ALBERT

Description: Temporal and spatial patterns in the architecture of the Franciscan Complex provide valuable insights into the subduction processes through which such patterns arise. The Nacimiento Franciscan belt is an allochthonous sliver of subduction assemblages in the central California Coast Ranges displaced either: (1) from southern California by 〉300 km of Neogene dextral slip along the San Andreas fault system or (2) from central California to southern California and back again, by 〉500 km of Late Cretaceous–Paleocene sinistral slip along the Sur-Nacimiento fault followed by San Andreas–related motion. New U-Pb detrital zircon data from 20 (meta)clastic samples indicate that the Nacimiento Franciscan section was assembled between ca. 95 and 80 Ma. Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic and Proterozoic zircon grains point to a southern California origin for Nacimiento Franciscan protoliths, precluding significant sinistral strike-slip along the Sur-Nacimiento fault. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from subduction complexes in southern California that were emplaced during Laramide shallow subduction (i.e., Sierra de Salinas, Portal Ridge, Quartz Hill, Rand, San Emigdio, and Tehachapi schists). Hence, the Nacimiento Franciscan is distinct from Franciscan rocks in central and northern California and more likely represents an outboard element of the Late Cretaceous southern California low-angle subduction system. Upon restoring the Nacimiento block to its Late Cretaceous position, an inboard-younging trend is apparent in the composite Nacimiento–southern California schist belt, suggesting that progressively younger accretionary materials were underplated farther inboard by tectonic erosion. We posit that arc and forearc elements absent from southern California were removed by a combination of physical and tectonic erosion attending shallow subduction, interleaved in the subduction complex, and recycled into the mantle. Steepening of the Laramide slab was marked by a phase of crustal extension in the overriding plate. During this phase, the Sur-Nacimiento fault likely functioned as a segment of a low-angle normal fault system spanning the southern Sierra Nevada batholith to the Nacimiento accretionary system.

Description: The Upper Triassic Chinle Formation in southwestern Laurentia is the oldest distinctive record of Early Mesozoic Cordilleran arc magmatism, in the form of detrital zircons and volcanic clasts. Initial deposition of the basal Shinarump and Mesa Redondo members, herein collectively called the Shinarump conglomerate, began in Late Triassic time, yet the earliest known arc magmatism is older by as much as 40 m.y. Analysis of detrital zircons from eight sites in southeastern Nevada, southern Utah, and northeastern Arizona and volcanic-clast zircons from four of these sites provides a basis for understanding the evolution of the Early Mesozoic arc. Most Permian and Triassic detrital zircons from the Shinarump conglomerate have ages from ca. 260 to 220 Ma with rare grains as old as 280 Ma. These ages are compatible with derivation from sources in the magmatic arc to the west and southwest, including plutons of corresponding age in the Mojave Desert. Volcanic clasts are uniformly in the range 232–224 Ma; their age and zircon geochemistry argue against a source in currently exposed Mojave Desert Triassic plutons. As a further test, we compared Th/U ratios of clast and detrital zircons with those of possible sources to the west. Th/U values of many detrital grains support their derivation from Triassic Mojave Desert plutons. Some detrital grains and those from the clasts, however, have Th/U values that are uniformly higher than those in Permo-Triassic Mojave Desert plutons and therefore argue for a different, unexposed source. We propose that the early arc lay offshore of western Laurentia. Over time, plutons were emplaced across a range of continental crustal thicknesses that likely increased toward the east. At approximately 235–230 Ma, a land connection between the arc and retro-arc areas was established and fluvial sedimentation began. The observation that the youngest grain ages in our detrital samples are variable suggests that this land connection was tenuous for perhaps 10 m.y. until well into Chinle Formation sedimentation.

Description: Correlation of lithotectonic packages across major transcurrent structures is critical to understanding the tectonic evolution of the North American continental margin. Detrital zircon geochronology of uppermost Proterozoic to Lower Paleozoic miogeoclinal strata from the White-Inyo Mountains permits evaluation of: (1) the age and provenance of these metasediments and (2) a model for truncation of the passive margin along a postulated large-magnitude Cretaceous dextral shear zone, i.e., the Mojave–Snow Lake fault. U-Pb ages of detrital zircons from the Neoproterozoic Wyman Formation, the uppermost Proterozoic Reed Dolomite (Hines Tongue Member), and clastic strata of the Lower Cambrian Deep Springs, Campito (Montenegro Member), Poleta, and Harkless formations reflect ultimate derivation from the adjacent 1.7–pre-1.8 Ga Mojavia terrane and/or 1.7–1.8 Ga Yavapai continental basement, with subsidiary sources in both the ca. 1.4 Ga Yavapai-Mazatzal anorogenic granitoids and the 〉2.5 Ga North American craton, and a small proportion of 1.0–1.3 Ga grains, most likely reworked from Grenville clastic wedge deposits. Detrital zircon age spectra from the Lower Cambrian Andrews Mountain Member of the Campito Formation are unique in comparison with the remainder of the studied section, containing a major age peak centered at ca. 1.1 Ga and a subsidiary Lower Cambrian age peak, permitting calculation of a ca. 527 ± 12 (2) Ma maximum depositional age. These features, in addition to abundant detrital magnetite-ilmenite grains, reflect a distal source for these rocks, most likely from the ca. 1.1 Ga Pikes Peak batholith and/or the Midcontinent rift and ca. 0.53 Ga bimodal intrusions of the Oklahoma-Colorado aulacogen. In terms of zircon age distribution, allochthonous metamorphic pendants in the Snow Lake terrane of the central Sierra Nevada batholith are most similar to those of stratigraphically equivalent units in the Death Valley region and, to lesser degrees, the White-Inyo section, and the Mojave Desert region. Given the similarity and relative proximity of Death Valley facies assemblages to the Snow Lake terrane, we suggest that the latter was not transported northward from the Mojave Desert region and instead represents footwall assemblages of a late Early to early Middle Jurassic low-angle normal fault system, probably along the outer transform-truncated margin of the Last Chance thrust stack. This model implies a few tens of kilometers of offset, in contrast to the hundreds of kilometers required by the Mojave–Snow Lake fault hypothesis.

Description: In the central Klamath Mountains, the English Peak plutonic complex (EPC) invaded the faulted contact between the outboard Eastern Hayfork and inboard North Fork terranes of the Western Paleozoic and Triassic Belt (WTrPz). This calc-alkaline igneous complex is composed of two small, ~1–2-km-diameter, relatively mafic satellitic plutons peripheral to the younger, much larger, ~10–15-km-diameter English Peak zoned granitic pluton. The EPC magmas were mantle derived and reflect temporary residence and mixing at various depths in the overlying crust, with initial storage and modification near the Moho, and uppermost crustal emplacement at 5–10 km depths. Phase assemblages suggest pre-emplacement magma storage at a depth of ~20–25 km for the early satellitic plutons, versus ~15–20 km for samples from the larger zoned granitic pluton. We obtained zircon U-Pb geochronologic results (reported as internal and external weighted-mean 207 Pb-corrected 206 Pb/ 238 U ages, 95% confidence level) from seven samples in the complex via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The 172.3 ± 2.0 [3.7] Ma Uncles Creek and 166.9 ± 1.6 [3.4] Ma Heiney Bar satellitic plutons range from gabbro–quartz diorite to granodiorite in bulk-rock composition. The main English Peak pluton consists of an early stage of gabbro-tonalite (three samples: 160.4 ± 1.1 [3.1] Ma, 158.1 ± 1.1 [3.1] Ma, and 158.0 ± 1.2 [3.1] Ma) and a late stage (two samples: 156.3 ± 1.3 [3.1] Ma and 155.3 ± 1.2 [3.0] Ma) passing inward from tonalite through granodiorite to a central zone of granite. The 172 Ma age of the Uncles Creek pluton makes it coeval with Middle Jurassic Western Hayfork arc magmatism. In contrast, Heiney Bar and the main English Peak igneous ages overlap some of the oldest and youngest components, respectively, of the Middle to Late Jurassic Wooley Creek plutonic suite. Study of this multiple-intrusion complex provides an illuminating example of the gradual intermediate-to-felsic modification of the upper crust in the central Klamath Mountains. Inherited zircon ages of ca. 172 Ma in two other EPC samples indicate potential Middle Jurassic crustal sources or contaminants. Geochronologic correlation of the EPC with geologic histories of other Klamath terranes provides fresh insights for understanding spatial and temporal elements of Middle to Late Jurassic arc magmatism in the Klamath Mountains sector of the Cordilleran margin. This igneous activity illuminates some petrotectonic processes whereby accreted ophiolitic basement terranes were modified and incorporated into the evolving Jurassic continental crust. It took place prior to the earliest Cretaceous onset of westward transport of the stack of Klamath allochthons relative to the active Jura-Cretaceous Sierran calc-alkaline arc.

Description: Monazite ([LREE]PO 4 , where LREE stands for light rare earth element) and xenotime (Y[HREE]PO 4 , where HREE stands for heavy rare earth element) occur in ore-grade concentrations within the Pinto Gneiss in the Music Valley region of southern California. However, both the age and petrogenesis of this potentially economically significant rare earth element (REE) deposit remain uncertain. New petrologic and geochronologic data enable assessment of the textural and temporal relationships between REE-bearing minerals and the host rock and development of a petrogenetic model for REE mineralization. Ore-forming monazite and xenotime are typically restricted to biotite folia within the host Pinto Gneiss, with greatest modal abundances occurring within a few meters of contacts between the host gneiss and a metadiorite intrusive unit, the latter of which is crosscut by pegmatite veins generated by partial melting of the Pinto Gneiss. Ore-forming monazite and xenotime preserve complex internal elemental zonation defining two distinct textures: (1) oscillatory zoning interpreted to represent primary crystallization, overprinted by (2) irregular embayed textures inferred to be the result of fluid-mediated dissolution re-precipitation reactions. The altered domains in monazite consist of primary monazite replaced by secondary monazite along with xenotime and uranothorite [(U,Th)SiO 4 ] inclusions. Similarly, primary xenotime is replaced by secondary xenotime with monazite and uranothorite inclusions. Localized breakdown of monazite, anorthite, and biotite to apatite and allanite provides further evidence for postmineralization metasomatism of the ore bodies. In situ monazite and xenotime U-Pb geochronology constrains the timing of primary REE mineralization to ca. 1.71 Ga, consistent with zircon dates obtained from the Pinto Gneiss. Based on the similarity in ages of monazite, xenotime, and zircon in the Pinto Gneiss, along with relict "igneous" zoning in the ore-bearing phosphate minerals, REE mineralization is inferred to have occurred during crystallization of the igneous protolith to the Pinto Gneiss. Metadiorite emplacement at ca. 1.4 Ga and pegmatite genesis at ca. 165 Ma both postdate the main phase of REE mineralization but likely played a role in fluid-assisted alteration, breakdown, and partial resetting of monazite and xenotime U-Pb systematics in the Pinto Gneiss.

Description: We image the lithospheric and upper asthenospheric structure beneath the central and eastern parts of the northern Gulf of Aden rifted passive continental margin with 59 broadband stations to evaluate the role of transform fault zones on the evolution of magma-poor continental margins. We used teleseismic tomography to compute a relative P wave velocity model in eastern Yemen and southern Oman down to 400 km depth. Our model shows low-velocity anomalies located in the vicinities of five major fracture zones and regions of recent volcanism. These low-velocity anomalies are likely caused by localized asthenospheric upwelling and partial melting, caused by small-scale convection promoted by gradients in the lithosphere-asthenosphere boundary topography near the fracture zones. In addition, low velocities underlie regions of elevated topography between major sedimentary basins. We suggest that locally buoyant mantle creates uplift and dynamic topography on the rift margin that affects the course of seasonal rivers and the sedimentation at the mouth of those rivers. Our new P wave velocity model suggests that the dynamic topography and recent volcanism in the central and eastern Gulf of Aden could be due to small-scale convection at the edge of the Arabian plate and/or in the vicinity of fracture zones.

Description: Models of arc-continent accretion often assume that the period of subduction of continental lithosphere before plate boundary reorganization is fairly short lived, yet the timescale of this period is poorly constrained by observations in the geologic record. The island of Timor is the uplifted accretionary complex resulting from the active collision of the Banda volcanic arc with the Australian continental margin. The exposure of underplated and exhumed Australian strata on Timor allows for the characterization of the structural history of accretion of uppermost Australian crust and the quantification of subduction of its original continental lithospheric underpinnings. New structural mapping in East Timor (Timor-Leste) reveals that duplexing of a 2-km-thick package of Australian continental strata has built the majority of the structural elevation of the Timor orogen. Coupling new structural observations with previous thermochronology results reveals the sequence of deformation within the orogen, the presence of subsurface duplexing below the hinterland slate belt, and motion along a foreland subsurface thrust ramp. Construction of balanced cross sections allows for the quantification of the amount of shortening in the orogen, and from that, the length of the subducted Australian continental lithosphere. Two balanced cross sections in East Timor reveal 326–362 km of shortening and that 215–229 km of Australian continental lithosphere have been subducted below the Banda forearc. These results highlight the fact that considerable amounts of continental lithosphere can be subducted while accreting only a thin section of uppermost crust. Continental subduction may have been favorable at Timor because of fast subduction rates, old oceanic crust at the consumed Australian margin, and subduction of some length of transitional crust. These results provide quantitative constraints for future numerical modeling of the geodynamics of continental subduction and arc-continent collision.

Description: Anthropogenic warming may promote rapid permafrost thaw in the Arctic and alter the global carbon cycle. Although several studies suggest increased thermo-erosion as a result of recent warming, a long-term context is necessary to assess the linkages of thermokarst processes with climate variability. We analyzed sediment cores from two lakes on the Alaskan North Slope (USA), one with (Lake NE14) and one without (Perch Lake) watershed thermo-erosion. Distinct geochemical and lithological characteristics provide evidence for sedimentary input from carbonate-rich permafrost soils associated with past retrogressive thaw slumping at Lake NE14 but not at Perch Lake. These characteristics include increases in Ca:Sr, Ca:K, carbonate:[feldspar + clay minerals], percent CaCO 3 , and 13 C, and decreases in 87 Sr: 86 Sr. At least ten episodes of thermo-erosion occurred over the past 6000 yr at Lake NE14. Most of these episodes coincided with periods of elevated summer temperatures, but moisture variation and geomorphic factors likely played a role in driving their occurrence. Our results suggest that positive feedbacks facilitate reactivation of thermo-erosion in ice-rich terrain, adding to the growing body of evidence that these Arctic landscapes are unstable in a changing climate.