Fluid inclusion microthermometric data from five temporally controlled vein sets in the southeastern Piedmont province record changes in fluid composition and deformation conditions during regional exhumation and cooling related to postorogenic Mesozoic rifting. In general, compositions of postmetamorphic fluids are remarkably consistent across the southeastern Piedmont, indicating regional fracture connectivity during fluid-trapping events between the end of the Alleghanian collisional orogeny and the opening of the Atlantic Ocean. Late Paleozoic P 1 and P 2 quartz veins associated with post-Alleghanian stress relaxation contain metamorphic CO 2 -H 2 O-NaCl fluids that were trapped under fluctuating lithostatic (125–240 MPa) to hydrostatic pressures (20–100 MPa) at burial depths of up to 9.3 km. Maximum depths are similar to emplacement depths of post-kinematic plutons, suggesting a period of rapid isobaric cooling. Fluids trapped in veins in the Modoc fault zone give similar trapping pressures, but significantly higher temperatures, suggesting fluid migration from depth along the zone. Triassic TR 1 quartz veins related to early failed rifting record regional decompression as fracturing above the brittle-to-ductile transition allowed regional pore-fluid pressure to drop to hydrostatic levels. During this time, extensive fluid mobility occurred due to opening of regional fractures associated with initiation of rifting and tapping of deep, hot, saline fluid reservoirs. Calcite in TR 2 veins, which are related to the development of Triassic basins, and later TR 3 veins related to initial Jurassic dike emplacement host common H 2 O inclusions. These inclusions are evidence for complete opening of the hydrologic system and for convective circulation of meteoric water, which resulted in dilution of “in situ” fluids, and ultimately to a pure-H 2 O system. These fluids continued to be trapped in vein minerals through the Early Cretaceous.

We combine horizontal Global Positioning System (GPS) velocities from a new compilation of published and new GPS velocities, results from an interferometric synthetic aperture radar (InSAR) study, and paleoseismic data to evaluate the postseismic response of historic earthquakes in the Central Nevada seismic belt. We assume that GPS velocity has contributions from time-invariant (i.e., steady permanent crustal deformation) and transient (i.e., time varying and associated with the seismic cycle) processes that are attributable to postseismic viscoelastic relaxation of the crust and upper mantle. In order to infer the viscosity structure of Basin and Range lower crust, η LC , and upper mantle, η UM , we apply three objective criteria to identify rheological models that fit both geodetic and geologic data. The model must (1) improve the apparent mismatch between geodetically and geologically inferred slip rates, (2) explain the InSAR-inferred vertical uplift rate, and (3) not imply time-invariant contractions anywhere in the extending province. It is not required for the postseismic deformation field to resemble the time-invariant velocity field in pattern, rate, or style. We find that the InSAR and horizontal GPS velocities form complementary constraints on the viscoelastic structure, excluding different parts of the model space. The best-fitting model has a lower crust that is stronger than the uppermost mantle, with η LC = 10 20.5 Pa·s and η UM = 10 19 Pa·s, a finding consistent with the majority of similar studies in the Basin and Range. The best-fitting viscosity model implies that the majority of Central Nevada seismic belt deformation is attributable to postseismic relaxation, and hence that western Basin and Range time-invariant deformation north of 39°N latitude is more tightly focused into the northern Walker Lane than would be inferred from uncorrected GPS velocities. However, significant deformation remains after correction for postseismic effects, consistent with Central Nevada seismic belt faults slipping at rates intermediate between the Walker Lane belt and the central Basin and Range.