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Subsidence in foreland basins is modulated by the size of the flexural load, the elastic thickness of the foreland lithosphere, the nature of the sedimentary fill, and the rate of convergence. Basal 'forebulge unconformities' are well known from these basins, but here we focus on a major unconformity in the Himalayan foreland, which removed much of the Oligocene--lower Miocene. This is a critical time in the Himalaya because it spans the period of initial rapid exhumation of high-grade metamorphic rocks and the start of motion on the Main Central Thrust. We show that the synchronous timing of unconformity and rapid Greater Himalayan exhumation may be explained by the same trigger, enhanced erosion. We estimate that accelerating erosion between 24 and 20 Ma would have removed ~1.5 km more rock from the Greater Himalaya than would have been the case had erosion remained at the slower Paleogene rates. During this time erosion must have temporarily exceeded rock uplift rates. By transferring rock from the Himalaya to the Indian Ocean the load is reduced and the Indian Plate is flexed less, so that the depth of the foreland basin shallows to a new equilibrium state. We use a flexural model to estimate that erosional unloading could have caused uplift of 400-500 m of the basin at the range front, extending 70-150 km into the basin. Intensification of the Asian summer monsoon around the Oligo-Miocene boundary (~24 Ma) is the most likely trigger of the stronger erosion. Similar unconformities are seen at 15-16 Ma and in the Pleistocene also, linked to faster erosion at these times. doi: 10.1029/2010TC002711.

Chinese loess is regarded as one of the most detailed and long-term archives of climate on land. However, there is still significant controversy over the deposit's origin, limiting interpretation of the sedimentological and paleoclimatic mechanisms responsible for its emplacement. Here this is addressed through morphology and the first laser ablation--inductively coupled plasma--mass spectrometer (LA-ICP-MS) U-Pb ages of detrital zircons from loess (last glacial age; northern Loess Plateau). These are compared to zircon U-Pb age spectra from desert and sandy lands surrounding the Loess Plateau. Surface samples were taken from the Tengger and Mu Us deserts, as well as the Horqin and Otindag sandy lands. The results demonstrate that zircon U-Pb ages can discriminate between potential source areas and highlight both similarities and differences in age spectra for the desert and sandy land samples. Most significantly, the loess age spectrum shows no single affinity to any of these regions and exhibits zircon ages associated with granitoid rocks representing tectonic events in both west and east northern China. Furthermore, and in contrast to proximal deserts, a significant proportion of zircons from the loess show affinities with rocks cropping out in the Qilian Mountains. The euhedral form of many of these grains further suggests direct transport from these crystalline source rocks, in contrast to previously hypothesized production or storage in deserts. Thus, dust-transporting storms tracked from the west during the last glacial maximum, although this does not explain all the zircon variability and implies multiple sources and storm-track variation over the depositional period. doi: 10.1130/B30102.1

Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U--Pb zircon dating yields ages of 493 [+ or -] 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 [+ or -] 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc--continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian--Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc--continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.

Climate is one of the principal controls setting rates of continental erosion. Here we present the results of a provenance analysis of Holocene sediments from the Indus delta in order to assess climatic controls on erosion over millennial time scales. Bulk sediment Nd isotope analysis reveals a number of changes during the late Pleistocene and early Holocene (at 14-20, 11-12 and 8-9 ka) away from erosion of the Karakoram and toward more sediment flux from the Himalaya. Radiometric Ar-Ar dating of muscovite and U-Pb dating of zircon sand grains indicate that the Lesser Himalaya eroded relatively more strongly than the Greater Himalaya as global climate warmed and the summer monsoon intensified after 14 ka. Monsoon rains appear to be the primary force controlling erosion across the western Himalaya, at least over millennial time scales. This variation is preserved with no apparent lag in sediments from the delta, but not in the deep Arabian Sea, due to sediment buffering on the continental shelf. Keywords: erosion, Himalayas, monsoon, rivers, provenance.

Marine seismic and drilling data from the Andean margin of Peru and northern Chile show trench retreat and tectonic erosion since at least 20 Ma. However, the onshore coastal zone has undergone modest regional uplift since ca. 2 Ma. This argues against subduction erosion of the margin by steady-state retreat, such as recognized in Tonga or South Sandwich. Instead we identify a new style of slow tectonic erosion. Uplifted forearc sediments indicate underplating of subducted material beneath the base of the coastal forearc, synchronous with tectonic erosion under the trenchward part of the forearc. These observations require slow rates of forearc crust subduction in the region during this time period, averaging no more than 13 [km.sup.3]/m.y./km since 20 Ma as the forearc wedge taper increased. Isostatic volume estimates suggest that as much as 85% of subducted material was underplated under the forearc since 330 ka and does not contribute to crustal growth under the Andes. The switch in tectonic style parallels that in the southern Andes, but cannot be triggered by increased sediment flux to the trench, as proposed for that area. Keywords: subduction, tectonics, subsidence, sedimentation, Andes, tectonic erosion.

[1] India-Asia collision has resulted in the largest orogenic event on Earth since at least 500 Ma, and this has had a profound influence on the drainage patterns of Asia, on the Asian monsoon climate, on global oceanography, as well as on faunal extinctions. Despite this importance, understanding of the Paleogene evolution of this system is limited. U-Pb and Lu-Hf isotopic analyses of detrital zircons from the clastic sedimentary rocks of the Indus Molasse, located within the Indus Suture of the Ladakh Himalaya, India, show that most have Mesozoic-Cenozoic ages and juvenile Hf isotopic compositions. This suggests that the main source of these sediments is the Transhimalayan batholith, with minor flux from the Lhasa-Karakoram terrane and Indian plate. The lower Indus Molasse (Chogdo Formation) is inferred to have been deposited at 60-49 Ma, whereas the upper Choksti Formation was deposited later than 45-41 Ma and contains a greater contribution from the Lhasa terrane. The Transhimalayan batholith was eroded throughout Indus Molasse sedimentation, but increasing exhumation of Lhasa-Karakoram terrane indicates its progressive uplift after initial collision, and requires formation of a paleo-Indus River shortly after collision.

The Jurassic Talkeetna volcanic arc of south-central Alaska is an oceanic island arc that formed far from the North American margin. Geochronological, geochemical, and structural data indicate that the arc formed above a north-dipping subduction zone after ca. 201 Ma. Magmatism migrated northward into the region of the Talkeetna Mountains ca. 180 Ma. We interpret this magmatism as the product of removal of the original forearc while the arc was active, mainly by tectonic erosion. Rapid exhumation of the arc after ca. 160 Ma coincided with the sedimentation of the coarse clastic Naknek Formation. This exhumation event is interpreted to reflect collision of the Talkeetna arc with either the active margin of North America or the Wrangellia composite terrane to the north along a second north-dipping subduction zone. The juxtaposition of accreted trench sedimentary rocks (Chugach terrane) against the base of the Talkeetna arc sequence requires a change from a state of tectonic erosion to accretion, probably during the Late Jurassic (before 150 Ma), and definitely before the Early Cretaceous (ca. 125 Ma). The change from erosion to accretion probably reflects increasing sediment flux to the trench due to collision ca. 160 Ma. Keywords: Alaska, terrane accretion, subduction, tectonic erosion, collision.

The Early Jurassic Talkeetna Volcanic Formation forms the upper stratigraphic level of an oceanic volcanic arc complex within the Peninsular Terrane of south-central Alaska. The section comprises a series of lavas, tufts, and volcaniclastic debris-flow and turbidite deposits, showing significant lateral facies variability. There is a general trend toward more volcaniclastic sediment at the top of the section and more lavas and tuff breccias toward the base. Evidence for dominant submarine, mostly mid-bathyal or deeper (>500 m) emplacement is seen throughout the section, which totals ~7 km in thickness, similar to modern western Pacific arcs, and far more than any other known exposed section. Subaerial sedimentation was rare but occurred over short intervals in the middle of the section. The Talkeetna Volcanic Formation is dominantly calc-alkaline and shows no clear trend to increasing Si[O.sub.2] up-section. An oceanic subduction petrogenesis is shown by trace element and Nd isotope data. Rocks at the base of the section show no relative enrichment of light rare earth elements (LREEs) versus heavy rare earth elements (REEs) or in melt-incompatible versus compatible high field strength elements (HFSEs). Relative enrichment of LREEs and HFSEs increases slightly up-section. The Talkeetna Volcanic Formation is typically more REE depleted than average continental crust, although small volumes of light REE-enriched and heavy. REE-depleted mafic lavas are recognized low in the stratigraphy. The Talkeetna Volcanic Formation was formed in an intraoceanic arc above a north-dipping subduction zone and contains no preserved record of its subsequent collisions with Wrangellia or North America. Keywords: sedimentology, arc volcanism, subduction, geochemistry, isotope geology.

[1] The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U-Pb ages of zircons spanning 3 Gyr show that only ~5% of the eroding .crust has been generated since India-Asia collision.' The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The [sup.39]Ar/[sup.40]Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0-15 Ma. Modern average exhumation rates are estimated at ~0.6 km/m.y. or less, and have slowed progressively since the early Miocene (~20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (~1.0 km/m.y.) driven by climate change. The [sup.39]Ar/[sup.40]Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 1625 Global Change: Geomorphology and weathering (1824, 1886); 1094 Geochemistry: Instruments and techniques; 1035 Geochemistry: Geochronology; 9320 Information Related to Geographic Region: Asia; KEYWORDS: Himalaya, Indus River, fission track, exhumation, erosion, thermochronology. Citation: Clift, P. D., I. H. Campbell, M. S. Pringle, A. Carter, X. Zhang, K. V. Hodges, A. A. Khan, and C. M. Allen (2004), Thermochronology of the modern Indus River bedload: New insight into the controls on the marine stratigraphic record.

A new analysis of Deep Sea Drilling Project (DSDP) Leg 84 data demonstrates that the dominant process controlling the Guatemala margin tectonic evolution since ca. 25 Ma is subduction-erosion. Data from benthic foraminifera, assemblages from upper-slope DSDP Sites 568, 569, and 570 indicate long-term, progressive subsidence from upper to middle bathyal depths (600-1000 m) ca. 19 Ma to modern abyssal depths (>2000 m). Rapid subsidence migrated landward starting at the Oligocene-Miocene boundary time under the current middle slope, where it increased sharply ca. 19 Ma, reached the current upper slope by ca. 15 Ma, and arrived at the uppermost slope ca. 2 Ma. Subsidence indicates crustal thinning by basal tectonic erosion of mass from the underside of the upper plate. Under the assumption that, in the Miocene, the morphology of the forearc was similar to that of today, landward migration of the trench was at a rate of 0.8-0.9 km/m.y. This linear rate corresponds to a tectonic erosion rate of the submerged forearc of 11.3-13.1 [km.sup.3] x [m.y..sup.-1] x [km.sup.-1]. The evolution of arc magmatism and superfast spreading at the East Pacific Rise since early Miocene time may have caused slab shallowing and tectonic erosion that readjusted the forearc geometry. Keywords: subduction-erosion, convergent margins, benthic foraminifera, Guatemala, Middle America Trench.

Because magmatism associated with subduction is thought to be the principal source for continental crust generation, assessing the relative contribution of pre-existing (subducted and assimilated) continental material to arc magmatism in accreted arcs is important to understanding the origin of continental crust. We present a detailed Nd isotopic stratigraphy for volcanic and volcaniclastic formations from the South Mayo Trough, an accreted oceanic arc exposed in the western Irish Caledonides. These units span an arc-continent collision event, the Grampian (Taconic) Orogeny, in which an intra-oceanic island arc was accreted onto the passive continental margin of Laurentia starting at ~475 Ma (Arenig). The stratigraphy corresponding to pre-, syn- and post-collisional volcanism reveals a progression of [[epsilon].sub.Nd(t)] from strongly positive values, consistent with melt derivation almost exclusively from oceanic mantle beneath the arc, to strongly negative values, indicating incorporation of continental material into the melt. Using [[epsilon].sub.Nd(t)] values of meta-sediments that represent the Laurentian passive margin and accretionary prism, we are able to quantify the relative proportions of continent-derived melt at various stages of arc formation and accretion. Mass balance calculations show that mantle-derived magmatism contributes substantially to melt production during all stages of arc continent collision, never accounting for less than 21% of the total. This implies that a significant addition of new, rather than recycled, continental crust can accompany arc-continent collision and continental arc magmatism. Keywords: Grampian Orogeny, western Ireland, continental crust, Nd isotopes, Laurentia, Iapetus Ocean.

[1] Subsidence of Lima Basin, part of the Peruvian forearc, is controlled by tectonic erosion by the subducting Nazca plate. Multichannel seismic reflection data coupled with age and paleowater depth constraints derived from Ocean Drilling Program (ODP) coring now allow the rates of erosion to be reconstructed through time. In trenchward locations the forearc has experienced limited recent relative uplift (700-850 m) likely due to preferential basal erosion under the center of Lima Basin. Long-term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin have been lost at an average rate of up to 3.1 km [myr.sup.-1]. Appoximately 110 km of that total appears to be lost since 11 Ma, implying much faster average rates of trench retreat (10 km [myr.sup.-1]) since collision of the Nazca Ridge with the Lima Basin at 11 Ma. Although there is no clear subsidence event at ODP Site 679 during the time at which Nazca Ridge was subducting beneath this part of the forearc (4-11 Ma), the more trenchward ODP Sites 682 and 688 show significant deepening after 11 Ma indicating that subduction of the ridge accelerates tectonic erosion. Long-term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins are net sinks of continental crust. INDEX TERMS: 3025 Marine Geology and Geophysics: Marine seismics (0935); 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8015 Structural Geology: Local crustal structure; 0935 Exploration Geophysics: Seismic methods (3025); KEYWORDS: Peru, subduction, tectonics, subsidence. Citation: Clift, P. D., I. Pecher, N. Kukowski, and A. Hampel, Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision, Tectonics, 22(3), 1023, doi:10.1029/2002TC001386, 2003.

The evolution and resulting morphology of a contourite drift system in the SE Pacific oceanic basin is investigated in detail using seismic imaging and an age-calibrated borehole section. The Nazca Drift System covers an area of 204 500 km2 and stands above the abyssal basins of Peru and Chile. The drift is spread along the Nazca Ridge in water depths between 2090 and 5330 m. The Nazca Drift System was drilled at Ocean Drilling Program Site 1237. This deep-water drift overlies faulted oceanic crust and onlaps associated volcanic highs. Its thickness ranges from 104 to 375 m. The seismic sheet facies observed are associated with bottom current processes. The main lithologies are pelagic carbonates reflecting the distal position relative to South America and water depth above the carbonate compensation depth during Oligocene time. The Nazca Drift System developed under the influence of bottom currents sourced from the Circumpolar Deep Water and Pacific Central Water, and is the largest yet identified abyssal drift system of the Pacific Ocean, ranking third in all abyssal contourite drift systems globally. Subduction since late Miocene time and the excess of sediments and water associated with the Nazca Drift System may have contributed to the Andean orogeny and associated metallogenesis. The Nazca Drift System records the evolution in interactions between deep-sea currents and the eastward motion of the Nazca Plate through erosive surfaces and sediment remobilization. [ABSTRACT FROM AUTHOR]

Sediment transport through the Mississippi River affects the lives and economies of millions of people along its course, so that understanding the controls on this process is of scientific and societal importance. Detrital U‐Pb geochronology, supported by grain size and major element data, can be a robust tool for constraining sediment provenance in clastic sedimentary systems that has been applied to the Mississippi. However, sediment storage and reworking can complicate interpretation, and this can be further exacerbated by anthropogenic alteration via the construction of levees, dams, locks and river diversion projects. In this study, we date zircons from the modern Mississippi River and compare them to previously acquired data to illustrate the difference between samples taken from the same catchment in order to better understand the downstream propagation of the modern detrital zircon signal. The modern Mississippi River and tributary systems show distinct similarities and are comparable between studies when samples are not too far separated (<100 km). We estimate that the Arkansas River is more important than previously proposed, at least in terms of sand supply, supplying 7%–11% of the total load. The largest supplier of sediment to the Mississippi is the Missouri River (33%–43%), which derives much of its sediment from sedimentary rocks in the foredeep deposited during the Sevier and Laramide events. Anthropogenic alteration of the modern river system can be seen in the downstream propagation of a Red River cut‐off signal following construction of the "River Control Structure", implying slow zircon transport rates (<2.8 km/year). Differences in the degree of recycling caused by sampling locations and low numbers of grains introduce significant uncertainties to mixing calculations. [ABSTRACT FROM AUTHOR]

The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program recovered a discontinuous erosional record for the Indus River drainage dating back to at least 9.8 Ma, and with a single sample from 15.6 Ma. We dated detrital zircon grains by U‐Pb geochronology to reconstruct how erosion patterns changed through time. Long‐term increases in detrital zircon U‐Pb components of 750–1,200 and 1,500–2,300 Ma record increasing preferential erosion of the Himalaya relative to the Karakoram between 8.3–7.0 and 5.9–5.7 Ma. The average contribution of Karakoram‐derived sediment to the Indus Fan fell from 70% of the total at 8.3–7.0 Ma to 35% between 5.9 and 5.7 Ma. An increase in the contribution of 1,500–2,300 Ma zircons starting between 2.5 and 1.6 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon age spectra is consistent with bulk sediment Nd isotope data. The initial change in spatial erosion patterns at 7.0–5.9 Ma occurred during a time of drying climate in the foreland. The increase in ILH erosion postdated the onset of dry‐wet glacial‐interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the ILH thrust duplex, especially during the Pliocene, also played an important role, while the influence of the Nanga Parbat Massif to the total sediment flux was modest. Plain Language Summary: Mountain belts are constructed by tectonic forces from within the solid Earth modulated by surface processes. The Himalaya represents the type example of such interactions, where the growth of the mountains and the Asian monsoon interact. Sediments from the Arabian Sea allow the long‐term erosion patterns of the NW Himalaya to be reconstructed. We use the crystallization age of zircon sand grains in sediment dating back to 15.6 Ma to see how the erosion has changed in response to the evolving climate. There is a trend toward more erosion from the Himalaya and less from the Karakoram starting around 8.3–7.0 Ma, which was a time of transition toward a drier climate in the NW Himalaya. Furthermore, we see a sharp increase in erosion from the Inner Lesser Himalaya starting between 2.5 and 1.6 Ma. This was caused by the formation of a thrust duplex pushing up these ranges, which in turn focused orographic precipitation. The timing of this exposure is much younger than understood from studies of sedimentary rocks in the foreland basin onshore. Our study highlights the role that tectonics plays in controlling what parts of the mountain belt are subject to the fastest erosion. Key Points: Geochronologic analyses show increased relative erosion from the Himalaya compared to the Karakoram between 8.3–7.0 Ma and 5.9–5.7 MaChanging patterns of erosion correlate with climatic drying at ∼7.7–5.9 Ma and relate to solid Earth tectonic forces building topographyErosion from the Inner Lesser Himalaya increased between 2.5 and 1.6 Ma on a regional scale [ABSTRACT FROM AUTHOR]

Dalradian meta-sediments of the Laurentian margin and mafic intrusions thereof in SW Connemara, Ireland, tectonically overlie meta-rhyolites of the Delaney Dome Formation. The two units are separated by the Mannin Thrust. A new U--Pb age of 474.6 [+ or -] 5.5 Ma shows that the Delaney Dome Formation is a temporal equivalent of arc volcanic rocks preserved in the adjacent South Mayo Trough: the Tourmakeady Volcanic Group, erupted during the collision of an oceanic island arc with the Laurentian margin in the Grampian Orogeny. New rare earth and high field strength element data show that the Delaney Dome Formation and Tourmakeady Volcanic Group are chemically similar and arc-like in character. This suggests that the Delaney Dome Formation is an along-strike equivalent of the Tourmakeady Group, strike-slip faulted south of the South Mayo Trough during or after the Grampian Orogeny. Further correlation of these units with northern Appalachian rhyolites is also possible. The Delaney Dome Formation is an extrusive temporal equivalent of intrusions that penetrate the Connemara Dalradian. Thus, movement along the Mannin Thrust brought mid-crustal plutons and Dalradian country rocks tectonically above the extrusive volcanic sequence. The Mannin Thrust is identified as a major imbricating structure within a continental arc, but not a terrane boundary. Keywords: Ireland, Grampian Orogeny, U--Pb, geochemistry, subduction, plate collision.

The convergent margins of the western Pacific are marked by widespread tectonic erosion of the overriding plate by the subducting oceanic lithosphere. At Ocean Drilling Program Site 841 on the mid-trench slope of the Tonga forearc, rates of tectonic trenchward tilting and subsidence of the basement appear to be linked, supporting models of basal erosion focused near the trench. Rates of subsidence significantly exceed those predicted from plate cooling models and require crustal thinning to account for the difference in this extensional environment. Estimated extension at Site 841 is 280%. Tilting and subsidence rates peaked ca. 32-34 Ma, just after spreading in the South Fiji Basin at 35 Ma. However, the trenchward direction of tilt and higher extension on the trench slope than on the Tonga platform suggest a link to tectonic erosion near the trench rather than extension focused in the arc edifice. Since this time, ca. 135 km has been eroded from the edge of the plate, of which 80 km is related to the ongoing collision of the Louisville Ridge. A probable paleo-collision event at 16 Ma caused the forearc at Site 841 to be uplifted and tilted arcward, forming a major unconformity due to mass wasting. Steady-state erosion averages