the end of slip on most of the West Salton detachment fault.

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