ABSTRACT A growing body of evidence suggests that continental arc lower crust and underlying mantle wedge assemblages native to the Mojave Desert (i.e., the southern California batholith) were displaced eastward during Laramide shallow-angle subduction, and reattached to the base of the Colorado Plateau Transition Zone (central Arizona) and farther inboard. On this field trip, we highlight two xenolith localities from the Transition Zone (Camp Creek and Chino Valley) that likely contain remnants of the missing Mojave lithosphere. At these localities, nodules of garnet clinopyroxenite, the dominant xenolith type at both studied localities, yield low jadeite components in clinopyroxene, chemically homogeneous “type-B” garnet, and peak conditions of equilibration at 600–900 °C and 9–28 kbar. These relations strongly suggest a continental arc residue (“arclogite”), rather than a lower-plate subduction (“eclogite”), origin. Zircon grains extracted from these nodules yield a bimodal age distribution with peaks at ca. 75 and 150 Ma, overlapping southern California batholith pluton ages, and suggesting a consanguineous relationship. In contrast, Mesozoic and early Cenozoic igneous rocks native to SW Arizona, with age peaks at ca. 60 and 170 Ma, do not provide as close a match. In light of these results, we suggest that Transition Zone xenoliths: (1) began forming in Late Jurassic time as a mafic keel to continental arc magmas emplaced into the Mojave Desert and associated with eastward subduction of the Farallon plate; (2) experienced a second ca. 80–70 Ma pulse of growth associated with increased magmatism in the southern California batholith; (3) were transported ~500 km eastward along the leading edge of the shallowly subducting Farallon plate; and (4) were reaffixed to the base of the crust at the new location, in central Arizona. Cenozoic zircon U-Pb, garnet-whole rock Sm-Nd, and titanite U-Pb ages suggest that displaced arclogite remained at elevated temperature (>700 °C) for 10s of m.y., following its dispersal, and until late Oligocene entrainment in host latite. The lack of arclogite and abundance of spinel peridotite xenoliths in Miocene and younger mafic volcanic host rocks (such as those at the San Carlos xenolith locality), and the presence of seismically fast and vertically dipping features beneath the western Colorado Plateau, suggest that arclogite has been foundering into the mantle and being replaced by upwelling asthenosphere since Miocene time.

ABSTRACT The Sierra Nevada batholith is an ~600-km-long, NNW-trending continental arc generally exposed from epizonal to mesozonal levels and showing a distinct strike-perpendicular zonation in structural, lithologic, petrologic, geochronologic, and isotopic patterns. South of 35.5° N, in the southern Sierra Nevada–northern Mojave Desert region, the depth of exposure increases markedly and a tectonostratigraphy consisting of three distinct, fault-bounded assemblages is observed. From high to low structural levels, these units are (1) fragments of shallow-level eastern Sierra Nevada batholith affinity rocks, (2) deeper-level western to axial zone rocks, and (3) subduction accretion assemblages (e.g., the Rand schist). This multi-tiered core complex is the product of shallow subduction that occurred over ~500 km of the plate margin in Late Cretaceous time. Slab shallowing was accompanied by intense contractile deformation within the crust and along the subduction megathrust; crustal thickening, uplift, and denudation of the residual arc to midcrustal levels; removal of the forearc and frontal arc by subduction erosion; and replacement of sub-batholithic mantle with underplated subduction assemblages. As the slab reverted to a “normal” trajectory, previously thickened crust no longer compensated at depth by a shallowly dipping slab became gravitationally unstable and underwent a profound phase of extensional collapse. Two subparallel shear zones, one separating assemblages 1 and 2 (the southern Sierra detachment) and the other juxtaposing units 2 and 3 (the Rand fault), comprise an integrated Late Cretaceous detachment system that accommodated extensional collapse. These Late Cretaceous events preconditioned the southern California crust for imprints of subsequent tectonic regimes. For example, subduction of the Pacific-Farallon slab window in early Neogene time created an extensional stress regime in the overriding plate, facilitating high-angle normal faulting across the previously extended region and volcanism associated with upwelling astheno-spheric material. The invasion of hot and buoyant asthenosphere destabilized dense sub-batholithic root material still affixed beneath the central Sierra Nevada batholith, leading to Pliocene–Quaternary delamination of the high-density rocks. Replacement of dense sub-batholithic root materials with asthenosphere has led to ~1 km of uplift across the southern Sierra Nevada and into the eastern San Joaquin Basin. The purpose of this trip is to highlight structural and petrologic records of multiple phases of tectonism in the southern Sierra Nevada–Mojave Desert region, illustrating the profound and lasting effect that shallow subduction may have on a continental margin.

Abstract Petrologic, structural, geochronologic, and geochemical data from rocks exposed in the western foothills of the Sierra Nevada batholith just south of the western Foothills metamorphic belt (the Stokes Mountain region) provide new insight regarding several poorly understood aspects of the development of the compound Sierra Nevada arc. Exposures of three different metasedimentary packages together document transitions in the depositional environments present along the outboard edge of this arc segment throughout the Mesozoic Era, culminating in the emergence of the Cretaceous continental margin arc. Exposures of mafic cumulates and associated differentiates of the Early Cretaceous batholith permit investigation into the earliest stages of differentiation of depleted-mantle–derived arc magmas. Together with the surrounding Kings-Kaweah ophiolite belt, these Mesozoic plutonic and metasedimentary rocks are proposed to form an analog for the crystalline basement underlying much of the eastern half of the Great Valley forearc basin.

Regional spatial variation patterns in igneous emplacement pressures, initial 87 Sr/ 86 Sr (Sr i ) values, zircon U/Pb ages, and pluton bulk compositions of the Sierra Nevada batholith are disrupted by the ~130-km-long proto–Kern Canyon fault, a Late Cretaceous ductile shear zone in the southern Sierra Nevada batholith. Vertical displacement and horizontal shortening across the proto–Kern Canyon fault in its early history are roughly constrained by the disruption of a regional primary batholithic structure that is recorded in petrologic and geochemical spatial variation patterns. The disruption of these patterns suggests that the proto–Kern Canyon fault underwent (1) subvertical west-directed reverse faulting that was instrumental in the exhumation and deep exposure of the southern part of the Sierra Nevada batholith, and (2) southward-increasing reverse/thrust displacement. The disruption of otherwise smoothly varying geobarometric gradients across the central part of the proto–Kern Canyon fault suggests up to ~10 ± 5 km of east-side-up reverse displacement across the shear zone. Southward from this area, the proto–Kern Canyon fault truncates, at an oblique angle, the petrologically distinct axial zone of the Sierra Nevada batholith, which suggests that up to ~25 km of normal shortening occurred across the southern part of the proto–Kern Canyon fault. Normal shortening is further supported by the coincidence of the Sr i = 0.706 isopleth with the proto–Kern Canyon fault from the point of initial truncation southward. Zircon U/Pb ages from plutons emplaced along the shear zone during its activity indicate that this shortening and vertical displacement had commenced by 95 Ma and was abruptly overprinted by dominantly dextral displacement with small east-side-up reverse components by 90 Ma. Conventional structural and shear fabric analyses, in conjunction with geochronological data, indicate that at least ~15 km of dextral shear slip occurred along the zone between 90 and 86 Ma, and another 12 ± 1 km of dextral slip occurred along the northern segment of the zone between 86 and 80 Ma. This later 12 ± 1 km of dextral slip branched southwestward as the ductile-brittle Kern Canyon fault, abandoning the main trace of the shear zone near its central section. Dextral shearing in the ductile regime was replaced by brittle overprinting by 80 Ma. The timing of initiation and the duration of reverse-sense displacement along the proto–Kern Canyon fault correspond closely with the shallow flat subduction of the Franciscan-affinity Rand schist along the Rand fault beneath the southernmost Sierra Nevada batholith. In its southern reaches, the proto–Kern Canyon fault flattens into the Rand fault system, suggesting that it behaved like a lateral ramp. Post–90 Ma dextral shear along the proto–Kern Canyon fault is suggested to have partitioned at least part of the Farallon plate’s tangential relative displacement component during an increase in subduction obliquity. Late-stage dextral ductile shear and early phase brittle overprints on the Kern Canyon fault system are coeval with tectonic denudation of the southernmost Sierra Nevada batholith. Geometric relations of the system’s terminal ductile and early brittle history with orthogonal extensional structures pose the possibility that the southern segment of the proto–Kern Canyon fault, along with the younger Kern Canyon fault, behaved as a transfer system during the extensional phases of tectonic denudation of the southernmost Sierra Nevada batholith, leading to exposure of the oblique crustal section we see today.

The western Jurassic belt of the Klamath Mountains represents one of the Earth's best-preserved exposures of ancient marginal ocean basin lithosphere and chiefly consists of the coeval Rogue–Chetco volcanic-plutonic oceanic arc and Josephine ophiolite. This Late Jurassic ocean basin is hypothesized to have formed in response to rifting that initiated at ca. 165 Ma along the western margin of North America, disrupting a Middle Jurassic arc that had been constructed on older Klamath terranes and forming a marginal ocean basin with an active arc, inter-arc basin, and remnant arc. Previous workers characterized a “rift-edge” facies in the remnant-arc region. This chapter describes field, age, and geochemical data that suggest that a similar rift-edge facies exists in the vicinity of the active arc, on the opposite side of the marginal basin. The rift-edge facies in the active arc setting consists of two main lithotectonic units, herein named informally as the Onion Camp complex and Fiddler Mountain olistostrome. The Onion Camp complex is partly composed of a characteristic metabasalt and red chert association. Red chert yielded scarce radiolarians of Triassic(?) and Early Jurassic age. A distinct chert-pebble conglomerate occurs at scarce localities within metasedimentary rocks. Concordant, composite bodies of amphibolite and serpentinized peridotite represent another distinctive feature of the Onion Camp complex. The metamorphic and lithologic features of the Onion Camp complex are similar to the lower mélange unit of the Rattlesnake Creek terrane, and the units are interpreted to be correlative. The Fiddler Mountain olistostrome is composed of Late Jurassic (Kimmeridgian?) pelagic and hemipelagic rocks interlayered with ophiolite-clast breccia and megabreccia, similar in character to olistostromal deposits associated with the rift-edge facies of the remnant arc. The occurrence of the Rattlesnake Creek terrane and an associated olistostromal deposit within the western Jurassic belt of southwestern Oregon may therefore represent the rift-edge facies in the active arc setting, at the transition between the Rogue–Chetco arc and Josephine ophiolite, further corroborating previous models for the Late Jurassic tectonic evolution of the Klamath Mountains.

Abstract Southeastward beyond the southern termination of the Sierran Foothills metamorphic belt, metamorphic pendants of Paleozoic ophiolitic basement are intruded by Middle Jurassic to Early Cretaceous, mafic-to-intermediate plutonic rocks. These rocks constitute a record of the various plutonic environments that were active along the western North American margin during the middle Mesozoic: a Middle Jurassic ensimatic arc, a Late Jurassic, Nevadan-age, transpressional-transtensional regime, and an emergent, Early Cretaceous continental-margin arc. In detail, these plutonic suites reveal the roles that both pre-and synmagmatic structures—such as Paleozoic transform faults, Nevadan-age regional sutures, and localized Cretaceous crustal tears—played in focusing magmatism. Taken together, outcrops of the Kings River ophiolite, the Owens Mountain dike swarm, and the Stokes Mountain ring dike complexes reveal a sequence of tectonic and magmatic processes through which accreted oceanic lithosphere was transformed into continental crust .

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Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.