Pre-existing Normal Faults Research Articles

Basin inversion and fault reactivation in laboratory experiments

A series of analogue models have been performed to get insights into the relationships between the reactivation of pre-existing normal faults and the orientation of a successive compressive stress field. The models were deformed in two phases: (1) a first extension phase ( σ h ≈ σ 3) generating a graben, and (2) a successive compressive phase with the orientation of the shortening direction ( σ h ≈ σ 1) trending between 0° and 90° (obliquity angle α) to the faults bounding the graben. The deepening of this graben was accompanied by syn-tectonic sedimentation that included a basal ductile layer (simulating salt in nature) overlain by a brittle sand pack. The experimental results suggest that the development of compressive structures is strongly controlled by the pre-existing normal faults and by the obliquity angle α. The experiments have invariably shown that the extensional faults were reactivated during shortening whatever the value of the angle α. Whereas strike-slip reactivation of major normal faults is favoured by low obliquity angles α, mostly dip-slip reactivation of pre-existing structures is instead promoted for high obliquity angles α. The analysis of model results highlights the important role played by a ductile layer at the base of the basin fill, and the obliquity angle α in controlling both strain partitioning and brittle–ductile decoupling.

Review and New Insights on Foreland Tectonics in Western Taiwan

Taiwan is located on the boundary between the Eurasian and Philippine Sea plates. As a result, foreland tectonics in western Taiwan can be divided into two domains: pre-orogenic extensional structures and those of the outer part of the fold-and-thrust belt that mingled with syn-orogenic normal fault reactivation. This paper proposes a synthetic model for foreland tectonics in western Taiwan, and advances possible mechanisms by which pre-existing normal faults might have affected the evolving thrust tectonics in foreland areas of western Taiwan. The sedimentary basins of pre-orogenic extensional tectonics are of two types—Paleogene and Neogene—which reflect two stages of continental rifting. Results from several studies have been synthesized to provide a tectonic map displaying the regional distribution of tectonic settings at different stages and the trends of normal faults in the basins. The similarity of the en echelon patterns of arrangement for both the Neogene and Paleogene tectonic and structural settings, as shown by the tectonic map, strongly suggests that the entire foreland area was influenced by regional dextral shear. We also provide a detailed description of structures in each tectonic setting, and propose a tectonic evolution model for Cenozoic basin architecture in western Taiwan. Among the pre-orogenic sedimentary basins, the Neogene ones, in which normal faults extend to the frontal areas of the fold-and-thrust belt in western Taiwan, open northeastward. Structural analysis of the thrust fault geometry indicates that, during development of the fold-and-thrust belt on the rifted continental margin in western Taiwan, the pre-existing normal faults in northwestern Taiwan were reactivated to form inversion structures of various types on different scales, depending on the angle between the strike of the normal faults and the direction of maximum compressive stress field. In southwestern Taiwan, where normal fault reactivation is absent from the eastern part of the foreland areas, pre-existing normal faults interacted with developing low-angle thrusts in the inner part of the fold-and-thrust belt. Normal fault reactivation, regardless of how it occurs, thus plays an important role in forming the deformation front of the fold-and-thrust belt. Based on this view, we propose that the orocline or tectonic arc of the island has been influenced more by normal fault reactivation than by the morphology of basement highs.

Seismic stratigraphy, structure and tectonic evolution of the Levantine Basin, offshore Israel

Abstract Multi-channel seismic reflection data and borehole information were used to study the structure and stratigraphy of the Levantine basin, offshore Israel. A new, 2D seismic survey that covers the southeastern Mediterranean Sea from the Israeli coast to the Eratosthenes Seamount shows the entire Phanerozoic sedimentary fill down to a depth of 14–16 km. The basin-fill is subdivided into six seismo-stratigraphic units interpreted as low-order, major depositional cycles (supersequences A-F). Correlation and mapping of these units allowed an investigation of the geological history of the basin and the analysis of two important tectonic phases: Neotethyan rifting, and Syrian Arc inversion and contraction. The Neotethyan rifting phase is recorded by the strata of supersequences A and B. Faulting took place during the Anisian (Mid-Triassic), continued through the Liassic and ceased during the Mid-Jurassic. The basin opened in a NW-SE direction, between the Eratosthenes Seamount and the Levant margin of the Arabian Massif, at an angle of about 30° to the present-day shoreline. No indications for sea-floor spreading were found in the present study. Late Triassic to Liassic volcanic rocks of assumed intraplate origin accumulated in the northeastern part of the basin. It is hypothesized that the basin originated as an intracontinental rift associated with the nucleation of an oceanic spreading centre, but reached only an early magmatic phase. An inversion and contraction phase, associated with closing of the Neotethyan ocean system, is recorded by supersequences C and D. The contractional structures of the Syrian Arc extend in a wide and elevated fold belt along the eastern edge of the deep-marine basin. These structures were formed by the inversion of pre-existing normal faults. The folding occurred in several pulses starting in the Senonian and ending in the Miocene. The western limit of the main fold belt, located 50–70 km west of the coastline, is defined by a transition in crustal properties. Supersequences E and F record the Late Cenozoic history of the basin. A Messinian, evaporitic basin was limited to the east by the elevated and uplifted Syrian Arc fold belt composed of older, Oligocene to Mid-Miocene strata. During highstand episodes, the Messinian evaporites were deposited on the entire slope and within canyons incised into the shelf. High sedimentation rates of Nilotic and locally derived sediments during the Plio-Pleistocene resulted in the development of extensive submarine deltas and basinward progradation of the Levant shelf break.

Evaluating sand and clay models: do rheological differences matter?

Dry sand and wet clay are the most frequently used materials for physical modeling of brittle deformation. We present a series of experiments that shows when the two materials can be used interchangeably, document the differences in deformation patterns and discuss how best to evaluate and apply results of physical models. Extension and shortening produce similar large-scale deformation patterns in dry sand and wet clay models, indicating that the two materials can be used interchangeably for analysis of gross deformation geometries. There are subtle deformation features that are significantly different: (1) fault propagation and fault linkage; (2) fault width, spacing and displacement; (3) extent of deformation zone; and (4) amount of folding vs. faulting. These differences are primarily due to the lower cohesion of sand and its larger grain size. If these features are of interest, the best practice would be to repeat the experiments with more than one material to ensure that rheological differences are not biasing results. Dry sand and wet clay produce very different results in inversion models; almost all faults are reactivated in wet clay, and few, if any, are significantly reactivated in sand models. Fault reactivation is attributed to high fluid pressure along the fault zone in the wet clay, a situation that may be analogous to many rocks. Sand inversion models may be best applied to areas where most faults experience little to no reactivation, while clay models best fit areas where most pre-existing normal faults are reactivated.

Sandbox experiments on basin inversion: testing the influence of basin orientation and basin fill

Analogue modelling experiments using brittle materials are performed to study the inversion of extensional structures. Asymmetric grabens of two different orientations are first created during a phase of extension and progressively filled. They are subsequently shortened in the same direction. The aim of our experiments is to determine factors affecting the style of deformation during inversion. We specifically investigate variations in thickness and distribution of strong and weak layers constituting the graben fill and in initial basin orientation. The main advantage of our experimental set-up is that we have a complete control on graben location, width, infill and orientation before inversion. The experiments show that shortening results only in limited reactivation of pre-existing normal faults. In general, forward thrusts and backthrusts cut across normal faults into the footwall of the graben. The forward thrusts either propagate parallel to the enveloping surface of faulted blocks or they cut across basin-limiting normal faults at various angles. The graben fill is mechanically extruded by displacement along forward thrusts that accommodate most of the shortening. Both pre-existing faults and weak graben fill act as zones of weakness during inversion and determine the orientation and location of both backthrusts and forward thrusts. The results of our experiments conform well to natural examples of inverted graben structures.

Fault reactivation in brittle–viscous wrench systems–dynamically scaled analogue models and application to the Rhine–Bresse transfer zone

Analogue experiments on oblique rifting and subsequent transpressional reactivation were performed with two-layer slabs of sand and silicone. In this brittle–viscous system, transtensional and transpressional wrench faulting was induced by movements of a basal rigid plate. The dynamically scaled analogue models are confronted with the structural evolution of the Rhine–Bresse transfer zone (RBTZ) that linked Palaeogene rifting in the Upper Rhine and Bresse Grabens and that appears to have been transpressively reactivated in Neogene to recent times. Fault patterns produced in the sand layer above the basal silicone layer are compared with structural elements in the sedimentary cover, separated from the basement by an evaporitic décollement layer. In order to investigate strain-rate dependence of fault reactivation and graben inversion, transpressional shortening was performed under different displacement rates. Experimental results suggest that the reactivation of pre-existing structures in a brittle cover above a viscous décollement is strongly dependent on the strain rate within the viscous layer. Under low to intermediate displacement rates (2.6 cm h −1), deformation concentrates within the basal viscous layer and former normal faults within the cover are not reactivated. The reactivation at higher displacement rates (5 cm h −1) results in a complete inversion of graben structures within the cover. Ongoing shortening produces lobed thrust fronts, which crosscut pre-existing normal faults. Late Pliocene to recent en-échelon aligned folds and isolated thrust faults in the cover of the RBTZ are attributed to thick-skinned reactivation of basement faults. A comparison of natural and experimentally obtained structures suggests that fault reactivation occurred under low displacement rates (<1 mm/a). This results in a low mechanical coupling between basement and cover in areas with significantly thick décollement layers, providing an explanation for decoupled stresses between basement and cover, such as observed in the northern Jura Mountains.

The role of extensional structures on experimental calderas and resurgence

The structure and shape of collapses and resurgences is often controlled by pre-existing discontinuities, such as normal faults in rift zones. In order to study the role of extensional structures on collapse and resurgence, we used analogue models. Dry sand simulated the brittle crust; silicone, located at the base of the sand-pack, simulated magma. In the experiments, regional extension pre-dated collapse or resurgence, forming normal faults in a graben-like structure; the graben was filled with additional sand, simulating post-rift deposits. A piston then moved the silicone downward or upward, inducing collapse or resurgence within the previously deformed sand. The collapses showed an ellipticity (length of minor axis/length of major axis) between 0.8 and 0.9, with the major axis parallel to the extension direction. The partial reactivation of the pre-existing normal faults was observed during the development of the caldera reverse faults, which, conversely to what was expected (from experiments without pre-existing extension), became partly inward dipping. Resurgence showed an elongation of the uplifted part, with the main axis perpendicular to the extension direction. At depth, pre-existing normal faults were partly reactivated by the reverse faults formed during resurgence; these locally became outward dipping normal faults. A total reactivation of pre-existing faults was also observed during resurgence. The experiments suggest that the observed elongation of calderas and resurgences is the result of the reactivation of pre-existing structures during differential uplift. Such a reactivation is mainly related to the loss in the coefficient of friction of the sand. The results suggest that elliptic calderas and resurgences in nature may develop even from circular magma chambers.

Structures associated with inversion of the Donbas Foldbelt (Ukraine and Russia)

The Donbas Foldbelt is part of the Prypiat–Dnieper–Donets intracratonic rift basin (Belarus–Ukraine–southern Russia) that developed in Late Devonian times and was reactivated in Early Carboniferous. To the southeast, the Donbas Foldbelt joins the contiguous, deformed Karpinsky Swell. Basin “inversions” led first to the uplift of the Palaeozoic series (mainly Carboniferous but also syn-rift Devonian strata in the southwesternmost part of the Donbas Foldbelt, which are deeply buried in the other parts of the rift system), and later to the formation of the fold-and-thrust belt. The general structural trend of the Donbas Foldbelt, formed mainly during rifting, is WNW–ESE. This is the strike of the main rift-related fault zones and also of the close to tight “Main Anticline” of the Donbas Foldbelt that developed along the previous rift axis. The Main Anticline is structurally unique in the Donbas Foldbelt and its formation was initiated in Permian times, during a period of (trans) tensional reactivation, during which active salt movements occurred. A relief inversion of the basin also took place at this time with a pronounced uplift of the southern margin of the basin and the adjacent Ukrainian Shield. Subsequently, Cimmerian and Alpine phases of tectonic inversion of the Donbas Foldbelt led to the development of flat and shallow thrusts commonly associated with folds into the basin. A fan-shaped deformation pattern is recognised in the field, with south-to southeast-vergent compressive structures, south of the Main Anticline, and north- to northwest-vergent ones, north of it. These compressive structures are clearly superimposed onto the WNW–ESE structural grain of the initial rift basin. Shortening structures that characterise the tectonic inversion of the basin are (regionally) orientated NW–SE and N–S. Because of the obliquity of the compressive trends relative to the WNW–ESE strike of inherited structures (major preexisting normal faults and the Main Anticline), in addition to reverse displacements, right lateral movements occurred along the main boundary fault zones and along the faulted hinge of the Main Anticline. The existence of preexisting structures is also thought to be responsible for local deviations in contractional trends (that are E–W in the southwesternmost part of the basin).

The control of pre-existing extensional structures on the evolution of the southern sector of the Aconcagua fold and thrust belt, southern Andes

The Aconcagua fold and thrust belt, located in the Andean mountains at 32°30′ to 34°S, has been described as a classic model of a thin-skinned thrust belt. However, new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple Mesozoic extensional phases, overprinted by structural inversion, as well as thin- and thick-skinned tectonics. Two major superimposed extensional structural styles have been identified for the Mesozoic characterized by distinctly oriented stress fields. A key role in the evolution of this part of the fold and thrust belt was played by a Late Triassic to Early Jurassic depocentre and by Late Jurassic block faulting. Shortening was accommodated by a combination of inversion of pre-existing normal faults, development of footwall short cuts and both thin and thick-skinned thrusting. Synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults, with further shortening along newly formed thin-skinned thrust faults. The geometry of thin-skinned fault systems is controlled by the architecture of the rift basin, competent footwalls forming barriers to the lateral propagation of detachments.

Three-dimensional structural model of the Rhourde el Baguel field, Algeria

The Rhourde el Baguel field, located in the Ghadames basin, is one of the largest oil fields in Algeria. It produces oil from fractured Cambrian sandstones at subsea depths of 2300–3070 m. The structure is reinterpreted as a compressive fold related to two reverse faults, which cut and deform a preexisting normal fault. The structure formed during three main episodes of deformation: (1) Triassic to Liassic extension resulting in normal faults having significant throw; (2) Middle Jurassic flowage of Liassic salt, resulting in low-amplitude structures in this unit; and (3) Austrian (Early Cretaceous) compression resulting in a basement-involved fault-propagation fold. The revised structural model explains the complex fault patterns and anomalous thickness variations of units observed in a number of deep wells on the west flank of the structure. The model has been used to sidetrack some existing deep wells into locations within the field and to optimize the location of new wells in the field.

Post mid-Cretaceous inversion tectonics in the Danish Central Graben – regionally synchronous tectonic events?

Structural analysis of the Upper Cretaceous to Palaeogene succession in the Danish Central Graben suggests continuous inversion heralded in the Late Hauterivian and continuing into Palaeogene times. The following phases of increased intensity are identified: 1) latest Santonian, 2) Mid Campanian, 3) late Maastrichtian, 4) Late Paleocene – Eocene, and 5) Early Oligocene. Phases 1 through 3 are Sub-Hercynian, phase 4 is Laramide, and phase 5 is Pyrenean according to Alpine Orogen nomenclature. A temporal change in structural style is noted from early inversion confined to narrow zones associated with reverse faulting along pre-existing normal faults to late inversion dominated by gentle basinwide flexuring and folding. Inversion phases in the Danish Central Graben seem to be synchronous with inversion phases along the Sorgenfrei-Tornquist Zone. The location of inversion is generally spatially linked to Upper Jurassic – Lower Cretaceous depocentres, whereas older depocentres generally have remained intact. The origin of the compressional stress field is generally based on suggested compressional stresses transmitted into the foreland from the Alpine Orogen. In the Sub-Hercynian phase, orogenic compression dominated the Eastern Alps and Northern Carpathians to produce a likely NW oriented compression. However, structures in Denmark rather suggest a transpressional environment resulting from NNE–SSW compression. Furthermore, transmission of Alpine orogenic stresses into the foreland commenced in the Turonian, a considerable time after the Late Hauterivian and later inversion precursors. Ridge-push forces transmitted from sea-floor spreading south of the Charlie-Gibbs fracture zone, particularly from the Goban Spur SW of Ireland, acting in conjunction with Alpine orogenic stresses are suggested as the cause for the stress field.

Thick‐skinned tectonics in the external Apennines, Italy: New evidence from magnetic anomaly analysis

We discuss the tectonic implications of a new residual magnetic map of the Apennine belt/Adriatic‐Apulian foreland obtained by integrating ground and offshore data sets [Chiappini et al., 2000a]. Negative anomalies are documented over the Adriatic‐Apulian foreland areas, whereas the external Apennine belt is characterized by a ubiquitous low‐amplitude (&lt;30 nT), long‐wavelength positive anomaly. In the central northern Apennines, three ∼100 km wide more intense (100–200 nT) round‐shaped anomalies are superimposed to the long‐wavelength feature. Finally, in the Tyrrhenian Sea and margins, high‐intensity, short‐wavelength positive‐negative couplets coincide with magmatic outcrops or bodies at shallow depth. The low‐amplitude anomaly pattern over Italy suggests that the magnetic basement beneath the Triassic evaporites is ubiquitously incorporated in the external belt compressive fronts, implying a thick‐skinned tectonic style for the external Apennines. The new residual magnetic map resolves the inconsistency between previous aeromagnetic data [AGIP SpA. Italia, 1981], which suggested a lack of basement involvement in the Apennine belt, and recent seismic data, which imaged deep reflectors penetrating the basement. Two magnetic models along NE‐SW transects in the northern and southern Apennines suggest consistent structural styles. In the northern Apennines, positive anomalies roughly coincide with the external compressive fronts, although there are local second‐order differences between the belt front and the edges of the anomaly. Here the magnetic data show that the basement rises southwestward along the thrust fronts from 6–7 km depth in the Adriatic foreland to 2–3 km depth in the axial belt, where some exploration wells have penetrated basement. Within the belt front, basement exhumation is inferred to occur along high‐angle, low‐displacement thrust faults inverting preexisting normal faults. In the southern Apennines, a remarkable positive magnetic anomaly is parallel with and tens of kilometers southwest of the belt front. Seismic data and oil wells show that the basement surface cannot be shallower in the belt than in the foreland. Therefore the observed magnetic anomaly is produced by strongly magnetic basement beneath the belt, likely an internal crustal wedge tectonically interposed between the Apulian carbonate sequences and basement.

Models of passive margin inversion: implications for the Rhenohercynian fold-and-thrust belt, Belgium and Germany

Positive tectonic inversion is related to the transmission of compressional stresses along a décollement into the foreland of an orogenic zone. This stress and strain concentration in regions remote from the main orogenic front is commonly related to the presence of pre-existing rheological heterogeneities such as normal syn-depositional faults. During inversion, these pre-existing normal faults are reactivated as reverse faults. Tectonic inversion in the Rhenohercynian fold-and-thrust belt during the Variscan Orogeny shows that inversion is likely synchronous with the onset of collision in the hinterland. Here, we present the results of a simplified thermo-mechanical model (STM) which allows one to study strain partitioning between two orogenic zones. We show that, if the two orogenic zones have the same mechanical properties, the viscosity of the décollement, which links them, controls the initial strain partitioning. During subsequent finite shortening, erosional processes determine the partitioning of strain rate. The presence of a weak structure in the inverted zone and of a low-viscosity décollement leads to initial strain concentration in the inverted track rather than in the collision zone and a progressive decrease in strain partitioning between the two orogenic zones. The STM results are in good agreement with results of a 2D finite-element model. We conclude that, in the western part of the Rhenohercynian Massif, simultaneous uplift and deformation within the Mid-German Crystalline Rise (the main collision zone) and the Ardenne Anticlinorium (the inverted zone) lead to interpreting this orogenic event as a case of vice tectonic rather than the propagation of a ‘wave of folding’ towards the Variscan front, as suggested by previous authors.

The Pampean flat-slab of the Central Andes

Late Cenozoic Andean deformation in the Pampean segment of Argentina and Chile (27°00′–33°30′S) provides an exceptional opportunity to study the orogenic effects of flat subduction in an active convergent margin. At these latitudes, constraints derived from oceanic geomorphic structures indicate that collision of the Juan Fernandez Ridge in the Nazca plate with the Chilean trench propagated from north to south from ca. 18 to ca. 11 Ma. In this region, Cenozoic tectonics have resulted in the development of the Principal and Frontal Cordilleras, the Cuyo Precordillera, and the associated Sierras Pampeanas in the eastward foreland region. The analyses of different structural transects across the Sierras Pampeanas between 27 and 33°S latitudes show a southward diachronic beginning of basement block uplift from 7.6 to 6 Ma in the Sierra de Aconquija (27°S), 4.5 to 4.19 Ma at Sierra de Famatina (29°S), 5.5 to 4.7 Ma at Sierra de Pocho (31°S), and ca. 2.6 Ma at Sierra de San Luis (33°S). Uplift times are constrained by the age of synorogenic deposits, fission-track data on apatite, and hydrogen isotopes in alunite, where available. These time constraints are compared with the migration and expansion of the magmatic activity into the foreland. The relation of these two data sets shows a striking coincidence, after a residence time of approximately 4–2.6 Ma, between magmatic activity and failure of the crust that resulted in basement block uplift. Thermal weakening of the crust associated with eastward migration of arc magmatism acted to elevate brittle–ductile subsurface décollments, thus leading to thick-skinned basement uplift of the Sierras Pampeanas. The evolution of the Sierras Pampeanas is linked to that of the main Andes at 33°S latitude. The main deformation phases and uplift of the thin- and thick-skinned fold and thrust belts of Principal Cordillera occurred between 20 and 8.6 Ma. Arc volcanism migrated eastward at this latitude between 16 and 15.8 Ma. This expansion was accompanied by deformation and uplift with the development of a foreland basin. Soon after the shallowing of the subduction zone, orogenic shortening and basin formation were followed by a migration of the volcanic front to the foreland. Propagation rates of the thrust front from 2.5 mm/a in the early stage (15–9 Ma) accelerated to 13.3 and 13.7 mm/a during the middle (9–6 Ma) and late (5–2 Ma) stages of shallowing. Uplift of the Frontal Cordillera overlapped the last stages of Principal Cordillera deformation at 9 Ma. This time is also constrained by magnetostratigraphic studies of the foreland deposits that record a rapid increase in subsidence rates at the same time that arc magmatism expanded to the Sierras Pampeanas, between 9.5 and 6.4 Ma. This rapid propagation of the thrust front and migration of the arc magmatism occurred 2 m.yr later than the 11 Ma collision of the Juan Fernandez Ridge at this latitude. Uplift of the Precordillera by tectonic inversion of pre-existing normal faults and basement uplifts of the Sierras Pampeanas in the vicinity of 33°S occurred from 2.6 Ma to the present. Neotectonic studies, as well as GPS measurements, indicate active deformation in the thrust front of the Precordillera, as well as in the Sierras Pampeanas. Magmatic activity ended in the Principal Cordillera at approximately 8.6 Ma and in the Sierras Pampeanas at 1.9 Ma. The last subduction-related volcanism occurred more than 750 km east of the trench. Analyses performed along four different transects show close relations between eastward propagation of the magmatic arc, formation of new brittle–ductile transition zones as a function of heat input, and orogenic deformation. Uplift and deformation in Sierras Pampeanas, thus clearly follows the eastward propagation of arc magmatism. This relation implies that uplift and deformation are more likely related to thermal weakening and crustal anisotropy than to fluctuations in horizontal compressive stress. Eastward propagation of magmatism to the foreland over the last 6 m.yr plays an important role in the thermal crustal weakening and development of brittle–ductile transitions that allowed uplift of Sierras Pampeanas basement-cored blocks.

Cenozoic inversion and uplift of southern Britain

Abstract Whereas significant exhumation of northern Britain took place during Paleocene time, probably as a consequence of uplift caused by a mantle plume, Paleogene basin inversion and uplift in southern Britain appears to be a consequence of Alpine tectonism. Recent publications demonstrate that inversion of Mesozoic basins across southern Britain was accompanied by subsidence of flanking basins in areas that had previously remained stable. Structures observed on seismic sections across the Weald Basin in SE England reveal that inversion occurred locally by north-directed reverse movements on pre-existing normal faults that cut down at a low angle deep into the basement. The overall effect of inversion of the Weald Basin, however, is a bulk deformation that produced a domal uplift, flanked by subsidence of the London and Hampshire-Dieppe basins. A 2D finite element thermomechanical model of continental lithosphere containing a region of reduced strength in the crust simulates Jurassic-Early Cretaceous extension to form the Weald Basin, followed by compression during the Tertiary to produce its inversion and the flanking basins. The timing of tectonic events across southern Britain correlates with times when Alpine stresses were transmitted into the foreland to the north sufficiently well to link them. Through most of Tertiary time, the landscape of southern England was of relatively low elevation and low-energy surface processes. However, late Neogene uplift, generally greater in the west, appears to have been part of a larger-scale uplift of land areas with hard rock at surface, which has no obvious tectonic explanation.

Pliocene-Quaternary transpressional evolution of the Anzi-Calvello and Northern S. Arcangelo basins (Basilicata, Southern Apennines, Italy) as a consequence of deep-seated fault reactivation

In this paper we report the results of a study of the Plio-Pleistocene Anzi-Calvello Basin and the northern sector of the S. Arcangelo Basin (Southern Apennines, Italy). Our work suggests that the evolution of the Anzi-Calvello Basin is related to the activity of a deep-seated, roughly NNE-trending, dextral transpressional fault zone (“Camastra Transcurrent Fault Zone”, CTFZ). Structural control of sedimentation by this structure is indicated by the geometry of the unconformity-bounded stratigraphic units as well as by the deformation of the basin fill. En-echelon N–S-trending thrusts, N- to NE-trending dextral transpressional faults, and NW- to NNW-striking folds affecting the substratum, constitute the surface response to lateral displacement along the CTFZ. The S. Arcangelo Basin is bounded to the north by a deep-seated, roughly WNW/NW-trending, sinistral transpressional structure (“Valsinni Lateral Ramp”, VLR) that terminates northwestward against the CTFZ. At the surface, the VLR is dominated by back thrusts and by transpressional fault zones. The VLR has been interpreted as the sinistral lateral ramp of the large-scale Valsinni thrust anticline and it is inferred to have strongly controlled local deposition within the S. Arcangelo Basin. The deep-seated structures (CTFZ and VLR) probably represent reactivated pre-existing normal and transfer faults affecting the rigid Apulian Platform that were generated during passive margin evolution, as well as during the flexural bending of the Apulian lithosphere. During the Pliocene–Pleistocene, these structures were reactivated as lateral ramps accommodating the thrusting within the Apulian Platform. Transpression along the lateral ramps in the Apulian Platform propagated upwards through the nappe pile, giving rise to a complex surface structural pattern and the local development of flower-like structures.

Listric thrusts in the western Transverse Ranges, California

Research Article| July 01, 2000 Listric thrusts in the western Transverse Ranges, California Leonardo Seeber; Leonardo Seeber 1Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar Christopher C. Sorlien Christopher C. Sorlien 2Institute for Crustal Studies, University of California, Santa Barbara, California 93106, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (7): 1067–1079. https://doi.org/10.1130/0016-7606(2000)112<1067:LTITWT>2.0.CO;2 Article history received: 02 Feb 1997 rev-recd: 28 Sep 1998 accepted: 21 Jul 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Leonardo Seeber, Christopher C. Sorlien; Listric thrusts in the western Transverse Ranges, California. GSA Bulletin 2000;; 112 (7): 1067–1079. doi: https://doi.org/10.1130/0016-7606(2000)112<1067:LTITWT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Some of the main faults accommodating current shortening in the western Transverse Ranges are probably listric because (1) they are associated with progressive tilting, and (2) they may be preexisting normal faults that accommodated Miocene extension. These faults have been reactivated in the Pliocene–Quaternary transpressive regime. We propose a listric thrust model where slip is proportional to backlimb dip. This model requires relatively little fault slip to account for progressive tilting and for wide (in the dip direction) and gently dipping backlimbs. In contrast, widely applied fault-bend fold and fault-propagation fold models relate fault slip to limb width alone and typically predict more shortening by the blind thrusts that can be accounted for by folding in the cover above them. We trace the southernmost structural high in the Transverse Ranges from the Santa Monica Mountains through the southern Santa Barbara Channel. The north-dipping backlimb of this anticline is 20–30 km wide and 220 km long; its presence suggests a very large north-dipping thrust that could generate very large earthquakes. The slip rate for this fault, however, is substantially lower for a listric thrust model than for a single-step ramp-flat model. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Listric thrusts in the western Transverse Ranges, California

Some of the main faults accommodating current shortening in the western Transverse Ranges are probably listric because (1) they are associated with progressive tilting, and (2) they may be preexisting normal faults that accommodated Miocene extension. These faults have been reactivated in the Pliocene–Quaternary transpressive regime. We propose a listric thrust model where slip is proportional to backlimb dip. This model requires relatively little fault slip to account for progressive tilting and for wide (in the dip direction) and gently dipping backlimbs. In contrast, widely applied fault-bend fold and fault-propagation fold models relate fault slip to limb width alone and typically predict more shortening by the blind thrusts that can be accounted for by folding in the cover above them. We trace the southernmost structural high in the Transverse Ranges from the Santa Monica Mountains through the southern Santa Barbara Channel. The north-dipping backlimb of this anticline is 20–30 km wide and 220 km long; its presence suggests a very large north-dipping thrust that could generate very large earthquakes. The slip rate for this fault, however, is substantially lower for a listric thrust model than for a single-step ramp-flat model.

Modelling hanging wall accommodation above rigid thrust ramps

Experimental models are used to study the role of material rheology in hanging wall accommodation above rigid flat–ramp–flat thrust footwalls. The deformation in the hanging wall was accomplished by forwards sliding along a rigid basal staircase trajectory with a variable ramp angle, α, ranging from 15° to 60°. We model different ramp angles to examine hanging wall accommodation styles above thrust ramps of overthrust faults ( α ranging from 15° to 30°), as well as above pre-existing normal faults ( α ranging from 45° to 60°). For the hanging walls we used stratified frictional (sand) and viscous (silicone putty) materials. In this paper we study three types of models. Type 1 models represent purely frictional hanging walls where accommodation above thrust ramps was by layer-parallel thickening and by generating a series of back thrusts. Type 2 and 3 models represent stratified frictional/viscous hanging walls. In these models, accommodation was by a complex association of reverse and normal faults, mainly controlled by the rheological anisotropy as well as by the ramp inclination angle α. In Type 2 models the silicone covered only the lower flat, while in Type 3 models it also covered the rigid ramp. For α≤30° in Type 2 models and α≤45° in Type 3 models, the viscous layer inhibited the development of back thrusts in the frictional hanging wall, instead the silicone thickened to develop a ‘ductile ramp’. For α-values higher than 30° in Type 2 models and α=45° in Type 3 models, back thrusts develop in response to the bulk compression. The experiments simulate many structures observed above natural thrust ramps with α≤30° and pre-existing normal faults with α≥45°. The models emphasise the importance of a basal ductile layer, which allows the hanging wall to step-up over the rigid ramp by building up its own ductile ramp. The models also emphasise that foreland-directed normal faulting can develop at a thrust front in the case that the vertical stress due to gravity exceeds the horizontal stress due to end-loading within a thrust wedge.

Quaternary faults and seismicity in the Umbro-Marchean Apennines (Central Italy): evidence from the 1997 Colfiorito earthquake

Analyses of structural and geomorphological data combined with remote sensing interpretation confirm previous knowledge on the existence of an extensional Quaternary tectonic regime in the Colfiorito area (Umbro-Marchean Central Apennines). This is characterized by a maximum principal axis of finite strain oriented approx. NE–SW, which is the result of a progressive deformation process due to pure and radial extension. Surface geological data, the crustal tectonic setting (reconstructed using a CROP 03 seismic reflection profile), and seismological data relative to the autumn 1997 Colfiorito earthquake sequence constrain the following seismotectonic model. We interpret the seismogenic SW-dipping low-angle normal fault pictured by seismic data as an inverted thrust ramp located in the basement at depth between 5 and 10 km. The surface projection of this seismogenic structure defines a crustal box within which high-angle normal faults are responsible for the deformation of the uppermost crust. The regional patterns of pre-existing basement thrusts therefore control the seismotectonic zoning of the area that cannot be directly related to the high-angle normal fault systems which cut through different crustal boxes; the latter system records, in fact, re-shear along pre-existing normal faults. Moreover, Quaternary slip-rates relative to high-angle normal faults in the Central Apennines are closely related to seismic hazard within each crustal box.