PLIOCENE (U-TH)/HE COOLING AGES FROM THE WEST SALTON DETACHEMENT SYSTEM (WSDS), SALTON TROUGH

The WSDS is a low-angle normal fault that bounds the eastern Peninsular Ranges and roots E beneath sedimentary and intrusive rocks of the Salton Trough. Slip on the WSDS began in the latest Miocene, based on syndetachment fluvial and marine deposits in the Fish Creek basin. The WSDS contains several ESE-plunging arches and striae trends range from N to ESE.

Preliminary (U-Th)/He apatite and zircon apparent ages from the footwall record rapid Pliocene cooling. Footwall and hanging wall apatite ages record a significant thermal break across the detachment. Two Pinyon Ridge footwall samples, vertically separated by 180 m, record rapid cooling through ~70°C between 5.0±0.2 and 4.4±0.2 Ma. Yaqui Ridge samples yield apatite ages of 4.6±0.2 to 2.2±0.1 Ma. About 1 km away from the youngest of these, three samples from a ~150 m vertical transect in the hanging wall have apparent ages of 38.9±1.9 to 33.1 ±1.7 Ma, probably recording prolonged residence in the apatite partial retention zone (PRZ). Six zircon samples also record this thermal break: footwall ages range from 51.4±2.6 Ma to 34.8±1.7 Ma but one hanging wall sample is 72.7±3.6 Ma. Two apatite ages of 9.7±0.5 Ma and 6.3±0.3 from the footwall at NE Whale Peak probably record residence at the bottom of the PRZ followed by rapid Pliocene exhumation. Preliminary estimates from comparison of apatite and zircon ages suggest a minimum of ~2 km of exhumation since ~3 Ma and exhumation rates on the order of 0.5-1.5 mm/yr (assuming a geothermal gradient of 30°C/km).

In general, ages are younger on the N limb of Yaqui Ridge and older on the S limb, and ages also decrease from W to E. The preliminary 4.2 Ma isochron trends WNW at Pinyon and Yaqui ridges, suggesting generally NE-ward unroofing at that time. Also, the rapid Pliocene cooling at Yaqui and Pinyon ridges, compared to the late Miocene apatite PRZ ages from Whale Peak, suggest a northward component of unroofing. Future dating will hopefully further constrain these patterns.