Foreland Lithosphere Research Articles

Lithospheric underthrusting beneath the northern Eastern Cordillera plateau, Colombian Andes: Insights from analyzing tomographic inversion using local earthquake travel time and gravity data

The northern Eastern Cordillera in the Colombian Andes is a wide (~200 km) high-elevation (>2.5 km) subduction-related asymmetric plateau. This plateau is currently subjected to shortening and magmatic addition, and yet the nature of the lowermost crust and its transition to the underlying mantle remains poorly constrained. To improve our knowledge about the structure of the deep crust beneath the EC plateau, we conducted a joint inversion of travel times of local earthquakes and gravity data. Gravity contributes to up to 0.5% in dVp and dVs with no affectation of the shape or location of the discussed anomalies. Along the latitudinal profile with the highest resolution (~5.7°N), two anomalies are identified at depths of 40–60 km beneath the plateau. A western low-velocity anomaly is interpreted as crustal material underthrust eastward beneath the northwestern EC. This process is triggering the abrupt change in topography between the adjacent low-elevation basin and the orogenic plateau. Additionally, a high-velocity anomaly beneath the eastern flank of the EC is likely related to mantle metasomatism and westward underthrusting of the foreland lithosphere. This metasomatism is either related to the interaction between mafic magmas and the uppermost mantle or silica enrichment that occurred during a past episode of flat subduction. Evidence for foreland underthrusting includes relocated earthquakes at 33–42 km depth within the thrust system between the EC and the foreland, along with increased shortening in the eastern part of the plateau, an increase in exhumation rates, the eastward migration of deformation towards the foreland, and dominated thick-skinned deformation in the upper crust of the foreland with an eastward vergence of thrusts and related folds. Furthermore, within the central part of the plateau, our results show low velocities between 30 and 40 km depth, consistent with previous constraints, suggesting magmatic underplating beneath the Paipa-Iza volcanic complex.

Stepwise Widening of the Central Andes—The Role of the Lower Crust

AbstractThe outward growth of many orogens occurs through pulsed migration of the strain front. During Cenozoic shortening of the central Andes, the strain front abruptly migrated ∼400 km eastward across the Altiplano, isolating the plateau interior from major deformation. In contrast to the traditional critical wedge model that focuses on shallow crust deformation, our lithosphere‐scale numerical models show the ductile lower crust plays a key role in the dynamics of the orogen. As the orogen grows upward, the lower crust flows outward and causes stepwise expansion of the orogen through landward‐migrating deformation. The step length of the strain‐front migration increases with decreasing lower crustal strength. Topographically, a weaker crust promotes the formation of a smooth plateau and underthrusting of stronger foreland lithosphere. Our results indicate that the seemly unrelated physiography, deep‐lithosphere structure, and plateau‐wide strain migration in the Altiplano may all be the result of a weak lower crust.

Late Cryogenian and late Paleozoic ice ages on the São Francisco craton, east Brazil

The miniature paleocontinent in the region of the São Francisco River valley, in eastern Brazil, holds the record of two different glacial epochs. The late Cryogenian Jequitaí Formation from the Bambuí Group is up to 100 m thick and covers areas mainly in the central São Francisco craton. Evidences for glacial sedimentation are beautifully preserved E-W grooves and striations, dropstones within fine-grained rocks, and a full set of diamictites enclosing a rich and complex depositional history. The Jequitaí Formation is in close link with the tectonic evolution of the São Francisco paleocontinent and the West Gondwana amalgamation. From west, the precocious Paranapanema and São Francisco blocks collision in late Cryogenian flexured the foreland lithosphere and created depozones that were infilled by glacial sediments. Toward east, the rifting and opening of the Adamastor Ocean allowed thick glacial and non-glacial deposits to form through subaqueous gravitational sedimentation. From west to east, proximal and distal glaciomarine, glaciocontinental, and non-glacial resedimentation are identified and linked to the evolving continental masses and climate during the Cryogenian and beginning of Ediacaran. The late Paleozoic Santa Fé Group is the youngest record of glaciation on the São Francisco craton. It is 60–80 m thick and yields consistent and confident glacial evidences such as N-S striations on top of Cambrian sandstones, ice-rafted debris, and rain-out diamictite, all preserved in small and patchy areas in the west-central São Francisco craton. Paleocurrents suggest a northern ice center and sedimentary facies indicate deposition in continental lakes and rivers. Although late Paleozoic, its age is poorly constrained and likely correlated with the uppermost Itararé Group (Taciba Formation) of Paraná Basin in south Brazil. Deglaciation and strong isostatic adjustments make up the termination of the Santa Fé Group sedimentary record and depict a glaciocontinental system evolved on an interior stable continental crust. The late Neoproterozoic Jequitaí Formation and the late Paleozoic Santa Fé Group are parts of the earth’s sedimentary history preserving a rich record of climate, tectonic, and surface processes in part controlled by the evolving continental masses on the São Francisco craton.

Development of Foreland Intracratonic Plateaus (Ozark Plateau and Appalachian Plateaus): A Consequence of Topographic Inversion Due To Erosion of Adjacent Fold‐Thrust Belts

AbstractUnlike well‐known plateaus associated with Cenozoic orogens, the Appalachian and Ozark Plateaus of the eastern United States fringe the foreland side of a long inactive and deeply eroded orogen. These foreland intracratonic plateaus (FIPs), which are underlain by sub‐horizontal cratonic‐platform strata and, in places, foreland‐basin strata, now lie 0.5–1.2 km above sea level, notably higher than adjacent fold‐thrust belts. An escarpment lies at or near the boundary between the FIPs and the fold‐thrust belts. Why did the topographic inversion leading to the development of the FIPs take place? To address this question, we built a numerical model, using Landlab, to simulate how topography evolves as foreland lithosphere flexes upward when post‐tectonic erosion causes unloading. In this model, flat‐lying cap‐rock strata (sandstone and limestone) of the foreland have greater resistance to erosion than do the deformed, tilted, cleaved, and fractured strata of the fold‐thrust belt, especially where the fold‐thrust belt contains argillaceous facies. We tested the model by characterizing the development of the Ozark Plateau in the foreland of the Ouachita fold‐thrust belt. Results demonstrate that regional isostatic uplift due to erosion, given reasonable differences in resistance to erosion between the fold‐thrust belt and the foreland, can generate the observed topographic inversion and a distinct escarpment, yielding a plateau. This model may help explain the post‐Paleozoic evolution of the Catskill Mountains, the Deep Valleys Province, and the Cumberland Plateau, highlands which border the Appalachian fold‐thrust belt.

The Miocene deep‐seated spontaneous coal‐seam fire in the Upper Silesian Coal Basin (S Poland) and its geotectonic trigger mechanism

AbstractThe deep‐seated spontaneous coal‐seam fire occurred in the southwestern part of the Upper Silesian Coal Basin (S Poland), confirmed by oxygenated compounds emission to over‐ and underlying rocks. The fire was dated at 19.2–19.8 Ma using the 39Ar‐40Ar method and palaeomagnetism. It was initiated in the Carpathian foreland thanks to a normal faulting in response to thrust‐loaded deflections of the Carpathian foreland lithosphere during collision of tectonic plates (Alcapa and stable Europe). The orogenic loading caused this normal faulting in the foreland and triggering the flexural mechanism of foredeep subsidence and forebulge uplift. Thanks to the dating of rocks affected by the spontaneous coal fire initiated during the rapid uplift of coal‐bearing rocks, when they became exposed to the aeration zone, the age of these mountain‐building processes in the Carpathians has been precisely defined.

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.

Contrasting along-strike deformation styles in the central external Dinarides assessed by balanced cross-sections: Implications for the tectonic evolution of its Paleogene flexural foreland basin system

The Cenozoic foreland fold and thrust belt of the external Dinarides along the eastern shore of the Adriatic Sea shows an overall NW-SE structural strike that changes to E-W in its central part, between Zadar and Mostar. This change is associated with a dextrally transpressive zone, named Split-Karlovac Fault. Segments of this fault straddle the Eo-Oligocene flexural foreland basin system of the external Dinarides. Two balanced cross-sections across the northern and southern segments of this transverse fault zone document highly contrasting styles of deformation and amount of shortening. The northern segment across Velebit Mountain is characterized by a triangle structure with a minimum shortening of 44 km. This triangle structure is formed by a set of thick-skinned top-SW thrust duplexes, topped by a thin-skinned passive top-NE roof backthrust. By contrast, the southern segment, traversing north of Split, shows a thin-skinned top-SW to top-S directed nappe stack, associated with a minimum shortening of 127 km and rooting into a basal detachment in the Paleozoic basement. To better understand parameters controlling these contrasting styles of deformation, we modelled the flexural response of the foreland lithosphere that provided the accommodation space for the Eocene to Oligocene syn-orogenic Promina Beds, a prevailingly carbonate-clastic and conglomerate-bearing sequence making a part of the foreland basin fill. Tectonic load exerted by the hinterland nappe stack was estimated using the two balanced cross-sections. Estimates of tectonic load allowed modelling the amount of flexural response that yielded depth and shape of the basin containing the Promina Beds. The best fitting modelling results provided additional confidence in the validity of the balanced cross-sections. It is shown that shortening in the two structurally contrasting segments of the thick-skinned thrust belt were contemporaneous with dextral strike-slip along the thick-skinned Split-Karlovac Fault and coeval with sedimentation of the Promina Beds. We propose that the Velebit triangle structure acted as a back-stop that prevented the propagation of the deformation front towards the SW. Instead shortening was transferred along the Split-Karlovac Fault into the southern transect, where deformation propagated towards the foreland along a successively shallowing stepped detachment. The contrasting style of Eo-Oligocene deformation probably resulted from the interplay of Permo-Triassic evaporites with the reactivation of inherited Middle Triassic normal faults to both sides of the transpressive zone. • Nappe stack vs. triangle structure in the external Dinarides fold and thrust belt. • Substantial along-strike difference of crustal shortening. • Effects of mechanically weak lithologies on the style of deformation. • Lithosphere deflection modelling in combination with balanced cross-sections. • Impacts of a transverse zone on the flexural foreland basin system geometry.

Delamination in the Betic Range: Deep structure, seismicity, and GPS motion

Subduction comes to an end when all oceanic lithosphere is consumed and continental or transitional crust, with positive buoyancy, becomes involved. During continental collision, the final stage of subduction may be characterized by slab breakoff, limited continental subduction, and delamination below the fold-and-thrust belt. We report receiver function images, constructed from a dense array of seismic broadband stations, that reveal the underthrusting of Iberia beneath the Alboran plate. The underthrusted crust is delaminating beneath part of the Betic mountain range, still connected to the foreland lithosphere on one side, and interrupted discontinuously along the other side. Intermediate deep earthquakes associated with the delamination process do not respond to downdip stresses like subduction earthquakes. The GPS velocity field and source mechanisms for shallow earthquakes demonstrate ongoing active shortening on top of the hinge of delamination, and localized perpendicular extension on top of the edge of delamination, introducing independent motion of the overriding crustal block. Delamination beneath the Betic Range drives seismicity by inducing fundamental changes in deformation patterns within the collision orogen.

Modes and timing of fracture network development in poly-deformed carbonate reservoir analogues, Mt. Chianello, southern Italy

Structural and paleostress analyses carried out on a kilometre-sized outcrop of allochthonous shallow-water carbonate units of the southern Apennines allowed us to unravel a superposed deformation pattern associated with plate convergence. The reconstructed tectonic evolution involves: (i) early extensional faulting and fracturing associated with bending of the foreland lithosphere during forebulge and foredeep stages (including the development of both ‘tangential’ and ‘radial’ normal fault and tensile fractures; Early-Middle Miocene); (ii) large-scale thrusting and folding (Late Miocene); (iii) transcurrent faulting (including two distinct sub-stages characterized by different remote stress fields; Pliocene-Early Pleistocene), and (iv) extensional faulting (late Quaternary). Stage (i) normal faults – generally occurring as conjugate sets – and related fractures and veins are variably deformed and overprinted by later horizontal shortening. Despite having experienced such a long and complex structural history, the studied carbonates are characterized by a ‘background’ fracture network – including two joint/vein sets orthogonal to each other and to bedding – that appears to be associated with the early fault sets that formed during the first (foredeep/forebulge-related) deformation stage. Therefore, away from younger (Late Miocene to Quaternary) fault zones, the permeability structure of the studied carbonates appears to be essentially controlled by the early, inherited fracture network. As a similar fracture network is likely to characterize also the buried Apulian Platform carbonates, representing the reservoir units for major oil fields in southern Italy, our results also bear possible implications for a better understanding of fluid flow in the subsurface and related hydrocarbon production.

A climatic trigger for a major Oligo-Miocene unconformity in the Himalayan foreland basin

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

High-resolution seismic imaging of theMw5.7, 2002 Molise, southern Italy, earthquake area: Evidence of deep fault reactivation

[1] We investigate the seismic structure of the Mw5.7, 2002 Molise earthquake area in order to understand the role of E–W trending strike-slip faults in the tectonics of the southern Apennines. We apply an innovative seismic migration technique to a high quality data set of earthquakes recorded at a dense local network. SP-converted waves are migrated in depth to image the high impedance contrast of the Apulian Platform top buried under the Apennines allochthonous cover. The continuity of the migrated seismic horizon is broken by vertical steps that we systematically picked along 200 cross sections. The best location points of these structures define two main tectonic features. The first one is related to NW–SE oriented normal faults and is consistent with the SW flexure of the foreland lithosphere beneath the orogenic belt. The second one indicates that shallow E–W oriented trans-tensional faults are concentrated directly above the deeper (10–20 km) strike-slip fault, delineating the geometry of a negative flower-type structure. This fault system delimits a depressed sector of the Apulian Platform, whose geometry is consistent with a pull-apart basin inherited from a previous left-lateral strike-slip tectonic regime. The buried structure is analogous to those outcropping in the Apulian foreland and in the Adriatic offshore, to the east. This correlation brings new support to the hypothesis of a regional E–W trending shear zone cutting the Adria plate and suggests that other earthquakes could occur on this or on parallel E–W trending strike-slip faults.

Long lasting interactions between tectonic loading, unroofing, post-rift thermal subsidence and sedimentary transfers along the western margin of the Gulf of Mexico: Some insights from integrated quantitative studies

After Jurassic rifting, numerous carbonate platforms (i.e., the Orizaba, Cordoba and Golden Lane–Tuxpan platforms) developed during the Lower and Middle Cretaceous episode of thermal subsidence along the western passive margin of the Gulf of Mexico, with intervening basinal domains (i.e., the Tampico-Misantla, Zongolica, Veracruz and Deep Gulf of Mexico – DGM – basins). During the Late Cretaceous–Paleocene, the east-verging Sierra Madre Oriental thrust belt developed, resulting in tectonic uplift and unroofing of the allochthonous units (i.e. tectonic units made up of former Orizaba and Cordoba platforms and Zongolica Basin series). This new topography provided also an important source of clastics to feed the adjacent foredeep, where coeval tectonic loading accounted for the bending of the foreland lithosphere. However, shallow water facies or even emersion persisted until the Eocene in the forebulge area (at the present location of the Golden Lane), preventing locally the clastics to reach the DGM. This topographic barrier was ultimately bypassed by the clastics only during the Oligocene and Neogene, once (1) the prograding clastic wedge had exceeded accommodation, and (2) the long lasting thermal subsidence of the passive margin could overpass the effect of the bending and force the former bulge to sink. Numerous paleo-thermo-meters (Tmax, Ro), paleo-thermo-barometers (fluid inclusions), PVT and coupled forward kinematic and thermal modeling have been used to calibrate and date the progressive unroofing of the thrust belt. Coupled tectonic and sedimentologic modeling was applied in the foreland to predict the distribution of sand versus shale ratios in the Oligocene to Plio-Quaternary clastic sedimentary wedge of the passive margin, where gravitational gliding of post-Eocene series occurred during the Neogene along major listric faults. Mantle dynamics are advocated as the main process accounting for post-orogenic uplift and regional tilting of the basement, which initiated a massive transfer of sediments from the Cordillera towards the Gulf of Mexico, from Oligocene onward, resulting in a destabilization and gravitational collapse of the western slope of the Gulf of Mexico in Neogene times.

An Early Ordovician (Finnmarkian?) foreland basin and related lithospheric flexure in the Scandinavian Caledonides

Early Ordovician (479 - 455 Ma) graywackes overlying Cambrian passive margin successions in the Baltica palaeocontinent-related Lower AUochthon of the Scandinavian Caledonides document early Caledonian tectonic activity. Such rocks occur from the Jamtland area (Sweden) northwards along the eastern Caledonian margin, and imply a wide regional extent of a foreland basin in Early Ordovician times. This foreland basin was subsequently incorporated into a fold-and-thrust belt during the final Caledonian, Scandian tectonic episode (c. 425 - 390 Ma). Based on published cross section data, this late deformation is restored. The results and other available information are compiled into a palaeogeographic map and sections on the geometry and evolution of the Early Ordovician foreland basin. Graywacke/turbidite sedimentation (Follinge Formation) started in western (internal) areas already in Early Arenig times with a main phase from Llanvirn to Late Caradoc (c. 472 - 461 Ma). There, the Follinge Formation rests with an erosional unconformity on older beds. Towards east, however, it overlies successively younger beds related to a carbonate domain. It is suggested that the areas of the sub-turbidite erosional unconformity represent the early location of a flexural forebulge, which subsequently migrated eastwards towards the margin of the carbonate domain of palaeocontinent Baltica. The restored foreland-basin geometry is compared with numerical models in order to derive some characteristics of the flexure of the foreland lithosphere. Time and lithospheric constraints make it possible to test the viability of the restoration and of foreland basin models. Relative to the available lithospheric strength data, the width of the basin is too large, the depth relatively small, and the large wavelength of the flexure is difficult to explain with simple orogenic loading. Graywacke sedimentation occurred after Finnmarkian (515 - 475 Ma) HP metamorphism, perhaps as a consequence of exhumation and consequent loading of the Baltican margin. Graywacke sedimentation ended at the time of Jamtland phase (460 -440 Ma) HP metamorphism and relatively deep water conditions. It is speculated that eclogitization of crustal material produced an additional load, which caused a relatively large down flexure of the lithosphere.

Magnetic and structural constraints for the noncylindrical evolution of a continental forebulge (Hyblea, Italy)

The Hyblean Plateau in southeastern Sicily, Italy, consists of an isolated and elevated forebulge structure formed since the early Miocene time by bending the foreland lithosphere below the advancing Maghrebian thrust‐fold belt. The Hyblean forebulge is presently located in front of an orogenic recess and partly surrounded by two orogenic salients. We analyzed magnetic (anisotropy of magnetic susceptibility) and structural (joints, faults, slickenside lineations, and bed attitudes) fabric data from Neogene carbonate rocks exposed atop the Hyblean forebulge. Results showed unidirectional fabrics developed in response to a NW‐SE extension at the end the of early Miocene time and beginning of middle Miocene time, and duple‐to‐multiple fabrics developed in response to major NW‐SE and NE‐SW extensions from the Langhian time onward. We interpreted these results as the evidence for the growth of a doubly plunging forebulge due to the presence of foreland crustal heterogeneities, which enhanced differential retreating processes of the foreland along the subduction zone and the subsequent formation of orogenic salients and recesses. A semianalytical model shows that the observed brittle deformations potentially involved a significant thickness of the carbonate sedimentary cover, thereby reducing the lithosphere flexural rigidity and enhancing its bending aptitude.

Quantifying different modes of the late Cenozoic shortening in the central Andes

Abstract Since the late Miocene, the two main segments of the central Andean high plateau, Altiplano and Puna, demonstrate different styles and magnitudes of tectonic shortening. Through numerical simulation of thermomechanical processes, we show that different shortening modes—pure and simple shear accompanied by thin-or thick-skinned tectonics—might be controlled by strength of the foreland uppermost crust and by temperature of the foreland lithosphere. Mechanical weakening and failure of the thick Paleozoic sediments overlying the cold lithosphere in the Altiplano foreland at 13–9 Ma explains the transition from pure to simple shear shortening accompanied by broad thin-skinned thrusting, started before the major uplift of the plateau. However, the high strength of the uppermost crust combined with a relatively warm lithosphere results in the thick-skinned shortening typical for the foreland of the Puna. Failure of Paleozoic sediments in the Altiplano foreland significantly reduces the force required to shorten the lithosphere, which may be the reason for the increased bulk shortening rate in the late Miocene.

Reply to comments by L. Michon and O. Merle on “Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere” by P. Dèzes, S.M. Schmid and P.A. Ziegler, Tectonophysics 389 (2004) 1–33

Reply to comments by L. Michon and O. Merle on “Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere” by P. Dèzes, S.M. Schmid and P.A. Ziegler, Tectonophysics 389 (2004) 1–33

Discussion on “Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere” by P. Dèzes, S.M. Schmid and P.A. Ziegler, Tectonophysics 389 (2004) 1–33

The evolution and origin of the European Cenozoic Rift System (ECRIS) is a matter of debate for several decades (e.g., Tapponnier, 1977; Bergerat, 1987; Ziegler, 1992; Michon et al., 2003). This rift system was characterized by the development of several grabens in the Pyrenean and Alpine forelands and by a magmatic activity starting at the K/T transition. Dezes et al. (2004) propose an additional reappraisal and interpret the ECRIS formation and the associated volcanism as resulting from the Alpine and Pyrenean collision and the emplacement of a mantle plume at depth below western Europe. Our remarks on this paper will be focused on three different topics which make the final conclusions of Dezes et al. (i.e., origin of the extension in the ECRIS) highly questionable.

Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere

The evolution of the European Cenozoic Rift System (ECRIS) and the Alpine orogen is discussed on the base of a set of palaeotectonic maps and two retro-deformed lithospheric transects which extend across the Western and Central Alps and the Massif Central and the Rhenish Massif, respectively. During the Paleocene, compressional stresses exerted on continental Europe by the evolving Alps and Pyrenees caused lithospheric buckling and basin inversion up to 1700 km to the north of the Alpine and Pyrenean deformation fronts. This deformation was accompanied by the injection of melilite dykes, reflecting a plume-related increase in the temperature of the asthenosphere beneath the European foreland. At the Paleocene–Eocene transition, compressional stresses relaxed in the Alpine foreland, whereas collisional interaction of the Pyrenees with their foreland persisted. In the Alps, major Eocene north-directed lithospheric shortening was followed by mid-Eocene slab- and thrust-loaded subsidence of the Dauphinois and Helvetic shelves. During the late Eocene, north-directed compressional intraplate stresses originating in the Alpine and Pyrenean collision zones built up and activated ECRIS. At the Eocene–Oligocene transition, the subducted Central Alpine slab was detached, whereas the West-Alpine slab remained attached to the lithosphere. Subsequently, the Alpine orogenic wedge converged northwestward with its foreland. The Oligocene main rifting phase of ECRIS was controlled by north-directed compressional stresses originating in the Pyrenean and Alpine collision zones. Following early Miocene termination of crustal shortening in the Pyrenees and opening of the oceanic Provençal Basin, the evolution of ECRIS was exclusively controlled by west- and northwest-directed compressional stresses emanating from the Alps during imbrication of their external massifs. Whereas the grabens of the Massif Central and the Rhône Valley became inactive during the early Miocene, the Rhine Rift System remained active until the present. Lithospheric folding controlled mid-Miocene and Pliocene uplift of the Vosges-Black Forest Arch. Progressive uplift of the Rhenish Massif and Massif Central is mainly attributed to plume-related thermal thinning of the mantle-lithosphere. ECRIS evolved by passive rifting in response to the build-up of Pyrenean and Alpine collision-related compressional intraplate stresses. Mantle-plume-type upwelling of the asthenosphere caused thermal weakening of the foreland lithosphere, rendering it prone to deformation.

Distinguishing fault reactivation from flexural deformation in the distal stratigraphy of the Peripheral Blountian Foreland Basin, southern Appalachians, USA

AbstractReactivation of intraplate structures and weak zones within the foreland lithosphere disrupt the modelled geometry and pattern of migration of the flexural wave in foreland basins. In the southern Appalachians (USA), the Middle Ordovician unconformity, irregular Middle Ordovician distal foreland deposition and backstepping of Middle–lower Upper Ordovician carbonate strata have been related to migration of the flexural wave. However, integration of stratigraphy, tectonic subsidence history and composition of palinspastically restored distal foreland strata, using a map of subsurface basement structures as reference, allows us to distinguish an early event of inversion from two events of flexural migration. Sections restoring at very short distances outside the boundaries of a former basement graben have the youngest passive‐margin strata preserved beneath Middle Ordovician (∼466 Ma) peritidal to deep lagoonal carbonates with gravel‐size chert clasts. In contrast, sections restoring inside the graben record >470 m of truncation of pre‐Middle Ordovician passive‐margin strata, late onset of deposition (∼456 Ma), and subaerial features in carbonate and siliciclastic strata. The lacuna geometry and early patterns of distal foreland uplift and carbonate deposition indicate that inversion of a basement graben in response to Middle Ordovician convergence, rather than a migrating or semi‐fixed forebulge, was the primary control on the early evolution of the distal foreland. Drowning of the carbonate platform in more proximal settings, northeastward onset of deposition on upthrown blocks, and thick accumulation of carbonates in downthrown blocks record northwestward and northeastward flexural wave migration at the Middle–Late Ordovician boundary. In early Late Ordovician, the overall shoaling of carbonate and siliciclastic depocentres and the rise of tectonic subsidence curves indicate hinterlandward migration of flexural uplift. Both events of flexural migration were accompanied by influx of volcanic ash and synorogenic sediments.

Early to middle Tertiary foreland basin development and the history of Andean crustal shortening in Bolivia

A >2.5-km-thick succession of Tertiary strata in the Eastern Cordillera of southern central Bolivia consists of predominantly fluvial and lacustrine deposits. The age of the base of the succession is middle Paleocene, and its upper part (which is erosionally truncated) is probably late Oligocene-early Miocene. The lower 50-130 m of the succession consists of interbedded fluvial sandstone and mudstone (including minor paleosols) of the Santa Lucia Formation. These strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the Santa Lucia Formation and lower part of the Impora Formation that may represent much of late Paleocene-Eocene time. Above the paleosol zone is a thin zone (∼10 m) of lacustrine carbonate and clastic rocks in the uppermost Impora Formation, and this in turn is overlain by a >2000-m-thick, upward-coarsening succession of clastic fluvial deposits in the Cayara, Camargo, and Suticollo Formations. Paleocurrent data indicate that fluvial channels that deposited the Santa Lucia, Impora, and Cayara Formations flowed mainly westward, whereas channels responsible for the Camargo and Suticollo Formations flowed generally eastward. Modal sandstone petrographic data show a long-term evolution from subarkosic (during Santa Lucia deposition) to quartz-arenitic (during Impora and Cayara deposition) to sublitharenitic (during Camargo and Suticollo deposition) compositions. We argue that the lithofacies and sediment-accumulation history of this succession are most consistent with deposition in an eastward-migrating foreland basin system. E -mail: decelles@geo.arizona.edu. Critical to this interpretation is the zone of extreme stratigraphic condensation in the lower Impora Formation, the inferred upper Paleocene-Eocene part of the succession. The abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous, postrift thermal subsidence resulted in a gradual, continuous decay of early Tertiary sediment-accumulation rates. Stratigraphic condensation is, however, consistent with passage of the forebulge through the region, an interpretation that also can be reconciled with the record of foreland basin development in the Altiplano to the west and gross sediment accumulation patterns in the Subandean zone and modern foreland basin to the east. Simple flexural modeling and palinspastic reconstruction of the complete Cenozoic migration history of the foreland basin system suggest that ∼1000 km of foreland lithosphere has been underthrust westward beneath the Andean orogenic belt at the latitude of southern Bolivia. The amount of middle and lower crust that would have been added to the Andean infrastructure is sufficient to explain the present crustal thicknesses in Bolivia.