Basement Ramp Research Articles

Oligocene to Pliocene structural evolution of the frontal Kirthar fold and thrust belt of Pakistan

The spatio-temporal evolution for the frontal part of the Kirthar fold and thrust belt in Pakistan is constrained for the first-time using apatite (U–Th-Sm)/He (AHe) and apatite fission track (AFT) dating of samples from the Oligocene Nari Formation in combination with a published balanced cross-section. The AHe ages appear to be fully reset, allowing us to date the timing of exhumation above ramps. Comparison of partially reset AFT ages (∼22–24 Ma) with previously published zircon fission track (ZFT) ages of the Nari Formation from nearby areas suggest that rocks in the frontal zone were not buried deeper than ∼4 km after deposition. The close range of AFT ages have implications for the timing of hinterland exhumation and Oligocene age of the Nari Formation. The AHe ages suggest that deformation along the major basement ramp was active since ∼7 Ma, forming a major topographic step (∼1.5 km) in the frontal part of the Kirthar fold and thrust belt. Combined analysis of the structural cross-section and thermal modeling of the samples suggest that major cooling occurred between ∼7 Ma and ∼5 Ma. This cooling was due to the erosion of Oligocene to Miocene strata from above the samples when Precambrian to Miocene strata was thrust above the pre-existing normal fault that acted as a basement ramp. The temporal structural evolution suggests that deformation in the frontal part of the Kirthar fold and thrust belt was characterized by faster rates (∼2.5 km/Ma) of orogenic growth and exhumation between ∼7 and 5 Ma, followed by slower shortening rates (<1 km/Ma) since ∼5 Ma.

Space and Time Variability of Detachment‐ Versus Ramp‐Dominated Thrusting: Insights From the Outer Albanides

AbstractDespite their markedly different structural setting, the northern and southern outer Albanides share a common tectonic evolution from detachment‐dominated to ramp‐dominated, basement‐involved thrusting. The former process (mainly Oligocene to Miocene) is essentially related with the occurrence of a thick décollement level represented by Triassic evaporites, while the latter involves basement ramps splaying out from a middle crustal décollement. As this weak crustal layer is inherited from the Mesozoic rifting stage, the original continental margin architecture is interpreted to strongly influence subsequent convergent deformation. The profoundly different nature of the two dominant décollements in the study area controlled the structural style of the fold and thrust belt. The decoupling capacity of the upper décollement is strongly dependent on the thickness of the Triassic evaporites. Where this is significant (≫1 km; southern outer Albanides), the occurrence of such a thick incompetent layer at the base of competent carbonate units favored the development of break‐thrust folds and imbrication of the sedimentary cover. Fold and thrust belt propagation was instead hindered where original stratigraphic variations resulted in a reduced thickness (≤1 km) of Triassic evaporites. On the other hand, the deeper middle crustal décollement is controlled by basement rheology. Its reactivation during plate convergence was assisted by collision‐related thermal weakening of the crust. This process governed late‐stage (&lt;5 Ma) crustal‐scale tectonic inversion and plays a major role in controlling present‐day seismicity.

Analogue model of the Bala Rud Fault, Zagros: an oblique basement ramp in a fold-and-thrust belt

The role of pre-existing structures during the building of the Zagros fold-and-thrust belt is a matter of debate. The orientation of the pre-existing basement faults varies in different parts of the Zagros belt. Major directions of the basement faulting in the Arabian plate and Zagros are N–S, NE–SW, NW–SE and E–W. Reactivation of these inherited pre-existing faults occurred at the Cretaceous. In this study, analogue modelling was constructed to test the effect of various orientations of the pre-existing basement faults on the deformation style in the northern boundary of Dezful Embayment. The model consisted of sand layers that partly overlay a viscous layer of silicon above wooden plates. Model results explain the development of deflection in the deformation front and the incompatibility location of faults in the basement and surface. The presence of the basal decollement horizon between the basement and overlying sedimentary sequences separated faulting geometry in the basement from sedimentary cover. Model results suggest that the flexure of the Zagros Mountain Front Faults (ZMFF) in the north part of Dezful Embayment is resulted by reactivation of pre-existing Bala Rud fault (BRF) which is an oblique ramp. The close similarity between the major structures produced during the analogue experiments and those observed in nature, suggests that BRF as a pre-existing basement fault has a major role in the deformation of the structures in the study area.

Ductile nappe extrusion in constrictive strain at the origin of transverse segments of the Allochthon Boundary Thrust in the Manicouagan Imbricate Zone (Central Grenville Province, Québec)

Separating the Parautochthonous and Allochthonous belts, the Allochthon Boundary Thrust (ABT) is a major lithotectonic boundary that generally runs parallel with the length of the Grenville Province, but with some transverse segments. In this contribution, we investigate one of these segments, the Manicouagan Imbricate Zone (MIZ), by adding new field-based data supplemented by new U–Pb deformation ages to a compilation of previous research conducted on both sides of the ABT in the region. Our results reveal striking differences between the footwall and hanging wall of the ABT in terms of strain and structures and concludes that the transverse orientation of the ABT is not related to folding. An updated strain analysis allows us to determine that deformation of the MIZ was dominated by a constrictive-type of strain. We build on a previously proposed conceptual model to document ductile extrusion over basement ramps driven by tectonic forcing and lateral density contrast in the upper crust at ca. 990 Ma. The insertion of the hot ductile nappe into partially molten metasediments of the surrounding Parautochthonous Belt was accommodated by constrictive strain within the former and flattening and transverse folding in the latter.

Upper crustal structure of NW Iran revealed by regional 3-D Pg velocity tomography

SUMMARYWe present the result of a 3-D Pg tomography in NW Iran to better understand the relationship between seismicity and velocity structure within the young continental collision system. In this regard, we have collected 559 07 Pg traveltime readings from 3963 well located earthquakes recorded by 353 seismic stations including 121 stations from four new temporary seismic networks. The most prominent feature of our Pg velocity model is a high correlation between the location of majority of large magnitude events and the location of low velocity regions within the seismogenic layer. The large instrumental and historical earthquakes with some limited exceptions tends to happen close to the borders of the low velocity regions. The Lorestan arc of Zagros has the thickest (∼20 km) low velocity region and Central Iran has the thinnest (less than 10 km) low velocity region where little seismicity is observed. Despite the relative increase of thickness of low velocity region in the uppermost part of the upper crust of Alborz, the average Pg velocity of the upper crust increases from Central Iran towards Alborz and reaches to its climax in the northern hills of Alborz, where the catastrophic Rudbar-Tarom 1990 event happened. The Pg velocity map shows presence of a low angle basement ramp in the Lorestan arc at the depth range of ∼10–20 km. The large low angle thrust Ezgele-Sarpolzahab 2017 earthquake and medium size high angle thrust events happened at the base and updip part of the velocity ramp, respectively. The calculated Pg velocity map shows low velocity regions at depths deeper than 11 and 20 km beneath the Sahand and Sabalan volcanoes, respectively.

The effects of lateral density gradients, slopes and buoyancy on channel flow: 1D analytical solutions and applications to the SE Canadian Cordillera

We present 1D analytical solutions for channel flow in orogens driven by various types of pressure gradients. Our calculations demonstrate that lateral density gradients in the upper crust, such as would occur across a suture zone separating arc rocks from pericratonic sediments provide a driving force for Poiseuille flow as large as topographic gradients observed in modern mountain belts. For cases for which the gradients are external (topographic and lateral density gradients) and internal (e.g. partial melting of channel material) to the channel, inclination decreases and increases the Poiseuille component of the average flow-velocity within the channel by the cosine and sine of the slope, respectively. The magnitude of the pressure gradient consequent upon the buoyancy generated by partial melting of metapelites in a channel with a 30° slope, such as would occur above an underthrusting basement ramp, is similar to that of topographic or lateral density gradients. Channel flow up a ramp could thus constitute an important exhumation mechanism in large hot orogens. Our calculations indicate that mid-crustal channel flow was a highly likely process in the Late Cretaceous-Paleocene setting of the southeastern Canadian Cordillera. The flow was first driven by the lateral density contrast between pericratonic sediments and the arc-related Intermontane terrane, then by combined effect of topographic gradient and melt-induced buoyancy of the Lower Selkirk Allochthon (part of the Shuswap Complex). Flow up the underthrusting basement ramp resulted in exhumation from mid- to upper-crustal levels. Channel flow then migrated downward to involve basement and overlying cover sequence rocks. Our results indicate that syn-convergent channel flow was a viable and very likely process in the southeastern Canadian Cordillera.

Kinematic evolution of a fold-and-thrust belt developed during basin inversion: the Mesozoic Maestrat basin, E Iberian Chain

AbstractThe Maestrat basin was one of the most subsident basins of the Mesozoic Iberian Rift system, developed by a normal fault system which divided it into sub-basins. Its Cenozoic inversion generated the N-verging Portalrubio–Vandellòs fold-and-thrust belt in its northern margin, detached in the Triassic evaporites. In the hinterland, a 40 km wide uplifted area, in the N–S direction, developed, bounded to the N by the E–W-trending, N-verging Calders monocline. This monocline is interpreted as a fault-bend fold over the ramp to flat transition of the E–W-trending, N-verging Maestrat Basement Thrust, and also indicates the transition from a thick-skinned (S) to a thin-skinned (N) style of deformation. This paper presents a kinematic evolutionary model for the northern margin of the basin and a reconstruction of the Maestrat Basement Thrust geometry, generated by the inversion of the Mesozoic normal fault system. It contains a low-dip ramp (9°) extended southwards more than 40 km, attaining a depth of 7.5 km. As this thrust reached the Mesozoic cover to the foreland, it propagated across the Middle Muschelkalk evaporitic detachment, generating a nearly horizontal thrust which transported northwards the supra-salt cover, and the normal fault segments within it, for c. 11–13 km. The displacement of the basement in the hanging-wall of the low-dip basement ramp generated the 40 km wide uplifted area, while the superficial shortening was accumulated in the northern margin of the basin – which contains the thinnest Mesozoic cover – developing the Portalrubio–Vandellòs fold-and-thrust belt.

Redefined crustal structure of the Buchan Rift, northeast Victoria: evidence from potential field modelling of newly acquired land-based gravity data

ABSTRACTThe Buchan Rift, in northeastern Victoria, is a north–south-trending basin, which formed in response to east–west crustal extension in the Early Devonian. The rift is filled mostly with Lower Devonian volcanic and volcaniclastic rock of the Snowy River Volcanics. Although the structure and geometry of the Buchan Rift and its major bounding faults are well mapped at the surface, a discrepancy exists between the surface distribution of the thickest rift fill and its expected potential field response. To investigate this variation, two new detailed land-based gravity surveys, which span the rift and surrounding basement rocks in an east–west orientation, have been acquired and integrated with pre-existing government data. Qualitative interpretation of the observed magnetic data suggests the highly magnetic rocks of the Snowy River Volcanics have a wider extent at depth than can be mapped at the surface. Forward modelling of both land-based gravity data and aeromagnetic data supports this interpretation. With the Snowy River Volcanics largely confined within the Buchan Rift, resolved geometries also allow for the interpretation of rift boundaries that are wider at depth. These geometries are unusual. Unlike typical basin inversions that involve reactivation of rift-dipping faults, the bounding faults of the Buchan Rift dip away from the rift axis and thus appear unrelated to the preceding rifting episode. Limited inversion of previous extensional rift faults to deform the rift-fill sequences (e.g. Buchan Synclinorium) appears to have been followed by the initiation of new reverse faults in outboard positions, possibly because the relatively strong igneous rift fill began to act as a rigid basement ramp during continued E–W crustal shortening in the Middle Devonian Tabberabberan Orogeny. Overthrusting of the rift margins by older sediments and granite intrusions of the adjacent Tabberabbera and Kuark zones narrowed the exposed rift width at surface. This scenario may help explain the steep-sided geometries and geophysical expressions of other rift basins in the Tasmanides and elsewhere, particularly where relatively mechanically strong basin fill is known or suspected.

Pomeranian Caledonides, NW Poland – A collisional suture or thin-skinned fold-and-thrust belt?

A main goal of this study was to understand the character of the Caledonian Deformation Front in Pomerania, NW Poland and its relationship to the adjoining Teisseyre-Tornquist Zone (TTZ), in the context of the Avalonia-Baltica early Palaeozoic collision. Since the Pomeranian Caledonides are concealed beneath 1–4km thick platform cover of upper Palaeozoic and Mesozoic sediments, we used a combination of potential field, seismic and well data to investigate the basement architecture and structure of the lower Palaeozoic rocks. Starting from a qualitative review of gravity and magnetic data, we built a 2D gravity and magnetic model upon the PolandSPAN™ PL1-5600 seismic reflection line and applied 3D gravity inversion for a depth-to-basement study. Using well tops, a top lower Palaeozoic horizon and lower Palaeozoic isopach map were created. We found out that the Pomeranian Caledonides represent a thin-skinned fold-and-thrust belt involving Ordovician and Silurian sediments of the Caledonian foreland basin. The deformation front was developed due to the buttressing effect of a basement ramp occurring directly above the TTZ. The latter corresponds to a Precambrian suture zone overprinted by successive extensional tectonics and buried beneath the foreland basin. In northern Poland, the suture is defined by a remnant crustal keel still preserved underneath the TTZ, probably resulting from Precambrian collision during formation of the Rodinia paleocontinent. We propose that the extension of the Thor suture separating Baltica and Avalonia must exist west from the TTZ, probably in the Rügen area. In our interpretation, the TTZ represents an intra-cratonic crustal discontinuity comparable to its NW prolongation, the Sorgenfrei-Tornquist Zone that runs across the Baltic Sea, Sweden and Denmark. Consequently, the basement underlying the Palaeozoic Platform in NW Poland may be considered to have been part of Baltica in early Palaeozoic times.

Structure of an inverted basin from subsurface and field data: the Late Jurassic-Early Cretaceous Maestrat Basin (Iberian Chain)

The Maestrat Basin experienced two main rifting events: Late Permian-Late Triassic and Late Jurassic-Early Cretaceous, and was inverted during the Cenozoic Alpine orogeny. During the inversion, an E-W-trending, N-verging fold-and-thrust belt developed along its northern margin, detached in the Triassic evaporites, while southwards it also involved the Variscan basement. A structural study of the transition between these two areas is presented, using 2D seismic profiles, exploration wells and field data, to characterize its evolution during the Mesozoic extension and the Cenozoic contraction. The S-dipping Maestrat basement thrust traverses the Maestrat Basin from E to W; it is the result of the Cenozoic inversion of the lower segment–within the acoustic basement–of the Mesozoic extensional fault system that generated the Salzedella sub-basin. The syn-rift Lower Cretaceous rocks filling the Salzedella sub-basin thicken progressively northwards, from 350m to 1100m. During the inversion, a wide uplifted area –40km wide in the N-S direction– developed in the hanging wall of the Maestrat basement thrust. This uplifted area is limited to the North by the E-W-trending Calders monocline, whose limb is about 13km wide in its central part, dips about 5oN, and generates a vertical tectonic step of 800-1200m. We interpreted the Calders monocline as a fault-bend fold; therefore, a flat-ramp-flat geometry is assumed in depth for the Maestrat basement thrust. The northern synformal hinge of the Calders monocline coincides with the transition from thick-skinned to thin-skinned areas. The vast uplifted area and the low-dip of the monocline suggest a very low-dip for the basement ramp, rooted in the upper crust. The Calders monocline narrows and disappears laterally, in coincidence with the outcrop of the Maestrat basement thrust. The evaporitic Middle Muschelkalk detachment conditioned the structural style. Salt structures are also related to it; they developed during the Late Triassic extension, as deduced from the Keuper seismic reflectors that onlap the folded Upper Muschelkalk and form growth strata above some basement normal faults.

Rift to drift transition in the South Atlantic salt basins: A new flavor of oceanic crust

The crustal structure of the continent-ocean transition zone in the South Atlantic salt basins is poorly understood. Current interpretations place the limits of oceanic crust at the distal salt limits, with sub-salt crust consisting of rifted continental crust and, in some versions, varying amounts of exhumed mantle. Plate reconstructions that map these limits of oceanic crust onto appropriate-age restorations show poor geometric fits, with unexplained gaps and overlaps. One possible reason for the poor fits is that the distal salt limits are not the real limits of oceanic crust. In this paper we investigate this option by mapping rift basins and seaward-dipping reflectors whose seaward edges mark significant structural boundaries as much as 300 km inboard of the distal salt limits. We interpret these boundaries, which match geometrically in a salt-age (Aptian) plate reconstruction, to be the limits of oceanic crust. We suggest that salt was deposited as seafloor spreading commenced and that, as the South Atlantic opened, salt flowed over the ridge axis, sealing off the extrusive component of oceanic crust, resulting in formation of intrusive oceanic crust. Seafloor spreading eventually broke through the thinning salt, forming breakthrough volcanoes preserved today as basement ramps at the distal salt limits. These ramps formed diachronously, so the distal salt limits are not isochrons, explaining the poor fit of these features in plate reconstructions.

Geometry and kinematics of the Andean thick-skinned thrust systems: Insights from the Chilean Frontal Cordillera (28°–28.5°S), Central Andes

The structure of the Chilean Frontal Cordillera, located over the Central Andes flat-slab subduction segment (27°–28.5°S), is characterized by a thick-skinned deformation, affecting both the pre-rift basement and the Mesozoic and Cenozoic infill of the NNE-SSW Lautaro and Lagunillas Basins, which were developed during the Pangea-Gondwana break-up. The compressive deformation show a complex interaction between Mesozoic rift structures and thrust systems, affecting a suite of Permo-Triassic (258–245 Ma) granitic blocks. We used a combination of geological mapping, new structural data, balanced and restored cross sections and geochronological data to investigate the geometry and kinematics of the Andean thick-skinned thrust systems of the region. The thrust systems include double-vergent thick-skinned thrust faults, basement-cored anticlines and minor thin-skinned thrusts and folds. The presence of Triassic and Jurassic syn-rift successions along the hanging wall and footwall of the basement thrust faults are keys to suggest that the current structural framework of the region should be associated with the shortening of previous Mesozoic half grabens. Based on this interpretation, we propose a deformation mechanism characterized by the tectonic inversion of rift-related faults and the propagation of basement ramps that fold and cut both, the early normal faults and the basement highs. New U–Pb ages obtained from synorogenic deposits (Quebrada Seca and Doña Ana formations) indicate at least three important compressive pulses. A first pulse at ∼80 Ma (Late Cretaceous), a second pulse related to the K-T phase of Andean deformation and, finally, a third pulse that occurred during the lower Miocene.

Jurassic evolution of the Gulf of Mexico salt basin

We present a new hypothesis for the Jurassic plate-tectonic evolution of the Gulf of Mexico basin and discuss how this evolution influenced Jurassic salt tectonics. Four interpretations, some based on new data, constrain the hypothesis. First, the limit of normal oceanic crust coincides with a landward-dipping basement ramp near the seaward end of the salt basin, which has been mapped on seismic data. Second, the deep salt in the deep-water Gulf of Mexico can be separated into provinces on the basis of position with respect to this ramp. Third, paleodepths in the postsalt sequence indicate that salt filled the Gulf of Mexico salt basin to near sea level. Fourth, seismic data show that postsalt sediments in the central Louann and the Yucatan salt basins exhibit large magnitudes of Late Jurassic salt-detached extension not balanced by equivalent salt-detached shortening. In our hypothesis, Callovian salt was deposited in preexisting crustal depressions on hyperextended continental and transitional crust. After salt deposition ended, rifting continued for another 7 to 12 m.y. before sea-floor spreading began. During this phase of postsalt crustal stretching, the salt and its overburden were extended by 100 to 250 km (62–155 mi), depending on location. Sea-floor spreading divided the northern Gulf of Mexico into two segments, separated by the northwest-trending Brazos transform. The eastern segment opened from east to west, leaving the Walker Ridge salient in the center of the basin as the final area to break apart. In some areas, salt flowed seaward onto new oceanic crust, first concordantly over the basement as a parautochthonous province, then climbing up over stratigraphically younger strata as an allochthonous province.

Wadi Fatima Thin-Skinned Foreland FAT Belt: A Post Amalgamation Marine Basin in the Arabian Shield

Wadi Fatima fold-thrust (FAT) belt is a distinctive foreland FAT belt in the Arabian-Nubian Shield (ANS) involving unmetamorphosed to slightly metamorphosed sedimentary sequence of Fatima Group, deposited over a metamorphic/igneous basement, comprising ortho-amphibolites, orthoand para-schists (with chaotic unmappable blocks of marbles, pyroxenites and metagabbros), older granite (773 ± 16 Ma) and younger granite. The basement exhibits structural fabrics, such as attenuated tight isoclinal folds, sheared-out hinges, NE-SW penetrative foliation and subhorizontal stretched and mineral lineations, related to an oldest prominent dextral shearing phase affected the main Wadi Fatima during the Neoproterozoic. In Wadi Fatima FAT belt, the style of deformation encompasses flexural-slip folding forming mesoscopicand map-scales NE to ENE plunging overturned antiforms and synforms, and a thrust duplex system bounded by floor thrust and sole thrust (basal detachment) dipping gently towards the hinterland (SE to SSE direction) and rises stratigraphically upwards towards the foreland. Such style is affiliated to thin-skinned deformation. Several lines of evidence, such as geometry of interacting outcropand map-scale folds and thrusts, patterns of thrust displacement variations and indications for hinge migration during fold growth, strongly suggest that folding and thrusting in Wadi Fatima FAT belt are geometrically and kinematically linked and that thrusting initiated as a consequence of folding (fold-first kinematics). Thrusts frequently show flat-ramp-flat geometry, and every so often give an impression that they are formed during two main sub-stages; an older sub-stage during which bedding sub-parallel thrusts were formed, and a younger sub-stage which generated younger ramps oblique to bedding. Thrust ramps with SE to SSE dipping regularly show sequential decrease in dip or inclination (due to piggy-back imbrication) into their transport direction which is proposed to be towards NW to NNW. Evidence indicating this transport direction of Wadi Fatima FAT belt embrace NW to NNW oriented stretching lineations recorded along thrust planes, NW to NNW folding vergence, and diminishing of the intensity of deformation and thrust stacking and imbrication from SE to NW; i.e. from hinterland to foreland. The tectonic transport vector is congruent with the mean orientation of slickenline striae formed by layer-parallel slipping along folded bedding planes. The mean orientation of slickenline lineations, after their host beds were rotated to horizontal about their strikes, is found to be N25°W - S25°E. Two tectonic models are proposed to unravel the structural history of the study area and to illustrate the tectonic evolution of Wadi Fatima FAT belt which represents one of interesting foreland FAT belts recorded worldwide. In the first model, the area was evolved from dextral shearing during the early convergence and amalgamation between East and West Gondwana, to emplacement of the older granite during a period of crustal cessation and relaxation, NNW SSE extension and extrusion of dyke swarms, emplacement of younger granite, deposition of Fatima Group over an ancient peneplain, layer parallel shortening, folding and fold tightening and overturning, thrusting, NE-SW (to NNE-SSW) shortening, and eventually NE tilting accompanied with Red Sea rifting (?). The second model suggests the presence of basement ramps (pre-existing normal faults), with NW to NNW dipping, have a strong effect on overlying Fatima Group which was evolved throughout gravitational, soft-sediment slumping and deformation.

Testing modes of exhumation in collisional orogens: Synconvergent channel flow in the southeastern Canadian Cordillera

This study investigates exhumation processes in collisional orogens. A critical test between three modes of exhumation is presented based on a review of quantitative numerical and analogue modeling studies. The test is applied to the large tract of migmatites surrounding the Monashee Complex in the southeastern Canadian Cordillera. It reveals that the extensive and multidisciplinary database of this region is entirely compatible with the synconvergent channel-flow mode but not with critical wedge and gravitational collapse modes. We propose that a partially molten channel decoupled from its lid and base and started to flow in the middle crust toward the foreland at 100–90 Ma. A steady-state channel-flow system was established for the following ∼30 m.y., during which rocks at the front of the channel were exhumed to upper-crustal levels as they flowed above an underthrusting basement ramp. Flow was accommodated by oppositely verging shear zones bounding the channel, by internal ductile deformation, and by shortening in the foreland belt. The locus of flow migrated downward to exhume rocks of the Monashee Complex between 60 and 50 Ma by a similar process. The southeastern Canadian Cordillera thus constitutes an excellent natural analogue for the channel-flow model. In contrast to the commonly held view of large-magnitude extension and core complex formation, the role of extension was limited to the final ∼10–15 km of exhumation after 50 Ma.

The effects of basement ramps on deformation of the Prebetics (Spain): A combined field and analogue modelling study

In this study we test whether pre-existing basement faults control deformation in the Prebetic zones of the Betic Cordillera. We focus on an area in the northern part of the Prebetics which is dominated by a transpressional fault system consisting of a NNE–SSW striking fault zone and E–W striking small-scale thrusts. Field observations show that both sets of faults formed in a regional NW–SE to NNW–SSE directed compressional stress field during the Miocene. We propose that the faults formed on top of a basement containing basement ramps overlain by a décollement of Late Triassic evaporites and clays. These basement ramps have either an E–W or NNE–SSW orientation and result from extensional faulting prior to the compressional phase. The style of faulting in the overlying sediments is controlled by the variation in orientation of the pre-existing basement ramps in a uniform stress field. Where the ramps have an E–W orientation small-scale thrusting and folding occurs, where the ramps are oriented NNE–SSW the fault zone acts as an oblique-slip fault. The model was tested by analogue tectonic experiments incorporating a ductile layer, representing the décollement, overlain by a brittle layer representing the overlying sediments. The pre-existing basement ramps were represented by rigid material. The close similarity between the structures produced during the analogue experiments and those observed in the field suggests that pre-existing basement faults exert a major role on deformation of the Prebetics.

Regional-scale lateral variation and linkage in ductile thrust architecture: the Oykel Transverse Zone, and mullions, in the Moine Nappe, NW Scotland

Abstract Sharp lateral changes in structural geometry of ductile thrust stacks are not widely reported. A regional-scale lateral culmination wall forms the southern boundary of the Cassley Culmination in Moine rocks in the Caledonides of Sutherland, Northern Scotland. This culmination wall is part of the Oykel Transverse Zone (OTZ), a kilometre-scale shear zone characterized by constrictional finite strain fabrics aligned sub-parallel to the regional WNW-directed thrust transport direction. Main phase folds and fabrics in the transverse zone hanging wall are folded by main phase folds and fabrics in the footwall, thus recording foreland-propagating ductile deformation. South of the Cassley Culmination, shortening occurred uniformly, without development of discrete subsidiary thrusts; distributed deformation (fold development) alternated with localized thrusting within the culmination. The classic ESE-plunging mullions at Oykel Bridge are an integral part of the OTZ and were generated by constriction aligned sub-parallel to the transport direction. Constriction is attributed to differential, transtensional movement across the OTZ during culmination development. Subsequent formation of the underlying Assynt Culmination further accentuated upward-bulging of the Cassley Culmination, amplifying the lateral change across the transverse zone. The OTZ aligns with a pronounced gravity gradient on the south-western side of the Lairg gravity low. Interpretive modelling relates this gradient to a buried basement ramp that possibly controlled the location of the transverse zone.

Geological and isotopic constraints on the structure of the Bilbao anticlinorium (Basque–Cantabrian basin, North Spain)

The structure of the Bilbao anticlinorium is re-evaluated based upon new geological and isotopic data on Mesozoic evaporites and related groundwaters. A detailed geological survey to guide and monitor the construction of a 3.3 km tunnel provided new data on Early Cretaceous rocks with evaporite intercalations. S and O isotopes from evaporite minerals and from sulfate dissolved in groundwaters permitted us to ascribe the sampled subsurface rocks to the Early Cretaceous, rather than to the Triassic, and trace their distribution. We present a new structural map of the anticlinorium compartmented by four major branching thrusts. New and reinterpreted cross sections resolve its deep structure. Cover rocks extend to depths below 6–8 km. Hangingwall anticlines occur above hangingwall and footwall thrust ramps and the ensemble of slices forms an antiformal stack directly above a south dipping basement ramp. Shortening of ca. 15–20 km was accompanied by alongstrike sinistral displacements. Currently, the Bilbao anticlinorium relates to major geophysical anomalies and is regarded as the surface expression of an indenting wedge of deep crustal rocks at mid-crustal depths.

Stratigraphic record of translation down ramps in a passive-margin salt detachment

Ramp syncline basins filled with synkinematic sediments record the incremental translation history and translation rates of strata transported over basement ramps. On passive margins, movement of strata over a stratabound salt detachment is stratigraphically recorded as an isopach thick on the seaward side of a monoclinal ramp. This growth syncline gradually moves seaward down and then off the monocline, as if on a conveyor belt. This shift creates accommodation space for a new isopach thick to form just landward of the older one. Repetition of this cycle creates a shingled series of isopach thicks within the ramp syncline basin. Kinematic forward models and restored cross sections from the Kwanza Basin, Angola, show that the stratigraphic pattern of ramp syncline basins is most influenced by three factors. (1) The relative rates of translation and aggradation control the curvature of the axial trace of the growth synclines. (2) Bathymetric scarps above basement ramps can create spectacular seaward-dipping onlap surfaces >30 km long downdip; adjoining ramps generate stacked, interfering onlap surfaces. (3) Salt diapirs or anticlines are commonly shortened at the top of the ramp, especially where this coincides with the base of the continental slope. Diapir shortening provides a buffer to absorb sliding from its landward side and impede sliding on its seaward side. This buffering can have a major control on the entire gravity-spreading system containing the diapirs.

ABSTRACT: Minibasins Translating Down a Basement Ramp in the Deepwater Monocline Province of the Kwanza Basin, Angola

ABSTRACT: Minibasins Translating Down a Basement Ramp in the Deepwater Monocline Province of the Kwanza Basin, Angola