Rapid uplift and rotation of mylonitic rocks from beneath a detachment fault: Insights from potassium feldspar 40Ar/39Ar thermochronology, northern Snake Range, Nevada

摘要

The thermal histories of mylonitic rocks from the footwall of the northern Snake Range decollement (NSRD), Nevada, were calculated using multiple diffusion domain analyses of potassium feldspar Arrhenius data and 40Ar/39Ar age spectra in order to characterize the cooling (exhumation) history of these mylonitic rocks and thereby constrain the origin and movement history of the NSRD. The calculated thermal histories, along with reported apatite fission track ages, indicate the three following episodes of rapid cooling (10–55°C/m.y.) related to extensional denudation: (1) middle Eocene (48–41 Ma), (2) late Oligocene (30–26 Ma), and (3) early Miocene (20–16 Ma). The first episode of rapid cooling may be related to the earliest period of normal faulting in east central Nevada and initiation of slip along the NSRD. The second episode signals a pulse of slip along the NSRD, and the third episode marks the cessation of lower plate mylonitic deformation and the onset of the major pulse of uplift and rotation of lower plate rocks from beneath the NSRD and/or younger east dipping normal faults. The thermal history data provide evidence for a temperature difference across the range within the same structural horizon; the northwestern flank of the range dropped below 300°C by 46 Ma and reached 115°C by 20 Ma, whereas lower plate rocks in the eastern parts of the range were at temperatures above 300°C until 19 Ma. If isotherms were subhorizontal, these relations suggest that either lower plate units had an eastward dip prior to the onset of Tertiary mylonitic deformation or that layering dipped eastward as a result of top-to-the-east shear within an east dipping shear zone. Either case results in differential cooling (uplift) of lower plate rocks from beneath the NSRD. A temperature-distance-age graph, constructed based upon the calculated thermal histories, shows differential uplift and rotation of moderately to steeply east dipping lower plate units from beneath the NSRD. Furthermore, this reconstruction shows that the dip of lower plate units increases and then decreases in time and space. This reconstruction implies that the NSRD initiated at moderate to steep angles that shallowed at depth, listric style, into a shallow dipping shear zone. These reconstructions provide a spectacular example of the rolling hinge/isostatic rebound model.