Description: Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Tectonics 29 (2010): TC3001, doi:10.1029/2009TC002541.

Description: Seismic profiles of several modern arcs have identified thick, low-velocity midcrustal layers (Vp = 6.0–6.5 km/s) that are interpreted to represent intermediate to felsic plutonic crust. The presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs. However, direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs. The accreted Talkeetna arc, south central Alaska, exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a Jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust. Previous research on the arc, which has provided insight into a range of arc processes, has principally focused on western exposures of the arc in the Chugach Mountains. We present new U-Pb zircon dates, radiogenic isotope data, and whole-rock geochemical analyses that provide the first high-precision data on large intermediate to felsic plutonic exposures on Kodiak Island and the Alaska Peninsula. A single chemical abrasion–thermal ionization mass spectrometry analysis from the Afognak pluton yielded an age of 212.87 ± 0.19 Ma, indicating that the plutonic exposures on Kodiak Island represent the earliest preserved record of Talkeetna arc magmatism. Nine new dates from the extensive Jurassic batholith on the Alaska Peninsula range from 183.5 to 164.1 Ma and require a northward shift in the Talkeetna arc magmatic axis following initial emplacement of the Kodiak plutons, paralleling the development of arc magmatism in the Chugach and Talkeetna mountains. Radiogenic isotope data from the Alaska Peninsula and the Kodiak archipelago range from $\varepsilon$Nd(t) = 5.2 to 9.0 and 87Sr/86Srint = 0.703515 to 0.703947 and are similar to age-corrected data from modern intraoceanic arcs, suggesting that the evolved Alaska Peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material. The whole-rock geochemical data and calculated seismic velocities suggest that the Alaska Peninsula represents an analogue for the low-velocity middle crust observed in modern arcs. The continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the Talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs.

Description: A central debate for the evolution of the Himalayan orogen is how the Greater Himalayan Crystalline complex in its core was emplaced during the Cenozoic Indo-Asian collision. Addressing this problem requires knowledge of the structural relationship between the South Tibet detachment fault (STD) and the Main Central thrust (MCT) that bound these rocks from above and below. The fault relationship is exposed in the Himachal Himalaya of northwestern India, where they merge in their updip direction and form a frontal branch line that has been warped by subsequent top-to-the-southwest shear deformation. To elucidate how the two major crustal-scale faults evolved in the western Himalaya, we conducted integrated geologic research employing field mapping, pressure-temperature (P-T) analyses, U-Pb zircon geochronology, trace and rare earth element (REE) geochemistry, and thermochronology. Our field study reveals complex geometric relationships among major thrusts with large-magnitude shortening within each thrust sheet. Three successive stages of top-to-the-southwest thrust development are recognized: (1) imbricate stack development, (2) translation of large thrust sheets along low-angle detachments and backthrusting along the STD, and (3) development of duplex systems via underplating. This kinematic process can be quantified by our new analytical data: (1) P-T determinations show 7-9 kbar and 450-630 {degrees}C conditions across the STD. The lack of a metamorphic discontinuity across the fault is consistent with a backthrust interpretation. (2) U-Pb zircon geochronology yields ca. 830 Ma and ca. 500 Ma ages of granitoids in the MCT hanging wall, ca. 1.85 Ga ages of granitic gneisses in both the MCT hanging wall and footwall, and 8-6 Ma ages of granitic pegmatites in the MCT footwall. These ages help define regional chronostratigraphy, and the youngest ages reveal a previously unknown intrusion phase. (3) Trace element and REE geochemistry of 1.85 Ga, 830 Ma, and 500 Ma granitoids are characteristic of remelted continental crust, constraining the protolith tectonic setting. (4) U-Pb geochronology of detrital zircon reveals that siliciclastic sedimentary sequences above the STD, below the MCT, and between these two faults have similar age spectra with Neoproterozoic youngest age peaks. This result implies that the STD and MCT each duplicated the same stratigraphic section. (5) Th-Pb geochronology of monazite included in MCT hanging-wall garnet yields Paleozoic and early Tertiary ages, indicating Paleozoic and early Tertiary metamorphism in these rocks. (6) The 40Ar/39Ar thermochronology of the K-feldspar from southern MCT hanging-wall rocks evinces cooling below 220-230 {degrees}C ca. 13-19 Ma or later, constraining the thrust development history. We use these results to derive a tectonic model of crustal shortening across the Himachal Himalaya involving early thickening, tectonic wedging emplacement of the Greater Himalayan Crystalline complex between the MCT and STD, and continued growth of the Himalayan thrust wedge by accretion of thrust horses from the Indian footwall.

Description: Shallow subduction of the Farallon plate beneath the western United States has been commonly accepted as the tectonic cause for the Laramide deformation during Late Cretaceous through Eocene time. However, it remains unclear how shallow subduction would produce the individual Laramide structures. Critical information about the timing of individual Laramide uplifts, their paleoelevations at the time of uplift, and the temporal relationships among Laramide uplifts have yet to be documented at regional scale to address the question and evaluate competing tectonic models. The Wind River Basin in central Wyoming is filled with sedimentary strata that record changes of paleogeography and paleoelevation during Laramide deformation. We conducted a multidisciplinary study of the sedimentology, detrital zircon geochronology, and stable isotopic geochemistry of the lower Eocene Indian Meadows and Wind River formations in the northwestern corner of the Wind River Basin in order to improve understanding of the timing and process of basin evolution, source terrane unroofing, and changes in paleoelevation and paleoclimate. Depositional environments changed from alluvial fans during deposition of the Indian Meadows Formation to low-sinuosity braided river systems during deposition of the Wind River Formation. Paleocurrent directions changed from southwestward to mainly eastward through time. Conglomerate and sandstone compositions suggest that the Washakie and/or western Owl Creek ranges to the north of the basin experienced rapid unroofing ca. 55.5-54.5 Ma, producing a trend of predominantly Mesozoic clasts giving way to Precambrian basement clasts upsection. Rapid source terrane unroofing is also suggested by the upsection changes in detrital zircon U-Pb ages. Detrital zircons in the upper Wind River Formation show age distributions similar to those of modern sands derived from the Wind River Range, with up to [~]20% of zircons derived from the Archean basement rocks in the Wind River Range, indicating that the range was largely exhumed by ca. 53-51 Ma. The rise of these ranges by 51 Ma formed a confined valley in the northwestern part of the basin, and promoted development of a meandering fluvial system in the center of the basin. The modern paleodrainage configuration was essentially established by early Eocene time. Carbon isotope data from paleosols and modern soil carbonate show that the soil CO2 respiration rate during the early Eocene was higher than at present, from which a more humid Eocene paleoclimate is inferred. Atmosphere pCO2 estimated from paleosol carbon isotope values decreased from 2050 {+/-} 450 ppmV to 900 {+/-} 450 ppmV in the early Eocene, consistent with results from previous studies. Oxygen isotope data from paleosol and fluvial cement carbonates show that the paleoelevation of the Wind River Basin was comparable to that of the modern Great Plains ([~]500 m above sea level), and that local relief between the Washakie and Wind River ranges and the basin floor was 2.3 {+/-} 0.8 km. Up to 1 km of post-Laramide regional net uplift is required to form the present landscape in central Wyoming.

Description: During the Late Cretaceous to early Cenozoic, southern California was impacted by two anomalous tectonic events: (1) underplating of the oceanic Pelona-Orocopia-Rand schists beneath North American arc crust and craton; and (2) removal of the western margin of the arc and inner part of the forearc basin along the Nacimiento fault. The Pelona-Orocopia-Rand schists crop out along a belt extending from the southern Sierra Nevada to southwestern Arizona. Protolith and emplacement ages decrease from 〉90 Ma in the northwest to

Description: New lithostratigraphic and chronostratigraphic, geochronologic, and sedimentary petrologic data illuminate the history of development of the North American Cordilleran foreland basin system and adjacent thrust belt from Middle Jurassic through Eocene time in northwestern Montana. The oldest deposits in the foreland basin system consist of relatively thin, regionally tabular deposits of the marine Ellis Group and fluvial-estuarine Morrison Formation, which accumulated during Bajocian to Kimmeridgian time. U-Pb ages of detrital zircons and sandstone modal petrographic data indicate that by ca. 170 Ma, miogeoclinal strata were being deformed and eroded in hinterland regions. Sandstones of the Swift and Morrison Formations contain detrital zircons derived from the Intermontane belt. The Jurassic deposits probably accumulated in the distal, back-bulge depozone of an early foreland basin, as suggested by the slow rates of tectonic subsidence and tabular geometry. A regional unconformity separates the Jurassic strata from late Barremian(?) foredeep deposits. This unconformity possibly resulted as a combined effect of forebulge migration, decreased dynamic subsidence, and eustatic sea-level fall. The late Barremian(?)-early Albian Kootenai Formation is the first unit that consistently thickens westward, as would be expected in a foredeep depozone. The subsidence curve at this time begins to show the convex-upward pattern characteristic of foredeeps. By Albian time, the fold-and-thrust belt had propagated to the east and incorporated Proterozoic rocks of the Belt Supergroup, as indicated by sandstone compositions, detrital zircon ages in the Blackleaf Formation, and by crosscutting relationships in thrust sheets involving Belt Supergroup rocks in the thrust belt. A major episode of marine inundation and black shale deposition (Marias River Shale) occurred between the Cenomanian and mid-Santonian, and was followed by a regressive succession represented by the Upper Santonian-mid-Campanian Telegraph Creek, Virgelle, and Two Medicine Formations. Provenance data do not resolve the timing of individual thrust displacements during Cenomanian-early Campanian time. The Upper Campanian Bearpaw Formation represents the last major marine inundation in the foreland basin. By latest Campanian time, a major episode of slip on the Lewis thrust system had commenced, as recorded in the foreland by the Willow Creek and St. Mary River Formations in the proximal foredeep depozone. The final stage in the evolution of the Cordilleran fold-and-thrust belt and foreland basin system is recorded by the Paleocene-early Eocene Fort Union and Wasatch Formations, which were preserved in the distal foreland region. Regional extensional faulting along the fold-and-thrust belt began during the middle Eocene. The results presented here enable the establishment of links between previous geological work in Canada and the better known parts of the Cordilleran foreland basin in the United States.

Description: Insights about lateral variations in the age, composition, and structure of the central Tibetan crust are provided by geologic investigations of metamorphic rocks in the Qiangtang terrane. Previous studies have shown that a tectonic melange of Triassic age with blueschist- and eclogite-bearing blocks within a greenschist-facies matrix is exposed over an E-W distance of [~]600 km in the central Qiangtang terrane. New mapping shows that the melange extends over a N-S distance of [~]150 km, nearly to the trace of the early Mesozoic Jinsha suture in the north. The melange, exposed structurally beneath Upper Paleozoic to Mesozoic strata in the footwalls of early Mesozoic normal faults, is composed mostly of Paleozoic metasedimentary and crystalline rocks. These findings support the hypothesis that a large part of the central and northern Qiangtang terrane crust is composed of supracrustal rocks. The Duguer Range,

Description: The McCoy Mountains Formation consists of Upper Jurassic to Upper Cretaceous siltstone, sandstone, and conglomerate exposed in an east-west-trending belt in southwestern Arizona and southeastern California. At least three different tectonic settings have been proposed for McCoy deposition, and multiple tectonic settings are likely over the [~]80 m.y. age range of deposition. U-Pb isotopic analysis of 396 zircon sand grains from at or near the top of McCoy sections in the southern Little Harquahala, Granite Wash, New Water, and southern Plomosa Mountains, all in western Arizona, identified only Jurassic or older zircons. A basaltic lava flow near the top of the section in the New Water Mountains yielded a U-Pb zircon date of 154.4 {+/-} 2.1 Ma. Geochemically similar lava flows and sills in the Granite Wash and southern Plomosa Mountains are inferred to be approximately the same age. We interpret these new analyses to indicate that Mesozoic clastic strata in these areas are Upper Jurassic and are broadly correlative with the lowermost McCoy Mountains Formation in the Dome Rock, McCoy, and Palen Mountains farther west. Six samples of numerous Upper Jurassic basaltic sills and lava flows in the McCoy Mountains Formation in the Granite Wash, New Water, and southern Plomosa Mountains yielded initial {varepsilon}Nd values (at t = 150 Ma) of between +4 and +6. The geochemistry and geochronology of this igneous suite, and detrital-zircon geochronology of the sandstones, support the interpretation that the lower McCoy Mountains Formation was deposited during rifting within the western extension of the Sabinas-Chihuahua-Bisbee rift belt. Abundant 190-240 Ma zircon sand grains were derived from nearby, unidentified Triassic magmatic-arc rocks in areas that were unaffected by younger Jurassic magmatism. A sandstone from the upper McCoy Mountains Formation in the Dome Rock Mountains (Arizona) yielded numerous 80-108 Ma zircon grains and almost no 190-240 Ma grains, revealing a major reorganization in sediment-dispersal pathways and/or modification of source rocks that had occurred by ca. 80 Ma.

Description: New mapping in eastern Bhutan, in conjunction with U-Pb detrital zircon and {delta}13C data, defines Lesser Himalayan tectonostratigraphy. The DalingShumar Group, 2-6 km of quartzite (Shumar Formation) overlain by 3 km of schist (Daling Formation), contains [~]1.8-1.9 Ga intrusive orthogneiss bodies and youngest detrital zircon peaks, indicating a Paleoproterozoic deposition age. The Jaishidanda Formation, 0.5-1.7 km of garnet-biotite schist and quartzite, stratigraphically overlies the Daling Formation beneath the Main Central thrust, and yields youngest detrital zircon peaks ranging from [~]0.8-1.0 Ga to ca. 475 Ma, indicating a Neoproterozoic-Ordovician(?) deposition age range. The Baxa Group, 2-3 km of quartzite, phyllite, and dolomite, overlies the Daling-Shumar Group in the foreland, and yields ca. 0.9 Ga to ca. 520 Ma youngest detrital zircon peaks, indicating a Neoproterozoic-Cambrian(?) deposition age range. Baxa dolomite overlying quartzite containing ca. 525 Ma detrital zircons yielded {delta}13C values between +3{per thousand} and +6{per thousand}, suggesting deposition during an Early Cambrian positive {delta}13C excursion. Above the Baxa Group, the 2-3 km thick Diuri Formation diamictite yielded a ca. 390 Ma youngest detrital zircon peak, suggesting correlation with the late Paleozoic Gondwana supercontinent glaciation. Finally, the Permian Gondwana succession consists of sandstone, siltstone, shale, and coal. Our deposition age data from Bhutan: (1) reinforce suggestions that Paleoproterozoic ([~]1.8-1.9 Ga) Lesser Himalayan deposition was continuous along the entire northern Indian margin; (2) show a likely eastward continuation of a Permian over Cambrian unconformity in the Lesser Himalayan section identified in Nepal and northwest India; and (3) indicate temporal overlap between Neoproterozoic-Paleozoic Lesser Himalayan (proximal) and Greater Himalayan-Tethyan Himalayan (distal) deposition.

Description: The Chinese Loess Plateau of central Asia is composed of interbedded loess and paleosol layers, deposited during glacial and interglacial cycles, respectively, during the past [~]2.5 m.y. Understanding the provenance of loess is fundamental to reconstructing wind patterns during Quaternary glacial periods. We determined and compared U-Pb ages on zircon crystals from Loess Plateau strata and potential source areas. The results indicate that the loess was largely derived from the Qaidam Basin and the northern Tibetan Plateau to the west, both of which exhibit spatially extensive geomorphic landforms indicative of past (interpreted as pre-Holocene) wind erosion and/or deflation by westerly winds. This challenges the current paradigm that the loess of the Chinese Loess Plateau was largely sourced from deserts located to the northwest, as observed in the modern interglacial climate. We propose that during glacial periods, the mean annual positions of the polar jet streams were shifted equatorward, resulting in more southerly tracks for dust-generating storms and suppression of the East Asian monsoon by inhibiting the subtropical jet from shifting northward across the Tibetan Plateau.