Basal Thrust Research Articles

The continent-ocean (Seve-Köli) boundary in the Sarek-Padjelanta Mts. revisited: Swedish Caledonides

ABSTRACT In the Swedish Caledonides, the continent-ocean boundary is represented by the basal thrust of the Iapetus-derived Köli Nappe Complex (KNC) which overlies the Seve Nappe Complex (SNC) derived from the outer rifted margin of Baltica and the continent-ocean transition (COT). Current maps and literature subdivide the Seve teconostratigraphy in northern Sweden (Sarek-Padjelanta region) into three mega-lenses, the lowest of which (Vaimok lens) is composed of quartz-feldspathic gneisses and subordinate marble, all hosting eclogites. It is overlain by the Sarek lens composed of a COT dyke complex and, along its western margin, a narrow belt of phyllites, schists, quartzite, marbles and greenschists. Overlying this belt is Tsäkkok lens of the same composition as the Vaimok lens. The situation with an intermediate-pressure and often excellently preserved sheeted-dyke complex intercalated between two lenses of complexly deformed high-pressure rocks invited to intricate models of accretion, including out-of-sequence stacking of slices of the continental margin during early Ordovician subduction. Based on a detailed study of a select area and examination of the evolution of previous concepts, this study reinterprets the narrow belt along the western margin of the Sarek Lens as typical of the lower KNC. The favoured explanation of the presence of intermediate-pressure Köli rocks below high-pressure Seve rocks is recumbent folding and/or mega-sheath folding of the Seve-Köli nappe sequence during Scandian (Siluro-Devonian) accretion, by analogy with the situation in places in the central Swedish Caledonides. Previous models of early out-of-sequence stacking during subduction of the continental margin need reconsideration.

Mesozoic intraplate deformation in the southern part of the Central European Basin - Results from large-scale 3D modelling

The Central European Basin is an intracontinental basin that initially formed due to long-lasting thermal subsidence accompanied with several tectonic phases of extension during Mesozoic times. Locally, thick evaporites were incorporated in the deformation and led to the formation of detached structures, salt pillows and diapirs. At the end of the Late Cretaceous, the formation of (sub-) basins and (half-) grabens became interrupted by a short-term event of contraction. Especially along the northern and southern edges of the basin, lithospheric shortening resulted in basin inversion and the uplift of large basement blocks. Hence, the present day's structural framework shows a complex pattern of kinematically and chronologically variable structures. Herein, we unravelled these structures for the Altmark region in northern Saxony-Anhalt (Central Germany). We used regional depth maps, reflection seismics and borehole data analysed by use of large-scale subsurface mapping and 3D modelling techniques to completely re-evaluate the structures in the Late Palaeozoic to Cenozoic sedimentary basin succession. Our results show a high variability of tectonic structures: narrow and distributed, in parts reactivated normal fault zones, huge inverted basins, thin-skinned thrust faults, detachment folds and large basement thrusts. The style of deformation significantly changed during the region's evolution. This is indicated by particular structural, kinematic, thermal and rheological conditions, which existed for the individual phases of basin formation and inversion and probably persist until today.

Angara–Selenga Imbricate Fan Thrust System

Abstract —We study Late Jurassic thrusting of the Archean craton basement over Jurassic sediments in Siberia, with the Khamar-Daban terrane as a rigid indenter. The study focuses on deformation and secondary mineralization in Archean and Mesozoic rocks along the thrusting front and the large-scale paleotectonic thrust structure. The pioneering results include the inference that the Angara, Posol’skaya, and Tataurovo thrusts are elements of the Angara–Selenga imbricate fan thrust system and a 3D model of its Angara branch. The history of the Angara–Selenga thrust system consists of three main stages: (I) detachment and folding of the basement under the Jurassic basin and low-angle synclinal and anticlinal folding in the sediments in a setting of weak compression; (II) brecciation and mylonization under increasing shear stress that split the Sharyzhalgai basement inlier into several blocks moving in different directions; formation of an imbricate fan system of thrust sheets that shaped up the thrusting front geometry, with a greater amount of thrusting in the front because of the counter-clockwise rotation of the Sharyzhalgai uplift; (III) strike-slip and normal faulting associated with the origin and evolution of the Baikal rift system, which complicated the morphology of the thrust system.

Topographic and Tectonic Evolution of Mountain Belts Controlled by Salt Thickness and Rift Architecture

AbstractThe role of the heterogeneous rheological architecture of rifted margins in the building of mountain belts has challenged our view of how a collisional orogen formed. We show, using two‐dimensional numerical experiments of collision built by the inversion of rifted margins, that a weak pre‐extensional evaporitic layer delays the growth of topography and the onset of syn‐orogenic sedimentary record in foreland basins. In tectonic models lacking a weak décollement layer, the orogen grows by progressive accretion of basement thrusts. With a 2‐km‐thick salt layer, the orogen develops an antiformal stack in the deep crust that is kinematically connected to a shallow thin‐skin thrust belt in the foreland as observed in many orogens. A 5‐km‐thick weak layer leads to the quasi‐absence of topography and widespread salt tectonics and obliterates thrusts propagation while promoting crust and mantle underthrusting. During convergence, the preservation of a marine seaway emphasizes that collision may involve a significant period of submarine continental accretion, which duration is controlled by the thickness of the weak décollement layer. From these experiments we infer that the thicker the salt layer, the longer the delay between the onset of far‐field shortening and the formation of the orogen. Our study explains first‐order features recognized in many orogens controlled by variable salt thickness and extension.

Geometry and development of a hybrid thrust belt in an inner forearc setting: Insights from the Potrerillos Belt in the Central Andes, northern Chile

Thick- and thin-skinned “hybrid” thrust belts are typically developed in areas where crustal shortening was preceded by extension and normal faulting. They have been recognized in many places within the Central Andean backarc regions of Perú, Argentina, and Bolivia. In contrast, these structures have not been broadly studied in forearc regions. To understand the geometry and kinematics of this style of tectonism in a forearc setting related to Andean-type subduction zones, we used field and industrial 2D seismic data to study the Potrerillos thrust and fold belt and the Salar de Pedernales Basin in northern Chile (26°S). Our results indicate that the structure in this region consists of a dominantly east-verging contracting system composed of large basement thrusts, Mesozoic inverted normal faults, and shallow thrust-related folds. Large basement thrust ramps and imbricated wedges represent the most prominent basement structures, which accommodated major crustal shortening. Reactivated and tectonically inverted Mesozoic extensional structures also constitute an important structural component, especially beneath the Salar de Pedernales Basin. These exhibit asymmetrical inverted anticlines with arrowhead shapes on the hanging walls of the inverted normal faults. The subsidiary shallow thrust-related folds commonly display fault bend and propagation folds that primarily accommodated the shortening of the Mesozoic and Cenozoic stratified deposits overlying the basement thrusts and cored anticlines. The age of the contraction in this region is poorly constrained; however, many of the oldest sequences accumulated in the top of the syn-rift sequences and related to synorogenic deposits, can be correlated with those identified in neighboring regions (e.g., Salar de Atacama, Salar de Punta Negra basins and Frontal Cordillera, among others), which have reported Upper Cretaceous ages. Based on this stratigraphic correlation we suggest that orogenesis may have begun in the Late Cretaceous with basin inversion, and then continued during the Cenozoic with basement thrusting.

The Decollement Depth of Active Thrust Faults in Italy: Implications on Potential Earthquake Magnitude

AbstractThrust fault ruptures during earthquakes do not often propagate down to the brittle‐ductile transition. Lithological variations control the behavior and depth of regional basal thrusts and decollement planes. Thrust fronts may be discontinuous along strike, limiting the dimension of single coseismic ruptures. These factors control the maximum expected magnitude in one region. This is the case of Italy where the convergence of few millimeter per year in the Apennines accretionary prism and along the retrobelt of the Alps generates compressional earthquakes with moderate to strong magnitudes. Here, using geological and geophysical data, we first compile a map of the undulated active basal thrust decollement for Italy that occurs from 1 to 17‐km depth. Then, we verify the relationship between the length of seismogenic ruptures in thrust faults (Lf) and the maximum depth of thrust faulting (zmax) of related earthquakes and find that their ratio (Lf/zmax) ranges between 2 and 4. Finally, we compute the potential seismogenic volume and estimate the maximum magnitude using an empirical relationship that multiplies the decollement depth and the Lf/zmaxratio. Maximum calculated magnitude is 6.7 ± 0.37 (depending on Lf/zmaxand fault dip angle), consistent with the largest magnitude of thrust‐related earthquakes recorded in Italy (6.5–7.0). Lower magnitudes are predicted in the Ionian Seas at the external front of the Apennines where smaller crustal volumes are involved, whereas higher magnitudes are expected in the southern Po Basin, the western Adriatic Sea, Sicily offshore, and the Southern Alps where the decollement is deeper and the brittle volumes are far greater.

Carbonate ramp drowning caused by flexural subsidence: The South Pyrenean middle Eocene foreland basin

A detailed sedimentological and magnetostratigraphic analysis of three selected sections of the Eocene carbonate platforms of the South Pyrenean Jaca-Pamplona foreland basin has been carried out in the Aragonese External Sierras (Huesca, Spain). The stratigraphic record spans from the middle-late Lutetian (Chron 20r, SBZ15) to the Bartonian (Chron 18r, SBZ17). The Lutetian-Bartonian transition was characterized by a major paleogeographic change, from a turbiditic foredeep with associated peripheral carbonate ramps to deltaic sedimentation. In most of the outcrops of the External Sierras, the peripheral carbonate platforms (Guara Formation) are overlained by prodelta and outer ramp marls (Arguis Formation). The drowning unconformity between these two formations in the outcrops located in the westernmost tip of the External Sierras is documented herein for the first time. In this area, the Guara Formation is overlain by glauconitic limestones deposited in a carbonate ramp setting. Between these units a characteristic unconformity is observed, with hard-ground formation and local evidence of subaerial exposure in the shallowest domains of the study area. The abundant glauconite found on top of this unconformity records a starvation period in the basin, followed by the development of a new carbonate ramp system during Bartonian times (Santo Domingo Member of the Arguis Formation). Magnetostratigraphic and sedimentological data show that tectonic tilting occurred in latest Lutetian times, prior to the drowning process. This suggests the influence of the flexural subsidence in the cratonic margin of the foreland as a result of the advance of a basement thrust in the hinterland. Nevertheless, the correlation of the uppermost Lutetian drowning unconformity with a basinwide regional sea level rise, which may have further been global, suggests an interplay between tectonic and eustatic processes in the drowning of the late Lutetian South Pyrenean carbonate ramps. In this work we point out a narrowing of the carbonate ramp, together with a relative sea level rise, as the processes responsible for the drowning of the Upper Guara Mb carbonate ramp. These processes could be responsible for the development of drowning unconformities in cratonic margins of other foreland basins. As carbonate ramps have a high sediment production potential, the narrowing of the carbonate factory could provide the conditions for the drowning of the carbonate ramp system in a subsequent sea level rise.

Initiation and development of the Pennine Basal Thrust (Swiss Alps): a structural and geochronological study of an exhumed megathrust

The Pennine Basal Thrust (PBT) is an exhumed megathrust developed during continental collision from late Eocene to Miocene. To trace its evolution, five samples, with indications for up to three microstructurally diachronous white-mica generations, were investigated by laser in-situ and step-heating 40Ar–39Ar dating. Three deformation-related crystallization ages can be distinguished: (1) D1, characterized in the PBT hanging wall by an S1 foliation defined by white mica + chloritoid, began at or before ∼38.0 Ma; (2) D2 formed a pervasive S2 cleavage and synchronous white-mica rich veins dated at ∼27 Ma; (3) D3 produced an S3 crenulation cleavage and chlorite + white-mica veins dated at ∼23 Ma. Older ages of ∼96 Ma (footwall) and ∼115 Ma (hanging wall) are interpreted as minimum ages for the detrital component. Finally, discrete faulting produced fault gouge, with an illite K–Ar age of ∼19 Ma. A simplified back-restored reconstruction provides a tectonic context for the dated structures. In this framework, D1 occurred during middle to late Eocene tectonic accretion. After late Eocene initiation of continental collision, D2 reflects Oligocene top-to-NW shearing, with both in- and out-of-sequence thrusting. D3 then developed from 23 to 19 Ma during the progressive deactivation of the PBT.

Kinematic, flexural, and thermal modelling in the Central Andes: Unravelling age and signal of deformation, exhumation, and uplift

Quantifying age, rate, and lateral variation of deformation and exhumation in convergent systems relies on integration of geologic map patterns, ag, and locations of reset thermochronometer systems, and synorogenic sediment distribution. The central Bolivian Andes provide an ideal location to examine the influence of variations in shortening and stratigraphic architecture on the structural evolution of the mountain range due to differential age and rates of shortening and distinct sedimentary basin geometries along strike. To quantify age and rate of shortening, we link thermokinematic modelling of sequentially deformed, forward-modelled, balanced cross-sections to synorogenic and thermochronologic histories. The preserved basin history in the Altiplano and Eastern Cordillera argues for an early fold-and-thrust belt located in the now-Western Cordillera, with subsequent propagation of shortening eastward around 40–50 Ma. Flexural modelling incorporating isostasy and erosion requires multiple basement thrust sheets with 35–97 km of displacement. A temporally evolving effective elastic thickness, as well as imposed subsidence in the foreland and uplift in the hinterland, is required to reproduce the surface geology, increase Subandean foreland basin depth, limit Altiplano sedimentation, and facilitate Altiplano uplift to modern elevation. Thermokinematic modelling is compatible with initiation of deformation at 50–40 Ma in a marked increase in Subandean velocities from ~5.5 to 8–10 mm/yr from ~12–10 Ma to present. Out-of-sequence thrusting at the westernmost limit of the Subandes is required to match measured young and partially reset zircon helium ages. Out-of-sequence faulting is supported by high Ksn values, indicative of active uplift, and was likely promoted by the abrupt eastern edge of the Paleozoic basin rocks, which limited forward propagation of structures, and/or increased erosion due to focused precipitation. Our results highlight the importance of incorporating detailed structural modelling in differentiating the geometry, kinematics, and timing of deformation to reproduce thermochronologic ages and basin histories.

Active bidirectional tectonic-tilting in a part of the Almora Klippe, Kumaun Lesser Himalaya, India: Insights from statistical analyses of geomorphic indices

The NW-SE trending, synformal Almora Klippe in central Kumaun Lesser Himalaya is actively uplifting along the basal thrust, which is known as North Almora Thrust (NAT) and South Almora Thrust (SAT) along the northern and southern flanks, respectively. Geomorphic indices of ‘Basin Asymmetry Factor (AF)’ and ‘Transverse Topography Factor (T)’ of 77 drainage basins of third-order streams developed on this tectonic block have been computed to decipher active tilt-block tectonics of a part of this vast klippe. Statistical analyses of the basins' geomorphic indices revealed differential, gentle to steep, bidirectional tilting of the Almora Klippe vis-à-vis lent insight into correct perspective which these must be used for such studies. The tectonic block of Almora Klippe is partitioned into two minor tectonic blocks along the hinge of the synform. Differential movements along the basal thrust of the klippe have caused westward down-tilting of the northern fold flank (‘AF’ having χ2 value of 7.556 for 4 azimuth classes at 90% confidence level, and ‘T’ having vector mean (θ¯v) of 260° and vector magnitude (r) of 0.25 at p-value of 0.1154), and eastward down-tilting of the southern fold flank (‘AF’ having χ2 value of 13.73 for 4 azimuth classes at 90% confidence level, and ‘T’ having vector mean (θ¯v) of 56° and vector magnitude (r) of 0.26 at p-value of 0.0585). The westward down-tilting of the northern fold flank is due to its faster uplift in the east, along the NAT. Similarly, the eastward down-tilting of the southern fold flank is due to its faster uplift in the west, along the SAT.

Dating the northern deposits of the Ebro foreland basin; implications for the kinematics of the SW Pyrenean front

Dating the sedimentary infill of the northern margin of the Ebro foreland basin informs about the orogen-basin evolution. A magnetostratigraphic section of ~5 km-thick combines new magnetostratigraphic results from the syntectonic alluvial Uncastillo Fm (Upper Oligocene-Lower Miocene, 1 km-thick, the Fuencalderas section) with previous (3.3 km-thick) and new (0.7 km-thick) magnetostratigraphic data from the underlying fluvial Campodarbe Fm (Luesia and San Marzal sections, Eocene-Oligocene). This composite section allows dating the entire basin infill in this sector, therefore bracketing the timing of the deformation in the southern margin of the Pyrenees after considering previous kinematic studies. Deformation recorded by the continental deposits spans from syn-Gavarnie nappe activity (Broto and Fiscal basement thrust sheets) from 31.3 to 24.55 Ma, Rupelian-Chattian (Oligocene) to the syn-Guarga thrust activity, from 24.55 to 21.2 Ma Chattian-Aquitanian (Oligocene-Miocene). The Accumulation rates vary from ca. 22 to ca. 39 cm/kyr between the genetic stratigraphic units in the Uncastillo Fm through the section. These rates are close to those of the underlying deposits of the Campodarbe Fm (average of ca. 36 cm/kyr). Deformation for the latest Pyrenean front (syn-Guarga thrust) is younger than previously assigned in ~5.4. ~1.5 and ~1.1 Myr of the beginning of the Punta Común thrust sheet (from 29.4 Ma to 24 Ma), Lower Riglos thrust sheet system (from 24 to 22.5 Ma) and the Upper Riglos thrust system (22.5 to 21.4 Ma) respectively. Other significant changes in accumulation rate and its derivative also indicate variations in the tectonic activity ca. 33 and 28 Ma.

Control of increased sedimentation on orogenic fold-and-thrust belt structure – insights into the evolution of the Western Alps

Abstract. We use two-dimensional thermomechanical models to investigate the potential role of rapid filling of foreland basins in the development of orogenic foreland fold-and-thrust belts. We focus on the extensively studied example of the Western European Alps, where a sudden increase in foreland sedimentation rate during the mid-Oligocene is well documented. Our model results indicate that such an increase in sedimentation rate will temporarily disrupt the formation of an otherwise regular, outward-propagating basement thrust-sheet sequence. The frontal basement thrust active at the time of a sudden increase in sedimentation rate remains active for a longer time and accommodates more shortening than the previous thrusts. As the propagation of deformation into the foreland fold-and-thrust belt is strongly connected to basement deformation, this transient phase appears as a period of slow migration of the distal edge of foreland deformation. The predicted pattern of foreland-basin and basement thrust-front propagation is strikingly similar to that observed in the North Alpine Foreland Basin and provides an explanation for the coeval mid-Oligocene filling of the Swiss Molasse Basin, due to increased sediment input from the Alpine orogen, and a marked decrease in thrust-front propagation rate. We also compare our results to predictions from critical-taper theory, and we conclude that they are broadly consistent even though critical-taper theory cannot be used to predict the timing and location of the formation of new basement thrusts when sedimentation is included. The evolution scenario explored here is common in orogenic foreland basins; hence, our results have broad implications for orogenic belts other than the Western Alps.

3D structure of subsurface thrusts in the eastern Jaca Basin, southern Pyrenees

This paper presents a new model of the subsurface structure of the eastern Jaca flexural basin of the west-central southern Pyrenees, by means of subsurface structural maps and four new balanced cross-sections. The study is based on the interpretation of a set of publicly available seismic reflection profiles tied to deep exploration well logs, which constitute a unique database in the southern Pyrenees associated to the gas discovery of the Serrablo field. Investigation of the deep basin structure highlights strong mechanical-stratigraphic contrasts between basement, a competent Upper Cretaceous-Eocene carbonate sequence in the deep basin and a weak infill of Eocene to lower Miocene synorogenic clastic deposits. These contrasts promote the occurrence of various decollement levels and a decoupled style of deformation between intervals of different competence. A contour map for the top of basement reveals a complex structure with lateral variations of the number of thrusts and the displacement on these and local transverse elements. Between the Gavarnie thrust at the southern edge of the Axial Zone and the Guarga thrust at the leading edge of the basement thrust system, three main other basement thrusts are defined below the north-eastern Jaca Basin, from South to North the Fiscal, Yesero and Broto thrusts. In the Meso-Cenozoic sedimentary cover, two low-angle thrusts are mapped in the subsurface across the Upper Cretaceous-Eocene carbonates: i) the deep Oturia thrust, connected upsection to the emerging Oturia thrust known at the surface, and ii) the deep Jaca thrust, drilled by the Serrablo wells, and connected to the emerging Jaca thrust and Yebra de Basa anticline through a zone of disharmonic deformation. The deduced subsurface geometrical relationships are consistent with the connection of the Gavarnie and Broto basement thrusts to the Priabonian-Rupelian Oturia and Jaca thrusts while the younger Yesero, Fiscal and Guarga basement thrusts emerge at the South Pyrenean thrust front of the Sierras Exteriores, active until the early Miocene. This study highlights the complex structural pattern that characterizes the deep structure of the South Pyrenean basin and the role of disharmonic deformation that challenges the resolution of the deeper thrust system without the help of seismic profiles.

Uplift analysis on a pericratonic region: An example in the Sierras de Córdoba (29°–34°S), Argentina

The Sierras de Córdoba (Argentina) is a mountain belt located in the most distal (pericratonic) part of the South-Central Andean foreland, in the easternmost end of the Sierras Pampeanas (Pampean ranges) broken foreland. Although this region is considered an excellent example for settings where basement thrusting generates relief and intermontane basin systems, hundreds of kilometers away from the Cordilleran front, the driving mechanisms and main controls on topography are still poorly understood. Given that the landscape, and particularly the rivers, react to changes in climate and tectonics on timescales between 103 and 106 yrs, we used DEM-based morphometry (elevation patterns, local relief, and analysis of 242 longitudinal stream profiles) to evaluate the main controls on modern topography. We focus on the morphologies of the main streams and their tributaries using metrics of the channel concavity and normalized channel steepness index (ksn). The distribution of local relief, concavity and ksn values was compared with the present-day topography, tectonic structures, rock-type and modern rainfall patterns. The stream longitudinal profiles show homogeneous concavities and ksn background values, with some local anomalies. Local structures (east and west vergent thrusts) in the Sierras de Córdoba seem to be the most fundamental controls on the formation of these anomalies, slope-break knickpoints as well as deep incision (high local relief). This suggests localized and accelerated rock uplift rates, which produce localized increases in the gradient along the longitudinal stream profile. It is important to notice that the highest ksn values were reported on the eastern range flanks, coincident with the highest values of mean annual rainfall. However, high ksn values appear to be maintained even with greater rainfall. Our study provides new insights on the Neogene Sierra de Córdoba uplift evolution, which will assist future sampling and modelling (cosmogenic nuclides analysis, thermochronology and GPS analysis).

Tectonic interpretation of the Raniya earthquake, Kurdistan, northern Iraq

The 23 August 2017 earthquake was the biggest recorded event in the Raniya region near the northeastern boundary of the Arabian plate in the Kurdistan region of Iraq over the last 37 years. It was felt in the city of Raniya and its surrounding areas in Erbil and Sulaimaniyah governorates. The Raniya earthquake was recorded by the USGS National Earthquake Information Centre with a magnitude of 5.1 (Mww). The rotational optimization method (Windows-Tensor software, version 5.8.8) was used to determine the present-day stress field by inversion of the fault plane solution. The results show that the Raniya earthquake was generated by the neotectonic reactivation of Late Precambrian northeast-dipping blind basement thrust fault, with a small amount of strike–slip motion, at a focal depth of 8 km. This indicates that the Raniya region is subjected to a present-day compressional deformation, with a northeast–southwest-oriented maximum horizontal stress axis (N47° E). The neotectonic activity along the Raniya basement thrust fault is in good agreement with the active shortening across the Zagros fold–thrust belt due to the Mid Miocene continental collision between the Arabian and Eurasian plates. Finally, a detailed seismic risk assessment is recommended for the Raniya region and surrounding areas.

The Wilson Cycle and Effects of Tectonic Structural Inheritance on Rifted Passive Margin Formation

AbstractThe parallelism between older collisional belts and younger rift systems is widely known and particularly well portrayed along the Atlantic Ocean. How tectonic inherited and new‐formed shear zones control rift nucleation and the final architecture of rifted conjugate passive margins is still poorly understood. Here we present lithospheric‐scale thermo‐mechanical numerical models that self‐consistently create extensional and contractional tectonic inheritance, where prior extension and contraction are systematically varied. Our results show that (1) initial reactivation occurs along the former lithospheric suture zones; (2) upper crustal thick‐skinned basement thrusts are partially or fully reactivated depending on the amount of prior contraction and size of the orogen; (3) with a small amount of contraction, thick‐skinned thrusts are efficiently reactivated in extension and provide the template for rifted margin formation; (4) with larger amounts of contraction, thick‐skinned thrusts distal to the lithospheric suture zone do not reactivate in extension; and (5) reactivation of prior contractional shear zones dominates during the early stages of rifting, while during the final stage of margin formation new‐formed extensional shear zones dominate. Force balance analysis predicts an inverse relation between midcrustal viscosity and the maximum offset for reactivation of weak upper crustal structures. Force balance also predicts that the degree of weakening or healing of the weak suture and the thermal thinning of the necking area control at which stage suture reactivation is deactivated and extension proceeds by mantle lithosphere thermal necking. Two rifted conjugate margins with orogenic inheritance in the North and South Atlantic are used for comparison.

Influence of Overlapping décollements, Syntectonic Sedimentation, and Structural Inheritance in the Evolution of a Contractional System: The Central Kuqa Fold‐and‐Thrust Belt (Tian Shan Mountains, NW China)

AbstractContractional deformation in the Kuqa fold‐and‐thrust belt (southern foreland of the Tian Shan Mountains, NW China) is recorded by well‐preserved syntectonic continental sequences. In addition, its structural evolution was strongly controlled by synorogenic salt (Eocene in age) and presalt décollements with varying spatial distribution. We present a balanced and sequentially restored cross section across the central part of this fold‐and‐thrust belt that provides a new interpretation of the structure beneath the evaporites, in which Paleozoic and Mesozoic strata are deformed by a thrust stack involving (i) a thin‐skinned thrust system detached on Triassic‐Jurassic coal units and (ii) an ensemble of south‐directed basement thrusts. The latter formed from the inversion of Mesozoic extensional faults such as those preserved both in the Tarim foreland basin and beneath the frontal part of the Kuqa fold‐and‐thrust belt. The constructed section shows a total shortening of 35 km from the Late Cretaceous to the present. The restoration depicts a three‐stage evolution for the Kuqa fold‐and‐thrust belt: (i) minor Mesozoic extension, (ii) an early compressional stage (Late Cretaceous to early Miocene) with low shortening and syntectonic sedimentary rates, and (iii) a later compressional stage (late Pliocene‐Pleistocene) characterized by a greater and progressively increasing shortening rate and rapid deposition. Our results are discussed in light of previous analogue and numerical modeling studies and demonstrate the control exerted by the interplay between syntectonic sedimentation, the inversion of inherited basement structures, and the nature and extent of Triassic/Jurassic and Eocene décollements.

Fault activity, tectonic segmentation, and deformation pattern of the western Himalaya on Ma timescales inferred from landscape morphology

The location and magnitude of Himalayan tectonic activity has been debated for decades, and several aspects remain unknown. For instance, the spatial distribution of crustal shortening that ultimately sustains Himalayan topography and the activity of major fault zones remain unknown at Ma timescales. In this study, we address the spatial deformation pattern in the data-scarce western Himalaya. We calculated catchment averaged, normalized river-steepness indices of non-glaciated drainage basins with tributary catchment areas between 5 and 200 km(2) (n = 2138). We analyzed the spatial distribution of the relative change of river steepness both along and across strike to gain information about the regional distribution of differential uplift pattern and relate this to the activity of distinctive fault segments. For our study area, we observe a positive correlation of averaged k(sn) values with long-term exhumation rates derived from previously published thermochronologic datasets combined with thermal modeling as well as with millennial timescale denudation rates based on cosmogenic nuclide dating. Our results indicate three tectono-geomorphic segments with distinctive landscape morphology, structural architecture, and fault geometry along the western Himalaya: Garhwal-Sutlej, Chamba, and Kashmir Himalaya (from east to west). Moreover, our data recognize distinctive fault segments showing varying thrust activity along strike of the Main Frontal Thrust, the Main Boundary Thrust, and in the vicinity of the steep topographic transition between the Lesser and Greater Himalaya. In this region, we relate out-of-sequence deformation along major basement thrust ramps, such as the Munsiari Thrust with deformation along a mid-crustal ramp along the basal decollement. We suggest that during the Quaternary, all major fault zones in the Western Himalaya experienced out-of-sequence faulting and have accommodated some portion of crustal shortening.

Neogene 3‐D Structural Architecture of The North‐West Apennines: The Role of the Low‐Angle Normal Faults and Basement Thrusts

AbstractThe northern Apennines of Italy are a classical site for studying fundamental issues in tectonic studies, such as ophiolite formation and emplacement, kinematics of thrust wedges, role of in‐sequence and out‐of‐sequence thrusting, and of along strike segmentation, synorogenic versus postorogenic extension, and interplay between tectonics, erosion, and sedimentation. Accordingly, the northern Apennines have been extensively studied since more than two centuries ago. Despite the huge amount of available data with different resolution, a 3‐D comprehensive regional view combining in a modern framework all available surface and subsurface information for contiguous sectors of the chain is still lacking. We performed such an attempt in the area framed between the Taro valley to the north and the northern termination of the Alpi Apuane to the south. The region includes the main morphostructural zones of the north‐west Apennines from the Tyrrhenian coast (SSW of La Spezia), through the main topographic divide of the Apennines, up to the foothills range of the chain to the north. The area has been investigated through a multidisciplinary approach that integrated surface geological data, collected during the last two decades of structural and stratigraphic field works, and subsurface geological data. The construction of two regional NE‐SW trending cross sections (the Levanto‐Pontremoli‐Parma to the north and the La Spezia‐ Sarzana‐North Apuane‐Cerreto to the south), intersected by a NW‐SE trending Taro‐Lunigiana‐Alpi Apuane composite section, allowed us to illustrate (i) the role of out‐of‐sequence blind thrusting in the basement and (ii) the presence of low‐angle normal faulting and its relationships with recent to active high‐angle normal faulting. Both extensional and contractional systems have relevant implications for the tectonics of the northern Apennines as well as the seismotectonics of the studied region.

The Structure of Southwest Taiwan: The Development of a Fold‐and‐Thrust Belt on a Margins Outer Shelf and Slope

AbstractThe southwest Taiwan fold‐and‐thrust belt is forming on the outer shelf and slope of the Eurasian continental margin. It comprises a roughly N‐S striking, west verging imbricate thrust system that has been developing since the Late Miocene. Here we present the results of new surface geological mapping from which we construct balanced and restored cross sections and along‐strike sections. From these we compile maps of the basal thrust, thrust branch lines and, where possible, stratigraphic cutoffs. To interpret the structure in the subsurface and beneath the basal thrust, we use a P wave velocity of 5.2 km/s as a proxy for the top of the Mesozoic basement. We divide the southwest Taiwan fold‐and‐thrust belt into a number of thrust sheets that form the basis of our description and interpretations. From these data we interpret the 3‐D structure of the fold‐and‐thrust belt and the influence that the structure and morphology of the continental margin is having on its development. We show that there is a significant along‐strike change in the structure. This change takes place across a transverse zone that is composed of a suite of structures at the surface. We suggest that this transverse zone has a causal relationship with variations in the geometry of the basal thrust which in turn is related to (possibly fault bounded) basement highs and lows that are inherited from the continental margin.