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The Sub-Andean retroarc region is a unique example of an active continental-scale retroarc foreland basin system. Heavily targeted for hydrocarbon exploration, the region hosts a large volume of subsurface data coupled to surface studies dedicated to refining its evolution in time and space. This paper presents a regional correlation of stratigraphic markers from seismic reflection and well logs across the Sub-Andean foothills at 23–21°S in southern Bolivia and northern Argentina, which reveals the contrasting along-strike history of Mesozoic to Cenozoic tectonics that preceded the foreland basin setting. Supported by published geochronological data and new zircon U-Pb maximum depositional ages, we describe the depositional transition from pre-Andean to Andean stratigraphy and discrete episodes of foreland basin subsidence and shortening. Based on interpreted stratigraphic breaks, we define the extent and stepwise evolution of this foreland basin, which was characterized by the progressive eastward migration of foreland basin depozones. Based on restored thickness profiles, we present flexural models of basin subsidence for the Sub-Andean foothills region. The modeling of discrete episodes of foreland basin subsidence refines the widely accepted bimodal elastic strength in the fore-land basin at 21–23°S, which is weaker in the western ranges (∼20 km effective elastic thickness) and stronger eastward (>40 km). Modeling results also reveal minimum values of subsidence rates (up to 1.2 mm/yr) in the sequential foredeep depozones and suggest that the modeled tectonic load migration—as constrained by the record of syntectonic strata—probably increased over time through the incorporation of Sub-Andean rocks into the orogenic wedge. [ABSTRACT FROM AUTHOR]

The Himalaya is known for dramatically rugged landscapes including the highest mountains in the world. However, there is a limited understanding of the timing of attainment of high elevation and relief formation, especially in the Nepalese Himalaya. Anomalous high-elevation low-relief (HELR) surfaces, which exhibit geomorphic antiquity and are possibly remnants of formerly widespread high-elevation paleosurfaces, provide a unique opportunity to assess the attainment of regional high elevation in the Himalaya. The Bhumichula plateau is one such HELR surface (4300-4800 m) in the western Nepalese Himalayan fold-thrust belt. The Bhumichula plateau is situated in the Dadeldhura klippe (also called the Karnali klippe), an outlier of Greater Himalayan Sequence highgrade metasedimentary/igneous rocks surrounded by structurally underlying Lesser Himalayan Sequence low-grade metasedimentary rocks. We assess the origin of the Bhumichula plateau by combining regional geological relationships and zircon and apatite (U-Th-Sm)/He and apatite fission track thermochronologic ages. The HELR surface truncates pervasive west-southwestward dipping foliations, indicating that it post-dates tilting of rocks in the hanging wall of the Main Central thrust above the Lesser Himalayan duplex. This suggests that the surface originated at high elevation by erosional beveling of thickened, uplifted crust. Exhumation through the ~180-60 °C thermal window occurred during middle Miocene for samples on the plateau and between middle and late Miocene for rocks along the Tila River, which bounds the north flank of the Bhumichula plateau. Cooling ages along the Tila River are consistent with erosional exhumation generated by early Miocene emplacement of the Main Central (Dadeldhura) thrust sheet, middle Miocene Ramgarh thrust emplacement, and late Miocene growth of the Lesser Himalayan duplex. The most recent middle-late Miocene exhumation took place as the Tila River and its northward flowing tributaries incised upstream, such that the Bhumichula plateau is a remnant of a more extensive HELR paleolandscape. Alpine glaciation lowered relief on the Bhumichula surface, and surface preservation may owe to its relatively durable lithology, gentle structural relief, and elevation range that is above the rainier Lesser Himalaya. [ABSTRACT FROM AUTHOR]

The Andes are the culmination of shortening and crustal thickening that commenced during Late Cretaceous time. First-order questions regarding the tectonic evolution of the central Andes include the magnitude and timing of shortening, and controls on the along-strike variability in observed styles of shortening and deformation. Along-strike differences in the time of surface uplift have spawned two contrasting hypotheses: (1) uplift is related to dynamic and isostatic processes accompanying lithospheric removal and is decoupled in space and time from crustal thickening and shortening, and (2) uplift is directly coupled with shortening and crustal thickening. Although considerable work has been done in Bolivia to address these hypotheses, work in northern Argentina has not yet produced a transorogenic balanced structural cross section from which the total amount and kinematic history of shortening can be evaluated. To help understand the evolution of the thrust belt in northernmost Argentina, we present a regional, retrodeformable cross section at 23°-24°S across the Puna and Eastern Cordillera. New apatite fission-track thermochronological data integrated with other geochronological, sedimentological, and structural data constrain incremental retrodeformation of the cross section between ca. 45 and 6.5 Ma. Regional shortening was facilitated by at least 12 major thrust systems, linked to a regional mid-crustal décollement. Deformation generally propagated eastward through time and involved two major episodes of eastward advance of the orogenic front, separated by periods of internal out-of-sequence shortening and kinematic stagnation of the orogenic front. A new minimum estimate of ~271 km of total shortening from the Cordillera de Domeyko to the eastern orogenic front explains crustal thickening at northern Puna latitudes. Together with previously published paleoaltimetry data, our new structural and thermochronologic data indicate that regional uplift in the northern Argentine Puna and Eastern Cordillera was synchronous with, and thus directly linked to, crustal shortening and thickening. [ABSTRACT FROM AUTHOR]

The article reports on the sedimentology, detrital zircon geochronology and stable isotopic geochemistry of the lower Eocene formations in the Wind River Basin in Wyoming. The Washakie Range and western Owl Creek Mountains reportedly underwent rapid unroofing at 55.5-54.5 million years (Ma), with the Wind River Range becoming the dominant sediment source terrane by circa 53-51 Ma. The article states that detrital zircon grains based on depositional age are absent in early Eocene sediment.

Our understanding of the processes involved in the Indo-Asian collision and the construction of the Tibetan Plateau are, in part, predicated on our understanding of the tectonic setting and crustal conditions of southern Tibet in the time period immediately preceding the Indo-Asian collision (Late Cretaceous). Several hypotheses have been proposed that describe the middle to Late Cretaceous tectonic and paleogeographic evolution of southern Tibet, each with different implications for when and how the Tibetan Plateau was uplifted. We examined the mid-upper Cretaceous Takena Formation of the Lhasa terrane in southern Tibet in order to reconstruct the middle to Late Cretaceous tectono-sedimentary history of this area and test the competing tectonic hypotheses. The Takena Formation consists of >2 km of sedimentary strata that include a lower marine limestone member (Penbo Member) and an upper member of clastic red beds (Lhunzhub Member). The Aptian-Albian Penbo Member consists of ∼250 m of orbitolinid-limestone beds that were deposited in a shallow-marine seaway during a time when there was little regional subsidence. The overlying Lhunzhub Member (>1500 m) consists primarily of fluvial strata that were deposited during a period of increased subsidence. Overall, the Lhunzhub Member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base, to multistory-multilateral braided stream deposits near the top. The fluvial sandstone units are lithic-rich and contain abundant plagioclase and volcanic grains, and paleo-current data record northwest-directed transport; these data indicate that the sediment was derived from the Gangdese volcanic arc that developed along the southern margin of the Lhasa terrane. Following deposition, but prior to ca. 70 Ma, the beds of the Takena Formation were folded and partially eroded. The sedimentary and stratigraphic characteristics of the Takena Formation are most consistent with deposition in a retroarc foreland basin setting. The limestone beds of the Penbo Member were deposited in a shallow-marine seaway that was eventually infilled by clastic sediment derived principally from the Gangdese volcanic arc. The low subsidence rate recorded in the lowermost strata of the Takena Formation, including the Penbo Member and the paleosol-rich interval of the Lhunzhub Member, is interpreted to be associated with the passage of a flexural forebulge. The overlying, upward-coarsening fluvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin. The Late Cretaceous folding of the Takena Formation indicates that the foreland basin strata were eventually incorporated into a fold-and-thrust belt. The upper Cretaceous sedimentary strata of the Lhasa terrane of southern Tibet predict that a north-verging fold-and-thrust belt existed along the southern margin of the Lhasa terrane during Late Cretaceous time, prior to the Indo-Asian collision. Collectively, the evidence indicates that the southern margin of the Lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the Indo-Asian collision. [ABSTRACT FROM AUTHOR]

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 wall) and Lesser Himalayan (footwall) metasedimentary rocks. εNd(0) values for lower Lesser Himalayan rocks typically range from -20 to -26, whereas Greater Himalayan rocks usually have εNd(0) values of -19 to -12. Lower Lesser Himalayan rocks yield detrital zircon s 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. 600 Ma. 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. [ABSTRACT FROM AUTHOR]

Examines the development of early to middle Tertiary foreland basin in Bolivia. History of Andean crustal shortening; Presence of interbedded fluvial sandstone and mudstone; Measurement of lacustrine carbonate and clastic rocks; Consistency of the lithofacies and sediment-accumulation history.

Documents the sedimentology of the Miocene-Pliocene Thakkhola graben fill in central Nepal. Reconstruction of basin evolution and paleoenvironment; Analysis of depositional systems, paleo-drainage patterns; Development of axial drainage in the basin.

Infers wedge behavior in the Sevier thrust belt of northeast Utah and southwest Wyoming through time using the time-space distribution of thrust faulting and sediment accumulation. Regional patterns of thrusting, erosion and sediment accumulation; Controls on sevier orogenic wedge.

Presents a reinterpretation of the distribution and timing of late Cretaceous through Paleocene thrust faulting in the Sevier thrust belt in Utah. Integration of cross-cutting structural relationships; Overlapping of sedimentary units; Conglomerate provenance data; Radiometric and palynological data; Eastward progression of deformation.

Documents the geometry and sedimentary facies of a well-exposed submarine canyon and associated paleoenvironments within the Stilo-Capo d'Orlando Formation along the southeastern coast of Calabria, southern Italy. Conglomeratic canyon fill; Thin-bedded turbidites; Sea-level changes; Local tectonic control.

Reliable ages of proximal conglomerates in the Cordilleran foreland basin that are associated with emplacement and erosion of major thrust sheets are essential for reconstructing the kinematic history of the Sevier fold-thrust belt. Although these conglomerates have been dated by palynology, their absolute ages have been difficult to determine because of their coarse-grained texture and a lack of marine interbeds and tuffaceous deposits. We collected sets of oriented samples from outcrops in northeastern Utah, USA, to construct an overall magnetic polarity stratigraphy that can be correlated to the geomagnetic polarity time scale (GPTS). We sampled fine-grained, hematitic interbeds in the Upper Cretaceous Echo Canyon Conglomerate and Weber Canyon Conglomerate. Common paleomagnetic and rock magnetic analyses were conducted, and several rock magnetic results indicated that the dominant magnetic carriers in these weakly magnetized rocks are hematite and very subordinate magnetite/titanomagnetite/maghemite and goethite. Demagnetization results show that hematitic, fine-grained sandstone to siltstone intervals carry a geologically stable magnetization with directions and polarity consistent with the Late Cretaceous geomagnetic field. A small percentage of samples carry a low laboratory unblocking temperature secondary overprint residing primarily in goethite. Magnetic polarity results indicate that the Echo Canyon Conglomerate is exclusively of normal polarity and that the younger Weber Canyon Conglomerate is of normal polarity in its lowermost part, reverse polarity in the middle, and normal polarity in the upper part of the sequence. The new data indicate that these coarse-grained strata were most likely deposited over the time span of the magnetic polarity Chron (C) 34n to C33r interval and the younger C33r to C33n interval; the former interval includes the Santonian-Campanian stage boundary (ca. 83.4 Ma/83.1 Ma). Palynological data suggest that these rocks span Coniacian-Santonian time (ca. 89--84 Ma); thus, the most parsimonious correlation of the normal polarity magnetozone of the Echo Canyon Conglomerate is with the youngest part of C34n Superchron, which is of ca. 30 Ma duration (ca. 115 Ma to 83.4 Ma/83.1 Ma). The normal polarity magnetozone of the lower part of the younger Weber Canyon Conglomerate likely correlates to the youngest part of C34n, whereas the reverse and normal magnetozone from the middle and upper parts of the Weber Canyon Conglomerate likely correlate to C33r and C33n, respectively. We infer that the Santonian-Campanian boundary resides in the lower Weber Canyon Conglomerate, which implies that deposition of the unit started prior to the C34n/C33r boundary age (ca. 83.4 Ma/83.1 Ma) and continued through the C33r and C33n chrons. Sediment provenance data and growth structures tie the Echo Canyon and Weber Canyon Conglomerates to emplacement of the Crawford thrust sheet. Based on the magnetic polarity data, as constrained by the biostratigraphic age estimates from these synorogenic deposits, we hypothesize that the principal displacement along the Crawford thrust started during the Coniacian (>C34n/C33r boundary age) and continued into the middle Campanian (

The timing of deformation within and adjacent to the Helena salient of west-central Montana is poorly constrained relative to other segments of the Sevier fold-and-thrust belt. This study presents low-temperature thermochronology data from the Little Belt Mountains, a basement-cored Laramide uplift that is juxtaposed with the leading edge of the salient. We analyzed eight samples of Paleoproterozoic basement for apatite fissiontrack (AFT) and zircon (U-Th)/He (ZHe) thermochronology. Four samples yielded AFT ages ranging from ca. 80 Ma to 73 Ma and associated long, unimodal confined track lengths, indicating rapid cooling and exhumation of Little Belt Mountains basement during the Late Cretaceous. The other four samples are characterized by younger AFT ages (ca. 55 Ma), which suggest a combination of prolonged residence in the apatite partial annealing zone and postexhumation magmatic reheating. In total, 20 new ZHe dates range from ca. 236 Ma to 28 Ma and show a correlation between date and effective uranium. Forward model results for ZHe data are consistent with upper-crustal residence during the Proterozoic followed by Phanerozoic burial and rapid Late Cretaceous cooling. Cross sections across the Little Belt Mountains display the geometry of the Volcano Valley fault zone, an array of down-to-the-south Proterozoic normal faults that profoundly influenced the development of the Cordilleran thrust belt. Our new constraints from the Little Belt Mountains when integrated with published kinematic constraints from the Helena salient reveal significant out-of-sequence deformation in this portion of the thrust belt between ca. 80 Ma and 55 Ma. A kinematic model is proposed that involves Late Cretaceous (ca. 80 Ma) exploitation of rheologically incompetent units at the base of the Belt Supergroup within the Helena Embayment, facilitating early exhumation in the Little Belt Mountains. Our new data and synthesis are consistent with previous interpretations in which an inherited stratigraphic and structural architecture of Proterozoic ancestry was the predominant control on the development of the Helena salient during Cretaceous-early Eocene time. [ABSTRACT FROM AUTHOR]

We present a new geologic map of western Nepal and three balanced regional cross sections in the Himalayan thrust belt. The minimum shortening between the South Tibetan detachment and the Main Frontal thrust is 485--743 km and suggests that total Himalayan shortening may exceed 900 km. All rocks involved in the thrust belt are of upper crustal affinity, implying that a comparable length of Indian lower crust and mantle lithosphere was subducted beneath Tibet. Major structural features are the Subhimalayan thrust system, Lesser Himalayan imbricate zone, Dadeldhura thrust sheet, Lesser Himalayan duplex, Ramgarh thrust sheet, Main Central thrust sheet, and a north-dipping normal-sense shear zone, possibly related to the South Tibetan detachment. These structures are continuous along the entire Nepalese portion of the Himalayan thrust belt. New 40Ar/39Ar ages from the Ramgarh thrust zone, Greater Himalayan rocks, and the lower part of the Tethyan sequence support a kinematic model in which major thrust systems in Nepal propagated southward from early Miocene time onward. The geometry and kinematic history of the thrust belt in western Nepal are not compatible with recent models for southward ductile extrusion of Greater Himalayan rocks in a mid-crustal channel. Instead, the thrust belt in western Nepal behaved like a typical forward propagating thrust system, involving unmetamorphosed, brittlely deformed rocks in its frontal part and ductilely deformed, higher-grade metamorphic rocks in its hinterland region. Although our results do not support published versions of the channel flow model, they provide additional geological and geo-chronological data that will assist future attempts to develop geodynamic models for the Himalayan-Tibetan orogenic system. [ABSTRACT FROM AUTHOR]

The Canyon Range, Pavant, Paxton, and Gunnison thrust systems in central Utah form the Sevier fold-and-thrust belt in its type area. The Canyon Range thrust carries an 12-km-thick succession of Neoproterozoic through Triassic sedimentary rocks and is breached at the surface by the Neogene extensional Sevier Desert detachment fault. The Pavant, Paxton, and Gunnison thrusts carry Lower Cambrian through Cretaceous strata and have major footwall detachments in weak Jurassic rocks. The Canyon Range thrust was active during latest Jurassic--Early Cretaceous time. The Pavant thrust sheet was emplaced in Albian time, formed an internal duplex beneath the Canyon Range during the Cenomanian, and then developed a frontal duplex during the Turonian. The Paxton thrust sheet was initially emplaced during the Santonian, and subsequently formed the Paxton duplex during the early to mid-Campanian. Some slip on the Paxton system was fed into a frontal triangle zone along the Sanpete Valley antiform. The Gunnison thrust system became active in late Campanian time and continued to feed slip into the frontal triangle zone through the early Paleocene. The Canyon Range and main Pavant thrust sheets experienced long-distance eastward transport (totaling >140 km) mainly because they are composed of relatively strong rocks, whereas the eastern thrust sheets accommodated less shortening and formed multiple antiformal duplexes in order to maintain sufficient taper for continued forward propagation of the fold-and-thrust belt. Total shortening was at least 220 km. Upper crustal thickening of 16 km produced crust that was >50 km thick and a likely surface elevation >3 km in western Utah. Shortening across the entire Cordilleran retroarc thrust belt at the latitude of central Utah may have exceeded 335 km. The Late Cretaceous paleogeography of the fold-and-thrust belt and foreland basin was similar to the modern central Andean fold-and-thrust belt, with a high-elevation, low-relief hinterland plateau and a rugged topographic front. The frontal part of the Sevier belt was buried by several kilometers of nonmarine and shallow-marine sediments in the wedge-top depozone of the foreland basin system. The Canyon Range thrust sheet dominated sediment supply throughout the history of shortening in the Sevier belt. Westward underthrusting of a several hundred-kilometer-long panel of North American lower crust beneath the Cordilleran magmatic arc is required to balance upper-crustal shortening in the thrust belt, and may be petrogenetically linked to a Late Cretaceous flare-up of the magmatic arc as preserved in the Sierra Nevada Batholith. [ABSTRACT FROM AUTHOR]

Thermochronologic dates of partially reset detrital grains are often difficult to interpret because of the potential for variation in inherited predepositional dates and kinetic behaviors of grains from diverse source terrains. In this study, we present several examples of complex detrital zircon (U-Th)/He date distributions from sedimentary rocks that have been heated to temperatures in or near the zircon He partial retention zone, leading to several types of date-eU correlations caused by variations in predepositional inherited age and radiation-damage--induced He diffusion kinetics. These examples are from three subvertical transects in mountain ranges in central Utah: the Wasatch Range near Provo, Utah, the Oquirrh Mountains, and the Stansbury Mountains. Each range lies in the hanging wall of one of three major thrust sheets that compose part of the Charleston-Nebo salient, a segment of the Cretaceous Sevier fold-and-thrust belt. Zircon grains from two of these transects (the Stansbury and Oquirrh Mountains) show large date variations that can be at least partially understood with a radiation-damage--based model for He diffusion in zircon. We combine the output from this model with a new approach for understanding partially reset data sets that relies upon the concept of an "inheritance envelope." The transect from the Wasatch Range is relatively simple and does not require an inheritance-based interpretation. We document a pulse of 5 km of exhumation in this range at ca. 100 Ma. Time-temperature (t-T) constraints from inheritance envelopes in the Oquirrh Mountains transect suggest a pulse of exhumation beginning at either 110 or 100 Ma. Finally, for the Stansbury transect, the inheritance envelope approach yields inconclusive results; some aspects of the model generate inheritance envelopes that match the measured data set, whereas others do not. Despite their complexity, the t-T histories for these nonideal data sets represent some of the only in situ constraints on the timing of Cretaceous exhumation of Sevier-style deformation in the U.S. Cordillera. In the case of the Oquirrh and Wasatch Range data sets, these direct measurements of thrust sheet t-T histories offer some insight into the evolution of the Sevier fold-and-thrust belt of central Utah. [ABSTRACT FROM AUTHOR]

New low-temperature thermochronology and geochronology data from Upper Jurassic-Upper Cretaceous strata from the North American Cordilleran foreland basin in Utah, Colorado, Wyoming, and South Dakota document rapid exhumation rates of the adjacent Cordilleran orogenic belt to the west. Both zircon (U-Th-[Sm])/He (zircon He) and apatite fission track (AFT) thermochronology were applied to proximal and distal synorogenic deposits in order to identify a thermochronometer suitable to record source exhumation during the North America Cordilleran orogeny. AFT lag times from Upper Jurassic-Upper Cretaceous deposits are 0-5 m.y. and indicate a relatively steady-state to slightly increasing exhumation rate between 118 Ma and 66 Ma. These lag-time measurements are consistent with active shortening and rapid exhumation rates of ~0.9->1 km/m.y. of the North American Cordillera throughout the Cretaceous. Double dating of the detrital AFT samples was performed on apatites with young AFT cooling ages, in order to test whether or not the young cooling ages represent a signal related to exhumation rather than volcanic activity. Maximum depositional ages using detrital zircon U-Pb geochronology match existing ages on basin stratigraphy. This study indicates that AFT is the most effective thermochronometer to resolve source exhumation from Lower to Upper Cretaceous foreland stratigraphy in the central Cordilleran foreland, and indicates that source material was exhumed from 4 to 5 km depths but was never buried more than a few kilometers (<4 km) since Cretaceous time. Zircon He dates indicate that the orogenic hinter land could not have been exhumed from depths >8-9 km. Double dating of apatites (with AFT and U-Pb) shows that volcanic contamination is a significant issue that can, however, be addressed by double dating. [ABSTRACT FROM AUTHOR]

The Upper Cretaceous Fort Crittenden Formation exposed in the Santa Rita and Huachuca Mountains of southeastern Arizona is a syntectonic deposit that has been associated with Laramide tectonic activity. However, the spatio-temporal relationships among Cretaceous sedimentation, magmatism, basement exhumation, and possible flat slab-related processes in the southern Laramide region remain poorly understood. Age controls for uplift and erosion of local topography and syntectonic deposition in response to deformation remain particularly poor. The Fort Crittenden Formation comprises 800-2500 m of locally derived fluvial to alluvial fan sedimentary rocks and records paleodrainage reorganization in response to active tectonics. Changes in sedimentary facies, provenance, and paleoflow suggest deposition in a tectonically partitioned intraforeland basin. New detrital zircon data constrain the timing of deposition of the Fort Crittenden Formation between ca. 86 Ma and ca. 76 Ma. The lack of depositional age zircons throughout the majority of the Fort Crittenden Formation is consistent with a magmatic lull in the Cordilleran arc between ca. 90 Ma and ca. 76 Ma. The overlying Salero Formation and Late Cretaceous intrusions are expressions of renewed magmatism in southeastern Arizona at ca. 75 Ma. New Lu-Hf data indicate that magmas evolved from contamination of old juvenile crust. When interpreted in conjunction with other regional data sets, our study indicates that the Laramide deformation front migrated eastward into southwestern New Mexico by 75 Ma. Thermal modeling of apatite fission track and (U-Th)/He data from granitic clasts are consistent with Late Cretaceous--Paleocene (ca. 76-55 Ma) heating related to magmatism and cooling and exhumation during the Eocene and Oligocene. [ABSTRACT FROM AUTHOR]

The Indus-Yarlung suture zone, within the larger Indo-Asia collision zone in southern Tibet, is characterized by a central depression with two oppositely directed axial rivers: the eastward-flowing Yarlung River and the westward-flowing Indus River. The axial valley is flanked by high-elevation ranges of the southern Lhasa terrane to the north and the Tethyan Himalaya to the south. This study analyzes the detrital geochronological and thermochronological signatures of rivers draining into the Indus-Yarlung suture zone as a proxy for the timing of tectonic, magmatic, and erosional processes in southern Tibet. Zircon U-Pb ages reflect source area crystallization ages, and their distribution is proportional to the relative area of source rocks exposed in the catchment areas. Rivers draining the northern side of the collision zone are dominated by ages between ca. 40 Ma and ca. 60 Ma, similar to ages of rocks in the Gangdese arc, whereas one river sample draining the southern side records a Tethyan Himalayan signature characterized by age clusters at ca. 500 Ma and ca. 1050 Ma. U-Pb zircon ages from the modern drainages are similar to signals preserved in the Oligocene-Miocene Liuqu and Kailas basins, suggesting that the geology of the Cenozoic drainages was similar to modern, albeit with significant erosion in the Miocene. New apatite fission-track (AFT) ages from some of the same rivers show an early Miocene regional exhumation signature, which is interpreted to record regional uplift-induced erosion coupled with efficient river incision by the Indus-Yarlung fluvial system as a result of renewed underthrusting (following rollback) of India under Asia. Late Miocene (ca. 8 Ma) apatite (U-Th)/He (AHe) ages are consistent with cooling and exhumation associated with E-W extension followed by a decrease in erosion after ca. 6 Ma. [ABSTRACT FROM AUTHOR]

Differential exhumation in the Puna Plateau and Eastern Cordillera of NW Argentina is controlled by inherited paleostructures and resulting paleotopography related to the Cretaceous Salta Rift paleomargins. The Cenozoic deformation front related to the development of the Andean retro-arc orogenic system is generally associated with >4 km of exhumation, which is recorded by Cenozoic apatite fission-track (AFT) and (U-Th-[Sm])/ He ages (He ages) in the Eastern Cordillera of NW Argentina. New AFT ages from the top of the Nevado de Cachi document Oligocene (ca. 28 Ma) cooling, which, combined with existing data, indicates exhumation of this range between ca. 28 Ma and ca. 14 Ma. However, some of the highest ranges in the Eastern Cordillera preserve Cretaceous ages indicative of limited Cenozoic exhumation. Samples collected from an ~3-km-elevation transect along the northern part of the Sierra de Quilmes paleorift flank (Laguna Brava) show AFT ages between ca. 80 and ca. 50 Ma and He ages between ca. 45 and ca. 10 Ma. Another set of samples from an ~1-km-elevation transect farther to the southwest (La Quebrada) shows Cretaceous AFT ages between ca. 116 Ma and ca. 76 Ma, and mainly Cretaceous He ages, in agreement with AFT data. Analysis of existing AFT and He ages from the area once occupied by the Salta Rift reveals a pattern characterized by Cretaceous ages along paleorift highs and Cenozoic ages within paleorift hanging-wall basins and later foreland basin depocenters. This pattern is interrupted by the Sierras Pampeanas at ~28°S, which record mid-Cenozoic ages. Our data are consistent with a complex inherited pattern of pre-Andean paleostructures, likely associated with paleotopography, which was beveled by the Cenozoic regional foreland basin and reactivated during the late Neogene (ca. <10 Ma), strongly controlling the magnitude of Cenozoic uplift and exhumation and thus cooling age distribution. This, combined with variable lithologic erodibility, resulted in an irregular distribution of themochronological ages. [ABSTRACT FROM AUTHOR]

Reconstruction of the timing and pattern of exhumation of basement-involved uplifts during the Laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region. We use reconstructions of the 87Sr/86Sr ratios of Late Cretaceous--early Cenozoic river water from fossil shells in six basins of the Rocky Mountains contextualized by modern riverwater Sr chemistry studies to trace the erosion of Precambrian basement cores in the Laramide ranges. The 87Sr/86Sr ratios and Sr concentration of modern river water in the Rocky Mountains are controlled by river bedrock lithology. Weathering of Precambrian silicate rocks in the cores of Laramide ranges produces high 87Sr/86Sr ratios in highland rivers. Weathering of Paleozoic and Mesozoic carbonates along the basin margins reduces the 87Sr/86Sr ratios and increases Sr concentration of rivers as they flow basinward. Lowland rivers that have headwaters in Precambrian basement mostly have 87Sr/86Sr ratios >0.711, whereas rivers con- fined to or with very long reaches in basins have 87Sr/86Sr ratios between 0.709 and 0.711. River-water d18O values do not change in response to changes in catchment elevation. Our results from fossil shells show that carbonate weathering dominated the river-water Sr chemistry in the Alberta Foreland, Williston, and Crazy Mountains Basins, and that Proterozoic low-grade metamorphic carbonates in the Belt-Purcell Supergroup were not exposed in the Canadian Rocky Mountains during Late Cretaceous--early Paleocene time. Silicate weathering influenced the Sr chemistry of surface waters in the Powder River Basin during the late Paleocene and the Washakie Basin during the early Eocene, suggesting Precambrian silicate basement rock was extensively exhumed and eroded. The observation that Precambrian basement was eroded earlier around the Powder River Basin than around the Washakie Basin is consistent with a previous study that suggests Laramide ranges in northeast Wyoming reached high elevation earlier than the ranges adjacent to the Sevier thrust belt. Widespread basement erosion in late Paleocene--early Eocene time was controlled by tectonic exhumation, and may have been intensified by the wet and warm global climate. [ABSTRACT FROM AUTHOR]

The provenance of sedimentary strata that accumulate in foreland basins record the growth and denudation of the adjacent orogen. We use U-Pb geochronology of detrital zircon and monazite, Sm-Nd isotope geochemistry, trace- and rare earth element geochemistry, and petrographic data from synorogenic clastic sedimentary rocks in the Cordilleran foreland basin of southwestern Alberta to provide new perspectives on the evolution of the orogen. Foreland basin clastic rocks comprise three major pulses of sediment delivery: (1) upper Fernie Formation-Kootenay Group (154-142 Ma), (2) Blairmore Group (115-103 Ma), and (3) Milk River Group-Porcupine Hills Formarion (78-58 Ma). Nd isotope data are dominated by εNd values of--7 to -12, interpreted to represent a well-mixed provenance from Devonian through Triassic strata and subordinate contributions from thrust-imbricated pre-Devonian strata of the Cordilleran miogeocline. Significant deviations to less negative (more juvenile) values between -5 and +1 represent periods when the foreland was flooded by juvenile detritus from oceanic arc sources such as Quesnel terrane and from syndepositional continental magmatic arcs of mid- and Late Cretaceous ages. Detrital zircon and monazite from the Fernie-Kootenay clastic pulse (pulse 1) indicate derivation from Triassic-Ordovician sandstones imbricated within the thrust-and-fold belt, consistent with the Nd tracer results and petrography. U-Pb zircon ages from the Blairmore Group (pulse 2) confirm a provenance from Triassic and Jurassic arc rocks of Quesnel terrane with only minor contributions from older miogeoclinal rocks; they also record the presence of syndepositional magmatic material. The upper part of the Blairmore Group shows a transition to less juvenile Nd isotopic signatures and the reappearance of detrital zircons of miogeoclinal derivation. A similar pattern occurs in the Milk River-Porcupine interval (pulse 3) with juvenile material occurring early in the sequence, accompanied by zircon grains from syndepositional volcanic sources and by more continental material in the upper part of the sequence. Pulse I records the erosion of thrust-imbricated miogeoclinal rocks during the creation and erosion of the foreland thrust-and-fold belt with no detectable material derived from the deeper parts of the hinterland. A significant unconformity of ∼27 m.y. duration led to redistribution of the foreland basin fill and erosion of the adjacent thrust-and-fold belt and corresponds to a period of magmatic and tectonic quiescence in the southern Canadian Cordillera. Renewed contraction within the erosionally modified thrust wedge led to development of out-of-sequence thrust structures which allowed juvenile terranes (Quesnel terrane) to become the dominant source for foreland (pulse 2) to the exclusion of miogeoclinal material. Reappearance of the miogeoclinal signature in upper pulse 2 is interpreted to record eastward propagation of the thrust-and-fold belt into miogeoclinal strata. The third pulse of sediment records significant input of windblown ash from juvenile sources in the Coast Belt mixed with bedload components derived from more local sources in the eastern Cordillera. The youngest deposits in the basin (ca. 58 Ma) are characterized by a cosmopolitan provenance that likely records cannibalization of older parts of the foreland as previously deposited foreland strata became important components of the thrust wedge. [ABSTRACT FROM AUTHOR]