Incremental40Ar/39Ar thermochronology of mylonitic rocks from the Northern Snake Range, Nevada

摘要

A40Ar/39Ar thermochronologic study of muscovite, biotite, and potassium feldspar from footwall rocks of the Northern Snake Range Decollement, Nevada, was undertaken in order to constrain the timing and temperatures of mylonitic deformation during extension and the subsequent cooling (uplift) history of these mylonitic rocks. Mylonitic deformation has been bracketed between 37 Ma and about 24 Ma, and minimum temperatures during ductile thinning were about 300°C. Mineral cooling ages along the east flank of the range suggest a minimum TMAX gradient of about 55°C/Ma. This steep thermal gradient is the result of a localized heat source and/or shearing of colder rocks with older cooling histories over hotter rocks with younger cooling histories. Although mylonitic deformation may have spanned a 13 m.y. period (37 Ma to 24 Ma), deformation was not synchronous throughout the range. On the west flank of the range, age spectra from muscovite and microcline samples indicate that rocks were at temperatures well below 325°C and deformation had ceased by 30 Ma, while on the east flank of the range, lower plate rocks were at temperatures above 325°C and ductile deformation was still ongoing. This is also reflected in chrontours of published K/Ar muscovite ages, along with40Ar/39Ar ages, which monotonically decrease in age from west to east. These data provide evidence for a temperature difference across the range within the same structural horizon from about 30 to 24 Ma. These relations suggest a lateral thermal gradient, the east flank of the range cooling more slowly than the west flank. Alternatively, the data suggest either an eastward dip of lower plate units prior to the onset of Tertiary mylonitic deformation or that layering dipped eastward due to deformation in an east dipping, top‐to‐the‐east shear zone. Both cases result in differential cooling of lower plate units as they pass through a series of subhorizontal isotherms. The40Ar/39Ar data, along with published finite strain, structural, and quartz petrofabric data, provide constraints on the formation and geometric evolution of metamorphic core complex detachme