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[1] Geochronology of fluvial deposits can be used to characterize provenance, the paleotopography of sediment source regions, and the development of regional drainage systems. We present U-Pb and (U-Th)/He ages of detrital zircon grains from Eocene gravels preserved in several paleoriver systems along the western flank of the central and northern Sierra Nevada. These ages allow us to trace the sourcing of detritus in paleorivers and to constrain the evolution of the Sierra Nevada range front. U-Pb zircon age distributions are bimodal, with a dominant peak between 1 10 and 95 Ma and smaller but significant peaks in the Middle to Late Jurassic, matching the predominant ages of the Sierra Nevada batholith. A small fraction (

U-Pb dating of 1035 detrital zircons from 12 spatially distributed samples of the Paleoproterozoic Vishnu Schist (Arizona, United States) reveals a bimodal [sup.207]pb/[sup.206]Pb age probability diagram with peaks at 1.8 Ga and 2.5 Ga. Surprisingly, only 13% of detrital zircon ages overlap with the published depositional age range of 1750-1741 Ma. The similarity of the age distributions in all samples constrains possible suturing of crustal blocks to pre-Vishnu Schist deposition rather than during the peak 1710-1680 Ma deformation. Of all grains analyzed, 15% overlap at 2[sigma] with the 1.84 [+ or -] 1 Ga Elves Chasm orthogneiss of the western Grand Canyon. This supports field evidence that the Vishnu Schist was deposited on 1.84 Ga arc basement rather than in a juvenile 1.75 Ga arc setting. Archean grains of 3.8-2.5 Ga compose 30% of all grains. A comparison of the ages older than 2.2 Ga from the Vishnu Schist (495 grains) with compilations of zircon ages from other cratons does not support provenances in the Wyoming, South China, or Siberian cratons; instead sources may be located in Gawler craton of Australia, North China craton, or Antarctica. If the detrital zircons were far-traveled, this is a new constraint for viable reconstructions of the Nuna supercontinent. However, given the high percentage of pre-1.8 Ga zircons, unexposed proximal basement sources are more likely, resulting in a model by which Vishnu sediments were derived from Mojave province crust that consists of Archean and 1.9-1.8 Ga crust, now in the subsurface, that was unroofed during Vishnu deposition. doi: 10.1130/G31335.1

[1] The stratigraphic record of Mesozoic arc accretion in the North American Cordillera is preserved in a discontinuous belt of clastic strata that are exposed inboard (cratonward) of the allochthonous Wrangellia composite terrane in southern Alaska, western Canada, and Washington State. LA-ICPMS analyses of eight samples (n = 714 detrital zircon grains) collected at different stratigraphic intervals from the Jurassic-Cretaceous Kahiltna assemblage in southern Alaska reveals a bulk U-Pb age distribution of Precambrian-Mesozoic age grains (Mz 74%, Pz 11%, Pc 15%). A comparison of U-Pb ages from older to younger stratigraphic intervals within the Kahiltna assemblage reveals three stages of exhumation and basin development during arc accretion. Stages include (1) an initial Late Jurassic-Early Cretaceous pre/early collisional phase during which detritus was derived almost solely from Middle-Late Jurassic and Early Cretaceous magmatic sources of the outboard Wrangellia composite terrane (Mz 100%, Pz 0%, Pc, 0%), (2) a second Early Cretaceous syncollisional phase that reflects the introduction of Paleozoic and Precambrian detritus from the inboard Intermontane belt (Mz 84%, Pz 11%, Pc 5%) and an upsection increase in older detrital zircon grains compared to Mesozoic age grains (Mz 65%, Pz 11%, Pc 24%), and (3) a final Early-Late Cretaceous late/postcollisional phase that represents continued detrital contributions from Precambrian-Mesozoic source areas (Mz 19%, Pz 22%, Pc 59%) located inboard and outboard of the Kahiltna basin. Similar bulk trends in detrital zircon age populations have been reported from along-strike, age-equivalent strata of the Gravina belt (Mz 74%, Pz 20%, Pc 6%) in southeastern Alaska suggesting that similar provenance trends may exist in basins along this >2000 km -long collisional zone. Citation: Hampton, B. A., K. D. Ridgway, and G. E. Gehrels (2010), A detrital record of Mesozoic island arc accretion and exhumation in the North American Cordillera: U-Pb geochronology of the Kahiltna basin, southern Alaska, Tectonics', 29, TC4015, doi: 10.1029/2009TC002544.

[1] The Sierra Valle Fertil Range in northwestern Argentina exposes a tilted crustal section through the Ordovician Famatinian arc, from >25 km to shallow crustal paleodepths. Fourteen new U-Pb zircon crystallization ages of magmatic rocks from Sierra Valle Fertil area show that this section of the arc was built over a short time interval during the Ordovician, between 485 and 465 Ma. Zircon rim ages demonstrate that high-grade metamorphism and migmatization were synchronous with magmatic emplacement. Inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the Famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto-South American continent in the Ordovician, which represents a part of Gondwana. Docking of the Precordilleran terrane outboard of proto-South America led to the cessation of arc magmatism in the Valle Fertil area and preservation of the arc in its early stages after

The U-Pb age spectrum of detrital zircons in sandstone of the fluvial Middle Jurassic Ionia Formation in the Michigan basin, United States, resembles the age spectra in Jurassic eolianites of the Colorado Plateau, except that a Neoproterozoic (725-510 Ma) subpopulation present in the eolianites is absent from the Michigan basin strata. The detrital zircon data are compatible with the hypothesis that the Ionia Formation was deposited by northern tributaries of a transcontinental Jurassic paleoriver system that transported detritus from headwaters in Atlantic rift highlands of the Appalachian region to floodplains and deltas in the present northern Rocky Mountains region. Previous detritai zircon studies have indicated that sedimented lowlands in that paleogeographic position were deflated by paleowinds to feed eolian sand southward (in present coordinates) into the Colorado Plateau ergs (sand seas). doi: 10.1130/G30509.1

U-Pb detrital zircon (DZ) ages (n = 807) from Lower Cretaceous and lowermost Upper Cretaceous synorogenic conglomerate and interbedded sandstone deposited in and near the foredeep of the Cordilleran foreland basin in central Utah indicate stratigraphic compositional variation among the deposits. Eight DZ age populations, ranging from Archean through Mesozoic, are present in the synorogenic foredeep deposits in varying proportions and permit definition of three compositional suites, termed here chronofacies. Chronofacies A, present in uppermost Neocomian--lowermost Aptian foredeep deposits, contains Archean through early Paleozoic DZ grains derived from Jurassic--Pennsylvanian strata of the thrust belt. Chronofacies B, in Aptian--Albian foredeep deposits, contains Archean and Paleoproterozoic grains with an age peak near 1850 Ma, a population distribution similar to that of lower Paleozoic quartzites of the thrust belt. Chronofacies C, in Albian--lower Cenomanian foredeep deposits, contains a trimodal population distribution of Paleoproterozoic and Mesoproterozoic grains similar to that of Cambrian--Neoproterozoic quartzite strata of the thrust belt. The chronofacies of the foredeep deposits thus record systematic erosional unroofing of the thrust belt during Early Cretaceous time. doi: 10.1130/G30684.1

[1] The late Miocene-Pleistocene Zhada basin in southwestern Tibet provides a record of subsidence and basin formation within an active collisional thrust belt. The >800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of Tethyan strata that were previously shortened in the Himalayan fold-thrust belt. Modal sandstone petrographic data, conglomerate clast count data, and detrital zircon U-Pb age spectra indicate a transition from detritus dominated by a distal, northern source to a local, southern source. This transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin-centric. At the same time the depositional environment in the Zhada basin changed from a large, braided fiver to a closed-basin lake. Sedimentation in the Zhada basin was synchronous with displacement on the Qusum and Gurla Mandhata detachment faults, which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin, respectively. These observations indicate that accommodation for Zhada basin fill was produced by a combination of tectonic subsidence and damming, as midcrustal rocks were evacuated from beneath the Zhada basin in response to arc-parallel slip on crustal-scale detachment faults. Citation: Saylor, J., P. DeCelles, G. Gehrels, M. Murphy, R. Zhang, and P. Kapp (2010), Basin formation in the High Himalaya by arc-parallel extension and tectonic damming: Zhada basin, southwestern Tibet, Tectonics, 29, TC 1004, doi: 10.1029/2008TC002390.

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avaionian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska-Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska-Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paieogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin.

Statistically indistinguishable U-Pb age spectra for detrital zircon populations from the quartzose petrofacies of the fluvial Lower Cretaceous Bisbee Group (southeastern Arizona) and from quartzose eolianites of the Middle to Upper Jurassic San Rafael Group (eastern Colorado Plateau) support the inference that quartzose detritus was reworked into the Bisbee basin of the border rift belt from older eolianites exposed along the uplifted rift shoulder to the north. The comparative data indicate that age spectra of detrital zircons can be reproduced faithfully during sediment recycling, and the possibility of zircon recycling should be considered for many provenance interpretations based on U-Pb ages of detrital zircons. Analogous relations between U-Pb age spectra of detrital zircons in the Lower Cretaceous basal McCoy Mountains Formation (California) and in Lower to Middle Jurassic eolianites of the western Colorado Plateau to the north suggest similar reworking of detritus from sedimentary cover of the rift shoulder into the keel of the rift belt farther west.

Sandstone petrography and detrital zircon U-Pb analysis of Upper Cretaceons--Paleogene sandstones in the foreland basin of the Sierra Madre Oriental in northeastern Mexico indicate long-distance sediment transport from arc, basement, and thrust-belt sources lying to the west, northwest, and south. The basin fill, termed the Difunta Group, consists of sublitharenites, litharenites, feldspathic litharenites, and lithic arkoses derived from mixed sources that included sedimentary rocks, magmatic arc rocks, and subordinate basement rocks. Six age populations comprise the detrital zircon content of the sandstones: Proterozoic (1900-900 Ma), early Paleozoic (500-400 Ma), late Paleozoic-Early Triassic (288-235 Ma), Jurassic (180-151 Ma), Early Cretaceous (150-111 Ma), and Late Cretaceous--Paleogene (110-54 Ma). These grains were derived from several arc terranes, ranging in age from Permian to Paleogene, in western Mexico and the southwestern United States, from sedimentary rocks and possibly interbedded tufts of the Sierra Madre Oriental orogen and from basement sources or their derivative sandstones of the southwestern United States. The petrographic and geochronologic provenance data corroborate existing models for derivation of much fore land detritus from arc sources to the west, identify the Sierra Madre orogen itself as an important source for sediment, and these data modify the Late Cretaceous--Paleogene paleogeography of Mexico to include a long, orogen-parallel fluvial system with headwaters in the southwestern United States. The difference in average ages of the youngest grains in the sandstones and their inferred depositional ages is 10.5 m.y., indicating that the initial coarse fill of the foreland basin was derived from early Laramide uplift and eastward arc migration in northwestern Mexico and the southwestern United States. Keywords: detrital zircons, Difunta Group, Mexico, foreland basin, Cretaceous, Paleogene, Laramide.

The hinterland of the Sevier orogenic belt of western North America is widely interpreted as a Cretaceous to Paleogene orogenic plateau. Although evidence for mid-crustal extension of Late Cretaceous age within the Sevier hinterland is widespread, coeval surface-breaking normal fault systems have not been documented. New 1:12,000-scale mapping within the type section of the latest Cretaceous to Eocene Sheep Pass Formation of east-central Nevada suggests that deposition occurred in response to normal fault movement recording up to 4 km of Late Cretaceous and Paleogene stratigraphic throw. Intrabasinal normal faulting caused lateral thickness variations within the basal Sheep Pass Formation, although upper members are largely unaffected. An extensional basin setting best explains the fanning of bedding dips, the deposition of megabreccia, and the presence of syndepositional normal faults within the Sheep Pass Formation. Deposition of the basal member of the Sheep Pass Formation is bracketed between ca. 81.3 [+ or -] 3.7 Ma and 66.1 [+ or -] 5.4 Ma, based on the (U-Th)/ He cooling ages of detrital zircons, and on a U-Pb carbonate age derived from the overlying lacustrine limestone member. These new data provide the strongest evidence to date for the existence of Late Cretaceous, surface-breaking normal faults in the Sevier hinterland. Normal faulting was coeval with mid-crustal hinterland extension and with continued contraction within the Sevier foreland to the east.

U-Pb ages for 1655 individual detrital zircon grains in 18 samples of eolian and associated marine and fluvial sandstones of the Glen Canyon and San Rafael Groups from the Colorado Plateau and contiguous areas shed light on patterns of Jurassic sediment dispersal within Laurentia. Most detrital zircon grains in Jurassic eolianites were derived ultimately from basement provinces older than 285 Ma in eastern and central Laurentia, rather than from rock assemblages of the nearby Cordilleran margin. The most prominent peaks of constituent age populations at 420 Ma, 615 Ma, 1055 Ma, and 1160 Ma reflect derivation from Paleozoic, Neoproterozoic, and Grenvillian sources within the Appalachian orogen or its sedimentary cover. Sediment was transported to a position upwind to the north of the Colorado Plateau by a transcontinental paleoriver system with headwaters in the central to southern Appalachian region, but subordinate non-Appalachian detritus was contributed by both northern and southern tributaries during sediment transit across the continent. Subordinate detrital zircons younger than 285 Ma in selected Middle to Upper Jurassic eolianites were derived from the Permian-Triassic East Mexico and the Mesozoic Cordilleran magmatic arcs. Lower Jurassic fluvial sandstones typically contain a mixture of detrital zircons redistributed from eolian sand and derived from the East Mexico arc, which lay up-current to the southeast. Zircons in marine Curtis sandstone were largely reworked from underlying Entrada eolianite, with minor contributions from the Jurassic backarc igneous assemblage of the Great Basin. Once mature quartzose detritus was dispersed widely across southwest Laurentia by a transcontinental paleoriver system and paleowinds, which deposited extensive Jurassic ergs, durable zircon grains were recycled by multiple intraregional depositional systems. Lower Jurassic fluvial sand is locally composed, however, of detritus derived from the nearby Cordilleran magmatic arc assemblage and its Precambrian basement. Keywords: Colorado Plateau, detrital zircon, eolianite, Glen Canyon Group, provenance, San Rafael Group.

An immense wedge of Carboniferous to Jurassic siliciclastic strata accumulated on the Verkhoyansk passive margin of the Siberian craton. U-Pb ages of detrital zircons from Pennsylvanian to Middle Jurassic sandstones are remarkably consistent and show a systematic change in the proportion of age populations. Most zircons originated from the southern margin of Siberia, which was tectonically active in the Paleozoic and early Mesozoic, and were transported to the Verkhoyansk margin by a major transcontinental river system that existed for ~200 m.y., the paleo-Lena River. Specific sources are the Angara-Vitim batholith of Transbaikalia (315 and 291-288 Ma age peaks), plutons of the Altay-Sayan region of the Central Asia fold belt (494-482 Ma), Proterozoic granitoids of northern Transbaikalia and the East Sayan Range (1888-1832 Ma), and minor contributions from the Siberian Platform and Aldan Shield (2900-2300 Ma). The paleo-Lena River met its demise when the Verkhoyansk margin was deformed in the Late Jurassic and Cretaceous, and sediment was diverted north to the Arctic Ocean. Thus, the life span of major transcontinental drainage systems can be comparable to that of the plate boundaries that surround them. Keywords: Siberia, Verkhoyansk, sedimentary provenance, detrital zircon.

Cenozoic Qaidam basin, the largest active intermountain basin inside Tibet, figures importantly in the debates on the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Asian collision. To determine when and how the basin was developed, we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin. Our geologic observations provide new constraints on the timing and magnitude of Cenozoic crustal thickening in northern Tibet. Specifically, our work shows that the southernmost part of the Qilian Shan-Nan Shan thrust belt and contractional structures along the northern margin of Qaidam basin were initiated in the Paleocene-early Eocene (65-50 Ma), during or immediately after the onset of the Indo-Asian collision. This finding implies that stress was transferred rapidly through Tibetan lithosphere to northern Tibet from the Indo-Asian convergent front located >1000 km to the south. The development of the thrust system in northern Qaidam basin was driven by motion on the Altyn Tagh fault, as indicated by its eastward propagation away from the Altyn Tagh fault. The eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of Qaidam basin, suggesting thrust loading was the main control on the basin formation and evolution. The dominant structure in northern Qaidam basin is a southwest-tapering triangle zone, which started to develop since the Paleocene and early Eocene (65-50 Ma) and was associated with deposition of an overlying southwest-thickening, growth-strata sequence. Recognition of the triangle zone and its longevity in northern Qaidam basin explains a long puzzling observation that Cenozoic depocenters have been located consistently along the central axis of the basin. This basin configuration is opposite to the prediction of classic foreland-basin models that require the thickest part of foreland sediments deposited along basin edges against basin-bounding thrusts. Restoration of balanced cross sections across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin suggests that Cenozoic shortening strain is highly inhomogeneous, varying from ~20% to >60%, both vertically in a single section and from section to section across the thrust belt. The spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of Cenozoic rotations in different parts of Qaidam basin. The observed crustal shortening strain also implies that no lower-crustal injection or thermal events in the mantle are needed to explain the current elevation (~3000-3500 m) and crustal thickness (45-50 km) of northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt. Instead, thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern Qaidam basin and the southern Qilian Shan-Nan Shan region. Keywords: Tibetan plateau, Qaidam basin. Qilian Shah, Nan Shan, thrust tectonics.

The Qiangtang metamorphic belt (QMB) in central Tibet is one of the largest and most recently documented high-pressure (HP) to near-ultrahigh-pressure (near-UHP) belts on Earth. Lu-Hf ages of eclogite- and blueschist-facies rocks within the QMB are 244-223 Ma, indistinguishable from the age of UHP metamorphism in the Qinling-Dabie orogen. Results of a U-Pb detrital zircon study suggest that protoliths of the QMB include upper Paleozoic Qiangtang continental margin strata and sandstones that were derived from a Paleozoic arc terrane that developed within the Paleo-Tethys Ocean to the north. We attribute QMB HP metamorphism to continental collision between the Qiangtang terrane and a PaleoTethys arc terrane. This collision, and the coeval South China-North China collision, may have slowed convergence between Laurasia and Gondwana-derived terranes and initiated Mediterranean-style rollback and backarc basin development within much of the remnant Paleo-Tethys Ocean realm. Keywords: Tibet, Qiangtang, eclogite, Paleo-Tethys, continental subduction, rollback.

New U-Pb zircon ages, geochemistry, and Nd isotopic data are presented from three localities in the Paleoproterozoic Mazatzal province of southern New Mexico, United States. These data help in understanding the source regions and tectonic setting of magmatism from 1680 to 1620 Ma, the timing of the Mazatzal orogeny, the nature of postorogenic magmatism, Proterozoic plate tectonics, and provide a link between Mazatzal subblocks in Arizona and northern New Mexico. The data indicate a period from 1680 to 1650 Ma in which juvenile felsic granitoids were formed, and a later event between 1646 and 1633 Ma, when these rocks were deformed together with sedimentary rocks. No evidence of pre-1680 Ma rocks or inherited zircons was observed. The igneous rocks have [[epsilon].sub.Nd](t) from -1.2 to +4.3 with most between +2 and +4, suggesting a mantle source or derivation from similar-aged crust. Nd isotope and trace element concentrations are consistent with models for typical arc magmatism. Detrital zircon ages from metasedimentary rocks indicate that sedimentation occurred until at least 1646 Ma. Both local and Yavapai province sources contributed to the detritus. All of the samples older than ca. 1650 Ma are deformed, whereas undeformed porphyroblasts were found in the contact aureole of a previously dated 1633 Ma gabbro. Regionally, the Mazatzal orogeny occurred mainly between 1654 and 1643 Ma, during final accretion of a series of island arcs and intervening basins that may have amalgamated offshore. Rhyolite magmatism in the southern Mazatzal province was coeval with gabbro intrusions at 1633 Ma and this bimodal magmatism may have been related to extensional processes following arc accretion. Keywords: Mazatzal province, New Mexico, Proterozoic, U-Pb geochronology, zircons, neodymium.

The Magdalena fan is an apparently beheaded submarine depocenter that has figured prominently in reconstructions of middle to late Miocene Pacific-North American plate interactions. The deposit accumulated rapidly at the base of the continental slope on top of newly formed oceanic crust of the Magdalena microplate from 14.5 to 13 Ma. Subduction of this crust ceased as the Pacific-Magdalena spreading center encountered the trench. The widely accepted two-phase kinematic model for the formation of the Gulf of California holds that ~300 km of dextral shear between the Pacific and North American plates occurred along faults west of Baja California prior to the onset of dextral-transtensional shearing in the gulf ca. 6 Ma. We measured 1796 detrital zircon U-Pb ages from 65 samples in an effort to characterize the provenance of the fan, determine its source region, and define the cumulative dextral slip along faults offshore of southwestern Baja California. Zircons from the fan are dominantly 120-65 Ma with subordinate 15-35 Ma grains. Excellent matches to the fan can be obtained by mixing Magdalena shelf strata and/or adding detritus from the west-draining portion of the Los Cabos block. The same cannot be accomplished with zircons from the east-draining portion of the Los Cabos block and mainland Mexico. Our results favor a western Baja source region for the fan and suggest that cumulative dextral slip along faults west of Baja was Keywords: Magdalena fan, detrital zircon, U-Pb, Baja California, Mexico, Integrated Ocean Drilling Program, Gulf of California, Neogene. doi: 10.1130/B26067.1

[1] In the distal region of the modern flat-slab segment in the southern Central Andes, an unusual stack of middle Miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west. These features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system. Sedimentary thickness farther east and far from the Cordilleran tectonic loads suggests accommodation and preservation driven by 'nonisostatic' dynamic subsidence. Regional overlapping relationships and basin modeling suggest that the Modern broken foreland (present Sierras Pampeanas) can be interpreted as a reactivation of a formerly partitioned broad forebulge. Citation: Davila, F. M., R. A. Astini, T. E. Jordan, G. Gehrels, and M. Ezpeleta (2007), Miocene forebulge development previous to broken foreland partitioning in the southern Central Andes, west-central Argentina, Tectonics, 26, TC5016, doi:l 0.1029/ 2007TC002118.

New thermobarometric and U/Pb and [sup.40]Ar [sup.39]Ar geochronologic data coupled with ages obtained from the Acatlan Complex, the basement of the Mixteco terrane of southern Mexico, reveal the existence of three distinctive high-pressure metamorphic events of early to middle Paleozoic age, each recorded in a separate lithological suite. Xayacatlan suite eclogites with oceanic affinity underwent peak metamorphism at 609-491[degrees]C and 13-12 kb during the Early Ordovician (ca. 490-477 Ma, U-Pb zircon), followed by a partial overprint at 600[degrees]C and ~9.6 kb and then at 500[degrees]C and ~6.7 kb. An overprinting event at 525-500[degrees]C and ~9.5 kb is ascribed to the Devonian. The pressure-temperature (P-T) path of the Xayacatlan suite indicates a subduction-exhumation process followed by tectonically related reburial. Ixcamilpa suite blueschists with oceanic affinity underwent epidote-blueschist metamorphism (T, 200-390[degrees]C; P, 6-9 kb) and then epidoteamphibolite (T, 390-580[degrees]C, P, 9-6 kb) events ascribed to the Late Ordovician--Early Silurian. Esperanza suite eclogites with continental affinity underwent peak metamorphism at 830-730[degrees]C and 17-15 kb. Amphibole from eclogite yields a 430 [+ or -] 5 Ma [sup.40]Ar/[sup.39]Ar age, dating the high-pressure (HP) event. P-T paths of high-temperature (HT) eclogites like those of the Esperanza suite have been related to the collision of continental blocks. Partial overprinting occurred at 690-640[degrees]C and 14-10 kb prior to 374 [+ or -] 2 Ma ([sup.40]Ar/[sup.39]Ar, phengite). The three HP suites were tectonically juxtaposed at different times before the Mississippian Period, resulting in the closure of the Iapetus Ocean. Phengite [sup.40]Ar/[sup.39]Ar geochronology reveals the existence of a widespread tectonothermal event between 345 and 323 Ma, which may be related to the juxtaposition of the HP-composed block and the Gondwanan-affinity Cosoltepec suite, causing the closure of the Rheic Ocean. The tectonothermal events in the Acatlan Complex coincide in time, physical conditions, and tectonic setting with events in the Appalachian-Caledonian orogen, suggesting their relation. On that basis the geology of the Acatlan Complex can lead to a more comprehensive understanding of the tectonic evolution of the Appalachian orogen and of the Gondwana-Laurentia interactions preceding the Pangean assembly. Keywords: eclogite, blueschist, Paleozoic, Appalachian, Acatlan Complex, Mexico.

The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intraoceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to subaerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of [sup.143]Nd/[sup.144]Nd and [sup.87]Sr/[sup.86]Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the are, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [[epsilon].sub. Nd](t) = 6.0-7.8 and [sup.87]Sr/[sup.86][Sr.sub.int] = 0.703379-0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5-168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [[[epsilon].sub.Nd] = 5.6-7.2 and [sup.87]Sr/[sup.86][Sr.sub.int]= 0.703383-0.7013624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [[[epsilon].sub.Nd](t) = 4.0-5.5 and [sup.87]Sr/[sup.86][Sr.sub.int] = 0.703656-0.706252] and contain inherited xenucrystic Carboniferous-Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust. Keywords: island arc, arc plutonism, Talkeetna, Chugach, Alaska, zircon.

Late Jurassic--Cretaceous are-related volcaniclastic rocks from the southern Guerrero and western Mixteca terranes of Mexico were analyzed by U-Pb detrital zircon geochronology (laser ablation-multicollector--inductively coupled plasma--mass spectroscopy) to place constraints on the depositional history and provenance of the rocks. Pre--Middle ,Jurassic basement rocks and sandstone from the Upper Cretaceous Mexcala Formation were also analyzed to define the origin and provenance of the prevolcanic substratum, and the time of accretion of Guerrero composite terrane sequences. Data from the Taxco-Taxco Viejo, Teloloapan, and Arcelia assemblages indicate that the youngest (129-141 Ma) zircon fraction in each sequence was derived from local volcanic sources, whereas older populations (ca. 247-317, 365-459, 530-617, 712-878, 947-964, 1112-1188, 1350-1420, 1842-1929, 2126-2439, and 2709-3438 Ma) show sediment influx from varied sources, most likely through grain recycling. The major zircon clusters in these sequences match the populations recorded in the nearby Acatlan Complex. In contrast, the Huetamo sample is dominated by Lower Cretaceous (ca. 126 Ma) zircons of local volcanic provenance, and the Zihuatanejo sample contains zircon clusters (ca. 259, ca. 579, and ca. 947-1162 Mat comparable to major populations recorded in the underlying Arteaga Complex. A sample from the Middle Triassic--Middle Jurassic Arteaga Complex at Tzitzio contains zircon clusters (ca. 202-247, ca. 424, ca. 600, ca. 971, and ca. 2877 Ma) consistent with an ultimate derivation from both North American and South American sources. The sample from the Las Ollas suite contains comparable zircon populations (ca. 376-475, ca. 575, ca. 988-1141, and ca. 2642-2724 Ma). and it is interpreted to be part of the prevolcanic basement. In contrast, the youngest zircon cluster (ca. 105 Ma) in the Mexcala Formation coincides with the major volcanic events in the Taxco-Taxco Viejo, Teloloapan, and Arcelia assemblages, whereas the older clusters (ca. 600, ca. 953, ca. 1215, ca. 1913, and ca. 2656-2859 Ma) broadly match the major populations recorded in rocks from the Acatlan Complex. These new data combined with available geochemical and isotopic data indicate that the Taxco-Taxco Viejo arc assemblage developed on continental crust. The Acatlan Complex is the most plausible candidate. The Teloloapan and Arcelia arc assemblages were developed on oceanic crust as offshore arcs facing the Acatlan Complex. The Zihuatanejo terrane assemblages were developed on the Arteaga Complex, and evidence no influence from the Acatlan Complex. This suggests that these assemblages were formed farther away or in a restricted basin. The Guerrero composite and Mixteca arc successions are coeval with the Alisitos arc of northern Mexico and in part with the Nevada and Klamath ranges of the southwestern United States, and with the arc series from the Greater and Lesser Antilles and northern South America. Data indicate that during late Mesozoic time, southwestern North America was a site of intensive volcanism in a complex arc-trench system similar to that of the east Pacific. Our data are consistent with a diachronic accretion of the Guerrero composite terrane sequences, beginning during late Cenomanian time with the amalgamation of the Teloloapan and probably the Arcelia assemblages, and finishing at the end of Cretaceous time with the accretion of the Zihuatanejo terrane assemblages. Keywords: U-Pb detrital zircon, Late Jurassic-Cretaceous arc successions, Guerrero terrane, Mixteca terrane, southern Mexico, southwestern North America, Caribbean region.

Exhumed sections of the middle and lower crust in western New Zealand reveal how deformation was partitioned within a thermally and rheologically evolving crustal column during Cretaceous continental extension. Structural data, P-Tdeterminations, and U-Pb geochronology from central Fiordland and the Paparoa Range in Westland show that extension initiated in the lower crust by ~114 Ma as a period of arc-related magmatism waned. Initially, deformation was localized into areas that were weakened by heat and magma. However, these hot, weak zones were ephemeral. During the period 114-111 Ma, lower crustal fabrics record a rapid progression from magmatic flow to high-temperature deformation at the gamet-granulite facies (T > 700[degrees]C, P = 12 kbar) to cooler deformation at the upper amphibolite facies (T = 550-650[degrees]C, P = 7-9 kbar). Lower crustal cooling and compositional contrasts between mafic granulites and hydrous metasedimentary material resulted in a middle crust that was weak relative to the lower crust. Between circa 111 and circa 90 Ma, focused subhorizontal flow and vertical thinning in a weak middle crust led to the collapse of the upper crust and the unroofing of midcrustal material. During this period, arrays of conjugate-style shear zones transferred displacements vertically and horizontally through the crust, resulting in a structural style that resembles crustal-scale boudinage. The New Zealand example of continental extension shows that a weak middle crust and a relatively cool, highly viscous lower crust can result in a localized style of extension, including the formation of metamorphic core complexes that exhume the middle crust but not the lower crust. doi:10.1029/2006TC002003.

A geological and geochronologic investigation of the Nima area along the Jurassic-Early Cretaceous Bangong suture of central Tibet (~32[degrees]N, ~87[degrees]E) provides well-dated records of contractional deformation and sedimentation during mid-Cretaceous and mid-Tertiary time. Jurassic to Lower Cretaceous ([less than or equal to] 125 Ma) marine sedimentary rocks were transposed, intruded by granitoids, and uplifted above sea level by ca. 118 Ma, the age of the oldest nonmarine strata documented. Younger nonmarine Cretaceous rocks include ca. 110-106 Ma volcanic-bearing strata and Cenomanian red beds and conglomerates. The Jurassic-Cretaceous rocks are unconformably overlain by up to 4000 m of Upper Oligocene to Lower Miocene lacustrine, nearshore lacustrine, and fluvial red-bed deposits. Paleocurrent directions, growth stratal relationships, and a structural restoration of the basin show that Cretaceous--Tertiary nonmarine deposition was coeval with mainly S-directed thrusting in the northern part of the Nima area and N-directed thrusting along the southern margin of the basin. The structural restoration suggests >58 km (>47%) of N-S shortening following Early Cretaceous ocean closure and ~25 km shortening (~28%) of Nima basin strata since 26 Ma. Cretaceous magmatism and syncontractional basin development are attributed to northward low-angle subduction of the Neotethyan oceanic lithosphere and Lhasa-Qiangtang continental collision, respectively. Tertiary syncontractional basin development in the Nima area was coeval with that along the Bangong suture in westernmost Tibet and the Indus-Yarlung suture in southern Tibet, suggesting simultaneous, renewed contraction along these sutures during the Oligocene-Miocene. This suture-zone reactivation immediately predated major displacement within the Himalayan Main Central thrust system shear zone, raising the possibility that Tertiary shortening in Tibet and the Himalayas may be interpretable in the context of a mechanically linked, composite orogenic system. Keywords: Tibet, plateau, thrust belt, Indo-Asian collision, suture zone, basin development.

The timing of protolith formation, ultra-high-pressure (UHP) subduction, and subsequent exhumation for the ultrahigh-pressure to high-pressure units across the eastern part of the Western Gneiss Region, Norway, were assessed using U/Pb zircon, Th/Pb monazite, and [sup.40]Ar/[sup.39]Ar white mica ages. U/Pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the Baltica basement before the entire mass was subducted to (ultra)high pressure. Eclogites within the allochthons across the entire Western Gneiss Region are Caledonian and show a degree of zircon (re)crystallization that increases with peak pressure, permitting the interpretation that the entire region underwent synchronous subduction. [sup.40]Ar/[sup.39]Ar white mica ages of 399 Ma indicate that the eastern part of the Western Gneiss Region had been exhumed to shallow crustal levels while UHP metamorphism was ongoing farther west, indicating a westward dip to the slab. The [sup.40]Ar/[sup.39]Ar white mica ages also show a clear east-to-west gradient across the entire Western Gneiss Region, indicating that the Western Gneiss Region rose diachronously to crustal levels from east to west between 399 and 390 Ma. Keywords: ultrahigh pressure, Western Gneiss Region, secondary ion mass spectrometry, [sup.40]Ar/[sup.39]Ar, exhumation, laser ablation inductively coupled plasma-mass spectrometry.

Although detrital zircon has proven to be a powerful tool for determining provenance, past work has focused primarily on delimiting regional source terranes. Here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance, and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers. For such an analysis to succeed for a given mountain, discrete intrarange source terranes must have unique U/Pb zircon age signatures and sediments eroded from the range must have well-defined depositional ages. Here we use ~1400 single-grain U/Pb zircon ages from northeastern Tibet to identify and analyze an area that satisfies these conditions. This analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete, punctuated changes in local source terranes. By tracking eroding rock units chronologically through the stratigraphic record, this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca. 8 Ma in the Laji Shan, a 10-25-km-wide range in northeastern Tibet with a unique U/Pb age signature. Keywords: detrital zircon, Tibet, provenance, unroofing, Miocene, U/Pb.

Abstract: Two samples of late Paleozoic grit and Late Mississippian quartzite-chert conglomerate collected from southeastern Yukon-Tanana terrane (YTT)--a composite thrust sheet resting structurally above North American parautochthonous strata and intervening [...]

[1] New U/Pb zircon and Th/Pb monazite ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane. Combined with Sm/Nd ages, Rb/Sr ages, inclusion relationships, and geologic relationships, they help define the timing of peak recrystallization, the timing of subsequent amphibolite-facies metamorphism, and the architecture of the Dabie-Sulu suture zone between the collided Sino-Korean and Yangtze cratons. The data indicate a ~15 Myr record of UHP recrystallization, the first clearly documented for a giant UHP terrane; this requires that continental subduction in the Dabie-Sulu orogen involved multiple UHP tectonic or recrystallization events. A 244-236 Ma 'precursor' UHP event, seen only in the Dabie Shan, was followed by a second, ~230-220 Ma 'main' UHP event, which was itself terminated by a 220-205 Ma amphibolite-facies overprint. Older eclogite-facies events seen in the Qinling segment of this orogenic belt raise the possibility that these rocks have undergone (U)HP metamorphism three or four times, but at present, there is no geochronological evidence in the Dabie-Sulu area to support this. The subduction of the lower, Yangtze plate did not proceed in a simple fashion: The ages of inherited zircon cores demonstrate that a ribbon continent of Yangtze affinity escaped subduction and became wedged against the Sino-Korean plate hanging wall. Citation: Hacker, B. R., S. R. Wallis, L. Ratschbacher, M. Grove, and G. Gehrels (2006), High-temperature geochronology constraints on the tectonic history and architecture of the ultrahigh-pressure Dabie-Sulu Orogen, Tectonics, 25, TC5006, doi: 10.1029/2005TC001937.

[1] To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum-Arctic region were dated by laser ablation-inductively coupled plasma-mass spectrometry (ICP-MS). The northern Verkhoyansk (NE Russia) has Permo-Carboniferous (265-320 Ma) and Cambro-Silurian (410-505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo-Carboniferous (280-330 Ma) and late Precambrian-Silurian (420-580 Ma) zircons in addition to Permo-Triassic (235-265 Ma), Devonian (340-390 Ma), and late Precambrian (1000-1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/ or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian-latest Precambrian (500-600 Ma) and 445-490 Ma zircons. Permo-Carboniferous and Permo-Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130-1240 Ma and older Precambrian zircons in addition to 430-470 Ma zircons. The most popular plate tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska--Chukotka microplate away from the Canadian Arctic margin. The dental zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic. Citation: Miller, E. L., J. Toro, G. Gehrels, J. M. Amato, A. Prokopiev, M. I. Tuchkova, V. V. Akinin, T. A. Dumitru, T. E. Moore, and M. P. Cecile (2006), New insights into Arctic paleogeography and tectonics from U-Pb detrital zircon geochronology, Tectonics, 25, TC3013, doi:10.1029/ 2005TC001830.

The U-Pb and [sup.40]Ar/[sup.39]Ar studies of a unique exposure of crystalline basement along the Jurassic-Early Cretaceous Bangong suture of central Tibet reveal previously unrecognized records of Mesozoic metamorphism, magmatism, and exhumation. The basement includes Cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185-170 Ma. The basement cooled to ~300 [degrees]C by 165 Ma and was exhumed to upper crustal levels in the hanging wall of a south-directed thrust system during Early Cretaceous time. We attribute Jurassic metamorphism and magmatism to the development of a continental arc during Bangong Ocean subduction, and Early Cretaceous exhumation to northward continental underthrustiug of the Lhasa terrane beneath the Qiangtang terrane. We speculate that a Jurassic arc extended regionally along the length of the Bangong suture, but in all other places in Tibet has been buried, either depositionally or structurally, beneath supracrustal assemblages. Keywords: Tibet, Bangong suture, terrane accretion, continental arcs, continental collision.

Using detrital zircon geochronology, turbidite deposystems fed from distinct sediment sources can be distinguished within the Songpan-Ganzi complex, a collapsed Middle to Late Triassic turbidite basin of central China. A southern Songpan-Ganzi deposystem initially was sourced solely by erosion of the Qinling-Dabie orogen during early Late Triassic time, then by Qinling-Dabie orogen, North China block, and South China block sources during middle to late Late Triassic time. A northern Songpan-Ganzi system was sourced by erosion of the Qinling-Dabie orogen and the North China block throughout its deposition. These separate deposystems were later tectonically amalgamated to form one complex and then uplifted as the eastern Tibet Plateau. Keywords: turbidite, collisional orogen, unroofing, detrital zircon, China.

The Unkar Group of the Grand Canyon Supergroup is one of the best-preserved remnants of Mesoproterozoic sedimentary rocks in the southwestern United States. It provides an exceptional record of intracratonic basin formation and associated tectonics kinematically compatible with protracted 'Grenville-age' NW-directed shortening. New U/Pb age determinations from an airfall tephra at the base of the Unkar Group dates the onset of deposition at ca. 1255 Ma, and [sup.40]Ar/[sup.39]Ar K-feldspar thermochronology in the Grand Canyon indicates that basement rocks cooled through 150 [degrees]C between ca. 1300 and 1250 Ma, refining exhumation rates of basement rocks just prior to Unkar deposition. Abrupt thickness and facies changes in conglomerate and dolomite of the Bass Formation (lower Unkar Group) associated with NE-striking monoclinal flexures indicate NW-directed synsedimentary contraction at ca. 1250Ma. A large disconlormity (~75 m.y. duration) is inferred between the lower and upper Unkar Group and is located below the upper Hakatai Shale, as documented by detrital zircons. A second style of Unkar Group deformation involved the development of half grabens and lull grabens that record NE-SW extension on NW-striking, high-angle normal faults. Several observations indicate that NW-striking normal faulting was concurrent with upper Unkar deposition, marie magmatism, and early Nankoweap deposition: (1) intraformational faulting in the Bass Formation, (2) intraformational faulting in the 1070 Ma (old Rb/Sr date) Cardenas Basalt and lower Nankoweap Formation, (3) syntectonic relationships between Dox deposition and 1104 Ma (new Ar/Ar date) diabase intrusion, and (4) an angular unconformity between Unkar Group and Nankoweap strata. The two tectonic phases affecting the Unkar Group (ca. 1250 Ma and ca. 1100 Ma) provide new insight into tectonics of southern Laurentia: (1) Laramide-style (monoclines) deformation in the continental interior at ca. 1250 Ma records Grenville-age shortening; and (2) ca. 1100 Ma detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate an Unkar Group source in the Grenville-age highlands of southwestern Laurentia during development of NW-striking extensional basins. We conclude that far-field stresses related to Grenville-age orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimentary and tectonic regime of southwestern Laurentia from 1250 to 1100 Ma. Keywords: Grenville, Unkar Group, Nankoweap Formation, Precambrian monocline, intracratonic rifting, Grand Canyon.

Five isotope-enhanced geologic transects in the southern Annapurna Range of central Nepal elucidate structural geometries near the Main Central thrust. Whole-rock Nd isotopes and U-Pb ages of detrital zircons unambiguously distinguish Greater Himalayan (hanging wail) and Lesser Himalayan 0botwall) metasedimentary rocks. [[epsilon].sub.Nd](0) values for lower Lesser Himalayan rocks typically range from -20 to -26, whereas Greater Himalayan rocks usually have [[epsilon].sub.Nd](0) values of -19 to -12. Lower Lesser Himalayan rocks yield detrital zircons with an age peak at ca. 1880 Ma and no detrital zircons younger than ca. 1550 Ma. In contrast, Greater Himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca. 1050 Ma and no detrital zircons younger than ca. 600Ma. The protolith boundary between Greater and Lesser Himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology. Field and microstructural observations indicate the presence of a top-to-the-south ductile shear zone superimposed on this boundary, confirming this shear zone as the Main Central thrust. No evidence exists for large-scale structural mixing of Greater and Lesser Himalayan rocks along the Main Central thrust in the Annapurna Range. Keywords: Himalaya, Nepal, tectonics, Nd-143/Nd-144, U/Pb, microstructure.

Salinia is an out-of-place granitic terrane in central coastal California whose debated origin is critical to understanding the tectonic history of southwestern North America. Salinian metasedimentary and sedimentary rocks that respectively host and cover its predominant are rocks should contribute important data about its origin and kinematic history, but pervasive intrusion, high-grade metamorphism and Cenozoic erosion of the Salinian block have inhibited their widespread characterization and correlation. To further address these problems, we report 605 U-Ph detrital-zircon geochronologic ages collected by laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) from seven Salinian metasedimentary framework (Sur Series) and sedimentary cover samples. Samples collected from the Sur Series contain Late Archean (2.5-2.9 Ga), late Paleoproterozoic (1.6-1.9 Ga), Mesoproterozoic (0.9-1.5 Ga), Neoproterozoic (0.65-0.8 Ga), Paleozoie (250-450 Ma), and possibly Mesozoic U-Pb detrital-zircon ages. Samples collected from Upper Cretaceous cover units have various age-peak distributions, which collectively include late Paleoproterozoic (I.6-1.8 Ga), early Mesoproterozoic (1.35-1.55 Ga), Permo-Triassic (220-290 Ma), and Jurassic-Cretaceous (80-190 Ma) peaks. From these data, several interpretations are made. (1) Maximum depositional ages of the Sur Series and cover intervals are 280-360 Ma and 78-90 Ma, respectively. (2) The presence of Late Archean, early Paleoproterozoic, and Neoproterozoic zircons in Salinian metasedimentary rocks suggest that uplift and erosion of adjacent basins recycled sediment onto Salinia. (3) The abundant pre-Mesoproterozoic detrital-zircon ages in Sur Series and cover units preclude the possibility that Salinia originated in southern Mexico, as has been previously suggested. (4) Five of six key detrital-zircon age peaks identified in Salinian basement and cover units are nowhere more closely arranged than in the Mojave Desert-Peninsular Ranges region of Baja and southern Alta California. (5) Paleozoic and early Mesozoic detrital zircons in Sur Series and cover units match the ages of several plutonic events that occurred along the western margin of North America--however, Permian ages favor a Mojave Desert origin over other candidates. Collectively, these and other data suggest that Salinia resided in the Mojave Desert-Peninsular Ranges region from the late Paleozoic until the Late Cretaceous, after which it was rapidly exhumed, deposited upon, and then translated outboard and northward to its current position. Keywords: tectonics, Cordillera, California, provenance, Sur Series, depositional age.

A late Oligocene-Miocene igneous complex south and west of Wrangell, Alaska, contains mafic dikes that yield a discordant paleomagnetic direction (inclination, I = 70.4[degrees]; declination, D = 39.3[degrees]; [[alpha].sub.95] = 4.8[degrees]; N = 72 sites). Combined with local and regional geobarometric, metamorphic, and structural observations, the discordant paleomagnetic direction indicates east-side-up tilt by 16[degrees] about a tilt axis with azimuth = 8[degrees]. Neogene tilt of crustal blocks in the Insular superterrane accounts for much of the paleomagnetic discordance in Cretaceous plutons without the coastwise translation of >1000 km, as suggested by the Baja British Columbia hypothesis. Keywords: Coast orogen, southeast Alaska, paleomagnetism, Neogene crustal extension.

Ages from U-Pb and [sup.207]Pb/[sup.206]Pb analyses of detrital zircons from synrift sandstone in the Lower Cambrian Cerro Totora Formation of the Argentine Precordillera have strong similarities to those from late synrift sandstones in the Lower Cambrian Rome Formation of southern Laurentia (Alabama). Ages of detrital zircons from the Cerro Totora sample cluster at 1160-970 Ma (60% of analyzed zircons), 1490-1300 Ma (24%), and 1890-1640 Ma (16%). Ages from two Rome samples cluster at 1240-970 Ma (32% of analyzed zircons), 1540-1270 Ma (31%), 1840-1610 Ma (14%), 1970-1890 Ma (5%), and 2930-2310 Ma (18%). The ages of detrital zircons from the Rome and Cerro Totora sandstones are consistent with sediment supply from the Grenville and older Proterozoic Laurentian provinces, and the older cluster in the Rome sandstones corresponds in age to the Laurentian Archean Superior province. Neither the Rome nor Cerro Totora samples include components younger than Grenville, and the lack of zircons from distinctly Gondwanan provinces is consistent with a Laurentian provenance. The detrital-zircon ages support previous interpretations that the Precordillera was rifted from the Ouachita embayment of Laurentia during Early Cambrian time and subsequently was transferred to Gondwana. Keywords: Argentine Precordillera, continental rifting, detrital-zircon geochronology, Laurentia, terrane transfer.

The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/ granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laser-ablation inductively coupled plasma-mass spectrometer (MC-LA-ICP-MS). About 20-30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30-42 km from the coast, yielded Grenvilleaged zircons (970-1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages (25-32 Ma) on undeformed plutons as well as mid-Mesozoic and Permian ages on gneisses. Most samples along the Puerto Escondido transect contain inherited ca. 1.1 Ga xenocrystals of zircons. The western transect (Acapolco) yielded Late Jurassic-Early Cretaceous ages (160-136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons, with no inherited Grenville ages. The older ages and xenocrystic zircons in arc-related Xolapa Complex mirror the crustal ages found in neighboring terranes (Mixteca and Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin of Xolapa with respect to its neighboring north-bounding terranes. The new data and previously published ages for Xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the Cenozoic. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa, and probably represent the more deeply exposed continuation of the Sierra Madre Occidental arc to the northwest. The available U-Pb data argue against the previously proposed eastward migration of magmatism between Acapulco and Puerto Angel during the Oligocene. Keywords: Xolapa Complex, arc magmatism, U-Pb zircon, geochronology, deformation.

The Pamir mountains are an outstanding example of extreme crustal shortening during continental collision that may have been accommodated by formation of a thick crust--much thicker than is currently thought--and/or by continental subduction. We present new petrologic data and radiometric ages from xenoliths in Miocene volcanic rocks in the southeastern Pamir mountains that suggest that Gondwanan igneous and sedimentary assemblages were underthrust northward, buried to >50-80 km during the early stage of the India-Asia collision, and then heated and partly melted during subsequent thermal relaxation before finally being blasted to the surface. These xenoliths, the deepest crustal samples recovered from under any active coilisional belt, provide direct evidence for early Cenozoic thickening of the Pamirs and lower-crustal melting during collision; the xenoliths also suggest that the present mountain range was a steady-state elevated plateau for most of the Cenozoic. Keywords: Pamir region, continental collision, subduction, partial melting, orogenic plateaus.