Miocene Foreland Basin Research Articles

Reconciling Spatial and Temporal Patterns of Cenozoic Shortening, Exhumation, and Subsidence in the Southern Bolivian Andes

The Bolivian Andes are an archetypal convergent margin orogen with a paired fold-thrust belt and foreland basin. Existing chronostratigraphic constraints highlight a discrepancy between unroofing of the Eastern Cordillera and Interandean Zone fold-thrust systems since 40 Ma and the onset of rapid sediment accumulation in the Subandean Chaco foreland after 11 Ma, previously attributed to Miocene climate shifts. New results from magnetostratigraphic, backstripping, erosional volumetric calculations, and flexural modeling efforts are integrated with existing structural and thermochronologic datasets to investigate the linkages between shortening, exhumation, and subsidence. Magnetostratigraphic and backstripping results determine tectonic subsidence in the Chaco foreland basin, which informs flexural models that evaluate topographic load and lithospheric parameters. These models show that Chaco foreland subsidence is consistent with a range of loading scenarios. Eroded volumes from the fold-thrust belt were sufficient to fill the Chaco foreland basin, further supporting the linkage between sediment source and sink. Erosional beveling of the Eastern Cordillera, local intermontane sediment accumulation after 30–25 Ma, and regional development of the high-elevation San Juan del Oro geomorphic surface from 25 to 10 Ma suggest that the western Eastern Cordillera did not store the large sediment volume expected from erosion of the fold-thrust belt, which arrived in the Subandean Zone after 11 Ma. Eocene to middle Miocene foreland basin accumulation was likely focused between the Eastern Cordillera and Interandean Zone, and has been almost completely recycled into the modern Subandean foreland basin. The delay between initial fold-thrust belt exhumation (early Cenozoic) and rapid Subandean subsidence (late Cenozoic) highlights the interplay between protracted shortening, underthrusting, and foreland basin recycling. Only with sufficient crustal shortening, accommodated by eastward advance of the fold-thrust belt and attendant underthrusting of Brazilian Shield lithosphere beneath the Subandes, did the Subandean zone enter proximal foreland basin deposystems after ca. 11 Ma. Prior to the late Miocene, the precursor flexural basin was situated westward and not wide enough to incorporate the distal Subandean Zone. These results highlight the interplay between a range of crustal and surface processes linked to tectonics and Miocene climate shifts on the evolution of the southern Bolivian Andes.

Timing of Contractional Tectonics in the Miocene Foreland Basin System of the Umbria Pre-Apennines (Italy): An Updated Overview

A large dataset of lithostratigraphic and biostratigraphic data, concerning the Early-Late Miocene turbidite succession of the Umbria pre-Apennines, is presented and analyzed. The data come from the study of 24 sections that are representative of all the main tectonic units cropping out between the front of the Tuscan allochthon and the Umbria-Marche calcareous chain. The sections have been dated using quantitative calcareous nannofossil biostratigraphy and, wherever possible, they were correlated through key-beds recognition. Such a multidisciplinary approach allowed us to reconstruct the evolution of the Umbria foredeep over time and to unveil the chronology of compressive deformations by defining: (i) the onset of the foredeep stage in each structural unit, (ii) the age of depocenter-shifting from a unit to the adjacent one, (iii) the progressive deactivation of the western sector of the foredeep due to the emplacement of allochthon units, and (iv) the internal subdivisions of the basin due to the presence of foreland ramp faults or thrust-related structures. A further original outcome of our study is having brought to light the Late Burdigalian “out-of-sequence” reactivation of the Tuscan allochthon which bounded westward the foredeep, and the subsequent protracted period of tectonic stasis that preceded the deformations of the Umbrian parautochthon.

Tracking the Growth of the Himalayan Fold‐and‐Thrust Belt From Lower Miocene Foreland Basin Strata: Dumri Formation, Western Nepal

AbstractNew data from the lower Miocene Dumri Formation of western Nepal document exhumation of the Himalayan fold‐thrust belt and provenance of the Neogene foreland basin system. We employ U‐Pb zircon, Th‐Pb monazite, 40Ar/39Ar white mica, and zircon fission track chronometers to detrital minerals to constrain provenance, timing, and rate of exhumation of Himalayan source regions. Clusters of Proterozoic–early Paleozoic (900–400 Ma) Th‐Pb monazite and 40Ar/39Ar white mica detrital ages provide evidence for erosion of a Greater Himalayan sequence protolith unaffected by high‐grade Eohimalayan metamorphism. A small population of ~40 Ma cooling ages in detrital white mica grains shows exhumation of low‐grade metamorphic Tethyan Himalayan sequence through the ~350 °C closure temperature along the Tethyan Frontal thrust (proto‐South Tibetan detachment) during the late Eocene. Dumri Formation detritus shows a ~12 Myr time difference between cooling of its source rocks through the ~350 and ~240 °C closure temperatures as recorded by ~40–38 Ma youngest peak cooling ages in 40Ar/39Ar detrital white mica and ~28–24 Ma youngest populations in detrital zircon fission track. Exhumation between circa 40 and 28 Ma is consistent with slip and exhumation along the Main Central Thrust. Combined with similar data from northwestern India, our study suggests west‐to‐east spatially variable exhumation rates along strike of the Main Central Thrust. Our data also show an increase in exhumation during middle Miocene–Pliocene time, which is consistent with growth of the Lesser Himalaya duplex.

The Spanish Petroleum Systems and the Overlooked Areas and Targets

The structural complexity of the Spanish territory hampered the development of large sedimentary basins with a time-space evolution that could have allowed the accumulation and preservation of large hydrocarbon deposits. In spite of this, including the few exploratory wells drilled (3.13 wells / 1,000 square miles), Spain (including on and offshore) has up to eight geological systems with present or past commercial hydrocarbon fields: the Gulf of Cadiz-Guadalquivir Miocene foreland basin, the Cretaceous of the Betic ranges, the Jurassic and Lower Miocene of the Western Mediterranean Sea, the Albian sands of the Rioja Paleogene foreland basin, the Platform-Basin of the onshore Cantabrian basin, the Ypresian of the Pyrenees thrust belts, and the westward continuity of the Aquitania foreland basin in the Gulf of Biscay. There are other remaining areas that supposedly contain an exploratory potential that could be much higher than the systems mentioned above: the deep waters of the Mediterranean and Cantabrian seas and the Atlantic Ocean of the Canary Islands, the Mesozoic covered by the olistostromes of the Betic ranges, the onshore Triassic sandy reservoirs and the onshore Carboniferous sub-basins found throughout the mainland. These areas have an even lower level of exploration (0.26 wells / 1,000 square miles).

Precambrian–Pleistocene tectno-stratigraphic evolution of the southern Llanos basin, Colombia

The Llanos basin, located in the eastern region of Colombia, northwestern South America, is an Andean foreland basin between the Eastern Cordillera (Colombian Andes) and the Guyana Precambrian shield. The basin is the latest stage of a complex multiphase evolution that began in the Paleozoic at the latest. A Paleozoic–Pleistocene basin evolution model is presented based on a regional, two-dimensional, industry seismic data set and well-log observations for the southern part of the basin. Five tectono-stratigraphic sequences were identified: (1) lower Paleozoic depocenters preserved along inverted Neoproterozoic basement blocks; (2) an upper Paleozoic marine sequence folded and faulted in the late Paleozoic during assembly of Pangea; (3) Upper Cretaceous–Paleocene shallow marine sediments deposited in a distal foreland basin related to uplift of the Western and Central Cordilleras of Colombia, the sequence pinches out against a Paleozoic hinge or foreland bulge area; (4) an Eocene–Miocene foreland basin related to uplift of the Eastern Cordillera resulting in a wedge geometry; and (5) Pliocene–Pleistocene fluvial deltaic rocks overfilling the foreland basin. Reactivation of Paleozoic structures occurs at the top of this sequence with the development of anticlinal structures. Present-day stress fields indicate that subduction of the Nazca plate beneath South America may be responsible for reactivation of Paleozoic structures. Inversion of north–south structures with the Neoproterozoic basement is interpreted to be responsible for the Paleozoic and Pleistocene deformation, whereas Cenozoic deformation is related to the two main stages of foreland development of the basin. To the east, where the Paleoproterozoic basement is present, no deformation is interpreted.

Late Cretaceous–Miocene sedimentary development of the Arabian continental margin in SE Turkey (Adıyaman region): Implications for regional palaeogeography and the closure history of Southern Neotethys

The Middle Cretaceous–Late Miocene sedimentary successions that are exposed on the Arabian continental margin (near Adıyaman) provide valuable insights into sedimentary, tectonic and climatic processes that affected the Southern Neotethys and its continental margins. Deposition in the region began with Aptian–Early Campanian shelf carbonates, followed by deepening during the Mid-Campanian. Lithoclastic–bioclastic turbidites accumulated along the downfaulted northern margin of the Arabian platform during the Campanian. A flexurally controlled foredeep developed in advance of the emplacement of Mesozoic allochthonous continental margin and oceanic rocks during the latest Campanian–Early Maastrichtian (e.g. supra-subduction zone Koçali ophiolite). Alluvial detritus was shed from the emplaced allochthon in non-marine to shallow-marine deltaic settings during latest Campanian–Early Maastrichtian. A marine transgression resulted in the establishment of a shallow-marine shelf during the mid-Maastrichtian. During the latest Maastrichtian, the platform submerged, initiating deeper-water hemipelagic sedimentation. Uplift took place during the Palaeocene resulting in sediment instability, slumping and formation of high-density, subaqueous gravity flows. In response to regional faulting and tilting, some areas in the north and the southwest emerged during the Early–Middle Eocene generating alluvial fans in a warm, humid climate that favoured oxidation and reddening. The likely control of the tectonic instability was far-field compression related to the latest stages of subduction of the Southern Neotethys or the initial stages of collision of the Arabian and Tauride continents. An Oligocene hiatus is likely to reflect the formation of a collision-related forebulge, followed by flexural subsidence to form an Early Miocene foreland basin. This was finally over-ridden by the northerly, active continental margin of the Southern Neotethys during Early–Mid Miocene. Regional suture tightening (Mid–Late Miocene) and related non-marine coarse clastic sedimentation was followed by westward ‘tectonic escape’ of the Anatolian microplate towards the Aegean region during Plio-Quaternary time.

Tectonic development of the North Patagonian Andes and their related Miocene foreland basin (41°30′‐43°S)

The Northern Patagonian Andes have been constructed through multiple mechanisms that range from tectonic inversion of extensional structures of Early to Middle Jurassic age in the Main Andes to Oligocene in the Precordilleran region. These have acted during two distinctive orogenic stages, first in late Early Cretaceous and later in Miocene times Late Oligocene extension separates these two contractional periods and is recorded by half‐grabens developed in the retroarc region. The last contractional stage coexists with an eastward foreland expansion of the late Miocene arc whose roots are presently exposed as minor granitic stocks and volcanic piles subordinately in the Main Andes, east of the present arc. As a consequence of this orogenic stage a foreland basin has developed, having progressed from 18 Ma in the main North Patagonian Andes, where the mountain front was flooded by a marine transgression corresponding to the base of the Ñirihuau Formation, to 11 Ma in the foreland area. Cannibalization of this foreland basin occurred initially in the hinterland and then progressed to the foreland zone. Blind structures formed a broken foreland at the frontal zone inferred from growth strata geometries. During Pliocene to Quaternary times most of the contractional deformation was dissipated in the orogenic wedge at the time when the arc front retracted to its present position.

Inner vs. outer wedge-top depozone “sequences” in the Late Miocene (late Tortonian–early Messinian) Sicilian Foreland Basin System; new data from the Terravecchia Formation of NW Sicily

The wedge-top depozone belongs to the innermost portion of a Foreland Basin System (FBS) (DeCelles and Giles, 1996) and includes all sediments, typically coarse-grained proximal facies, that bury the active frontal part of a fold and thrust belt. The Terravecchia Formation (Flores, 1959; Schmidt Di Friedberg, 1962, 1964–1965; Catalano, 1979) is a composite lithostratigraphic unit widespread in Sicily (southern Italy) which has been recently considered (Gugliotta, 2010) as a part of the stratigraphic record of the Late Miocene (late Tortonian to early Messinian) Sicilian wedge-top depozone and represent the main object of this paper. Two end-member wedge-top “sequences” the (i) the inner wedge-top sequence (IWS) and (ii) the outer wedge-top sequence (OWS), respectively, were recognized in the Terravecchia Formation outcropping in different sectors of NW Sicily and here described and compared on the base of both their depositional and deformative pattern. The differences existing between the IWS and OWS clearly reflect the tectono-depositional evolution of sedimentary basins located at different position across the wedge-top depozone. The more coarse grained and IWS was deposited filling narrow and often oversupplied basins located in the inner sectors of the wedge-top depozone. In these basins the sedimentary evolution has been strongly controlled by a syn-sedimentary transpressional tectonics which produced, since the late Tortonian, the development of “local scale” intraformational angular unconformities. Contemporaneously, the more fine-grained OWS was deposited filling relatively wide and mainly shallow-water marine basins, probably open to major marine areas, located in a less external position of the wedge-top depozone. In these external areas during the late Tortonian–early Messinian the transpressional tectonics was active but still confined to the deeper structural layers producing long wavelength deformation and “basin-scale” unconformities. Integrating all data an alternative tectono-depositional model of the Sicilian Late Miocene Foreland Basin System has been outlined. The model here presented substantially differs from those previously known from other authors and shows a complex wedge-top depozone articulated in a “inner” and “outer” sector whose evolution has been driven by the progressive activation of deep-seated structures.

Testing Miocene Remagnetization of Bey Dağları: Timing and Amount of Neogene Rotations in SW Turkey

Here we reassess the timing and amount of rotation of the eastern limb of the Aegean orocline, located in SW Turkey. The current model for this orocline involves a 25-30° counterclockwise (CCW) rotation of the Bey Dağları region, which exposes upper Cretaceous to Eocene platform carbonates and lower Miocene flysch deposits. In this model the rotation has to postdate a Miocene remagnetization, which would not exclude non-synchroneity between rotations of the western and eastern limbs of the Aegean orocline. To test this model a detailed palaeomagnetic study was conducted on lower Miocene strata in the Bey Dağları area. Two (composite) sections were sampled near Korkuteli and Doğantaş, spanning the lower Miocene foreland basin stratigraphy from the Aquitanian unconformity with the Bey Dağları limestones to the uppermost Burdigalian-lowermost Langhian, and a wide array of palaeomagnetic analyses were obtained. The natural remanent magnetization (NRM) in the Aquitanian limestones and Burdigalian blue clays resides dominantly in magnetite with minor greigite and haematite. A positive reversals test, a correlation of obtained polarity patterns to the geomagnetic polarity timescale, showing significant compaction following acquisition of the NRM, a scatter in NRM directions that can be confidently attributed to palaeosecular variation and a positive recently-proposed end-member model test of the acquisition curves of isothermal remanent magnetization (IRM) all show that the Miocene sediments on the Bey Dağları platform have not been remagnetized. Our new results, combined with a partial reassessment of existing data, imply that the Bey Dağları region underwent no rotation between the late Cretaceous and late Burdigalian, and 20° counterclockwise rotation between 16 and 5 Ma, i.e. during the middle to late Miocene and prior to deposition of previously reported non-rotated Pliocene sediments north of Antalya. The two limbs of the Aegean orocline thus rotated simultaneously: our new age constraints of 16-5 Ma in the east, compare with (largely) 15-8 Ma in the west, as published previously. The rotation of the Bey Dağları was probably bounded in the south at the plate boundary with Africa, and in the east by the Aksu thrust and Kırkkavak dextral strike-slip fault, which together partitioned dextral transpression induced by the rotating block. Accommodation of the western Anatolian rotations to the north and west of the Bey Dağları will be the subject of a future publication.

Tectonic control on sedimentary facies pattern and sediment accumulation rates in the Miocene foreland basin of the southern Alborz mountains, northern Iran

The southern Alborz mountains of northern Iran are an integral part of the Arabia/Eurasia collision zone. A magnetostratigraphic and rock magnetic investigation of the Eyvanekey stratigraphic section in the southern Alborz mountains reveals the spatiotemporal character of sedimentary facies migration in the Alborz foreland basin. The section constitutes three coarsening upward units (units 1, 2, and 3), comprising the Upper Red and Hezardarreh formations. Our data reveal that the Upper Red Formation was deposited between 17.5 and 7.5 Ma, while the depositional age of the top of the Hezardarreh Formation can be extrapolated to ∼6.2 Ma. Slow sediment accumulation rates correlate with sedimentary facies comprising prograding, coarsening‐upward units. This is likely the result of intraforeland uplift (units 1 and 2) and basin inversion, probably associated with a growth syncline located in the proximal foreland (unit 3). In contrast, fine‐grained strata at the bottom of each cycle are associated with faster sediment accumulation rates, testifying to enhanced flexural basin subsidence in the course of thrust loading. Progradation of coarse‐grained facies also occurred during relatively fast sediment accumulation (top of unit 2), suggesting that the influx of coarse‐grained sediment outpaced the storage capacity of the proximal foreland. Thus, despite an overall southward propagation of deformation into the southern Alborz foreland, the locus of active deformation must have migrated back and forth on a time scale of circa 0.7 to 2 Ma.

The deep seismic reflection MARCONI-3 profile: Role of extensional Mesozoic structure during the Pyrenean contractional deformation at the eastern part of the Bay of Biscay

The new MARCONI-3 deep seismic profile allows recognition of the upper crustal structure of the eastern part of the Bay of Biscay and the main features of its Alpine geodynamic evolution. It denotes that the easternmost part of the Bay of Biscay consists of a thick wedge of uppermost Cretaceous to Cenozoic synorogenic sediments lying unconformably on the top of a thinned continental crust with the Mesozoic Parentis Basin to the north and the coeval Landes High to the south. The Parentis Basin appears as a major half-graben bounded southwards by a north-dipping planar fault. It is filled by a thick sequence of Jurassic–Upper Cretaceous carbonates affected by salt domes and squeezed diapirs made up of Triassic evaporites and mudstones. These salt tectonic structures also affect the overlying uppermost Cretaceous to Lower Miocene synorogenic deposits which are folded upon these structures. The Landes High includes a thin pre-Upper Cretaceous cover tilted to the south. In the Basque shelf, it is deformed by a basement-involving thrust wedge emplaced during the Late Eocene–Miocene that constitutes the North-Pyrenean contractional front. Geometric relationships and thickness variations depict that this overall structure results from the following. • Mesozoic extensional stage which includes a Late Jurassic (?)–Late Aptian syn-rift stage in which the Parentis Basin formed; and an Albian–early Late Cretaceous post-rift stage in which diapirs of Triassic evaporites grew close to this major fault. • Compressive deformational stage coeval to the Pyrenean orogeny which led to (1) the development of a latest Cretaceous up to Middle Miocene foreland basin; and (2) from Late Eocene, the formation of a basement-involving thrust wedge in the innermost foreland basin, the squeezing of the diapiric structures formed previously in the Parentis Basin and, later (Middle Miocene), the partial inversion of some pre-existent faults located in the southern Parentis Basin margin. This geodynamic evolution together with the structure of the area evidences that the extensional structure resulting from the opening of the Bay of Biscay played an important role both in the location of the North-Pyrenean front and in the North-Pyrenean foreland contractional deformation features. Specially, the lack of significant inversion structures in the Parentis Basin, despite that it belongs to a severely thinned crustal area before the Alpine compression, denotes that the Mesozoic Landes High acted as an important buffer for the propagation of the Pyrenean contractional deformation to the north. This deformational buffer was active until Early Miocene and probably vanished afterwards during the last stages of Pyrenean orogen development when some basement faults reactivated in the Parentis Basin.

The Miocene of the Hatay area, S Turkey: Transition from the Arabian passive margin to an underfilled foreland basin related to closure of the Southern Neotethys Ocean

Miocene sediments of the Hatay area document the distal, southerly, part of an “underfilled” peripheral foreland basin related to the closure of the Southern Neotethys. The basin is underlain by latest Cretaceous and Eocene shallow-marine carbonates, interpreted as the youngest part of the Arabian passive margin succession. Unconformably overlying Lower Miocene conglomerates, sandstones and palaeosols (up to 250 m thick) represent braided river deposits derived from an uplifted area to the south. During the Middle Miocene, carbonate sediments accumulated in a variety of shallow-marine environments dominated by a northward-sloping ramp, with non-marine, peritidal, lagoonal and coral reef deposition in local areas. The Upper Miocene succession is mainly deeper-water hemipelagic marl with clastic interbeds. Messinian evaporites were deposited near the depocentre. The Early Miocene coarse fluvial sediments above the regional unconformity reflect the development of a flexural bulge, or re-activated basement structure. The Middle Miocene carbonates and Upper Miocene marls are interpreted as the lower and middle units of an “underfilled trinity”, considered typical of peripheral foreland basins. Similar Lower Miocene coarse clastic sediments and Mid-Miocene shallow-water carbonates unconformably overlie the Arabian platform succession c. 75 km to the north, where the succession continues upwards into thick shallow-marine to non-marine terrigenous sediments (upper unit of the “trinity”), sourced from the overthrust load. Comparable coarse clastic sediments are absent from the distal foreland basin in the Hatay area, probably reflecting its more southerly position, structural barriers in the foreland and the westward bypassing of sediment towards the Mediterranean Sea. Regional convergence halted by the end of the Miocene and was followed by strike–slip and westward “tectonic escape”. As a result the Miocene foreland basin was dismembered to form the Plio-Quaternary transtensional Hatay Graben. This like many other foreland basins shows some features which are not accommodated by ideal models of foreland basin evolution.

Seep-carbonates in a thrust-related anticline at the leading edge of an orogenic wedge: The case of the middle–late Miocene Salsomaggiore Ridge (Northern Apennines, Italy)

Seep-carbonates are important indicators of submarine fluid expulsion and dewatering of sediments along convergent margins and accretionary prisms. In the Northern Apennine compressional wedge, seep-carbonates usually occur in pelitic successions from different settings of the middle late Miocene foreland basin system. In Serravallian foredeep deposits of the Salsomaggiore area, seep-carbonates cemented and/or encrusted coarse-grained sandstones and conglomerates. Fluid expulsion, strictly related to variable intensities in tectonic stresses, generated seep-carbonate facies and sediment instabilities, depending on the position of the fluid-pathway feeders within the Salsomaggiore Ridge, a middle–late Miocene intra-basinal high, at the leading edge of the Apenninic orogenic wedge. This site provides a unique opportunity for analyzing depositional characters, and the three-dimensional distribution and evolution of a seepage system in a thrust-related anticline. The resulting evolutionary model shares common features with recent ridges in other accretionary wedges. In this model a strict relationship between tectonics, seepage and sediment instability during a major tectonic phase is outlined.

Topographic Controls on Production and Deposition of Tidal Cool-Water Carbonates, Uzes Basin, SE France

We document an example of a cool-water carbonate deposit interpreted as a flood-tidal delta formed during sea-level fall in the Uzes basin (French Miocene foreland basin). In the Miocene it was a peripheral basin connected to the open sea by a narrow, 15-km-long seaway. The studied Uzes Formation is a 50-m-thick tidal bioclastic carbonate body, formed at the junction between the Uzes basin and the seaway. It is composed dominantly of rhodalgal packstones that are interpreted to have been produced inside the basin. However, most of the deposit was built out by bidirectional hydraulic dunes, suggesting that accumulation of carbonate grains was not controlled by production but rather by tidal currents. The deposit is thickest and coarsest grained, and exhibits the larger dunes at the inlet of the seaway, where currents are expected to have been fastest. The deposit is thinner, finer-grained, and more bioturbated away from the inlet toward the basin; that is interpreted as a result of a decrease of current velocity due to flow expansion. The main deposit exhibits a general upward increase in thickness and grain size of cross-beds, together with a decrease of faunal diversity (from bivalve-rich to rhodalgal facies), consistent with progressive tidal flow acceleration in the seaway resulting from sea-level fall. Lime mud is pervasive in the matrix of even the highest-energy facies. This suggests that one cannot draw any average correlation between permeability and architecture of the deposit. Also, this could favor the observed early marine diagenesis of the deposit and give it a high preservation potential in the stratigraphic record.

Structural geology and regional tectonic significance of the Ramgarh thrust, Himalayan fold‐thrust belt of Nepal

The Ramgarh thrust is one of the major fault systems of the Himalayan thrust belt in Nepal and northern India. The Ramgarh thrust sheet is ∼0.2–2.0 km thick and can be traced along strike the entire length of the Himalaya in Nepal. The fault generally places the oldest Paleoproterozoic rocks in the Lesser Himalayan series upon younger Lesser Himalayan rocks or lower Miocene foreland basin deposits. Regional balanced cross sections suggest that the Ramgarh thrust had at least ∼120 km of initial south vergent displacement. Subsequently, the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying Lesser Himalayan rocks. Ramgarh hanging wall strata are greenschist‐grade phyllite, quartzite, and augen gneiss, all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top‐to‐the‐south sense of shear. Structural fabrics in the Ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet. Regional and local mapping of the Ramgarh thrust in Nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat, except where local lateral ramps complicate its geometry. Similarly, the structurally overlying Main Central thrust always places a hanging wall flat in Greater Himalayan series rocks upon the regionally flat Ramgarh thrust sheet. These geometric relationships preclude kinematic and thermal models that elevate Greater Himalayan and lower Lesser Himalayan rocks along high‐angle thrust ramps in the vicinity of the present traces of the Ramgarh and Main Central thrust faults. Instead, the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the Main Central thrust. The present steep dips of the Ramgarh and Main Central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the Lesser Himalayan zone. The Ramgarh thrust sheet overlaps a bed length of at least 100 km in lower Miocene foreland basin deposits, indicating that a significant amount of displacement on the thrust must have occurred after ∼15 Ma. Growth of the Lesser Himalayan duplex and additional slip on the frontal part of the Ramgarh thrust occurred from ∼12 to 5 Ma. The presence of a major greenschist‐grade metasedimentary thrust sheet composed of Lesser Himalayan rocks directly below the Main Central thrust suggests that the famous “inverted metamorphism” in this region is a result of structural inversion. Similarly, the concept of a broad zone of intense shear strain related exclusively to emplacement of the Main Central thrust sheet is probably invalid in Nepal.

Aptian to recent compressional deformation, foothills of the Neuquén Basin, Argentina

The Neuquén Basin lies in north-western Patagonia, on the eastern foreland of the Andes. It formed as a rift basin in the early Mesozoic and contains some 5000 m of Triassic to Aptian sediment, mainly marine. As a result of Andean orogeny, the western edge of the basin now forms part of the foothills. We describe some of the main compressional structures, drawing on surface and sub-surface data. We argue that regional eastward dips are due to west-verging thrust displacements on the Andacollo–Loncopué fault system. During a Late Cretaceous Peruvian phase (Aptian to Campanian), continental sediment accumulated in a retro-arc foreland basin, which was mainly of flexural origin. Nevertheless, more than 1500 m of Late Cretaceous sediment accumulated in the footwall of the east-verging Agua Amarga thrust. The Peruvian phase was responsible also for transpressional reactivation of a rift transfer zone, the Huincul Arch. The Incaic phase (Eocene) was responsible for north-westerly trending folds and thrusts, reactivation of Cretaceous thrusts, abundant andesitic lavas and high-level intrusions, and a swarm of north-easterly trending bitumen dykes. The Quechua phase (Miocene to Recent) produced northerly trending folds and thrusts and a Miocene foreland basin, containing some 1000 m of basalt lava flows. Structures that formed during the Incaic and Quechua phases are broadly compatible with the history of convergence at the Pacific margin of South America, as deduced from plate reconstructions.

Characteristics of repeated, detached, Miocene–Pliocene tectonic inversion events, in a large delta province on an active margin, Brunei Darussalam, Borneo

The Baram Delta province evolved during the Middle Miocene to present day from a foreland basin to a shelf margin. Episodic folding events affected the region, causing uplift of the hinterland, delta progradation, and inversion of gravity-related faults. Existing models place the N–S- and NE–SW-trending folds in a strike-slip transpressional setting. New geological mapping and re-interpretation of existing data suggest the region is better understood as the development of a west-verging thrust belt in a Middle Miocene foreland basin (filled by sandstones and shales of the Belait and Setap Formations), with key major folds (Jerudong and Belait anticlines, Belait syncline) forming during the Middle Miocene as fault bend and fault propagation folds. The deformation style is complicated by the predominantly shaley Middle Miocene–Pliocene Setap Formation becoming thicker and more overpressured from south to north. Onshore where the Setap Formation is thin or absent, the Belait formation is attached to the underlying Lower Miocene and older sequences. Offshore and in a narrow onshore strip the overpressured Setap Formation causes deformation in the Beltait Formation to be detached from the underlying Cretaceous–Tertiary accretionary prism ‘basement’. The detached style exhibits considerable structural complexity, including lift-off folds, growth faults, shale diapirs and pipes. Onshore thrust and inversion features are dominantly N–S-trending and began activity in the Middle Miocene; deformation is probably associated with an E–W maximum horizontal stress direction. In the Late Miocene (around 7.5 Ma; Watters et al., 1999) NE–SW striking inversion folds developed, located mostly over early counter-regional faults and associated reactive diapirs. Folds verge towards the NW when underlain by counter regional faults, and towards the S or SE when underlain by regional faults. Folds and diapirs along N–S trends were also reactivated. Continuation of this deformation into the Pliocene is largely confined to the offshore area. Onshore the N–S structures (no detachment) were not reactivated during the Pliocene.

Eocene‐early Miocene foreland basin development and the history of Himalayan thrusting, western and central Nepal

Sedimentologic, petrographic, and U‐Pb detrital zircon ages from middle Eocene through early Miocene sedimentary rocks in the Lesser Himalayan zone of western and central Nepal indicate that a peripheral foreland basin system had developed in the eastern Himalayan collision zone by middle Eocene time. The shallow‐marine, Eocene Bhainskati Formation accumulated in a back‐bulge depozone between a southward migrating forebulge and the Indian craton. Migration of the forebulge through this region during Eocene‐Oligocene time produced a regional unconformity that spans ∼15–20 Myr. By early Miocene time, the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone, represented by fluvial deposits of the Dumri Formation. Continued southward migration of the foredeep during the Neogene accommodated the fluvial Siwalik Group. Light mineral provenance data and U‐Pb detrital zircon ages suggest that the Bhainskati was derived partly from Tethyan sedimentary rocks of the Tibetan Himalayan zone during initial growth of the Himalayan fold‐thrust belt. The Dumri was derived from metasedimentary and crystalline rocks of the Greater Himalayan zone during emplacement of the Main Central thrust and contemporaneous tectonic unroofing by normal faulting along the South Tibetan detachment system. The Lesser Himalayan crystalline thrust sheets were emplaced soon after deposition of the Dumri Formation, ∼15–10 Ma. Paleocurrent and lithofacies data from the Dumri Formation indicate deposition by west‐southwestward flowing rivers that drained into the Indus portion of the Himalayan foreland basin system during the early Miocene. Thick channel sandstones in the lower Dumri may represent the early Miocene counterpart of the modern Ganges River. Eastward diversion of the Ganges drainage system to near its present location had occurred by ∼15 Ma, as the high‐standing Aravalli Range on the northern Indian shield approached the front of the fold‐thrust belt. Assuming reasonable values for the flexural rigidity of Indian lithosphere, the time span of the forebulge unconformity yields a velocity of ∼14–33 mm/yr for the southward migration of the fold‐thrust belt relative to India. This range of values is consistent with Neogene and present‐day estimates and suggests that only one third to one half of India‐Eurasia convergence has been accommodated by shortening in the Himalayan fold‐thrust belt since the onset of collision.

Lateral diapiric emplacement of Triassic evaporites at the southern margin of the Guadalquivir Basin, Spain

Abstract The Guadalquivir Basin is the Neogene foreland basin of the central and western Betic thrust belt in southern Spain. At the boundary between the basin and the outcrops of thrust nappes of Mesozoic limestones of the Prebetic and Subbetic is a broad belt of outcrops of Triassic evaporitic sediments with scattered younger rocks: the so-called ‘Olistostrome’ unit. This is highly deformed, in places chaotic, and its mode of emplacement has been attributed by various authors to olistostromal debris flow, diapirism, or tectonic melange. Studies of outcrop data in conjunction with seismic and well data, integrated using restorable cross-sections lead us to propose the following sequence of emplacement mechanisms. (a) Loading above a Triassic evaporite formation, probably in the Intermediate Units depositional zone, by north vergent thrusting of thick nappes of Mesozoic sediments, causes northward expulsion of evaporitic sediments between a basal thrust and the base of the limestones. (b) Continued thrust loading drives the diapiric body forwards ahead of the thrust belt, into the floor of the deepening Miocene foreland basin. The body includes blocks of Triassic rocks in normal stratigraphic sequence, as well as blocks of younger rocks broken off the leading hanging-wall cutoffs of the nappes. (c) When the diapiric body reaches the sea-floor of the basin, its top becomes subject to modification by sedimentary processes such as dissolution of evaporites leaving a cap rock and debris flow, both submarine and subaerial but rarely, if ever, forming true olistostromes. (d) At the leading edge of the diapir, northward compression of Miocene basin sediments results in thin-skinned thrusting within these sediments, and formation of duplex structures with a north-dipping monoclinal deformation front. Results from analogue and numerical modelling match the main geological features observed in the study area, thus supporting the plausibility of the proposed lateral diapiric emplacement of the chaotic unit.

Deformation history of a synorogenic sedimentary wedge, northern Cilento area, southern Apennines thrust and fold belt, Italy

In the northern Cilento area of the southern Italian Apennines, a deformed turbiditic succession of Langhian to lower Tortonian age (the Cilento unit) unconformably overlies tectonic units (Liguride and Sicilide) derived from an oceanic accretionary complex and, to the east, overlies Mesozoic–Cenozoic carbonates belonging to the Apulian continental paleomargin. The turbiditic succession, representing the sedimentary infill of a Miocene foreland basin developed over the flexured southwestern margin of the Apulian plate, shows a complete record of the deformation at the macroscopic and mesoscopic scales. Structures postdating synsedimentary (slump) features in the Cilento unit include Neptunian dikes and extensional veins perpendicular to bedding, all related to high pore-fluid pressures associated with burial. Early tectonic structures consist of minor thrust faults, often conjugate and at a low angle to bedding, produced by initial layer-parallel shortening. The main regional structure mapped in the area consists of a moderately inclined to recumbent (hinterland-vergent) backfold. This structure probably developed by shortening of the strata caught between shear surfaces that cut obliquely across bedding, within the framework of a noncoaxial progressive deformation. The main shear surface constituting the upper boundary to the shear zone is represented by a regional backthrust which brings Mesozoic-Cenozoic carbonates onto the terrains of the Cilento unit, forming a triangle zone where backfolding of the latter occurred. Minor open domes and basins deform both limbs of the main regional structure, probably as a result of progressive shear deformation and, possibly, of deformation above underlying thrust culminations. It appears that high pore-fluid pressures must have been important not only during the development of pretectonic (i.e., burial-related) structures, but also during the main tectonic deformation. Abnormally high fluid pressures, reducing the effective stress and favoring intergranular movements, suppressed cleavage formation and pervasive strain during the main folding event. Later in the deformation sequence, a decrease in pore-fluid pressure probably favored local cleavage development during refolding. The deformation sequence outlined above provides a case history of the structural evolution of synorogenic clastic deposits progressively deformed and incorporated within a thrust belt. This study, documenting the deformation of a sedimentary wedge dominated by backfolding associated with hinterland-vergent reimbrication, should contribute to a better understanding of the wide range of deformation sequences and processes characterizing the tectonic evolution of thrust-belt–foredeep systems.