The Franciscan subduction complex of California is considered a type example of a subduction-accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high-grade blocks within mélange. Many of the high-grade blocks record counterclockwise pressure-temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high-grade blocks show evidence for clockwise pressure-temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U-Pb ages from inclusions in garnet and Lu-Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at~176 Ma. Matrix zircon U-Pb ages and Lu-Hf estimates of final garnet growth ages record a barroisite-amphibolite assemblage overprint of eclogite at~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone.

The Mineral King pendant in the Sierra Nevada batholith (California, USA) contains at least four rhyolite units that record highsilica volcanism during magmatic lulls in the Sierran magmatic arc. U-Th-Pb, trace element (single crystal spot analyses via sensitive high-resolution ion microprobe-reverse geometry, SHRIMP-RG), and bulk oxygen isotope analyses of zircon from these units provide a record of the age and compositional properties of the magmas that is not available from whole-rock analysis because of intense hydrothermal alteration of the pendant. U-Pb spot ages reveal that the Mineral King rhyolites are from two periods, the Early Jurassic (197 Ma) and the Early Cretaceous (134-136 Ma). These two rhyolite packages have zircons with distinct compositional trends for trace elements and δ 18 O; the Early Jurassic rhyolite shows less evidence of crustal infl uences on the rhyolites and the Early Cretaceous rhyolite shows evidence of increasing crustal infl uences and crystal recycling. These rhyolites capture evidence of magmatism during two periods of low magmatic fl ux in the Sierran Arc; however, they still show that magmas were derived from interactions of maturing continental crust, increasing from the Early to Late Jurassic. This fi nding likely refl ects the transition of the North America margin from one of docking island arcs in the Early Jurassic to one of a more mature continental arc in the Early Cretaceous. This also shows the utility in examining zircon spot ages combined with trace element and bulk isotopic composition to unlock the petrogenetic history of altered volcanic rocks.

Documenting the processes that facilitate exhumation of ultrahigh-pressure (UHP) rocks at convergent margins is critical for understanding orogen dynamics. Here, we present structural and temperature data from the Himalayan UHP Tso Morari nappe (TMN) and overlying nappes, which we integrate with published pressure-temperature-time constraints to refine interpretations for their structural evolution and exhumation history. Our data indicate that the 5.5-km-thick TMN is the upper portion of a penetratively deformed ductile slab, which was extruded via distributed, pure shear-dominated, top-down-to-east shearing. Strain in the TMN is recorded by high-strength quartz fabrics (density norms between 1.74 and 2.86) and finite strain data that define 63% transport-parallel lengthening and 46% transport-normal shortening. The TMN attained peak temperatures of~500-600°C, which decrease in the overlying Tetraogal and Mata nappes to~150-300°C, defining a field gradient as steep as 67°C/km. Within the overlying nappes, quartz fabric strength decreases (density norms between 1.14 and 1.21) and transport-parallel lengthening and transport-normal shortening decrease to 14% and 18%, respectively. When combined with published 40 Ar/ 39 Ar thermochronometry, quartz fabric deformation temperatures as low as~330°C indicate that the top-to-east shearing that exhumed the TMN continued until~30 Ma. Peak temperatures constrain the maximum depth of the overlying Mata nappe to 12.5-17.5 km; when combined with published fission-track thermochronometry, this provides further support that the TMN was not underplated at upper crustal levels until~30 Ma. The long-duration, convergence-subnormal shearing that exhumed the TMN outlasted rapid India-Asia convergence by~15 Myr and may be the consequence of strain partitioning during oblique convergence.

No abstract

Despite playing a fundamental role in all models of Himalayan tectonics, minimal data constraining the structural evolution, metamorphic history, and offset magnitude of the South Tibetan detachment system (STDS) are available. Here, we integrate petrofabric, finite strain, and kinematic data with metamorphic and deformation temperatures to generate a structural model for the STDS in northwestern Bhutan. We divide the STDS into an ∼2-km-thick lower level that accommodated ∼6–13 km of thinning via ≥30–76 km of simple shear-dominant displacement within Greater Himalayan rocks, and an ∼3-km-thick upper level that accommodated ≥21 km of displacement via an upward decrease (from 44% to 2%) in transport-parallel lengthening within Tethyan Himalayan rocks. Peak metamorphic temperatures in the lower level are ∼650–750 °C, and two distinct intervals of telescoped isotherms in the upper level define a cumulative upward decrease from ∼700 to ∼325 °C. These intervals are separated by an abrupt upward increase from ∼450 to ∼620 °C, which we interpret as the result of post-STDS thrust repetition. Above the upper telescoped interval, temperatures gradually decrease upward from ∼325 to ∼250 °C through a 7-km-thick section of overlying Tethyan Himalayan rocks. Telescoped isotherms lie entirely above the high-strain lower level of the STDS zone, which we attribute to progressive elevation of isotherms during protracted intrusion of granite sills. This study demonstrates the utility of using gradients in fabric intensity and thin section-scale finite strain to delineate shear zone boundaries when field criteria for delineating strain gradients are not apparent.

No abstract

No abstract

No abstract