Foreland Fold Research Articles

Thick-skinned tectonics and basement-involved fold–thrust belts: insights from selected Cenozoic orogens

AbstractDefining the structural style of fold–thrust belts and understanding the controlling factors are necessary steps towards prediction of their long-term and short-term dynamics, including seismic hazard, and to assess their potential in terms of hydrocarbon exploration. While the thin-skinned structural style has long been a fashionable view for outer parts of orogens worldwide, a wealth of new geological and geophysical studies has pointed out that a description in terms of thick-skinned deformation is, in many cases, more appropriate. This paper aims at providing a review of what we know about basement-involved shortening in foreland fold–thrust belts on the basis of the examination of selected Cenozoic orogens. After describing how structural interpretations of fold–thrust belts have evolved through time, this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics, and emphasizes the key control exerted by lithosphere rheology, including structural and thermal inheritance, and local/regional boundary conditions on the occurrence of thick-skinned tectonics in the outer parts of orogens.

Paleofluid Evolution In Fault-Damage Zones: Evidence From Fault–Fold Interaction Events In the Jabal Qusaybah Anticline (Adam Foothills, North Oman)

Abstract Fault zones are known to play a key role in controlling fluid migration, including hydrocarbons, at local and regional scales. In this contribution physicochemical data from veins are used to document the extent and maintenance of fluid conductivity associated with fault-damage zones in an anticlinal structure occurring in the frontal part of a foreland fold and thrust belt. Migration of fluids during major deformational events has been recorded by several generations of calcite and localized dolomite cementation in veins in marine Cretaceous carbonates of the Jabal Qusaybah Anticline (North Oman). Applying detailed petrographic and paragenetic relations, geochemical analysis (stable carbon and oxygen isotopes, strontium isotopes, and trace elements), and microthermometry, coupled with structural studies of fault zones and associated calcite veins, made it possible to link fluid circulation to the structural evolution of the Jabal Qusaybah Anticline. Based on the structural framework and fault systems, three groups of veins were distinguished. Group 1 veins are interpreted to be linked to burial and early-tectonic deformation in association with E–W extensional fault zones. Calcite cementation in this group occurred in a closed to semiclosed fluid system, which originated as a result of intraformational fluid movement along a low-connectivity fracture network. The δ13C values of these veins range from 3‰ to 3.5‰, and δ18O values vary from –10.3 to –8.1‰ V-PDB. Group 2 veins are related to syntectonic deformation along NE–SW strike–slip and incipient N–S extensional faults linked to culmination of the Jabal Qusaybah Anticline. From this stage onward, fluids correspond to progressive fault connectivity and dilation related to interaction of fault–folding systems. The cements precipitated during this stage have more variable δ13C values ranging from 0.9 to 3.2‰ and possess a wide range of δ18O values varying between –11.2 and 0.3‰ V-PDB. The cements in Group 2 veins originated from fluids characterized by a relatively broad range of precipitation temperature (75–107°C) and salinities evolving from seawater to moderately saline fluids (3.4–12.8 eq. wt% NaCl), with H2O–NaCl composition. The source of salinity is inferred to be evaporitic brines that originated from the Ara Group, and the large range of salinity is interpreted as the result of mixing between those brines and modified seawater. Group 3 veins are considered to be related to late–tectonic deformation corresponding to late fold amplification of the anticline hinge, transition from NE–SW left-lateral strike–slip to N–S extensional faulting, progressive regional and local uplift, and exhumation. Stable-isotope data support the occurrence of bacterial methanogenesis or CO2 degassing at the end of this stage. Their associated cements are characterized by higher δ13C and δ18O values compared to other groups of veins, ranging from 0.6 to 8.0‰ and –3.6 to 1.5‰ V-PDB, respectively. Fluids responsible for calcite precipitation in this group evolved from a moderate saline to a more saline (8.5–16 eq. wt. % NaCl) H2O–NaCl–CaCl2 brine. The progressive 18O enrichment relative to seawater and preceding cements and its covariation with fluid salinity in the later stage suggest the higher contribution of the evaporitic brines. This hypothesis is further supported by the increase in their 87Sr/86Sr ratios compared to both middle to Upper Cretaceous marine carbonates and other groups of veins. Although during this episode calcite cementation occurred at temperatures below 50°C, their geochemical signatures reveal that the faults were sufficiently conductive to transfer extraformational fluids late in the deformation history of this structure. The interference between strike-slip and extensional fault systems and their interaction with fold amplification is invoked to be responsible for enhanced conductivity in fault-damage zones.

Growth structures, piggy-back basins and growth strata of the Georgian part of the Kura foreland fold–thrust belt: implications for Late Alpine kinematic evolution

Abstract The Kura foreland fold–thrust belt is located in the northern part of the active collisional Lesser Caucasus orogenic belt associated with Arabia–Eurasia convergence. This belt is the best example of mountain-building processes in late Alpine time. Seismic reflection profiles show that the Kura foreland fold–thrust belt of the eastern Caucasus is an active thin-skinned fold–thrust belt and is represented by fault-bend folds, fault-propagation folds and duplexes. Analysis of growth strata in seismic profiles and oil well data from the Kura foreland fold–thrust belt documents that the evolution of deformation has been continuing during the last c. 14–15 myr (since the Middle Miocene), together with the thrust system kinematics. The geometry of the growth strata is associated with footwall synclines and piggy-back basins. Compressional deformation on the Kura foreland began in the Middle Miocene (Chokrakian) and reached its maximum rate at the end of the Miocene ( c. 5 Ma).

The Madong Early Paleozoic fold–thrust belt in southern Tarim Basin

The Madong fold–thrust belt, which strikes NE–SW and thrusts southeastward, locates in the southern Tarim Basin. It is a part of the Kunlun Early Paleozoic foreland fold–thrust belt, and so is the Tangnan fold–thrust belt on the southeast of Madong. The Madong and Tangnan fold–thrust belts developed in Cambrian–Ordovician strata, and the Middle Cambrian gypsum–salt layer serves as the main décollement surface. The Middle Silurian and above strata unconformably overlie Madong while the upper Lower Silurian unconformably overlie Tangnan. On the basis of the facts that: (1) the Upper Ordovician is the youngest strata involved in the fold–thrust deformation, (2) the upper–Lower to Middle Silurian is the oldest strata unconformably overlying the foreland fold–thrust belt (including Madong and Tangnan), and growth strata exist in the upper part of the Upper Ordovician, we infer that the deformation time of the Kunlun Early Paleozoic foreland fold–thrust belt (including Madong and Tangnan) was during the Late Ordovician–Early Silurian. Tangnan is the residual of the major part of the foreland fold–thrust belt. Its northwestward thrust direction represents the main thrust direction of the foreland fold–thrust belt. Madong is the front belt of the foreland fold–thrust belt. It mainly thrusts southeastward and serves as the back-thrust belt of the Kunlun Early Paleozoic foreland fold–thrust belt. It is a triangle zone between Madong and Tangnan. The Madong fold–thrust belt is the best-preserved section of the Kunlun Early Paleozoic collisional orogenic belt, and thus is an important geological record of the Kunlun Early Paleozoic orogeny.

Archean–Proterozoic collision boundary in central Fennoscandia: Revisited

Archean–Proterozoic (AP) collision boundary in central Fennoscandia is a complex collage of Archean basement rocks, its Paleoproterozoic supracrustal cover rocks, and Svecofennian (1.93–1.78Ga) supracrustal and plutonic rocks. The apparent steep nature and complexity of the AP boundary has led to contradictory tectonic models varying from arc-continent collision via back-arc closure to accretion of arc terranes along a strike-slip shear zone. We present new geochronology data from detrital zircons combined with whole-rock geochemical and Sm–Nd isotopic data on Paleoproterozoic sedimentary and volcanic rocks across the boundary to revise its nature.The 2.06Ga Siilinjärvi volcanic complex and associated turbidites, with an inferred deposition age at 2.1–2.06Ga, are part of the cover sequence. The Siilinjärvi volcanic rocks derive from both plume (OIB) and sub-continental lithospheric mantle (SCLM) magma sources and are often contaminated by the Archean crust. The Sahara suite sedimentary cover rocks are derived from local 1.95–1.92Ga bimodal volcano-plutonic source, which source also explains the common age population found in younger sedimentary rocks. The Näläntöjärvi suite psammites, with maximum depositional ages of 1.97–1.93Ga, are continental slope to deep water sediments deposited at the continental margin. Contemporaneous mafic magmatism, seen as lavas and dykes, show a change from SCLM dominated (present east) to asthenospheric depleted mantle (DM) dominated (present west) melts. The interpretation of the Näläntöjärvi suite as cover rocks of the lower plate indicates that the Archean crust continues at least 50km more to the west than thought earlier. Parautochthonous sedimentary rocks from Salahmi, Eastern Kansanneva and Loutemäki with maximum depositional ages at 1.92–1.91Ga are inferred as collision-related foredeep deposits from local sources.Both the Lampaanjärvi suite and Pyhäjärvi suite sedimentary rocks have deposited at 1.89–1.88Ga or later, whereas the Lampaanjärvi suite rocks are dominated by a local source and the Pyhäjärvi suite rocks derive from a mixture of older sediments and 1.89–1.88Ga granitoids. At least two tectono-metamorphic stages separated with a major unconformity can be differentiated in the study area. The early deformation and metamorphic stage in the Näläntöjärvi suite has occurred before the intrusion of the late- to post-collisional pyroxene granitoids at 1.88Ga. The Pyhäjärvi suite sedimentary rocks have been deposited after the intrusion of these granitoids between 1880 and 1875Ma and then subsequently deformed and metamorphosed at ≤1875Ma.We propose a tectonic model where the continental breakup occurred at 2.1–2.06Ga followed by a new spreading within the pre-existing passive margin at 1.98–1.93Ga. During initial continent-arc collision the subducted slab steepened strongly at high angle, subduction ceased rapidly, and finally detached and shallow slab breakoff occurred at 1.92–1.91Ga followed subsequently by ophiolite obduction and a formation of a foreland fold and thrust belt. Mantle-crust decoupling of the Archean lithosphere occurred and a crocodile jaw formed, where the arc rocks are partly buried beneath the bounding Archean continent. Similar features have been recorded in other AP collision zones, which could indicate that the buoyant nature of the Archean lithosphere combined with denser Paleoproterozoic lithosphere may be a general cause for decoupling of Archean crust and mantle during arc-continent collision in Paleoproterozoic.

The Chaqupacha Mississippi Valley-type Pb–Zn deposit, central Tibet: Ore formation in a fold and thrust belt of the India–Asia continental collision zone

The newly discovered Chaqupacha Mississippi Valley-type (MVT) Pb–Zn deposit in central Tibet has been found to be helpful for understanding MVT ore formation relative to tectonic evolution of a foreland fold and thrust belt. The deposit lies in the Tuotuohe area of the western Fenghuo Shan-Nangqian fold and thrust belt of the India–Asia continental collision zone. It contains NNW-striking and folded Late Permian strata including an upper clastic unit and an underlying limestone unit. The strata overlie late Oligocene clastic rocks through a south-dipping reverse fault that is associated with regional northward thrusting during the Paleogene. The Late Permian and late Oligocene strata are unconformably overlain by flat-lying early Miocene marl and mudstone of the Wudaoliang Formation. Lead and zinc ores are mainly hosted by pre-ore dissolution and collapse breccias in the Late Permian limestone. The style of mineralization is epigenetic, as shown by replacement of the pre-ore dissolution breccia matrix and open-space-fill by galena, sphalerite, calcite, and minor barite and pyrite. δ34S values of the main sulfide galena range from −27.5‰ to +12.6‰. These features, together with the lack of magmatic activity during the mineralization, suggest that Chaqupacha is an MVT deposit. Subordinate mineralization is also present in the early Miocene Wudaoliang Formation marl and the paleokarst breccia which contains matrix compositionally equivalent to strata of the Wudaoliang Formation. The mineralization shares similar mineral associations and textures with the pre-ore dissolution breccia-hosted mineralization. Thus, the Pb and Zn mineralization in the entire deposit probably resulted from the same mineralizing event, which is younger than the youngest ore-hosting rocks (i.e., the early Miocene Wudaoliang Formation). Considering that thrusting in the Tuotuohe area had ceased prior to deposition of the Wudaoliang Formation host rocks, the mineralization at Chaqupacha post-dated the regional deformation. The Chaqupacha deposit thus provides a good example of MVT mineralization in a foreland fold and thrust belt that post-dates regional thrusting.

Detachment Surface Morphology and Shortening Distribution in the Foreland Folds of Iraq

Determinations of detachment surface depth and amount of local shortening were carried out in the more specific location foreland folds of Iraq. Twenty one traverses of anticlinal structures which are normally distributed on the study area were used for this application. Two types of deformation styles were found in the study area. They are thin-skinned and thick-skinned tectonics. Depth of the detachment surface generally increases towards east with as abnormal case appeared in Aqra Anticline. Westerlythe depth becomes shallower depth from the south towards Dohuk Anticline and it returns to increase toward the north. In Ain Zala, Butma West, Dohuk and Dahqan Anticlines and some parts of Bekhair Anticline, thin-skinned style was indicated. The shortening generally increases towards the north and northeast, with specific anomaly at Permam Dagh. It increases in the area after Permam Dagh to the north and northeast. However southeast of this area has low amount of shortening and the lowest appears in Qara chauq Anticline.

Closing the Canada Basin: Detrital zircon geochronology relationships between the North Slope of Arctic Alaska and the Franklinian mobile belt of Arctic Canada

Constraining the pre-opening paleogeography of the Canadian and Alaskan margins of the Canada Basin is a first-order objective in resolving the plate tectonic evolution of the Amerasia Basin of the Arctic Ocean. The most widely accepted model for opening of the Canada Basin involves counterclockwise rotation of Arctic Alaska away from Arctic Canada about a pole of rotation in the Mackenzie Delta region, although numerous other kinematic models have been proposed. The rotation model is tested using detrital zircon U-Pb geochronology of 12 samples from Middle Mississippian to Early-Middle Jurassic strata (Ellesmerian and lower Beaufortian megasequences) obtained from wells and outcrop along Alaska’s North Slope. These northerly-derived strata were deposited in fluvial to nearshore marine environments along the south-facing (present-day) shelf margin of the Arctic Alaska Basin and contain 360–390 Ma, 415–470 Ma, 500–750 Ma, 0.9–2.1 Ga, and 2.4–3.2 Ga zircon populations. Detrital zircon age populations in Ellesmerian and lower Beaufortian strata are remarkably similar to detrital zircon populations from Devonian foreland clastic wedge strata in the Canadian Arctic Islands and northern Yukon Territory. A paleogeographic setting in which Arctic Alaska received sediments recycled from the Devonian foreland clastic wedge and underlying Franklinian Basin strata is most consistent with the model of [Embry (1990)][1] in which northern Alaska lay within the foreland fold and thrust belt of the Franklinian mobile belt prior to the opening of the Canada Basin. The sequences that are inferred to have been the long-lived source region for Ellesmerian and lower Beaufortian strata were uplifted by Paleozoic (predominantly Late Devonian) deformation that has been documented along the Canadian and Alaskan margins. Triassic and Jurassic strata deposited along the Arctic Canada, Arctic Alaska, and northern Yukon shelves have detrital zircon ages that are significantly older than the youngest detrital zircon ages (Mesozoic) in coeval strata that were deposited west of Hanna Trough and north of the Sverdrup Basin axis, supporting continuity of these bathymetric features prior to opening of the Canada Basin. [1]: #ref-32

A comparison of the effectiveness of clast based finite strain analysis techniques to AMS in sandstones from the Sevier Thrust Belt, Wyoming

Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques, which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres. To evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility (AMS) in sandstone samples from the Wyoming salient. The Cordilleran Mountain Belt of North America is one of the world's classic foreland fold and thrust belts. The Sevier Belt represents the thin-skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of Devonian to Cretaceous strata eastward. Deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility (AMS) to clast-based strain analyses. Studies attempting to define the relationship between AMS and finite strain have been in vogue since the link between layer parallel shortening and AMS was first established. This relationship, despite proven strong correlations between the AMS tensors and tectonic directions, is generally complicated by competing sub-fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the AMS fabric. Even with these complications, we conclude that AMS is much more sensitive to incipient tectonic fabrics than clast based methods.

New geochronological and Sm–Nd constraints across the Pajala shear zone of northern Fennoscandia: Reactivation of a Paleoproterozoic suture

Altogether 1130 U–Pb analyses on zircons from 25 samples across the Pajala shear zone in northern Fennoscandia were performed by LA-ICPMS methodology to study the origin of the zone. The samples were also analyzed for their Sm–Nd isotope systematics and geochemically. The new data is tested with existing geological information to elaborate the provenance and maximum depositional ages, the stratigraphic position of studied samples, and finally to assess the tectonic evolution in the Pajala shear zone area.Chromium-enriched metasedimentary rocks are found within and to the east and west of the N-S trending Pajala shear zone. No material originating from Proterozoic felsic igneous (zircons) or mafic (Sm–Nd) rocks was found in these rocks and thus, only Neoarchean or very early Paleoproterozoic ages of deposition are possible. Our new results indicate that the bimodal 1.99–1.97Ga supracrustal rocks of the Rovaniemi Supersuite cover a large area east of the Pajala shear zone. Based on the absence of Archean zircons typical of the metasedimentary rocks covering the Archean basement, it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the Rovaniemi Supersuite. The metasedimentary rocks of the Uusivirka Supersuite, located within the Pajala shear zone, vary from hornblende-bearing metasedimentary rocks, metapelite, and meta-arkose to orthoquartzite. Their characteristic features include a dominant Proterozoic zircon population (65–87%), with a high proportion of analyzed grains yielding ages between 1.96Ga and 1.91Ga, and maximum depositional ages of 1.92–1.91Ga. We record at least three metamorphic events; at ca. 1.92–1.90Ga, ca. 1.85Ga and at 1.82–1.78Ga, of which the youngest can possibly be separated into two events at 1.83–1.82Ga and 1.79–1.77Ga, respectively. We propose a tectonic model in which continental breakup occurred at 2.1–2.05Ga, followed by the development of a magmatic arc (Kittilä arc) at ca. 2.0Ga above a subduction zone. Two Archean continental units (Norrbotten and Karelia) collided at ca. 1.92–1.91Ga, the Kittilä arc was obducted upon the Karelia continent as a foreland fold and thrust belt, and the metasedimentary rocks of the Uusivirka Supersuite were emplaced as foreland deposits. The proposed tectonic model is based on the absence of 2.44Ga layered intrusions, and especially 2.06Ga rocks and 1.99–1.97Ga rocks of the Rovaniemi Supersuite that are found only east of the Pajala shear zone. In our model the Pajala shear zone originated as a divergent plate boundary, and was multiply reactivated after continental collision with both lateral and vertical movements.

Effects of irregular basement structure on the geometry and emplacement of frontal thrusts and duplexes in the Quebec Appalachians: Interpretations from well and seismic reflection data

Irregular basement geometry may affect thrust propagation in foreland fold–thrust belts creating a perturbation in structural continuity of hydrocarbon fields. Here we investigate how the irregular pattern of normal faults, along with the presence of uplifts and transverse faults in the Grenvillian basement has influenced the geometry and emplacement of frontal thrusts and duplexes in the parautochthonous domain of the southern Quebec Appalachians during the middle–late Ordovician Taconian orogeny. Integration of data from surface geology, wells, and 2D and 3D seismic reflection surveys into a regional-scale structural model is used to reconstruct the 3D geometry and emplacement history of one- and two-horse duplexes in the Joly–Saint-Flavien gas storage area. The normal hinterland to foreland sequence of thrusting in this area is complicated by the differential emplacement of tectonic slices along strike of the orogenic front, starting in the SW and developing subsequently to the NE within each structural level. The shortening related to duplex emplacement decreases laterally over the distance of 20km from −49% in the SW (Joly area) to −31% in the NE (Saint-Flavien area). Duplex emplacement resulted from the differential forward propagation, deflexion and vertical-axis rotation of the roof thrust (Logan's Line), which, in turn, has been induced by the presence of frontal uplift and transverse faults in the basement. The structural–lithological analysis of tectonic slices and restoration of their initial location allows us to consider the lower slice of the Joly duplex as a structural trap comparable to the fractured reservoir in lower Ordovician dolomites of the Saint-Flavien duplex.

Physical analog (centrifuge) model investigation of contrasting structural styles in the Salt Range and Potwar Plateau, northern Pakistan

We use scaled physical analog (centrifuge) modeling to investigate along- and across-strike structural variations in the Salt Range and Potwar Plateau of the Himalayan foreland fold-thrust belt of Pakistan. The models, composed of interlayered plasticine and silicone putty laminae, comprise four mechanical units representing the Neoproterozoic Salt Range Formation (basal detachment), Cambrian–Eocene carapace sequence, and Rawalpindi and Siwalik Groups (Neogene molasse), on a rigid base representing the Indian craton. Pre-cut ramps simulate basement faults with various structural geometries.A pre-existing north-dipping basement normal fault under the model foreland induces a frontal ramp and a prominent fault-bend-fold culmination, simulating the Salt Range. The ramp localizes displacement on a frontal thrust that occurs out-of-sequence with respect to other foreland folds and thrusts. With a frontal basement fault terminating to the east against a right-stepping, east-dipping lateral ramp, deformation propagates further south in the east; strata to the east of the lateral ramp are telescoped in ENE-trending detachment folds, fault-propagation folds and pop-up structures above a thick basal detachment (Salt Range Formation), in contrast to translated but less-deformed strata with E–W-trending Salt-Range structures to the west. The models are consistent with Salt Range–Potwar Plateau structural style contrasts being due to basement fault geometry and variation in detachment thickness.

Magnetic Fabrics in Jingzhuba‐Shiwo Section of Foreland Fold Belt of the South Dabashan: Constraints on Its Tectonic Evolution

AbstractA detailed investigation of the structural geometry and magnetic fabrics together with rock magnetism has been carried out along Jingzhuba‐Shiwo section located in the foreland fold belt of the South Dabashan, which aims to shed light on the superimposed deformation and tectonic evolution. As a whole, the intensity of deformation decreases from the northeast to the southwest, and the axial planes dip northeastward and the hinges of large‐scale folds plunge northwestward at small angle. Moreover, the maximum principal stress indicates that the compression of the South Dabashan is dominant as per palaeostress analysis. The sampling horizons before the Late Jurassic are mainly characterized by magnetic fabrics in deformed rocks, whereas the sampling horizons ranging from the Late Jurassic to the Early Cretaceous are characterized by incipient magnetic fabrics in weakly deformed rocks. The magnetic lineations are dominantly oriented NW‐SE and remain almost parallel to the main tectonic structures of the section, suggesting that AMS can be ascribed to tectonic processes. Also there are two types of unusual magnetic fabrics revealed. The first one merely occurring in the northeast section is characterized by an oblique magnetic foliation with respect to bedding, which is related with parallel‐bedding simple shear during fold growth. The other one caused by tectonic superimposition is characterized by an oblique magnetic lineation with respect to bedding strike. The plunges of Kmin increase with decreasing deformation intensity from northeast to southwest along the section, which can be taken as a gauge to distinguish the deformation intensity of weakly‐deformed sedimentary rocks according to the relativity. Consequently it can be concluded that the superimposed structure of the section was formed by the combined compression which arose from the South Dabashan primarily and the Micangshan and the eastern Sichuan fold belt secondly during the Late Jurassic. Afterwards the compression from the South Dabashan was still dominant, the compression from the other two directions, however, became much weaker in the Early Cretaceous than in the Late Jurassic. Furthermore, the effects resulting from Qinling intracontinental orogeny during J3‐K1 and Xuefeng intracontinental deformation could be reflected by the results mentioned above, which can also provide evidences for exploration of continental dynamics.

Topography of the Variscan orogen in Europe: failed–not collapsed

The Variscan orogenic collage consists of three subduction-collision systems (Rheno-Hercynian, Saxo-Thuringian and Massif Central-Moldanubian). Devonian to early Carboniferous marine strata are widespread not only in the individual foreland fold and thrust belts, but also in post-tectonic basins within these foreland belts and on the Cadomian crust of peri-Gondwanan microcontinental fragments, which represent the upper plates of the subduction/collision zones. These marine basins preclude high elevations in the respective areas and also in their neighbourhood. Widespread late Carboniferous intra-montane basins with their coal-bearing sequences are likewise incompatible with high and dry plateaus. While narrow belts with high elevations remain possible along active margins within the orogen, comparison of the Variscides with the Himalaya/Tibetan plateau is unfounded. Plausible reasons for the scarcity of high Variscan relief include subduction of oceanic and even continental crust, subduction erosion, orogen-parallel extension and—most important—lithospheric thinning accompanied by high heat flow and magmatism. In many areas, timing and areal array of magmatism and HT metamorphism are not compatible with a model of tectonic thickening and subsequent gravitational collapse. It is suggested, instead, that lithospheric thinning and heating are due to mantle activities caused by the Tethys rift. The lower and middle crust were thermally softened and rendered unfit for stacking and isostatic uplift: in terms of topography, the Variscides represent a failed orogen. The HT regime also explains the abundance of granitoids and HT/LP metamorphic rocks typical of the Variscides. Melting in the HT regime extracted mafic components from Variscan and Cadomian crust as well as from Cadomian metasomatized lithospheric mantle, thus mimicking subduction-related magmatism. The onset of the HT regime at c. 340 Ma may also have triggered the final ascent of HP/UHP felsic metamorphic rocks.

Recent emerging paradigms in hydrocarbon exploration

Interesting points can be made about hydrocarbon volumes discovered in the past decade: - 350 bboe of 2P (Proved and Probable) discovered volumes were found, and 91 of the fields have 2P volumes in excess of 500 million boe (‘elephant’) totalling approximately 200 billion boe, based on publicly available information. - This compares with 120 bboe in the period 1991-2001, and 140 bboe in the period 1981-1991 (From AAPG Memoir data) - six petroleum plays can be defined and ranked against the corresponding volumes: - Carbonates below salt and Large Delta Systems account for more than 85% of the discovered volumes; - A new play concept: Abrupt Margins has been proven, with the Jubilee Field in Ghana (disc 2007). Foinaven Field (West of Shetlands, disc. 1991) also goes into this category; - New Exploration Techniques will be needed in mature plays such as Foreland fold and Thrust belts. These discovered volumes are far from negligible in sustaining current worldwide production (2013) of almost 90 mmboe/d and 400 bcf/d, rates that will increase to 100 mboe a day by 2020 for oil and to 500 bcf/d by 2030 for gas, depending on demand and investment rates.

Mesozoic deformations of the Dabashan in the southern Qinling orogen, central China

The Dabashan, a curved tectonic belt in the southern Qinling orogen, was formed as a result of the collision of the Qinling micro-plate and the Yangtze plate in the Late Triassic. This study shows that the Dabashan experienced multistage structural deformations. Two main deformational episodes are recognized, (1) foreland folding and thrusting due to the Late Triassic collision and (2) structural superposition during the Late Jurassic to Early Cretaceous. The northern Dabashan is typified by imbricate thrust sheets, whereas the southern Dabashan is characterized by occurrence of widespread superimposed folds. The early fold axis is orientated almost due north or NE–SW. The folding propagated southward and southwestward into the Sichuan basin from the Chengkou-Fangxian fault, which separates the southern from the northern Dabashan belts. It is concluded that the southern and northern Dabashan were two distinct domains, at least in early Mesozoic. This interpretation is important for understanding the tectonic evolution of the Qinling orogen and the collisional process of the North and South China plates.

Structure, paleogeographic inheritance, and deformation history of the southern Atlas foreland fold and thrust belt of Tunisia

Structural analysis of the southern Tunisian Atlas was carried out using field observation, seismic interpretation, and cross section balancing. It shows a mix of thick‐skinned and thin‐skinned tectonics with lateral variations in regional structural geometry and amounts of shortening controlled by NW‐SE oblique ramps and tear faults. It confirms the role of the Late Triassic–Early Jurassic rifting inheritance in the structuring of the active foreland fold and thrust belt of the southern Tunisian Atlas, in particular in the development of NW‐SE oblique structures such as the Gafsa fault. The Late Triassic–Early Jurassic structural pattern is characterized by a family of first‐order NW‐SE trending normal faults dipping to the east and by second‐order E‐W trending normal faults limiting a complex system of grabens and horsts. These faults have been inverted during two contractional tectonic events. The first event occurred between the middle Turonian and the late Maastrichtian and can be correlated with the onset of the convergence between Africa and Eurasia. The second event corresponding to the principal shortening tectonic event in the southern Atlas started in the Serravalian‐Tortonian and is still active. During the Neogene, the southern Atlas foreland fold and thrust belt propagated on the evaporitic décollement level infilling the Late Triassic–Early Jurassic rift. The major Eocene “Atlas event,” described in hinterland domains and in eastern Tunisia, did not deform significantly the southern Tunisian Atlas, which corresponded in this period to a backbulge broad depozone.

Formation of chocolate-tablet boudins in a foreland fold and thrust belt: A case study from the external Variscides (Almograve, Portugal)

Chocolate-tablet boudins of quartzite are restricted to steep limbs of D1-folds along the SW coast of Portugal. The chocolate-tablet geometry results from older vertical and younger horizontal quartz veins. Both sets of veins developed during similar conditions by extension fracture in pre-existing necked domains: (1) both veins show stretched crystal fibers; (2) the boudin aspect ratio is the same in vertical (3.0 ± 1.4) and in horizontal sections (2.9 ± 1.2); (3) temperatures obtained from fluid inclusions are similar (200 ± 20° for vertical and 230 ± 22 °C for horizontal veins) and are compatible with temperatures obtained from illite crystallinity (ca. 200–ca. 250 °C). Given thermal equilibrium between the host rock and the precipitating fluid, the chocolate-tablet boudins formed close to the metamorphic peak. We interpret that the vertical veins developed after the limbs of the D1-folds had attained their steep attitude and the orientation of the greatest and intermediate principal strain axes had interchanged. Subsequently, the initial strain field was restored and opening of horizontal veins led to the chocolate-tablet geometry. The direction of the main shortening direction was constant from the initial buckling stage via isoclinal folding and during all boudinage stages.

Sub-seismic fractures in foreland fold and thrust belts: insight from the Lurestan Province, Zagros Mountains, Iran

The Simply Folded Belt of the Zagros Mountains, Iran, is a spectacularly well-exposed example of a foreland fold and thrust belt. A regional analysis of the Cenomanian–Coniacian Sarvak and Ilam Formations, exposed in the southern Lurestan Province, is presented as a case study for sub-seismic fracture development in this type of compressive setting. The area is characterized by gentle to tight anticlines and synclines parallel to the NW–SE trend of the belt. In the Lurestan Province, the Cenomanian– Coniacian interval is exposed in the core of most of the outcropping anticlines. Fold style is intimately related to both vertical and lateral facies distribution. Geometry, kinematics and timing of sub-seismic fractures were characterized through extensive fieldwork, interpretation of orthorectified QuickBird imagery and interpretation of 3D photorealistic models derived from LiDAR. Data were collected from 12 anticlines covering an area of approximately 150
200 km. Key outcrops for understanding fracture geometry, kinematics and timing are presented. Field observations and interpretation of QuickBird and 3D photorealistic models reveal the complexity of fracture geometry and timing. Fractures record pre-, syn- and post-folding stages of deformation. Pre-folding structures include synsedimentary normal faults, and subsequent small-scale thrusts, systematic veins and stylolites. During folding, pre-existing fracture planes were re-activated and through-going fractures and reverse faults developed. Strike-slip faults typically postdate pre- and syn-folding structures and are probably related to the late stages of fold tightening. All structures are geometrically and kinematically consistent with the trend of the Arabian passive margin and its subsequent tectonic inversion.

Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt

Terra Nova, 23, 85–91, 2011AbstractVitrinite reflectance and Raman spectroscopy of carbonaceous material data are used to better resolve the thermal history of the Hsuehshan Range, which is accreted between the foreland fold–thrust belt and bulldozer hinterland units in the Taiwan mountain belt. The observed thermal data indicate that the strata in the northern Hsuehshan Range underwent dynamic metamorphism during the Neogene orogeny, while the strata in the southern Hsuehshan Range may have predominantly experienced burial metamorphism during Palaeogene sedimentation. Based on the thermal constraints, the Hsuehshan Range is interpreted to consist of nappe stacks, originating from the rifted Eurasian continental margin. This interpretation is consistent with well‐documented cases in the European Alps and the Himalayas and is also shown in physical modelling and thermo‐kinematic studies invoking underplating and erosion processes.