Foreland Sediments Research Articles

Early development of the Patagonian fold and thrust belt and the onset of foreland sedimentation in the Austral-Magallanes basin

Retroarc basins register the onset of compressional exhumation during the Cretaceous in the Andes, evidenced by sequences showing sedimentary provenance from the orogen. We present U–Pb detrital zircon geochronology from the Austral-Magallanes basin in the Southern Patagonian Andes (∼47–48°SL), where the foreland stage began in the Early Cretaceous. The littoral sandstones from basal foreland sequences of the Río Belgrano Formation yield ∼123 Ma maximum depositional ages, confirming previous studies that the basin infill is early Aptian. The detrital zircon ages showed two main clusters of 129-115 Ma and 151 -140 Ma, that are derived from the coetaneously exhumed Andean arc and earlier Jurassic magmatic sources. The zircons which overlap with ages of Upper Jurassic Andean synrift volcanics, confirms orogenic exhumation coeval with sedimentation. A sequence of retroarc volcanism dated between 119 and 97 Ma overlies the basal foreland littoral-continental deposits, and it is followed by a Late Cretaceous-Paleocene hiatus. Further analysis of detrital zircons ages distribution and geological considerations, indicates that the studied northern depocenter was a source region of sedimentary supply after its Cenomanian exhumation.

Cosmogenic Nuclide Tracking of Sediment Recycling From a Frontal Siwalik Range in the Northwestern Himalaya

AbstractThe Himalayan orogen exports millions of tons of sediment annually to the Indo‐Gangetic foreland basin, derived from both hinterland and foreland fold‐thrust belts (FTB). Although erosion rates in the hinterland are well‐constrained, erosion rates in the foreland FTB and, by extension, the sediment flux have remained poorly constrained. Here, we quantified erosion rates and sediment flux from the Mohand Range in the northwestern Himalaya by modeling and measuring the cosmogenic radionuclide (CRN) 10Be and 26Al concentrations in modern fluvial sediments. Our model uses local geological and geophysical constraints and accounts for CRN inheritance and sediment recycling, which enables us to determine the relative contributions of the hinterland and foreland FTB sources to the CRN budget of the proximal foreland deposits. Our model predictions closely match measured concentrations for a crustal shortening rate across the Mohand Range of 8.0 ± 0.5 mm yr−1 (i.e., approximately 50% of the total shortening across the Himalaya at this longitude) since Ma. This shortening implies a spatial gradient in erosion rates ranging from 0.42 ± 0.03 to 4.92 ± 0.34 mm yr−1, controlled by the geometry of the underlying structure. This erosion pattern corresponds to an annual sediment recycling of ∼2.0 megatons from the Mohand Range to the downstream Yamuna foreland. Converted to sediment fluxes per unit width along the Himalaya, the foreland FTB accounts for ∼21% ± 5% of the total flux entering the foreland. Because these sediments have lower 10Be concentrations than hinterland‐derived sediment, they would lead to ∼14% overestimation of 10Be‐derived erosion rates, based on Yamuna sediments in the proximal foreland.

Cenozoic subsidence‐driving mechanisms in the southernmost Patagonian basins of Tierra del Fuego and SW Atlantic

AbstractForeland basins are ideal laboratories to examine and quantify forces that contribute to Earth's topography. The interaction of these driving mechanisms (atmospheric, lithospheric and asthenospheric) affects the accumulation and preservation of strata in marine or terrestrial depocentres. For foreland basins that cover thousands of kilometres along orogens, geodynamic processes or lithospheric structure might differ and/or overlap differently along or across strike. The Magallanes‐Austral basin in the southernmost Patagonia serves as a good analogue to analyse the interactions between subcrustal forces and foreland sedimentation. While to the northern part of southern Patagonia, Cenozoic basins were predominantly terrigenous and above sea level; at the southernmost end of Patagonia, sedimentation in the island of Tierra del Fuego was mostly submarine. We analysed in this contribution the southernmost foreland of Patagonia by combining backstripping with reconstruction of flexural and dynamic subsidence. These results were compared with terrestrial records exposed further north of southern Patagonia. We found that, in addition to crustal contributions (as deformation and sedimentation), subcrustal forces are required to accommodate the proximal and distal foreland strata and explain the palaeoenvironmental and subsidence discrepancies that resulted after our analysis. When our models are compared with dynamic topographic curves, strong correlations are observed during the Palaeogene, whereas strong topographic differences occurred in the Neogene. Dynamic topography models in the Neogene have reproduced clear uplift, whereas our residual topography results show equilibrium (close to the orogen) to subsidence values (to the distal foreland). We propose that changes in the lithospheric mantle had to work together with the rest of the tectonics and dynamic forces to match 1‐D backstripping and flexural curves. This suggests that foreland basins in southern Patagonia were controlled differently along strike the southern Andes and that crustal deformation, asthenospheric flows and a heterogeneous lithospheric mantle structure affected the Cenozoic basin evolution.

Evidence of new diapiric structures in the southern Adria Plate (Eastern Margin of Tethyan Ocean): Implications for Triassic paleogeography and evaporites remobilization during subduction/collision (Northern Ionian Sea, Central Mediterranean)

It has long been recognized that the Adria Plate, as well as the Dinaric-Hellenic sector, contains a large volume of Triassic salt associated with diapirism, and it is well known that Triassic evaporites in the Mediterranean Sea region were deposited over epicratonic platforms surrounding the Mesozoic passive margin of Tethys Ocean. However, the real extent of these evaporites is still unknown; especially in the deep-water offshore area where publicly available seismic reflection profiles are scarce and petroleum exploration wells are missing. Several authors highlighted the presence of Triassic salt structures, mostly triggered by Meso-Cenozoic compressions and associated with fold-and-thrust belts. Both the lack of data and the later compressions contribute to hiding how a diapiric structure, formed in a passive margin and characterized by dome's geometry, is remoulded to assume a teardrop geometry typical of diapirs in fold-and-thrust belts. Two end members of diapiric structures are discussed: extensional structures formed in a passive margin, and compressional structures formed in a fold-thrust belt. The transformation of one end-member into the other is not well known. In the Northern Ionian Sea, the Apulian foreland, belonging to the southernmost termination of the Adria Plate and considered one of the largest preserved Mesozoic passive margin block of the Tethys Ocean, is a deep-water area associated with the opposing fronts of the nearby Calabrian Arc/Apennines and Dinaric-Hellenic Chains since the Cretaceous. Therefore, it represents an ideal area for investigating both the real extent of Triassic evaporites and the transformation of the diapiric structures under different regional geodynamic contexts.Based on seismic reflection profiles but no petroleum exploration wells, we present evidence of Triassic evaporites in the deep-water portion of the Apulian foreland, as indicated by the interpretation of two diapiric structures never described previously. These two new diapirs likely formed from inherited Mesozoic salt structures such as pillows and/or salt domes. The identification of halokinetic-related sequences affecting the Plio-Quaternary foreland sediments allows constraining the polyphaser evolution of the two diapirs. We suggest they are reactivated in response to the compressive stresses transmitted by the nearby Calabrian Arc/Southern Apennines and Hellenides. But, after the Middle Pleistocene, extensional tectonics, related to Adria Plate flexuring, acted again, and the diapirs were cross-cut by normal faults. These observations and findings are integrated into the geological context that controls the fundamental features of the diapirs, providing new insights on the fate of diapiric structures formed in a passive margin, later involved in a subducting and colliding plate, and before they become part of a fold-and-thrust belt. Furthermore, the study provides new constraints on the Triassic paleogeographic reconstruction of the Northern Ionian and Central Mediterranean Seas at the continental margin of the Tethys Ocean.

Phanerozoic Tectonic and Sedimentation History of the Arctic: Constraints From Deep‐Time Low‐Temperature Thermochronology Data of Ellesmere Island and Northwest Greenland

AbstractRocks exposed along both sides of the Smith Sound in Ellesmere Island and NW Greenland record the tectono‐sedimentary evolution of the whole Phanerozoic, including two periods of mountain building—the Palaeozoic Ellesmerian Orogeny and the Palaeogene Eurekan Orogeny—and the formation of two major sedimentary basins, the Franklinian and the Sverdrup Basins. We used geo‐ and thermochronology and apatite chemistry data to unravel this evolution. Apatite fission track and (U‐Th)/He dates vary strongly from >600 to <100 Ma. We present internally consistent thermal history models, which allow to explain the data variations by a unitized exhumation and burial history. Our models suggest that the cratonic areas were buried beneath a several km‐thick succession of Franklinian Basin deposits. During the Ellesmerian Orogeny, the craton acted as sediment source, as also suggested by the composition of apatite and by U‐Pb ages of zircon contained in Devonian foreland sediments. The Ellesmerian foreland was buried by up to 4–5 km thick strata on top of the preserved sedimentary rocks. During the Triassic, the Sverdrup Basin strongly widened and extended at least ∼370 km further toward the east, as compared with previous reconstructions of the basin based on the preservation of Triassic deposits. Thermal history modeling suggests Late Cretaceous to early Cenozoic reheating, which may be caused by deposition associated with the Eurekan Orogeny and/or enhanced heat flow associated with continental breakup. Our data also show that low‐temperature thermochronology is not suitable for resolving potential strike‐slip movements along the Wegener Fault.

Mechanisms of Subsidence and Uplift of Southern Patagonia and Offshore Basins During Slab Window Formation

AbstractSubduction of bathymetric anomalies (e.g., an active ridge) can alter the morphology of subducted slabs and their coupling to surface processes. A natural laboratory to study these effects is the subduction of the Oceanic Chilean Ridge beneath the South American plate, which led to the formation of the Patagonian slab window. Its formation and subsequent northward migration contributed to the regression of Patagoniense sea and exhumation of marine strata to their present elevation. To date, there is no quantitative analysis of the effects on the sediment routing system of the slab window. We modeled the Neogene topographic change and foreland sedimentary evolution from the Andean Cordillera to Atlantic margin. Our results show that subcrustal‐driven subsidence correlated with accelerated subduction of the Nazca plate is required to explain the timing of the Patagonian transgression and thickness and spatial extent of marine beds during the incursion. In other words, traditional mechanisms, such as foreland flexure and global sea‐level rise, are insufficient. The subsequent regression and accumulation of mid‐Miocene alluvial‐fluvial deposits were associated with the growth of the Cordillera and a possible flattening of Nazca subduction in the middle Miocene. Isostatic uplift of ∼1 km due to lithospheric thinning during slab window formation can explain the foreland exhumation, sediment bypass, and increases in the offshore sedimentation rate. However, spatial‐temporal varying dynamic uplift is required to explain the along‐strike variations in foreland sedimentation. Our study provides new insights into the interplay between slab window formation, crustal deformation, and landscape evolution.

Sediment dispersal and basin evolution during contrasting tectonic regimes along the western Gondwanan margin in the central Andes

A >15-20 km-thick succession in southern Bolivia forms the most-complete stratigraphic record in western Gondwana. Upper Neoproterozoic–Carboniferous clastic rocks record ∼300 Myr of marine, nonmarine, and glacially influenced sedimentation in diverse basin systems generated by variable tectonic regimes along the western edge of Gondwana during active and passive-margin conditions. New provenance results help resolve key uncertainties regarding source regions and sediment dispersal patterns. The findings are integrated with spatial variations in stratigraphic thicknesses to evaluate regional patterns of basin subsidence, magmatism, and deformation during long-term evolution of the western Gondwanan margin in the central Andes.Detrital zircon U–Pb geochronological data for 17 sandstone samples reveal sedimentary input from Precambrian cratonic basement provinces and pre-Andean basement, magmatic arc, and fold-thrust belt source regions. The basement age signatures indicate derivation from the flanking Brasiliano (900-560 Ma) and Pampean (650-500 Ma) provinces to the south and east, and the distal Rio de la Plata craton (2400-2000 Ma) along the eastern South American margin ∼1000–1500 km to the southeast. Although the greater Amazonian craton was not a major contributor, subordinate Amazonian signatures from the Sunsás (1300-950 Ma) province to the east and northeast selectively fed the northern basin regions of the central Andes. Despite the lack of Paleozoic igneous rocks in Bolivia, detrital zircons of Ordovician age attest to the pre-Andean influence of the subduction-related Famatinian magmatic arc. Limited Devonian-Carboniferous igneous material was contributed locally from western pre-Andean highlands or regionally by axial northward transport from selected igneous sources in Argentina and Chile. Episodic recycling of Neoproterozoic–Paleozoic basin fill, including a sharp reappearance of Famatinian-age detritus, can be linked to periods of Paleozoic crustal shortening and foreland sedimentation ascribed to Famatinian, Ocloyic, Chañic, or Gondwanide phases of deformation.The spatial distribution of sediment sources along with temporal shifts in sediment routing highlight several stages in the paleogeographic evolution of the western Gondwanan margin preserved in the central Andes. Initial regional subsidence spanned a multiphase Neoproterozoic to early Paleozoic history of Rodinia breakup, Brasiliano-Pampean orogenesis, and post-orogenic back-arc extension prior to final late Paleozoic amalgamation of Gondwana. The early Paleozoic onset of subduction and Famatinian arc magmatism led to high-magnitude subsidence (>10–15 km) likely driven by Ordovician slab rollback in an extensional back-arc basin. Thereafter, intermittent Paleozoic contraction in a poorly understood pre-Andean system (best expressed in the Eastern Cordillera of Bolivia and neighboring segments of northern Argentina and southern Peru) generated transient topographic loads that produced superimposed flexural foreland and successor basin systems.

Sverdrup Basin Composite Tectono-Sedimentary Element, Canadian Arctic Islands

Abstract The Sverdrup Basin Composite Tectono-Sedimentary Element (CTSE) covers 210 000 km 2 in the Canadian Arctic Archipelago. The CTSE was initiated in the Early Carboniferous by rifting of highly deformed Early Paleozoic strata and contains a maximum of 15 km of Carboniferous–Eocene strata. Eight phases of basin development have been recognized, with each being characterized by a specific combination of tectonic and depositional regimes. The phases are separated by intervals of uplift and tectonic reorganization, and each resultant first-order sequence is regarded as a separate TSE. Carbonate sedimentation was dominant in the Late Carboniferous and Early Permian, with clastic sedimentation becoming more common in the Middle Permian. Source areas lay to the east, south and north. In the Triassic, clastic sedimentation rates increased; and by the end of the Triassic, the central basin was filled. A shallow seaway was present throughout the Jurassic. In the Early Cretaceous, subsidence rates and clastic supply increased significantly, and basic volcanism occurred over most of the CTSE. Subsidence rate, sediment supply and volcanism decreased in the Late Cretaceous. The basin was progressively deformed in the Paleogene (Eurekan Orogeny), with local foreland deposits reaching 3 km. Seventeen oil and gas fields have been discovered on salt-cored, Eurekan anticlines. A combination of stratigraphic and structural traps, involving Triassic–Jurassic strata, has the greatest potential for future hydrocarbon discoveries.

Tectonic evolution of the eastern margin of the Southern Patagonian Andes fold-thrust belt: U-Pb detrital zircon geochronology and kinematic-structural modelling

We present a multidisciplinary study in the Austral-Magallanes basin combining U-Pb geochronology and provenance analysis with structural modelling to unravel the evolution of the fold-thrust belt and related foreland basin sedimentation. Our approach is innovative for the studied area, where the foreland stratigraphic record comprises a stack of marine deposits exceeding 5000 m in thickness. Using zircon U-Pb geochronology from 13 samples distributed through the foreland deposits in the Southern Patagonian Andes at 51°20′S latitude, we calculated maximum depositional ages and analyzed regional variations in provenance. The older sample, with an age of 96.0 ± 1.5 Ma, corresponds to the first sandstone beds of the Punta Barrosa Formation, whereas the uppermost sample, with an age of 17.9 ± 0.4 Ma stems from the Santa Cruz Formation. Furthermore, the presented data help to complete the geochronological information existing for the South Patagonian Andes, by informing the regional geodynamic framework and timing variations. Linking new and previously published data regarding detrital zircon provenance, maximum depositional ages analysis, thermochronology and stratigraphic data with a balanced structural cross section, we propose a step-by-step model combining the kinematic evolution of the fold-thrust belt orogenic front with the basin history. A wedge-top setting for the Late Cretaceous is recognized, as well as two minor uplift events by Eocene time, followed by a major Miocene tectonic exhumation event.

From rift to foreland basin: A case example from the Magallanes‐Austral basin, southernmost Andes

AbstractThis contribution characterizes primary lithologic and depositional components of the Magallanes‐Austral basin and defines infill geometries and stacking patterns from seismic and well data. An integrated seismic model is proposed for recognition of rifting, thermal sag and foreland tectono‐stratigraphic phases based on depositional geometries and its relation with the evolving deformational and geodynamic framework. Above a Middle–Late Jurassic extensional phase, evidenced by synrift depositional geometries, follow marine successions representing the subsidence thermal sag phase (Tithonian–Early Cretaceous) characterized by concordant and laterally extensive seismic reflectors. The following foreland phase is described through the evolution and lateral migration history of the foredeep depocentre and concomitant forebulge development. The foreland phase is represented by different stages characterized by asymmetric sedimentary wedges bounded by basal surfaces and/or major unconformities recording transitions from underfilled to overfilled conditions. The accumulated thickness due to lithospheric flexure reflects different foreland subsidence profile patterns across the southern depocentre of the Magallanes‐Austral basin, producing asymmetrical westward and southward thickening wedges. The first Foreland I stage (Coniacian?–Maastrichtian) is recorded as an asymmetric wedge infill, that thins cratonward, with a NW‐trending foredeep axis. The erosive basal foreland surface (BF) at its base deepens towards the west and south along the active margin of the basin, where subsidence was maximum. On top of it, along the western portion of the basin and with a source area from the north, deep‐marine slope deposits and turbiditic complexes were deposited; while on the forebulge to the east, a clastic platform developed. The Foreland II stage (early‐to‐middle Palaeocene–middle Eocene) is characterized by renewed uplift and flexure, and increasing tectonic subsidence rates, building a new clastic wedge‐shaped foreland succession next to the orogenic belt, and a well‐represented forebulge to the east. Subsequently, an extensive diachronous G7 unconformity was generated, eroding locally the previous foreland deposits towards the eastern margin. A pronounced and continuous NW‐SE trending deflection is established subtly to the east. The following Foreland III stage (middle to late‐Eocene–Oligocene) is characterized by a reduction in thrust load along the western active margin, and progradational systems towards the NE, a time during which the subsidence rate decreased and accommodation space was reduced. Deposition occurred within a wide and continuous NW‐SE trending foredeep without a marked forebulge. The top of this stage is the A1 unconformity, marking the beginning of the Foreland IV stage (early Miocene–Neogene), regarded as an overfilled basinal stage without a marked foredeep and major variations in thickness across the extent of the basin. The depositional pattern in this stage is largely conformal and tabular. The proposal model represents an evolutionary example for the internal geometry of deep‐marine foreland basin system, including variables such as tectonic load and flexural subsidence, accommodation space, sediment supply variation, and relative sea‐level fluctuations.

The Montello Thrust and the Active Mountain Front of the Eastern Southern Alps (Northeast Italy)

AbstractNew field and subsurface data are combined to define the geometry and evolution of the mountain front of the eastern Southern Alps. The Montello thrust (MT), the southernmost frontal structure, is reconstructed by recently published well‐located microseismicity and is connected at depth with the larger Bassano‐Valdobbiadene (BV) thrust. The latter started at 10–9 Ma, whereas the MT evolved at 8–6 Ma, as documented by growing unconformities in the foreland deposits and the nature of the natural gas in the anticline culmination. The latter, named Montello anticline, is growing from the upper Miocene to the Quaternary, as shown by surfaces of abrasion and deposition, including the Biadene windgap. The folded bottom of the windgap is a fan unit recently dated to the MIS 3. The MT is presently creeping and its deformation rates are much lower than previous estimates. Most of the late Miocene to Pleistocene deformation at the front of the eastern Southern Alps is accounted for the BV thrust. The reconstructed average shortening rates are in agreement with the geodetic velocity field from literature. The BV and MT are a unique active thrust system with variable slipping and locked patches, whose interactions should be further studied.

Hindered Trench Migration Due To Slab Steepening Controls the Formation of the Central Andes

AbstractThe formation of the Central Andes dates back to ∼50 Ma, but its most pronounced episode, including the growth of the Altiplano‐Puna Plateau and pulsatile tectonic shortening phases, occurred within the last 25 Ma. The reason for this evolution remains unexplained. Using geodynamic numerical modeling we infer that the primary cause of the pulses of tectonic shortening and growth of the Central Andes is the changing geometry of the subducted Nazca plate, and particularly the steepening of the mid‐mantle slab segment which results in a slowing down of the trench retreat and subsequent increase in shortening of the advancing South America plate. This steepening first happens after the end of the flat slab episode at ∼25 Ma, and later during the buckling and stagnation of the slab in the mantle transition zone. Processes that mechanically weaken the lithosphere of the South America plate, as suggested in previous studies, enhance the intensity of the shortening events. These processes include delamination of the mantle lithosphere and weakening of foreland sediments. Our new modeling results are consistent with the timing and amplitude of the deformation from geological data in the Central Andes at the Altiplano latitude.

Late Miocene unroofing of the Inner Lesser Himalaya recorded in the NW Himalaya foreland basin

AbstractTesting models that link climate and solid Earth tectonics in mountain belts requires independent erosional, structural and climatic histories. Two well‐preserved stratigraphic sections of the Himalayan foreland basin are exposed in NW India. The Jawalamukhi (13–5 Ma) and Joginder Nagar sections (21–13 Ma) are dated by magnetostratigraphy and span a period of significant climate change and tectonic evolution. We combine sediment geochemistry, detrital zircon U–Pb dating and apatite fission track analyses to reconstruct changes in the patterns of erosion and exhumation in this area from the Early Miocene to Pliocene. The provenance of the foreland sediments reflects a mixture of Tethyan and Greater Himalayan sources from 21 to 11 Ma, with influx from the Inner Lesser Himalaya starting after 11 Ma, and a strong increase in Crystalline Inner Lesser Himalayan erosion after 8 Ma. This distinct shift in provenance most likely reflects exhumation of the Kullu‐Rampur Window, as well as the northward motion of the Jawalamukhi section towards the Himalayas, drainage reorganization in the foreland, and/or tectonically driven drainage capture in the mountains. Prior to 10.5 Ma sediment came from a large river whose sources were Greater Himalaya and Haimanta dominated, likely a palaeo‐Sutlej, while after 8 Ma the source river was dominated by a more local drainage. Our work is consistent with Nd isotope and mica Ar‐Ar constraints from the same sections that demonstrate initial Inner Lesser Himalayan unroofing in this region from 11 Ma, earlier than the 2 Ma implied from the marine record and during a period of summer monsoon weakening when fission track data indicate very rapid cooling and erosion of the Lesser Himalaya sources from no later than 10 Ma. Tectonically driven rock uplift coupled with southerly migration of the maximum rainfall belt during a time of drying, may have focused erosion over the Lesser Himalayan Duplex and created the Kullu‐Rampur Window.

Controls of the Foreland Deformation Pattern in the Orogen‐Foreland Shortening System: Constraints From High‐Resolution Geodynamic Models

AbstractControls on the deformation pattern (shortening mode and tectonic style) of orogenic forelands during lithospheric shortening remain poorly understood. Here, we use high‐resolution 2D thermomechanical models to demonstrate that orogenic crustal thickness and foreland lithospheric thickness significantly control the shortening mode in the foreland. Pure‐shear shortening occurs when the orogenic crust is not thicker than the foreland crust or thick, but the foreland lithosphere is thin (<70–80 km, as in the Puna foreland case). Conversely, simple‐shear shortening, characterized by foreland underthrusting beneath the orogen, arises when the orogenic crust is much thicker. This thickened crust results in high gravitational potential energy in the orogen, which triggers the migration of deformation to the foreland under further shortening. Our models present fully thick‐skinned, fully thin‐skinned, and intermediate tectonic styles in the foreland. The first tectonics forms in a pure‐shear shortening mode whereas the others require a simple‐shear mode and the presence of thick (>∼4 km) sediments that are mechanically weak (friction coefficient <∼0.05) or weakened rapidly during deformation. The formation of fully thin‐skinned tectonics in thick and weak foreland sediments, as in the Subandean Ranges, requires the strength of the orogenic upper lithosphere to be less than one‐third as strong as that of the foreland upper lithosphere. Our models successfully reproduce foreland deformation patterns in the Central and Southern Andes and the Laramide province.

Determining the tempo of exhumation in the eastern Himalaya: Part 2. Integrating bedrock and detrital cooling ages through thermokinematic modelling

AbstractThe exhumation record of a fold‐thrust belt is preserved by thermochronologic minerals, such as zircon and apatite, both in exposed bedrock and in synorogenic sedimentary rocks in the foreland basin. Treating these as separate records can lead to potentially contrasting interpretations of a single exhumation history. Integrating the bedrock and detrital records with thermokinematic models of sequential deformation of a fold‐thrust belt can identify viable exhumation pathways of the bedrock and elucidate both how bedrock exhumation varies in space and time and how accurately the basin records exhumation changes in the source region. Predicted bedrock cooling ages and modelled basin thickness are used to estimate the amount and source of sediments supplied to the foreland basin during each increment of deformation to predict the detrital cooling signal over time. Applying this integrated bedrock‐detrital model to a cross‐section in Arunachal Pradesh, NE India demonstrates spatial and temporal variability in exhumation, with a background exhumation rate of <2 mm/yr, periods of rapid exhumation at rates of 3–7 mm/yr, and short pulses of 10–12 mm/yr rates during out‐of‐sequence thrusting. Our results predict that the detrital apatite fission track (DAFT) system records a constant lag time of 0.5–1 Myr. Although the response of the detrital zircon fission track (DZFT) system is more complex, the system records changes in lag time (1–5 Myr) as a function of the kinematics, deformation rates and thermal profile of the crust in the hinterland. However, the DZFT cooling signal is delayed by 1–2 Myr relative to the age of marked shifts in location, magnitude and rate of exhumation in the source region. Our models also highlight the importance of recycled foreland deposits in matching the ca. 20 Ma static peak in the DZFT record and ca. 14 Ma static peak in the DAFT record.

Stratigraphic response to fragmentation of the Miocene Andean foreland basin, NW Argentina

AbstractForeland basins are sensitive recorders of spatiotemporal variations in tectonic and climatic forcing associated with an approaching orogenic front. Thus, analysis of foreland deposits and their associated deformation patterns and provenance signals allows assessment of tectonic and sedimentary processes during orogeny, providing clues to past environmental conditions. The Calchaquí region in the southern part of the northwest Argentinian Eastern Cordillera (ca. 25–26°S lat) structurally evolved from a contiguous Paleogene foreland of the Andes into a broken foreland and finally an intermontane basin landscape. This history is recorded in the sedimentary sequences of the Mio‐Pliocene Angastaco and Palo Pintado Formations. We combine sedimentological methods, U‐Pb zircon and K‐Ar glass geochronology, clay mineralogy, and geochemical weathering indices with apatite fission track and (U‐Th‐Sm)/He thermochronology, structural data, and fault modelling to document the stratigraphic response to the fluvial and tectonic processes that followed the formation of the broken foreland. Our observations suggest that fluvial systems in the Calchaquí region repeatedly changed their location and geometry. These fluvial systems were associated with pro‐ and retrograding gravel wedges that most likely formed in response to the structural growth of the Aguas de Castilla and Altos de Viñaco ranges that bound the basin to the east. A compartmentalisation of the foreland with restricted fluvial networks must have occurred by ca. 9 Ma. Our results demonstrate that the reconstruction of stratigraphic architectures constitutes a powerful means to better understand intrabasin tectonics and surface uplift in foreland basins.

Closure of the Paleo‐Asian Ocean in the Middle‐Late Triassic (Ladinian‐Carnian): Evidence From Provenance Analysis of Retroarc Sediments

AbstractA major current debate in Asian tectonics concerns the time of closure of the Paleo‐Asian Ocean (PAO) and the collision between the Central Asian arcs and the Tarim and North China cratons (NCC). This has led to much uncertainty about the reconstruction of proto‐Asia and tectonic configuration of northeast Pangea. Here we report Triassic provenance change in the foreland of the Alxa Block by field‐based zircon U‐Pb and Hf isotopic analysis. Nonmarine sediments, unconformable on or in fault contact, with the Alxa basement yield Precambrian and Paleozoic–Triassic detrital zircon ages with maximum depositional ages of Middle‐Late Triassic (∼244–224 Ma). Hf isotopic data indicate that Middle Triassic sandstones were sourced from the NCC basement and a continental arc, whereas Late Triassic sediments received an additional input from a juvenile arc. This provenance change in the foreland sedimentation indicates that the final closure of the PAO was in the Middle‐Late Triassic.

Coupled Rapid Erosion and Foreland Sedimentation Control Orogenic Wedge Kinematics in the Himalayan Thrust Belt of Central Nepal

AbstractSpatial and temporal coincidence among rapid Pliocene‐Holocene bedrock exhumation, development of a topographic bight, abundant monsoonal precipitation, accumulation of anomalously thick proximal foreland basin deposits, and development of an opposite‐polarity salient‐reentrant couple on the two most frontal major thrust faults in the Himalayan orogenic wedge of central Nepal provide a basis for a model that links these diverse phenomena and could be operating in other parts of the frontal Himalaya. Rapid bedrock erosion is documented by a concentration of young (<5 Ma) low‐temperature thermochronologic ages in the Narayani River catchment basin. Where the river exits the Lesser Himalayan Zone, the Main Boundary thrust has a 15‐km‐amplitude reentrant. Directly south of the reentrant lies the ∼50 km wide Chitwan wedge‐top basin, which is confined by a large salient on the Main Frontal thrust. Rapid erosion and sediment flux out of the Narayani catchment basin, possibly due to anomalously intense monsoonal precipitation in this topographically depressed region of central Nepal, causes greater flexural subsidence and surface aggradation in the foreland, both of which increase initial wedge taper and render this region more susceptible to anomalous forward propagation of the thrust front. Analysis of the modern and post‐early Miocene taper history of the thrust belt suggests that rapid erosion hindered forward propagation of the contemporaneous Main Boundary thrust, but simultaneously produced conditions in the foreland that eventually elevated initial taper to a critical/supercritical value promoting forelandward propagation of the Main Frontal thrust. This analysis has implications for large damaging earthquakes in the Himalaya.

U-Pb ZIRCON DATING OF MIDDLE EOCENE CLASTIC ROCKS FROM THE GERCUS MOLASSE, NE IRAQ: NEW CONSTRAINTS ON THEIR PROVENANCE, AND TECTONIC EVOLUTION

The provenance of Middle Eocene clastic rock from the Gercus Molasse, NE Iraq was determined by detrital zircon (DZ) U-Pb geochronology. The Gercus Molasse in the Iraqi segment of the north-eastern Zagros Thrust Zone provides an ideal example of foreland system evolution with respect to the transition from passive margin to the accretionary complex terrene-flexural foreland basins. The DZ U-Pb age spectra from the Gercus Molasse suggest that the foreland sediments either influx from multiple provenances or are the result of recycling from the accretionary complex terrane. During pre-accretion, however, the radiolarite basin (Qulqula Radiolarite, 221 Ma) located along Arabian passive margin likely acted as an intermediate sediment repository for most or all of the DZ. Representative DZ U-Pb measurements revealed that the Gercus clastic rocks fall into several separable age population ranges of 92-102 (Albian-Cenomanian), 221 (Upper Triassic), 395-511 (Cambrian), 570- 645 (Neoproterozoic), 1111 (Mesoproterozoic), and lesser numbers of Paleoproterozoic (1622-1991 Ma) ages. The source of Proterozoic detrital Zircons is enigmatic; the age peaks at 1.1, 1.5, 1.6, and 1.9 Ga (Proterozoic) does not correspond to any known outcrops of Precambrian rocks in Iraq, and it may be useful to continue to search for such basement. The detrital zircons with age populations at 0.63–0.86 Ga probably originated from the Arabian-Nubian Shield. The age peak at 0.55 Ga correlates with Cadomian Magmatism reported from north Gondwana. The age peaks at ~0.4 Ga is interpreted to represent Gondwana rifting and the opening of Paleotethys. The youngest ages populations at 93 Ma indicate that fraction of DZ were transported directly from the contemporaneously active magmatic arc (Zagros Ophiolite segments). The paleogeography and tectonic evolution of the Neogene Zagros foreland basin were reconstructed and divided into two tectonic stages. The early stage is defined by the Campanian accreted terranes (i.e. orogenic wedge) form loads sufficient to produce flexural basin with a deepest part is situated next to the tip of the loads. This flexural basin is filled by the flysch clastics of the Maastrichtian– Early Eocene (i.e. referred to by the Tanjero-Kolosh flysch sequence). The late stage is marked by a synchronized modification of the clastics fill of the basin and changes in dip directions to compensate for the reduction of the load by both erosion and extension and the basin, therefore, was sealed by a shallowing upwards depositional sequence ending with the terrestrial Gercus Formation.

Evaluating chemical weathering of sediments using “R” programming environment – A possible alternate of spreadsheet applications

Evaluation of chemical weathering of sediments is the key factor for provenance study and characterization of various sedimentological processes involved in weathering. The various Weathering Indices (WIs) models are available to estimate major elemental geochemistry of the sedimentary package including CIW, CPA, CIA, PIA, CIX, ICV, WIP, R, V, STI and MWPI. The present investigation is to improve and correlation of the earlier models with our development of a computer program using R programming environment to establish various statistical measures, correlation of chemical indices of weathering and to plot different compositional variation diagrams and their analyses. The present results reveal that the correlation of Different correlation plots suggest a significant level of relationship and positive correlation coefficients in CIW – PIA, CIA – PIA, CPA – CIX, ICV – MWPI pairs while significant level of relationship and negative correlation coefficients in CIW – ICV, CIA – WIP pairs. Besides, boxplot of WIs indicates a skewed left or negatively-skewed distribution of CIW and CIA while a skewed right or positively-skewed distribution of CPA, CIX and WIP. Different compositional space diagrams (A – C – N, A – CN – K, A-K – C – N, M – F – W, A – CNK – FM, A – N – K) constrain sediments and rock/mineral composites major elemental geochemistry and demonstrate weathering profiles and source rock chemical makeup. R environment provides flexibility to alter the code as per the data and user requirement by altering different arguments in the code and to include further relevant charts & plots. The applicability of the programming tool is demonstrated by its performance in evaluating the chemical weathering of Surma and Barail sandstones, Mizoram Foreland Basin, NE India and sediments of Manasbal Lake, Kashmir, India, having contrasting provenance histories.