12 Tectonics, Volcanism, and Sediment Dispersal Within and Proximal to the Rio Grande Rift and Southern Rocky Mountains

This session integrates research on coupled tectonics, volcanism, and sediment dispersal in the Rio Grande Rift and southern Rockies. We examine how extensional faulting controls volcanic processes and reshapes sediment routing from Colorado through Texas.

Monday, 18 May 2026
1:30 PM - 4:30 PM
Alvarado A

From deposition to exhumation, documenting changes in the Orocopia Schist and included metasomatized rocks at Cemetery Ridge, southwest Arizona

Paper number: 12-4
Program: Technical Sessions
Time: 2:35 PM – 2:55 PM

Authors

Abstract

Orocopia Schist, trench sediments that were underplated and metamorphosed during Laramide flat-slab subduction, is exposed at Cemetery Ridge over an area of 5 km2 in southwest Arizona. Included in the dominant quartzofeldspathic schist are peridotite bodies and zones of extreme metasomatic alteration. The age of subduction-related metamorphism is constrained by detrital and depth-profiled metamorphic zircon U-Pb ages to be <65 Ma to approximately 40 Ma, when flat-slab subduction ceased. Thermodynamic modeling of garnet-bearing Orocopia Schist at Cemetery Ridge and in the southeastern Chocolate Mountains, California, document a subduction–related pressure-temperature path from 5.5 kbar and 520°C to 12.5 kbar and 585°C. Transport within the subduction channel was associated with isoclinal folding, fabric development, and mechanical mixing.   

Following underplating of the rocks at Cemetery Ridge along the subduction channel at a depth of >45 km, the rocks were exhumed to ~18 km depth (5.5-5 kbar). The exhumation is constrained to have occurred prior to or coeval with the intrusion of 20-22 m.y. granitic rocks along the margins of the Orocopia Schist exposure.  The intrusions caused contact metamorphism that heated the rocks to ca. 720°C. The contact metamorphism is dated by lower intercept Tera–Wasserburg titanite and rutile ages from 21.0 ± 1.0 to 21.90 ± 0.96 Ma and monazite 238U/206Pb ages 25-21 Ma. REE patterns on dated monazite grains display negative Eu anomalies and show LREE depletion and HREE enrichment with younger ages. The wide ranges of titanite and plagioclase δ18O values (+7 to -1‰ and +10 to +1‰, respectively) and fractionations (∆18Ottn-plag from 0‰ to -8‰) suggest that these minerals were not in isotopic equilibrium during metasomatism. The lowest feldspar and titanite δ18O values indicate interactions between variable amounts of meteoric water and high-temperature, rock-equilibrated magmatic or metamorphic fluid that was changing over time. The Cemetery Ridge exposure of Orocopia Schist is bounded at the west and south by extensional faults that are responsible for exhumation to the surface. The rocks at Cemetery Ridge experienced deposition in an oceanic trench, transport and metamorphism within a subduction channel, decompression associated with exhumation to the midcrust, contact metamorphism and then exhumation to the surface between 65 to 20 Ma, exemplifying the efficiency of tectonic processes and impressive ability for rocks to preserve their history. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410607

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