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- Volume 8, Issue 2, 1996
Basin Research - Volume 8, Issue 2, 1996
Volume 8, Issue 2, 1996
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Foreland basin systems
Authors Peter G. DeCelles and Katherine A. GilesA foreland basin system is defined as: (a) an elongate region of potential sediment accommodation that forms on continental crust between a contractional orogenic belt and the adjacent craton, mainly in response to geodynamic processes related to subduction and the resulting peripheral or retroarc fold‐thrust belt; (b) it consists of four discrete depozones, referred to as the wedge‐top, foredeep, forebulge and back‐bulge depozones – which of these depozones a sediment particle occupies depends on its location at the time of deposition, rather than its ultimate geometric relationship with the thrust belt; (c) the longitudinal dimension of the foreland basin system is roughly equal to the length of the fold‐thrust belt, and does not include sediment that spills into remnant ocean basins or continental rifts (impactogens).
The wedge‐top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge, including ‘piggyback’ and ‘thrust top’ basins. Wedge‐top sediment tapers toward the hinterland and is characterized by extreme coarseness, numerous tectonic unconformities and progressive deformation. The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the proximal flank of the forebulge. This sediment typically thickens rapidly toward the front of the thrust belt, where it joins the distal end of the wedge‐top depozone. The forebulge depozone is the broad region of potential flexural uplift between the foredeep and the back‐bulge depozones. The back‐bulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential flexural subsidence cratonward of the forebulge. This more inclusive definition of a foreland basin system is more realistic than the popular conception of a foreland basin, which generally ignores large masses of sediment derived from the thrust belt that accumulate on top of the orogenic wedge and cratonward of the forebulge.
The generally accepted definition of a foreland basin attributes sediment accommodation solely to flexural subsidence driven by the topographic load of the thrust belt and sediment loads in the foreland basin. Equally or more important in some foreland basin systems are the effects of subduction loads (in peripheral systems) and far‐field subsidence in response to viscous coupling between subducted slabs and mantle–wedge material beneath the outboard part of the overlying continent (in retroarc systems). Wedge‐top depozones accumulate under the competing influences of uplift due to forward propagation of the orogenic wedge and regional flexural subsidence under the load of the orogenic wedge and/or subsurface loads. Whereas most of the sediment accommodation in the foredeep depozone is a result of flexural subsidence due to topographic, sediment and subduction loads, many back‐bulge depozones contain an order of magnitude thicker sediment fill than is predicted from flexure of reasonably rigid continental lithosphere. Sediment accommodation in back‐bulge depozones may result mainly from aggradation up to an equilibrium drainage profile (in subaerial systems) or base level (in flooded systems). Forebulge depozones are commonly sites of unconformity development, condensation and stratal thinning, local fault‐controlled depocentres, and, in marine systems, carbonate platform growth.
Inclusion of the wedge‐top depozone in the definition of a foreland basin system requires that stratigraphic models be geometrically parameterized as doubly tapered prisms in transverse cross‐sections, rather than the typical ‘doorstop’ wedge shape that is used in most models. For the same reason, sequence stratigraphic models of foreland basin systems need to admit the possible development of type I unconformities on the proximal side of the system. The oft‐ignored forebulge and back‐bulge depozones contain abundant information about tectonic processes that occur on the scales of orogenic belt and subduction system.
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The Sub‐Balkan graben system of central Bulgaria
Authors Tz. Tzankov, D. Angelova, R. Nakov, B. C. Burchfiel and L. H. RoydenThe Sub‐Balkan graben system in central Bulgaria forms the present northern boundary of the Aegean extensional region. This east‐trending graben system lies along the southern flank of the Stara Planina range and consists mainly of half‐grabens. The sedimentary fill in the grabens ranges in age from late Miocene to Recent and records the initiation and evolution of the graben system. The sedimentary fill in the grabens is oldest in the central graben and becomes progressively younger to the west and east, indicating a diachronous development of the grabens. Grabens are formed in the hangingwalls of south‐dipping low‐angle normal faults which have been displaced by younger higher angle normal faults along the foot of the Stara Planina. Hangingwall rocks have been complexly faulted and rotated such that some graben fill has been rotated down‐to‐the‐north. The Sredna Gora range south of the grabens is part of a complexly faulted and rotated hangingwall block bounded on the south by south‐dipping normal faults forming the northern boundary of the Thracian Basin. The Stara Planina range has been formed by uplift and rotation due to footwall unloading along the low‐angle normal faults and forms the northern margin of the graben system. Most of the topography of Bulgaria south of the Sub‐Balkan graben system is the result of late Miocene to Recent extensional processes linked to the Aegean region that have been superposed on convergent features and earlier extensional features that extend back to late Eocene time.
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Flexural uplift of the Stara Planina range, central Bulgaria
Authors M. Roy, L. H. Royden, B. C. Burchfiel, Tz. Tzankov and R. NakovThe Stara Planina is an E–W‐trending range within the Balkan belt in central Bulgaria. This topographically high mountain range was the site of Mesozoic through early Cenozoic thrusting and convergence, and its high topography is generally thought to have resulted from crustal shortening associated with those events. However, uplift of this belt appears to be much younger than the age of thrusting and correlates instead with the age of Pliocene–Quaternary normal faulting along the southern side of the range. Flexural modelling indicates the morphology of the range is consistent with flexural uplift of footwall rocks during Pliocene–Quaternary displacement on S‐dipping normal faults bounding the south side of the mountains, provided that the effective elastic plate thickness of 12 km under the Moesian platform is reduced to about 3 km under the Stara Planina. This small value of elastic plate thickness under the Stara Planina is similar to values observed in the Basin and Range Province of the western United States, and suggests that weakening of the lithosphere is due to heating of the lithosphere during extension, perhaps to the point that large‐scale flow of material is possible within the lower crust. Because weakening is observed to affect the Moesian lithosphere for ≈10 km beyond (north of) the surface expression of extension, this study suggests that processes within the uppermost mantle, such as convection, play an active role in the extension process. The results of this study also suggest that much of the topographic relief in thrust belts where convergence is accompanied by coeval extension in the upper plate (or ‘back arc’), such as in the Apennines, may be a flexural response to unloading during normal faulting, rather than a direct response to crustal shortening in the thrust belt.
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Late‐ to post‐orogenic basins of the Pan‐African – Brasiliano collision orogen in southern Africa and southern Brazil
Authors P. G. Gresse, F. Chemale, L. C. Da Silva, F. Walraven and L. A. HartmannLate‐ to post‐orogenic basins formed on both sides of the Pan‐African – Brasiliano orogen when the Congo and Kalahari Cratons collided with the Rio de la Plata Craton during the formation of western Gondwana. Trace fossil evidence and radiometric age dating indicate that deposits on both sides are coeval and span the Cambrian–Precambrian boundary.
A peripheral foreland basin, the Nama Basin, developed on the subducting southern African plate. Lower, craton‐derived fluviomarine clastics are overlain by marine platform carbonates and deltaic flysch derived in part from the rising subduction complex along the northern (Damara Belt) and western (Gariep Belt) orogenic margins. Rare, thin volcanic ash layers (tuffs and cherts) are present. Upper sediments consist of unconformable red molasse related to collisional orogenesis. Orogenic loading from the north and west led to crustal flexure and the formation of a remnant ocean that drained to the south and closed progressively from north to south. During final collision SE‐, E‐ and NE‐verging nappes overrode the active basin margins. Although younger than most of the post‐orogenic magmatism, its setting on the cratonic edge of the subducting plate precluded marked volcanism or granitic intrusion, the only exception being the youngest intrusions of the Kuboos‐Bremen Suite dated at 521±6 Ma to 491±8 Ma.
Two foreland‐type basins, perhaps faulted remnants of a much larger NE–SW elongated retroarc foreland basin, are found west of the Dom Feliciano Belt on the edge of the Rio de la Plata Craton in southern Brazil. In the southern Camaqua Basin, basal fluvial deposits are followed by cyclical marine and coarsening‐up deltaic deposits with a notable volcanic and volcaniclastic component. This lower deformed succession, comprising mainly red beds, contain stratabound Cu and Pb–Zn deposits and is overlain unconformably by a fluviodeltaic to aeolian succession of sandstones and conglomerates (with minor andesitic volcanics), derived primarily from an eastern orogenic source and showing southerly longitudinal transport. In the northern Itajaí Basin, sediments range from basal fluvial and platform sediments to fining‐up submarine fan and turbidite deposits with intercalated acid tuffs. The Brazilian basins had faulted margins off which alluvial fans were shed. They also overlie parts of the Ribeira Belt. Thrust deformation along the orogenic margin bordering the Dom Feliciano Belt was directed westward in the Camaqua and Itajaí basins, but reactivated strike‐slip and normal faults are also present. Late‐ to post‐orogenic granitoids and volcanics of the Dom Feliciano Belt, ranging in age from 568±6 Ma to 529±4 Ma, occur in the foreland basins and are geochemically related to some of the synsedimentary volcanics.
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Plan‐view curvature of foreland basins and its implications for the palaeostrength of the lithosphere underlying the western Alps
More LessIn zones of continental collision, three methods can be used to calculate the strength of the underthrust lithosphere: (1) a forward model approach to the Bouguer gravity field, (2) an inverse model of the gravity and topography using admittance techniques, or (3) a forward model of the stratigraphic infill of the foreland basin to estimate the cross‐sectional profile of the downflexed plate. The use of reconstructed stratigraphy has the potential to yield values for the equivalent elastic thickness (Te) of the cratonic lithosphere at varying slices in geological time, and hence enable an insight into the longer term (10–50 Myr) mechanical behaviour of the continental lithosphere.
Calculations of Te based on isopachs of foreland basin stratigraphy use sea level as a reference line to estimate the basement deflection, and therefore are limited to using stratigraphy which records shallow marine or coastal sedimentation. A new empirical approach is applied to evaluating ancient Te values using the reconstructed palaeocurvature of the basin in plan view. The radius of curvature of 12 curvilinear foreland basins is plotted against their documented Te values and shows a linear relationship. The maximum Te value for a given radius of curvature can also be plotted as a straight line. The palaeocurvature of reconstructed basins can then be compared with the plots, and estimates of likely maximum Te values may be obtained.
During Eocene times, the underfilled foreland basin of the Alps was characterized on its cratonic edge by the deposition of Nummulite‐rich limestones. Palaeogeographical reconstructions of the Nummulitic Limestones enable estimates of the palaeocurvature of the cratonic margin of the Alpine foreland basin during the Eocene. By comparing this value with the curvature of documented basins, it is possible to suggest that the European lithosphere underlying the western Alps had an effective elastic thickness of no greater than 17 km during the Eocene. It has been suggested that the transition in the depositional state of the Alpine foreland basin from an underfilled to a filled state during middle Oligocene times was linked to a thickening of the continental lithosphere associated with the effective ramp of the Tethyan passive margin. The Te value of less than 17 km during the underfilled stage combined with a value of 10±5 km for the later filled stage at 17 Ma does not lend support to this hypothesis.
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Volumetric contraction during the compaction of mudrocks: a mechanism for the development of regional‐scale polygonal fault systems
Authors J. A. Cartwright and L. LonerganThis paper describes the geometry and strain characteristics of a complex system of small extensional faults affecting Lower Tertiary mudrock‐dominated successions throughout the central North Sea Basin. Structural mapping using three‐dimensional seismic data shows that the fault trace geometry is polygonal. The shallow origin of the faults is confirmed by the recognition of growth sequences developed in their hangingwalls. Line balancing techniques were used to measure the extensional strain in two survey areas. This was found to be radially isotropic in the map plane. Extension in any line of section was found to vary from 6 to 19%. Since the deformation is clearly layer‐bound and there is no evidence for displacement transfer to basement structures, it is argued that the only explanation for this apparent extension is by layer‐parallel volumetric contraction. This is believed to occur in response to fluid expulsion from the mudrocks during early compaction. The conditions for failure may be achieved through increased pore fluid pressure or through tensile stresses generated as a result of pore fluid loss, or a combination of these two processes. Far‐field tectonic stresses are not considered to be responsible for the formation of this fault system.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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Volume 2 (1989)
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Volume 1 (1988)