Abstract Isotopic and fluid inclusion analyses of veins and host rocks constrain the compositions, temperatures and sources of palaeofluids along the La Popa salt weld. Most veins formed after the salt was evacuated from the precursor salt wall; veins are generally more abundant on the downthrown side of the weld and near a significant bend in the trace of the weld. The spatial distribution of fluid types and temperatures suggests the weld served as a vertical fluid conduit and a horizontal baffle. Stable isotopes indicate there was significant fluid–rock interaction and little vertical fluid communication between rock units in areas away from the weld. Fluid temperatures along the weld ranged from 84 to 207 °C, salinities ranged from 4 to 25 wt% NaCl equiv. and methane was abundant in the weld zone and on the downthrown side of the weld. Strontium isotopes suggest that some of the vein-forming fluids were derived from the evaporites that once occupied the weld. Our results suggest the sealing potential of similar welds may be related to the presence of abrupt changes in weld geometry such as cusps or bends, the amount of shortening across the weld and the amount of vertical displacement across the weld.

Analysis of regional joint orientations, vein mineral paragenesis, and fluid inclusion microthermometry provides valuable insights into the deformation history of the Central Appalachians. These data indicate that deformation occurred as a continuum from hinterland to foreland, and they reflect a progressive rotation in shortening direction. However, separate deformation milestones punctuate the history. Prior to North Mountain thrust emplacement, the rocks in the Valley and Ridge Province were fractured under a NNW-directed shortening. The emplacement of the North Mountain thrust sheet occurred under a more NW-directed shortening and resulted in rapid loading of the Paleozoic section in the footwall by 7–10 km of rock that extended 20–30 km toward the hinterland from the North Mountain thrust ramp. The loading of the Paleozoic section resulted in overpressuring of Cambrian Waynesboro décolle-ment and the development of the Valley and Ridge duplex, and the formation of the Adams Run–Cacapon Mountain anticlinorium in the northern part of the study area. Growth of this anticlinorium shed more sediments toward the foreland, thereby driving addition horse formation. Continued NW- and then WNW-directed shortening resulted in the formation of the Wills Mountain duplex under the load of the North Mountain thrust sheet and sediments derived from the Adams Run–Cacapon Mountain anticlinorium. The growth of the Wills Mountain duplex resulted in duplication of the Cambrian–Ordovician carbonate section with coeval syntectonic erosion, shedding 3–4 km of sediments into the plateau, and triggering the formation of several thrusts in the Cambrian–Ordovician carbonate section. A final E-W–directed shortening event resulted in extensive regional mesoscale fracturing.

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.