Foreland Sediments Research Articles

Sedimentation in a foreland basin within synorogenic orocline: Palaeogene of the Isparta Bend, Taurides, SW Turkey

AbstractThe Palaeogene Isparta Basin of southwestern Anatolia formed between two convergent arms of the Isparta Bend orocline of the Tauride orogen. The origin of this tightening orocline is hypothetically explained in plate‐tectonic terms. Basin sedimentation commenced on a down‐warped Mesozoic carbonate platform of a crustal block accreted at the end of Cretaceous to the southern margin of the Anatolian plate. The basin earliest deposits are Palaeocene reddish mudstones with a fossil‐barren condensed basal part and increasingly interspersed with thin calcarenitic turbidites towards the top. The supply of turbiditic sediment to the basin plain subsequently increased, as the upper‐bathyal basin plain became surrounded from both sides by a narrow littoral shelf with an advancing turbiditic slope ramp. A major forced regression occurred at the end of Bartonian, causing incision of subaerial to submarine valleys up 600 m deep, filled in with gravelly to sandy turbidites and debrisflow deposits during the subsequent rise of relative sea level. The half‐filled valleys were re‐incised due to a Rupelian forced regression and were fully filled with fluvio‐deltaic bayhead deposits during a final marine transgression that re‐established the basin‐margin biocalcarenitic shelf. The littoral environment then expanded across the shallowing basin, as the basin axial zone was up‐domed and eroded to bedrock level at the end of Oligocene and the basin was tectonically inverted in Miocene. The pattern of intra‐orocline foreland sedimentation documented by this case study provides tentative criteria for the recognition of synorogenic oroclines and for their distinction from post‐orogenic oroclines.

Early Neogene foreland of the Zagros, implications for the initial closure of the Neo-Tethys and kinematics of crustal shortening

We study the transition from passive margin to foreland basin sedimentation now exposed in the High Zagros belt to provide chronological constraints on the initial stage of Arabia–Eurasia collision and closure of the Neo-Tethys. We performed magnetostratigraphy and strontium isotope stratigraphy along two sections near the Zagros suture which expose the oldest preserved foreland deposits: the Shalamzar section in the west and the Dehmoord section in the east. The top of the passive margin Asmari formation has an age of 28–29 Ma in the High Zagros and is overlain by foreland deposits with a major basal unconformity representing 7 Myr of hiatus. The base of the foreland deposits has an age of 21.5 Ma at Dehmoord and ca. 26 Ma at Shalamzar. The sedimentation rate increased from 30 m/Myr in the passive margin to 247 m/Myr in the foreland. Combined with available age constraints across the Zagros, our results show that the unconformity is diachronous and records the southwestward migration of the flexural bulge within the Arabian plate at an average rate of 24 ± 2 mm/yr over the last 27 Ma. The time evolution of sediment accumulation in the Zagros foreland follows the prediction from a flexural model, as the foreland is thrust beneath the orogenic wedge and loaded by the wedge and basin fill. We detect the onset of forebulge formation within the Asmari Formation around 25 Ma. We conclude that closure of the Neo-Tethys formed the Zagros collisional wedge at 27 ± 2 Ma. Hence, the Arabia–Eurasia collision was probably not the main driver of global cooling which started near the Eocene–Oligocene boundary (ca. 33.7 Ma). We estimate 650 km of forebulge migration since the onset of the collision which consists of 350 km of shortening across the orogen, and 300 km of widening of the wedge and increasing flexural rigidity of Arabia. We conclude the average rate of shortening across the Zagros to be ca. 13 mm/yr over the last 27 Myr; a value comparable to the modern rate. Palinspastic restoration of structural cross-sections and crustal volume conservation comprise only ca. 200 km of shortening across the Zagros and metamorphic Sanandaj–Sirjan belt implying that at least 150 km of the Arabian crust was underthrust beneath Eurasia without contributing to crustal thickening, possibly due to eclogitization.

Flexural bending of the Zagros foreland basin

We constrain and model the geometry of the Zagros foreland to assess the equivalent elastic thickness of the northern edge of the Arabian plate and the loads that have originated due to the Arabia–Eurasia collision. The Oligo-Miocene Asmari formation, and its equivalents in Iraq and Syria, is used to estimate the post-collisional subsidence as they separate passive margin sediments from the younger foreland deposits. The depth to these formations is obtained by synthesizing a large database of well logs, seismic profiles and structural sections from the Mesopotamian basin and the Persian Gulf. The foreland depth varies along strike of the Zagros wedge between 1 and 6 km. The foreland is deepest beneath the Dezful embayment, in southwest Iran, and becomes shallower towards both ends. We investigate how the geometry of the foreland relates to the range topography loading based on simple flexural models. Deflection of the Arabian plate is modelled using point load distribution and convolution technique. The results show that the foreland depth is well predicted with a flexural model which assumes loading by the basin sedimentary fill, and thickened crust of the Zagros. The model also predicts a Moho depth consistent with Free-Air anomalies over the foreland and Zagros wedge. The equivalent elastic thickness of the flexed Arabian lithosphere is estimated to be ca. 50 km. We conclude that other sources of loading of the lithosphere, either related to the density variations (e.g. due to a possible lithospheric root) or dynamic origin (e.g. due to sublithospheric mantle flow or lithospheric buckling) have a negligible influence on the foreland geometry, Moho depth and topography of the Zagros. We calculate the shortening across the Zagros assuming conservation of crustal mass during deformation, trapping of all the sediments eroded from the range in the foreland, and an initial crustal thickness of 38 km. This calculation implies a minimum of 126 ± 18 km of crustal shortening due to ophiolite obduction and post-collisional shortening.

A new Paleoproterozoic tectonic history of the eastern Capricorn Orogen, Western Australia, revealed by U–Pb zircon dating of micro-tuffs

The Earaheedy Basin is one of several Paleoproterozoic siliciclastic basins in the eastern part of the Capricorn Orogen in Western Australia. In the absence of an adequate chronostratigraphic framework, multiple conflicting models have been proposed for the tectonic history of the basin. We present in situ SHRIMP U–Pb zircon dates of 1971±11Ma, 1954±5Ma and 1949±10Ma from three thin tuff horizons (“micro-tuffs”) in the Yelma Formation at the base of the basin succession. These micro-tuffs are only slightly younger than the 1990±6Ma Imbin porphyry, which has been interpreted as a basement inlier. Our new dates, combined with field mapping, suggest that the Imbin porphyry is a volcanic–subvolcanic complex within the basin. Micro-tuffs in the Yelma Formation are more than 50m.y. older than c. 1890Ma micro-tuffs within the overlying Frere Formation. Deposition of the Yelma Formation was coeval with continent–continent collision farther west in the Orogen marked by the 2005–1950Ma Glenburgh Orogeny, and the formation is possibly correlative with peripheral foreland deposits of the Padbury Group. The Frere Formation was deposited in shallow marine conditions within an intracratonic basin about 60m.y. after the Glenburgh Orogeny. The Frere Formation is unconformably or disconformably overlain by the Miningarra Group, the youngest unit of which is the Mulgarra Sandstone. This sandstone was deposited after c. 1865Ma, and derived either from exposed Frere Formation or from a now-buried source within the Capricorn Orogen that was uplifted by inception of the 1820–1770Ma Capricorn Orogeny. Therefore, the Earaheedy Basin comprises at least three distinct, unconformity-bound successions deposited in contrasting tectonic settings.

40Ar–39Ar laser dating of ductile shear zones from central Corsica (France): Evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar–39Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P–T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~NNE–SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P–T conditions attained during shearing at ~300°C and minimum pressures of ~0.6GPa. In-situ 40Ar–39Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~45–36Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages at ~34Ma, but a hump-shaped age spectrum, with maximum ages of ~35Ma and intermediate- to high-temperature steps as young as ~33–32Ma. Results indicate that the studied samples consist of an earlier celadonite-poor white mica with a minimum age of ~46Ma, overgrown by a synshear high-celadonite white mica, developed at greater depth between ~37 and 35Ma; faint late increments in shearing occurred at ≤33–32Ma, when white mica incipiently re-equilibrated during exhumation. Results suggest that ductile shearing with a dominant strike-slip component pervasively deformed the Corsican basement complex during the emplacement and progressive thickening of the Alpine orogenic wedge and broaden the extent of the domain affected by the Alpine tectonometamorphic events. Integration of petrological modeling and geochronological data shows that the Variscan basement of central Corsica, close to the Alpine nappes, was buried during the late Eocene by ≥18km of Alpine orogenic wedge and foreland deposits. Our results, combined with previously published apatite fission-track data, imply an overburden removal ≥15km from the late Eocene (Priabonian) to the early Miocene (Aquitanian), pointing to a minimum average exhumation rate of 1.3–1.5mm/a.

Large along-strike variations in the onset of Subandean exhumation: Implications for Central Andean orogenic growth

Plate tectonics drives mountain building in general, but the space-time pattern and style of deformation is influenced by how climate, geodynamics, and basement structure modify the orogenic wedge. Growth of the Subandean thrust belt, which lies at the boundary between the arid, high-elevation Central Andean Plateau and its humid, low-elevation eastern foreland, figures prominently into debates of orogenic wedge evolution. We integrate new apatite and zircon (U–Th)/He thermochronometer data with previously published apatite fission-track data from samples collected along four Subandean structural cross-sections in Bolivia between 15° and 20°S. We interpret cooling ages vs. structural depth to indicate the onset of Subandean exhumation and signify the forward propagation of deformation. We find that Subandean growth is diachronous south (11±3 Ma) vs. north (6±2 Ma) of the Bolivian orocline and that Subandean exhumation magnitudes vary by more than a factor of two. Similar north–south contrasts are present in foreland deposition, hinterland erosion, and paleoclimate; these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms. Of particular interest is an abrupt shift to cooler, more arid conditions in the Altiplano hinterland that is diachronous in southern Bolivia (16–13 Ma) vs. northern Bolivia (10–7 Ma) and precedes the timing of Subandean propagation in each region. Others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded. These mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop, either through surface uplift or spatially variable erosion. In summary, we suggest that diachronous Subandean exhumation was driven by piecemeal hinterland uplift, orography, and the outward propagation of deformation.

MESOZOIC THERMAL EVENTS IN CENTRAL LOWER YANGTZE AREA AND IMPACTS ON HYDROCARBON GENERATION OF MARINE SOURCE ROCKS

AbstractTemperature is a key parameter of controlling hydrocarbon generation of source rocks. Elevated temperature resulted from tectonothermal events may significantly affect hydrocarbon generation. Based on geology, fission track analysis, apatite U‐Th/He and volcanic rock isotopic age data, it is found that there existed Indosinian (T3‐J2) and Yanshanian (J3‐K1) tectonothermal events in Lower Yangtze area and the sedimentary basins reached their peak heat flow at about 130∼110 Ma. The maximum heat flows obtained from thermal history reconstruction based on vitrinite reflectance are ∼94 mW·m–2 and ∼78 mW·m–2 in Jurong and Taixing areas respectively, with an increasing trend from east to west. Hydrocarbon generation history reconstruction based on the EASY%Ro model shows that the dominating gas generation periods of the Cambrian source rock are the Early Permian‐Late Triassic and the Late Triassic‐Early Cretaceous in Changzhou and Jurong areas respectively. Because of the dual effects from the foreland sediments deposited in T3‐J1–2 and the magmatism thermal event in the Early Cretaceous, the marine source rocks reached their maximum temperature at the end of Early Cretaceous. The temperature of the marine source rocks was no longer elevated owing to the decreased heat flow since the Late Cretaceous, though, in some parts of the Jurong, Taixing and Changzhou areas, subsidence occurred and burial depth increased during the K2‐E period. Therefore, the secondary hydrocarbon generation of the marine source rocks is not a widespread process in central Lower Yangtze area.

From the Hadean to the Himalaya: 4.4 Ga of felsic terrestrial magmatism

Detrital zircons as old as nearly 4.4 Ga offer insights into the earliest moments of Earth history. Results of geochemical investigations of these grains have been interpreted to indicate their formation in near-H2O saturated meta- and peraluminous magmas under a relatively low (15–30 °C/km) geotherm. A key feature in pursuing a petrotectonic model that explains the full spectrum of these observations is their seeming contrast to most Phanerozoic magmatic zircons, specifically their low Ti-in-zircon crystallization temperatures and inclusion assemblages. The ~22 Ma Arunachal leucogranites of the eastern Himalaya appear, however, to be a rare exception to this generality. They show large-ion lithophile covariance trends indicative of wet basement melting together with a normal distribution of magmatic crystallization temperatures about an average of 660 °C. In the same fashion as Hadean zircons, Arunachal leucogranite and host gneiss zircons are dominated by muscovite + quartz inclusions that yield formation pressures of 5–15 kbars. We suggest that the Arunachal leucogranites originated in the hanging wall of a megathrust that carried H2O-rich foreland sediments to depths of >20 km whereupon de-watering reactions released fluids that fluxed hanging wall anatexis. Modeling suggests the thermal structure of this continental collision environment may have been broadly similar to a Hadean ocean-continent subduction zone. The similarity of these two environments, separated by over 4 Ga may explain seemingly common features of the Hadean and Arunachal leucogranite zircons. Their key difference is the absence of metaluminous magmas in the continental collision environment, which is shielded from juvenile additions.

Tectono‐sedimentary evolution of the northern Iranian Plateau: insights from middle–late Miocene foreland‐basin deposits

AbstractSedimentary basins in the interior of orogenic plateaus can provide unique insights into the early history of plateau evolution and related geodynamic processes. The northern sectors of the Iranian Plateau of the Arabia–Eurasia collision zone offer the unique possibility to study middle–late Miocene terrestrial clastic and volcaniclastic sediments that allow assessing the nascent stages of collisional plateau formation. In particular, these sedimentary archives allow investigating several debated and poorly understood issues associated with the long‐term evolution of the Iranian Plateau, including the regional spatio‐temporal characteristics of sedimentation and deformation and the mechanisms of plateau growth. We document that middle–late Miocene crustal shortening and thickening processes led to the growth of a basement‐cored range (Takab Range Complex) in the interior of the plateau. This triggered the development of a foreland‐basin (Great Pari Basin) to the east between 16.5 and 10.7 Ma. By 10.7 Ma, a fast progradation of conglomerates over the foreland strata occurred, most likely during a decrease in flexural subsidence triggered by rock uplift along an intraforeland basement‐cored range (Mahneshan Range Complex). This was in turn followed by the final incorporation of the foreland deposits into the orogenic system and ensuing compartmentalization of the formerly contiguous foreland into several intermontane basins. Overall, our data suggest that shortening and thickening processes led to the outward and vertical growth of the northern sectors of the Iranian Plateau starting from the middle Miocene. This implies that mantle‐flow processes may have had a limited contribution toward building the Iranian Plateau in NW Iran.

Application of actualistic models to unravel primary volcanic control on sedimentation (Taveyanne Sandstones, Oligocene Northalpine Foreland Basin)

This work is focused on the Taveyanne Sandstones (Grés de Taveyanne), an Oligocene volcaniclastic turbidite sequence cropping out in the Northern Alpine Molassa between SE France and Central Switzerland, with the aim to investigate the temporal relationship between volcanic activity and sediment supply. Detailed stratigraphic, sedimentological, and petrographic (XRD analyses on mudstones and point counts on sandstones) studies conducted on three sections (Col de l'Oulette and Flaine in SE France, Taveyanne in SW Switzerland) allow a discrimination of three main facies, among which only one is extremely enriched in volcaniclastic detritus and characterized by features similar to those of disaggregated pyroclastic density current deposits. The other two facies are characterized by variable to no volcanic detritus but supplied by crystalline and sedimentary detritus. Such sediment trends are similar to those of modern, volcanically controlled source-to-sink systems. This allows a reinterpretation of the Taveyanne Sandstones as a syn-volcanic turbidite system, episodically supplied by large amounts of volcanic detritus, which periodically modified the drainage paths. Moreover, the well-known temporal and spatial persistence of such modifications in modern settings leads to conciliate the syn-volcanic supply with the location of the volcanic centers in the internal part of the Alps, without invoking particular climatic and tectonic conditions controlling foreland sedimentation.

Detrital-zircon geochronology and provenance of the Ocloyic synorogenic clastic wedge, and Ordovician accretion of the Argentine Precordillera terrane

The Precordillera terrane in northwestern Argentina is interpreted to be an exotic (Laurentian) continental fragment that was accreted to western Gondwana during the Ordovician. One prominent manifestation of the subduction and collision process is a Middle–Upper Ordovician clastic wedge, which overlies a passive-margin carbonate-platform succession in the Precordillera. U/Pb ages of detrital zircons from sandstones within the clastic wedge, as well as zircons from clasts within conglomerates, provide documentation for the composition of the sediment provenance. The ages of detrital zircons are consistent vertically through the succession, as well as laterally along and across strike of the Precordillera, indicating a single, persistent sediment source throughout deposition of the clastic wedge. The dominant mode (∼1350–1000 Ma) of the detrital-zircon ages corresponds to the ages of basement rocks in the Western Sierras Pampeanas along the eastern side of the Precordillera. A secondary mode (1500–1350 Ma) corresponds in age to the Granite-Rhyolite province of Laurentia, an age range which is not known in ages of basement rocks of the Western Sierras Pampeanas; however, detritus from Granite-Rhyolite-age rocks in the basement of the Precordillera was available through recycling of synrift and passive-margin cover strata. Igneous clasts in the conglomerates have ages (647–614 Ma) that correspond to the ages of minor synrift igneous rocks in the nearby basement massifs; the same ages are represented in a minor mode (∼750–570 Ma) of detrital-zircon ages. A quartzite clast in a conglomerate, as well as parts of the population of detrital zircons, indicates the importance of a source in the metasedimentary cover of the leading edge of the Precordillera. The Famatina continental-margin magmatic arc reflects pre-collision subduction of Precordillera lithosphere beneath the western Gondwana margin; however, no detrital zircons have ages that correspond to Famatina arc magmatism, indicating that sedimentary detritus from the arc may have been trapped in a forearc basin and did not reach the foreland. The indicators of sedimentary provenance for the foreland deposits are consistent with subduction of the Precordillera beneath western Gondwana, imbrication of basement rocks from either the Precordillera or Gondwana into an accretionary complex, and recycling of deformed Precordillera cover rocks.

Detrital zircon U–Pb age and Hf isotopic composition from foreland sediments of the Assam Basin, NE India: Constraints on sediment provenance and tectonics of the Eastern Himalaya

Synorogenic Palaeogene–Neogene sediments of the Assam foreland basin, were derived by erosion of adjacent crustal and orogenic sources following the Greater India–Eurasia collision since ∼55Ma. To constrain source sediment influx, and its relation to Himalayan tectonics, from pre- to post-collision time, detrital zircon U–Pb geochronology and their Hf isotopic compositions were carried out. The varying detrital zircon spectral patterns analyzed from the Paleogene Jaintia, Barail and Neogene Surma and Tipam Groups, with sediment petrography, track source sediment derived from cratonic India, Gangdese and eastern Transhimalayan batholiths and the eastern Himalaya. These sources are tested against Cenozoic paleopositions proposed for the northeastward motion of the Indian plate.Precollisional cratonic detritus to Middle to Late Eocene Sylhet Formation shifted to Tethyan Himalaya and arc sources of the Gangdese and eastern Transhimalayan batholiths to Late Eocene Kopili and Barail Formations, consistent with the proposed paleoposition proximal to the Indus-Yarlung suture. This Sylhet–Kopili Formation transition, within the Jaintia Group, reflects one of the earliest Himalayan hinterland exhumation stages during the Late Eocene. Major shift in provenance to Higher Himalayan Crystalline and arc detritus is recorded from the Surma Group, constraining Mid Miocene Himalayan tectonic exhumation from the eastern Himalaya. Late Miocene Tipam Group preserves sediment of Higher Himalayan Crystalline detritus, ophiolite and likely Lesser Himalayan rocks.

Plio-Pleistocene evolution of the north Alpine drainage system: new constraints from detrital thermochronology of foreland deposits

The evolution of drainage systems in and around active orogens may be strongly affected by climatic or tectonic processes. Information on the drainage evolution is stored in the sediments of the foreland depocentres. We investigated the provenance of two key deposits adjacent to the Central Alps, the Pliocene Sundgau gravels and the Pleistocene Hohere Deckenschotter by applying detrital thermochronology. Combined with provenance information from Rhine Graben deposits, we propose a reconstruction of the north Alpine drainage system since the middle Pliocene and discuss potential controlling mechanisms. Our data show that the Rhine Graben received detritus from the Alpine realm already during the Pliocene, indicating two different river systems—the proto-Rhine and the Aare–Doubs—draining the Alpine realm toward the North Sea and Mediterranean Sea. The investigated sediments contain detritus from two central Alpine sources, one showing a regional exhumational equilibrium and the other characterized by increasing exhumation rates. Discharge of the latter source ceased after ~2 Ma, reflecting a northward shift of the main Alpine drainage divide. Between ~2.0 and 1.2 Ma, the drainage system was affected by a major change, which we explain as resulting from a change in the Alpine stress field leading to tectonic exhumation and topography reduction in the area of the southern Aar massif. Generally, it seems that between ~4 and 1.2 Ma, the drainage system was mainly controlled by tectonic processes, despite first glaciations that already affected the north-Alpine foreland by ~2 Ma. The drainage system only seems to have reacted to the late Cenozoic climate changes after ~1.2 Ma, i.e., at the time of the most intense Alpine glaciation. At that time, the course of the Rhine River shifted toward the area of the Hegau volcanics, and the size of the Rhine River catchment became strongly reduced.

GEOCHEMISTRY AND HEAVY MINERALS ANALYSES ON "BLACK SANDS" OF THE LOWER CAMBRIAN FISH RIVER SUBGROUP (NAMA GROUP, NAMIBIA)

Heavy mineral rich-sandstones (“black sands”) of the Lower Cambrian upper Fish River Subgroup in Namibia were used to reveal the provenance of the detrital material. The host rocks of the black sands from the Haribes and Rosenhof Members are mainly composed of partially rounded quartz, feldspar (K-feldspar > plagioclase), polycrystalline quartz, few siltstone fragments and labile altered or metamorphosed volcanic lithoclasts. Geochemical proxies are similar to those from typical foreland deposits and point to slightly recycled upper continental crustal composition, e.g.: Th/Sc ratios between 1.23 and 1.95, Zr/Sc ranging from 24 to 38 and La/Sc from 4.8 to 10.8. Heavy mineral addition of up to 30% does not significantly change Th/Sc, La/Sc and La/Th values, but slightly changed the Zr/Sc ratios, which range from 38 to 53. The heavy mineral-enriched beds (50 to 75% of the whole rock), display nearly similar La/Sc ratios compared with other lithotypes. They are 9x enriched in rare earth elements and show a significantly stronger negative Eu-anomaly, and an enrichment of heavy over light rare earth elements, as well as higher Zr/Sc ratios (82 to 299), typical for reworked passive margin or foreland deposits. The chemical composition of separated heavy minerals point to a relatively strong influence of metapelitic and metamorphosed mafic sources, which were not identified using geochemical data. Mineral chemical analyses of chromian spinels point to island arc and mid-ocean ridge basalts as protoliths, whereas rutiles were derived from metamorphosed mafic and pelitic rocks, and abundant garnet (mainly almandine) derived from high-grade and low-grade metapelitic sources. Titanite shows dominance of felsic and mafic plutonic rocks as sources, while epidote point to low-grade metamorphic protoliths. Magnetite was eroded from felsic plutonic and/or mafic metamorphosed rocks. Previous works using U-Pb detrital zircon dating show typical Namaquan and Pan-African age populations as expected for a collisional tectonic setting. Palaeocurrents indicate that the main source region of the heavy and the host rock minerals is within the Damara Belt which resulted from the convergence of Congo and Kalahari Cratons. Although the exact age of thrusting is debated, the data presented suggest the exhumation of metamorphosed crustal fragments and plutonism related to the Damara orogeny during the deposition of the upper Nama Group (ca. 520 Ma).

Antecedence of the Yarlung–Siang–Brahmaputra River, eastern Himalaya

Abstract At the eastern terminus of the Himalayan orogen, distortion and capture of southeast Asian drainage basins reflects regional patterns of crustal strain due to the indentation of the Indian Plate into Eurasia. After flowing eastward > 1000 km along the southern margin of Tibet, the Yarlung–Siang–Brahmaputra River turns abruptly southward through the eastern Himalayan syntaxis rapidly exhuming a crustal scale antiform in an impressive > 2 km knickpoint. This conspicuous drainage pattern and coincidence of focused fluvial incision and rapid rock exhumation has been explained by the capture of an ancestral, high-elevation Yarlung River by headward erosion of a Himalayan tributary. However, recent observation of Tibetan detritus in Neogene foreland basin units complicates this explanation, requiring a connection from Tibet to the foreland prior to the estimated onset of rapid rock exhumation. We constrain the sedimentary provenance of foreland basin units deposited near the Brahmaputra River confluence in the eastern Himalayan foreland basin using detrital zircon U–Pb geochronology. We interpret the significant presence of Gangdese-age detritus in each foreland basin unit to indicate that connection of the Yarlung–Siang–Brahmaputra River was established during, or prior to foreland deposition in the Early Miocene. Our results indicate that connection of the Yarlung–Siang–Brahmaputra River precedes exhumation of the syntaxis, demonstrating the potential for the progressive coevolution of rock uplift and rapid erosion of the Namche Barwa massif.

Syn- and post-sedimentary controls on clay mineral assemblages in a tectonically active basin, Andean Argentinean foreland

In the northern part of the Calchaquí Valley (NW Argentina), Palaeogene Andean foreland sediments are represented by a 1400-metre-thick continental succession (QLC: Quebrada de Los Colorados Formation) consisting of claystones, siltstones, sandstones, and conglomerates representing sedimentation in fluvial-alluvial plains and alluvial fan settings. To understand the main syn- and postsedimentary variables controlling the clay mineral assemblages of this succession, we have studied the fine-grained clastic sediments by X-ray diffraction and electron microscopy, along with a detailed sedimentary facies analysis, for two representative sections. In the northern section, the whole succession was sampled and analysed by XRD, whereas in the second section, a control point 15 km to the south, only the basal levels were analysed. The XRD study revealed a strong contrast in clay mineral assemblages between these two sections as well as with sections in the central Calchaquí Valley studied previously. In the northernmost part of the study area, a complete evolution from smectite at the top to R3 illite/smectite mixed-layers plus authigenic kaolinite at the bottom, through R1-type mixed-layers in between, has been recognized, indicating the attainment of late diagenesis. In contrast, the clay mineral assemblages of equivalent foreland sediments cropping out only 15 km to the south contain abundant smectite and micas, subordinate kaolinite and chlorite, and no I/S mixed-layers to the bottom of the sequence. Early diagenetic conditions were also inferred in a previous study for equivalent sediments of the QLC Formation cropping out to the south, in the central Calchaquí Valley, as smectite occurs in basal strata. Burial depths of approximately 3000 m were estimated for the QLC Formation in the central and northern Calchaquí Valley; in addition, an intermediate to slightly low geothermal gradient can be considered likely for both areas as foreland basins are regarded as hypothermal basins. Consequently, the attainment of late diagenesis in the northernmost study area cannot be explained by significant differences in burial depth nor in geothermal gradient in relation to the section 15 km to the south nor with the central Calchaquí Valley. The formation of R3 mixed-layer I/S and authigenic kaolinite in the northern study area was most likely controlled by the circulation of hot, deep fluids along the reverse faults that bounded the Calchaquí valley. These faults were active during the Cenozoic, as evidenced by the syndepositional deformation features preserved in the studied sediments. Stress could also have been a driving force in burial diagenesis at the R3 mixed-layer I/S stage in these young continental sediments.

Influence of tectonics, sedimentation and aqueous flow cycles on the origin of global groundwater arsenic: Paradigms from three continents

Arsenic (As) is a trace element in the Earth’s crust. However, its presence in elevated concentrations in groundwaters of major aquifers around the world raises concern about its primary source(s). A close look at the global distribution of known As enriched areas reveals an intriguing systematic pattern, where most of the As enriched aquifers are parts of large sedimentary basins adjoining major orogenic belts, suggesting the existence of a large-scale geological process. Many of these sedimentary basins may be tectonically regarded as foreland basins that developed by lithospheric flexure at the time of mountain building processes (orogenesis) along convergent plate boundaries. Arsenic enrichment in the groundwater of these foreland basin aquifers may be ultimately sourced to crustal evolution processes related to plate tectonics, along with transportation of As-enriched magmatic rocks from the depth to surficial deposits, which subsequently release or mobilize the As to groundwater under conducive surficial biogeochemical processes. Circulating As-laden hydrothermal fluids may also be derived from magmas and form part of the discharge in the surficial hot springs in arc environments. These hydrothermal-genic deposits in orogenic belts may also act as the primary provenance for the As-laden sediments that are transported into foreland basins by wind, glacial erosion, and/or streams. Ultimately, the As-laden foreland sediments serve as modern-day aquifers, where the sediments release As into groundwater by water–rock interaction during various biogeochemical processes under conducive hydrogeochemical conditions. The significance of this hypothesis is that it proposes the existence of a common primary source for globally dispersed geogenic As-enriched aquifers, that have been so far mostly studied as individual occurrences. The proposed hypothesis can explain the widespread presence of As in areas as diverse as the Indus–Ganges–Brahmaputra basin (Himalayan orogen), the Chaco-Pampean basin (Andean orogen), Rocky mountain basin (Western Cordilleran orogen), New England and northeastern USA (the Applachian orogen).

The origin of oriented lakes: Evidence from the Bolivian Amazon

The presence of hundreds of rectangular and oriented lakes is one of the most striking characteristics of the Llanos de Moxos (LM) landscape in the Bolivian Amazon. Oriented lakes also occur in the Arctic coastal plains of Russia, Alaska and Canada and along the Atlantic Coastal Plain from northeast Florida to southeast New Jersey and along the coast of northeast Brazil. Many different mechanisms have been proposed for their formation. In the LM, Plafker's (1964) tectonic model, in which subsidence results from the propagation of bedrock faults through the foreland sediments, is the most accepted. However, this model has not been verified. Here, we present new results from stratigraphic transects across the borders of three rectangular and oriented lakes in the LM. A paleosol buried under mid-Holocene sediments is used as a stratigraphic marker to assess the vertical displacement of sediments on both sides of the alleged faults. Our results show that there is no vertical displacement and, therefore, that Plafker's model can be ruled out. We suggest that, among all the proposed mechanisms behind lake formation, the combined action of wind and waves is the most likely. The evidence from the LM provides new hints for the formation of oriented lakes worldwide.

Retroarc deformation and exhumation near the end of the Andes, southern Patagonia

The southern Patagonian Andes constitute the narrow, high-latitude end of the Andean orogen belt in South America, where inherited basin paleogeography, subduction processes, retroarc crustal thickening, and late Cenozoic glaciation have collectively influenced their unusual tectonic and physiographic evolution. New zircon and apatite (U–Th)/He thermochronology from the Patagonian Andes between 50°30′ and 51°30′S suggest concentrated exhumation across the retroarc (leeward) side of the orogen since early Miocene time. Zircon (U–Th)/He (ZHe) ages range from 44 to 10 Ma; oldest ages are recorded in the Patagonian batholith and along the far eastern frontal foreland monocline. Regionally-uniform ZHe ages between 22 and 18Ma, located across a ∼75km wide-zone of the Patagonian retroarc thrust-belt indicate widespread early Miocene cooling through the ZHe partial retention zone. Mesozoic sedimentary and volcanic rocks in this region have been exhumed from at least 5 to 6km depths. Early Miocene denudation of the thrust-belt, deformation, and increased foreland sedimentation rates coincided with opening of the Scotia Sea, suggesting a causal response of the foreland to changes in plate dynamics. The apatite (U–Th)/He (AHe) ages from a similar region range from 11 to 3Ma; the youngest ages (6–3Ma) are spatially clustered within the more deeply-exhumed central thrust domain. We interpret these AHe ages to record >1–2km of erosional denudation associated with late Cenozoic glaciation and fluvioglacial processes in Patagonia beginning ca. 7Ma.Zircon results indicate that since ca. 22Ma, long-term exhumation rates have been highest across the western and central thrust domains (0.22–0.37mmyr−1), and significantly lower along the eastern thrust front (0.10–0.17mmyr−1). Since ∼7Ma, apatite results from these same regions suggest comparable and slightly higher denudation rates (0.14–0.46mmyr−1), particularly within the eastern thrust domain, consistent with efficient erosional processes acting in the retroarc region. These results from the Patagonian retroarc region, particularly the predominance of <7–4Ma AHe ages, provide a new understanding for regional orogenic erosion models that are largely based on existing data from the windward regions. We suggest that retroarc denudation was enhanced by widespread Miocene structural uplift and unroofing of the fine-grained siliciclastic rocks of the marine Cretaceous Magallanes/Austral foreland basin. Furthermore, its location at the southern tip of the orogen, may have allowed additional moisture to reach the leeward side, leading to sustained late Cenozoic erosional denudation.

Tracing the Oligocene-Miocene Evolution of the Western Alps Drainage Divide with Pebble Petrology, Geochemistry, and Raman Spectroscopy of Foreland Basin Deposits

The formation of the western Alps topography is the result of continental collision between the Apulian and European plates. In this study, we trace the Early Oligocene to Early Miocene development of topography and the position of the drainage divide in the southern western Alps by analyzing the erosion products preserved in the pro-side (Montmaur and Barrême) and retro-side (Torino hills) foreland basins of this orogen. Using petrologic and geochemical analyses of basalt pebbles and Raman spectroscopy of serpentine sand grains and pebbles, we identify source lithologies, which are not easily detected with more commonly used detrital thermochronology. Lower Oligocene sediments of the pro-side foreland basin contain numerous basalt pebbles that share strong geochemical similarities with the Chenaillet obducted ophiolite (Montgenèvre massif). Other ophiolite-suite derived clasts, e.g., radiolarite or serpentinite appear widely in pro- and retro-side foreland sediments since about 30 Ma. This suggests a wider distribution of Chenaillet-type obducted ophiolite rocks in the western Alps during the Oligocene, but the exact locations, except for the Chenaillet, are unknown. Raman analysis on serpentine grains and pebbles from the retro-side foreland basin deposits documents a systematic trend from antigorite (high-grade metamorphic source rocks) to lizardite (low-grade metamorphic source rock) from the Early Oligocene to the Early Miocene. This trend is attributed to a westward growth of the paleo-Dora Riparia drainage basin in the southern western Alps. Ophiolite erosion and drainage divide shift were caused by the topographic evolution of the western Alps, which we suggest to be linked to the shift in convergence direction between the Apulian and Eurasian plates from N–S to E–W and the presence of the so-called Ivrea body mantle splinter acting as a vertical indenter beneath the western Alps at that time. The drainage patterns of the paleo-Durance and paleo-Doria Riparia Rivers seem to have remained stable since the Early Miocene. In comparison to the central Alps, the drainage divide shift in the southern western Alps occurred earlier than in the central Alps, but in both locations the trend from an internal to a more external position is the same.