ABSTRACT In this study, we determined the timing of burial and subsequent exhumation of Barrovian metamorphic rocks from the Chloride Cliff area of the Funeral Mountains in southeastern California by constraining the ages of different portions of a pressure-temperature ( P-T ) path. Using a split-stream laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) system, we analyzed 192 domains from 35 grains of monazite within five samples with a spot size of 8 µm to determine U-Pb ages and trace-element abundances from the same samples (same polished sections) that were analyzed to produce the P-T paths. Changes that took place within individual monazite grains reflect localized equilibrium and captured the changes in heavy rare earth element (HREE) abundances in the matrix reservoir that occurred as garnet grew, resorbed, and then regrew, thus constraining ages on different portions of the P-T path. The results show that garnet began growing ca. 168 Ma, began resorbing ca. 160 Ma, began retrograde regrowth ca. 157 Ma, and continued to regrow at least through ca. 143 Ma. The early garnet growth corresponds to a period of pressure increase along the P-T path. The subsequent partial resorption corresponds to the prograde crossing of a garnet-consuming reaction during decompression, and the retrograde garnet regrowth occurred when this same reaction was recrossed in the retrograde sense during further decompression. These results are consistent with previously determined ages, which include a Lu-Hf garnet age of 167.3 ± 0.72 Ma for the early pressure-increase portion of the P-T path, and 40 Ar/ 39 Ar muscovite cooling ages of 153 and 146 Ma in the lower-grade Indian Pass area 10 km southeast of Chloride Cliff. The 40 Ar/ 39 Ar muscovite ages document cooling at the same time as retrograde garnet regrowth was taking place at Chloride Cliff. The oldest monazite age obtained in this study, 176 ± 5 Ma, suggests that southeast-directed thrusting within the Jurassic retroarc was ongoing by this time along the California portion of the western North American plate margin, as a consequence of east-dipping subduction and/or arc collision. The Funeral Mountains were likely located on the east side of the northern Sierra Nevada range in the Jurassic, taking into account dextral strike-slip displacement along the Cretaceous Mojave–Snow Lake fault. The Late Jurassic timing of burial in the Funeral Mountains and its Jurassic location suggest burial was associated with the East Sierran thrust system. The timing of prograde garnet resorption during exhumation (160–157 Ma) corresponds to a change from regional dextral transpression to sinistral transtension along the Jurassic plate margin inferred to have occurred ca. 157 Ma. The recorded exhumation was concurrent with intrusion of the 148 Ma Independence dike swarm in the eastern Sierra Nevada and Mojave regions, which developed within a regime of northeast-southwest extension.

Jurassic tectonism in the northeastern Great Basin produced varied structures, many closely associated with widespread magmatism at ca. 155–165 Ma and with local metamorphism. Many of the plutons are of suitable mineralogy for Al-in-hornblende barometry, providing the potential for depth data. We have studied conditions of metamorphism in the Pilot Range and barometry for six Jurassic plutons across the northeastern Great Basin. All barometry results are in harmony with pressures estimated from stratigraphic data, requiring little or no tectonic thickening. On the basis of structural styles and barometric data, we divide the northeastern Great Basin into three Jurassic tectonic provinces. An eastern extensional province, largely in western Utah, is characterized by Paleozoic strata that were thrust faulted and then intruded by shallow plutons shortly after or during normal and strike-slip faulting. Extension was probably a short-lived event associated with magmatism, but its west trend indicates a total reorientation of stress at this time, perhaps within transtensional strike-slip zones. A central province of modest, and possibly locally extreme, Jurassic shortening in eastern Nevada is characterized by metamorphosed Paleozoic rocks and by thrusts and kilometer-scale southeast-vergent folds. Upper amphibolite facies, but low pressure (3–4 kbar) metamorphism is present near Jurassic plutons in the Pilot Range and Ruby Mountains, probably indicating metamorphism induced by heat from magmas. In contrast, metamorphism in other ranges, which is known only to be pre–Late Cretaceous, indicates thickening of 10–20 km. This thickening may have entirely postdated the Jurassic. A western province in north-central Nevada is characterized by preserved Jurassic volcanic rocks and shallow plutons, indicating that little erosion, and probably surface uplift, occurred during the late Mesozoic. Folds and thrust faults indicate minor Jurassic shortening but many structures are undated. The low-pressure upper-crustal conditions for demonstrably Jurassic events suggest that higher-pressure metamorphism recorded in the central province is younger (Cretaceous) in age. We suggest that Jurassic structures were caused by distributed minor crustal shortening, manifested mainly as small-scale thrust faults. Local thermal highs created by plutonism produced metamorphic zones in relatively shallow crust. Shortening in the east was manifested by zones of strike-slip, within which plutons were emplaced in tensile niches. Lack of a deep foreland basin and lack of evidence for massive erosion argue against high-relief mountain belts caused by significant crustal shortening. Paleozoic rocks metamorphosed at pressures far in excess of stratigraphic burial are restricted to narrow lenses exhumed during Late Cretaceous and Tertiary extension and are bordered by rocks that always have been part of the shallow crust. The abundant shallow-crustal rocks preserved across the region indicate that a conventional hypothesis of large-scale, regional crustal thickening causing many kilometers of surface uplift and consequent erosion is unlikely to have taken place in the Mesozoic.