INTERPRETATION AND TECTONIC IMPLICATIONS OF COOLING HISTORIES - AN EXAMPLE FROM THE BLACK MOUNTAINS, DEATH-VALLEY EXTENDED TERRANE, CALIFORNIA

In the Death Valley extended terrane of California, the Black Mountain s have long been considered unique because they largely lack the mioge oclinal cover rocks characteristic of the surrounding ranges. Fission- track ages presented here are combined with published Ar-40/Ar-39 ages and used to construct cooling path envelopes for samples of Precambri an crystalline basement and Miocene plutonic rocks collected across th e entire range. The cooling history reconstructions are used to differ entiate between contrasting Miocene unroofing histories proposed for t his range. Apatite and zircon fission-track ages from the southeastern portion of the range suggest unroofing occurred there at approximatel y 13-8.5 Ma from temperatures well below 300-degrees-C. Cooling age da ta from the central Black Mountains indicate major unroofing at 8.5-6. 0 Ma from temperatures greater than 300-degrees-C. Old cooling ages fr om directly beneath the highly extended Amargosa chaos rocks are consi stent with the chaos rocks being part of an allochthonous slice that w as tectonically transported from high crustal levels onto deeper crust al levels. Scenarios for the Miocene unroofing history of this range r ely heavily on interpretations of the depth of emplacement of Miocene plutons in the core of the range. Thermochronologic and geobarometric data and thermal modeling of intrusion cooling suggest emplacement of an 11.6 Ma pluton into the crystalline core at a depth of 10-15 km. Bo th the cooling-age data and considerations of the local geology seem t o preclude an unroofing history dominated by erosion of the overlying miogeoclinal section. The morphology of the cooling path envelopes con structed here are similar to those constructed for detachment fault te rranes. The data are most consistent with unroofing involving tectonic denudation (10-15 km) along a single, westerly dipping detachment zon e. Diachronous rapid cooling from southeast to northwest within the ra nge is interpreted as a result of the lower plate undergoing flexural deformation as it pulls out from underneath a relatively rigid, scoop- shaped hanging wall block (the 'rolling-hinge' model). Migration of un roofing and tilting within the range mimics the overall east to west s equential tilting and unroofing of range blocks in the Death Valley ex tended terrane. Similar thermochronologic evidence for sequential rang e-scale tilting has been obtained from the Lake Mead extended region, suggesting that this style of extension might provide an explanation f or strongly extended domains in the Basin and Range Province.