Reconnaissance Geology and K-AR Geochronology of the Trigo Mountains Detachment Terrane, Yuma County, Arizona

Thesis, (M.S.) Geological SciencesTwenty-seven new potassium-argon ages from the Trigo Mountains record geologic events that occurred from Jurassic through Miocene time. A well-developed detachment fault divides the range into upper and lower plates. The fault is folded into antiformal and synformal structures with lower-plate crystalline rocks exposed in denuded antiforms at the north end of the range. Upper-plate units are Precambrian crystalline and Tertiary volcanic units extended by a series of northwest-trending normal faults with exposures of the Pelona-Orcopia schist occurring in the south. The faults and tilted upper-plate volcanic units dip steeply into the detachment surface and appear to have been transported to the southwest. Lower-plate rocks are Precambrian metasediments, metaigneous rocks, and rapakivi granite. These rocks are nonmylonitic, but are chloritized and brecciated to varying degrees.\ud \ud Apparent ages of lower-plate crystalline terrane and upper-plate metamorphic terrane range from a biotite age of 54 .8 ?? 3.3 m.y. to a hornblende age of 152.2 ?? 9.2 m.y. and record resetting of ages during Jurassic and Cretaceous thermo-tectonic events. An unmetamorphosed granitic to granodioritic stock, which intrudes metamorphic rocks of the upper plate, gives biotite ages of 71.2 ?? 5.5 m.y. and 72.9 ?? 2.9 m.y., indicating the intrusion is synchronous with the Cretaceous thermal event. Three lower-plate ages, from locations 0 to 10 meters below the detachment surface, show no apparent resetting related to detachment faulting. This is in contrast to other ranges involved in detachment faulting where mylonitic terranes show strong Tertiary resetting near the detachment surface, and non-mylonitic terranes show at least some resetting. East-trending hypabyssal dikes (39.1 ?? 6.3 m. y., biotite) cut lower-plate rocks in the north. Allochthonous upperplate volcanic rocks give plagioclase ages from 19.6 ?? 5.9 m.y. to 29.1 ?? 3.5 m.y. and appear to correlate with the volcanic stratigraphy of the nearby Palo Verde Mountains, Picacho Area, Paymaster Quarry Site and Castle Dome Mountains