Extensional Features Research Articles

Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations

The area of the Vinodol Valley and Bakar Bay represents a NW‒SE oriented valley in the NW Adriatic characterised by prominent historical and instrumentally recorded seismicity. As part of the greater geodynamic domain including the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone, new geological and structural data addressing the tectonic evolution of the area were collected in order to better understand the focal mechanisms of previous earthquakes and to enable identification of potential seismogenic sources. Mapped informal lithostratigraphic units mostly correspond to the Upper Cretaceous, Palaeogene and Quaternary successions described in other parts of the External Dinarides. However, a shorter stratigraphic range of the Gornji Humac fm., the youngest Cretaceous unit in the study area, was determined and suggests that the uplifted area in the central NW part of the Adriatic Carbonate Platform already comprised several thousand km2 (from W and NW Istria to Krk and Vinodol area) at the end of the Turonian. Structural measurements of the fault planes in the study area generally correspond to the existing structural model of the tectonic evolution of the Dinarides. However, in contrast to the SW vergences typical of the Dinarides, NE-vergent reverse structures are common, especially along the SW margin of the Vinodol Valley. Cross-cutting relationships suggest that transpressional (NW–SE and NE–SW striking dextral and sinistral faults) and extensional features (NW–SE and NE–SW striking normal faults) are structurally concurrent or younger than the reverse faults, suggesting a change in the palaeostress field during the Neogene–Quaternary, with prevalent transpression and radial extension. Comparison of results of the palaeostress field analysis and the constructed synthetic focal mechanisms on one side, with available focal mechanism solutions for earthquakes within the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone on the other, shows a favourable orientation of the observed NW‒SE and NE‒SW striking faults with respect to the recent compressional/transpressional stress field (N‒S oriented P-axis), indicating these as potential seismogenic sources within the study area.

How to build an extensional basin in a contractional setting? Numerical and physical modelling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia

In Tunisia, Africa-Eurasia convergence resulted in the Tell fold-and-thrust belt (FTB), part of the Tell-Rif (or Maghrebides), associated, to the south, to the Atlas intra-continental belt. The Mejerda rift-basin is developed over the compressional boundary between the Tell and the Atlas. The main objective of this study is to identify the mechanism responsible for such a basin development. The abundance of Triassic salt structures in the northern Atlas, compared to the Tell, suggests a friction change along the basal décollement formed by the salt level. This frictional contrast is expected to produce a heterogeneous deformational response and complex structures. On this basis, numerical and analogue modelling were used to investigate the impact of a friction change on the deformation style. The kinematic approach of the limit analysis theory was applied here, via the software Optum G2. The onset of faulting in all numerical tests reveals extensional faults appear at the friction boundary. Beyond the onset of faulting, sandbox experiments show extensional features such as collapsing blocks and extensional faults. Overall, the adopted numerical and physical approaches showed evidences of extension. In particular, the experiment using sandpaper and Silicon putty (PDMS) for the décollement transition shows viscous deformation, mimicking diapiric movement. These observations of numerical and physical model behaviors highlight that compression can produce an extensional basin at the top of a thrust wedge, as expression of a strong basal friction change. Hence, the Mejerda basin is interpreted as a wedge-top basin. It developed, during the growth of the Tell and Atlas FTBs. The same configuration likely exists in other mountain belts: the case of the Molasse Basin between the Jura and the Western Alps is advocated.

Slump sheets as a record of regional tectonics and paleogeographic changes in South China

Slumps of various scales are recognized in the late Katian (late Ordovician) mixed carbonate and siliciclastic slope successions in the Changshan area, western Zhejiang Province, South China. In order to understand their deformational processes, triggering mechanisms, paleo-slope orientation and geological implications, a detailed sedimentological study was carried out on four outcrop sections. In the southern Changshan (the Putangkou, Zhoutang and Shikeng sections), we recognized three slumping episodes (ca. 3.5 m, 11 m, and 73 m in thickness in stratigraphically ascending order) and co-occurred debris flows. At the Sanqushan section (~11.5 km north of Changshan), thirteen slump units, each being tens of cm to 6 m in thickness, are recognized. Facies analyses and diagenetic characteristics of these slumps and their internal deformation structures suggest that they formed as a result of plastic and brittle deformation, and liquefaction and fluidization during slumping. The rip-up blocks, cracks and ruptured beds in the slumps of southern Changshan indicate extensional features, whereas recumbent and disharmonic folds in the slumps of northern Changshan represent compressional features. These spatially different features suggest that the slumps may have been generated on a paleo-slope dipping generally to the north. The case study of slumps and debris flows from the Putangkou section indicates a significant mass flow transition from a slumped mass into debris flow deposits. Autogenic triggering mechanisms of these slumps, e.g., rapid deposition, storm-wave loading, and tidal shear, are ruled out by the results of facies analysis. Instead, the slumps might have been triggered by relative sea-level changes or tectonically-induced, progressively steepening of the slope, which is likely related to the contemporaneous Kwangsian Orogeny.

Mesozoic – Cenozoic evolution of the Southern Patagonian Andes fold and thrust belt (47°–48°S): Influence of the Rocas Verdes basin inversion and onset of Patagonian glaciations

The Southern Patagonian Andes (SPA) between 47°–48°S are characterized by relatively high elevations (1500–4000 m) and a 200-km-wide, largely exhumed, basement domain along the backbone of the Cordillera, bounded by a narrow 30-km-wide fold and thrust-belt to the east. The tectonic evolution of the SPA includes widespread Jurassic extension during the opening of the Austral-Magallanes basin and the Rocas Verdes back-arc oceanic basin (RVB), followed by Cretaceous to Cenozoic shortening. Although the main phases of deformation that affected this Andean segment have been the focus of previous studies, its finite geometry and step by step structural and kinematic evolution have not been quantitatively determined. In this work, we present kinematically restored cross sections, showing the variable geometry, trough time, of the eastern basement domain and the fold and thrust-belt. A regional westward-dipping listric decollement, interpreted as formed during the Jurassic extensional event, was reactivated during the Andean deformation constituting the main detachment surface of basement structures and the fold and thrust belt. West verging faults systems are considered a consequence of inversion of north-south trending extensional features as passive roof high-angle west-vergent faults. Using regional data such as field relations, unconformities, and existing ages to differentiate deformational stages, three main phases of contractional deformation are proposed for the area, starting with an Aptian - Cenomanian? phase related to inversion of the extensional depocenter in inmediate response to the closure of northernmost RVB. A second, Miocene phase responsible for most of the shortening occurred between 18 Ma and 10 Ma. Finally, the onset of glaciations and glacial erosion might have triggered out-of-sequence thrusting in the interior of the orogen after 7 Ma.

Recent cryovolcanism in Virgil Fossae on Pluto

The Virgil Fossae region on Pluto exhibits three spatially coincident properties that are suggestive of recent cryovolcanic activity over an area approximately 300 by 200 km. Situated in the fossae troughs or channels and in the surrounding terrain are exposures of H2O ice in which there is entrained opaque red-colored matter of unknown composition. The H2O ice is also seen to carry spectral signatures at 1.65 and 2.2 μm of NH3 in some form, possibly as a hydrate, an ammoniated salt, or some other compound. Model calculations of NH3 destruction in H2O ice by galactic cosmic rays suggest that the maximum lifetime of NH3 in the uppermost meter of the exposed surface is ~109 years, while considerations of Lyman-α ultraviolet and solar wind charged particles suggest shorter timescales by a factor of 10 or 10000. Thus, 109 y is taken as an upper limit to the age of the emplacement event, and it could be substantially younger.The red colorant in the ammoniated H2O in Virgil Fossae and surroundings may be a macromolecular organic material (tholin) thought to give color to much of Pluto's surface, but probably different in composition and age. Owing to the limited spectral range of the New Horizons imaging spectrometer and the signal precision of the data, apart from the H2O and NH3 signatures there are no direct spectroscopic clues to the chemistry of the strongly colored deposit on Pluto. We suggest that the colored material was a component of the fluid reservoir from which the material now on the surface in this region was erupted. Although other compositions are possible, if it is indeed a complex organic material it may incorporate organics inherited from the solar nebula, further processed in a warm aqueous environment inside Pluto.A planet-scale stress pattern in Pluto's lithosphere induced by true polar wander, freezing of a putative interior ocean, and surface loading has caused fracturing in a broad arc west of Sputnik Planitia, consistent with the structure of Virgil Fossae and similar extensional features. This faulting may have facilitated the ascent of fluid in subsurface reservoirs to reach the surface as flows and as fountains of cryoclastic materials, consistent with the appearance of colored, ammoniated H2O ice deposits in and around Virgil Fossae. Models of a cryoflow emerging from sources in Virgil Fossae indicate that the lateral extent of the flow can be several km (Umurhan et al., 2019). The deposit over the full length (>200 km) of the main trough in the Virgil Fossae complex and extending through the north rim of Elliot crater and varying in elevation over a range of ~2.5 km, suggests that it debouched from multiple sources, probably along the length of the strike direction of the normal faults defining the graben. The source or sources of the ammoniated H2O are one or more subsurface reservoirs that may or may not connect to the global ocean postulated for Pluto's interior. Alternatives to cryovolcanism in producing the observed characteristics of the region around Virgil Fossae are explored in the discussion section of the paper.

Trace element systematics and ore-forming processes in mafic VMS deposits: Evidence from the Troodos ophiolite, Cyprus

The volcanogenic massive sulfide (VMS) deposits in the Troodos ophiolite (Cyprus) are ancient analogues for modern day seafloor massive sulfide mineralisation formed in a supra-subduction zone environment. In this study we present the first detailed in situ study of trace elements in sulfides from twenty VMS deposits hosted in the Troodos ophiolite to better understand factors that influence the distribution, enrichment and incorporation of trace elements in different sulfide minerals.On a mineral scale, trace elements exhibit systematic variations between pyrite, chalcopyrite and sphalerite. Pyrite preferentially incorporates As, Sb, Au and Te, whilst chalcopyrite is enriched in Co and Se. Sphalerite is trace element poor with the exception of Ag and Cd. Selenium averages 278 ppm (n = 150) in chalcopyrite but only 42 ppm (n = 1322) in pyrite. Bismuth and Te in pyrite show a weak positive correlation (R2 = 0.35) in some VMS deposits possibly linked with the occurrence of Bi-telluride inclusions. Trace element concentrations also vary between colloform and euhedral pyrite, with an enrichment of Au, As, Sb, Cu and Zn in colloform compared to euhedral pyrite.Time resolved laser ablation profiles reveal that the trace element distribution on a mineral scale is not uniform and varies with crystallographic effects, fluctuating physicochemical fluid conditions such as temperature, pH, fS2, fO2 and ligand availability during sulfide precipitation. Incorporation mechanisms in sulfides differ between elements in pyrite, Ag, As, Se and Pb are hosted in solid solution or as nanoscale inclusions, whilst Au, Sb and Te may form micro-scale inclusions.On a regional scale (20 km) the distribution of trace elements exhibits systematic variations between three major structural domains; namely the Solea, Mitsero and Larnaca grabens. The VMS deposits of the magmatic-tectonic Solea graben are enriched in Se, Co, Te, Au and Cu relative to Mitsero, which is a purely extensional feature. Therefore, we hypothesise that a variable magmatic volatile influx related to a) ‘magma’ volume, b) migration of the magmatic-hydrothermal crack front and associated brine liberation or c) a variation in protolith metal concentration are responsible for regional scale variations in VMS geochemistry. This is suggested to be intrinsically linked to the spreading architecture of Troodos.

Europa's ice tectonics: New insights from physical wax experiments with implications for subduction initiation and global resurfacing processes

Jupiter's Moon Europa has one of the youngest geological surfaces in our solar system with an age of 40–90 Ma, implying an intense history of resurfacing. The surface of Europa indeed shows abundant evidence of tectonic deformation related to extension, strike-slip, and shortening. However, observed features related to shortening are scarce compared with pervasive extensive extensional features such as dilational bands, and do not suffice as the sole mechanism for recycling aging terranes. Recently, evidence for potential plate tectonics, associated with subduction zones, has been discovered on Europa; this could be responsible for recycling most of Europa's surface. However, basic physical parameters needed to initiate subduction on Europa, such as thickness of the brittle layer, deformation rates, and orientation of pre-existing zones of weakness at which subduction could start, are not well understood. Here, we aim to better understand the process and the conditions that could lead to initiation of subduction on Europa through physical experiments, using wax to simulate Europa's two-layered (i.e. convective) icy crust. By deforming the wax, strain on Europa's surface—possibly caused by diurnal tides or its nonsynchronous rotation—is simulated. Our results indicate that subduction could initiate over a broad range of surface thicknesses and deformation rates above a minimum conductive layer thickness, but is strongly dependent on the orientation of the pre-existing zones of weakness. Very thin conductive layer experiments, however, result in a previously undescribed process that we term ductile rolldown, which creates surface features similar to double ridges observed on Europa. Thus, subduction and ductile rolldown represent physically plausible mechanisms that could play a critical role in resurfacing Europa throughout its geologic history. These results could yield significant implications for Europa's thermal history and evolution, habitability, and future spacecraft missions.

Evidence of transpressional tectonics on the Uruk Sulcus region, Ganymede

The surface of Ganymede is characterized by intense tectonic deformation. The two terrains, light and dark, show morphotectonic features, namely, grooves and furrows. These represent kilometric systems at the regional scale, interpreted as extensional structures (e.g., horst and graben systems). Currently, evidence of significant compression has not yet been recognized, leaving open the opportunity to clarify the tectonic balance of Ganymede's surface. Debate still exists on the geodynamics responsible for surface deformation, and the investigation of these features contributes to understanding the internal processes. To understand the deformation that shapes the surface of the satellite, we explore the tectonic setting of the light terrain region of Uruk Sulcus. Using structural geology methodologies by remote sensing and systematic analysis of grooves, we recognize three groove systems within the sulcus and one furrow system in the neighboring dark terrain. Uruk Sulcus is also characterized by the presence of two domains of lineaments, subtle linear texture anisotropies related to the influence of tectonic processes on morphology. Comparison of the combined results from the analyses of the recognized groove systems with the lineament domains allowed us to infer the stress field responsible for their development in the studied area. According to these results, an evolutionary tectonic scenario for Uruk Sulcus is proposed: a tectonic corridor characterized by dextral transpression, with 58% of the internal compressional component approximately N-S oriented and responsible for localized, E-W extensional features among crustal blocks of the shear zone.

Fault kinematics at the Hengill Triple Junction, SW-Iceland, derived from surface fracture pattern

The Hengill Triple Junction in Iceland is formed by the intersection of the Western Volcanic Zone (WVZ), Reykjanes Peninsula Oblique Rift (RPOR) and a transform zone, the South Iceland Seismic Zone (SISZ). The 19 × 13 km2 study area thus contains extensional structures related to volcanic systems of the two rift zones, Hengill and Hrómundartindur, as well as strike-slip structures characteristic of the transform zones. High definition aerial photographs, satellite images, and digital elevation models are used, supported by field studies, to accurately map and classify tectonic features in an effort to separate N-S trending strike-slip faults from the fissure swarms of the volcanic systems. A portion of the southern Hengill fissure swarm lies within the area. It trends NE-SW and exhibits extensional features characteristic of spreading ridges, i.e. normal faults, open fractures, and eruptive fissures. It is clearly delineated by surface fractures, and is traced a length of 28 km southwest from the central volcano, terminating before reaching the coastline. The fissure swarm is 2–3.5 km wide in the study area, widening southwards. Elevation profiles across the fissure swarm suggest a spreading rate comparable to the southern WVZ, which spreads at 4–5 mm/year. A portion of the Nesjahraun eruptive fissure is anomalously smooth and may represent secondary rifting. East-west shearing in the RPOR and SISZ is accomplished by many smaller and less conspicuous, northerly oriented strike-slip faults in a ‘bookshelf faulting’ manner. These parallel faults are overprinted on the volcano-tectonic structures of the RPOR. Evidence of 11 northerly striking strike-slip faults is found within the study area. The zone of bookshelf faulting is thus shown to be continuous across the triple junction area, from the SISZ to the RPOR. Finally, several structures (eruptive fissures, exposed dikes, and faults) strike near-perpendicular to the fissure swarm and may be the result of changes in the direction of maximum stress caused by rifting events, however, their tectonic significance remains uncertain.

Ice‐marginal forced regressive deltas in glacial lake basins: geomorphology, facies variability and large‐scale depositional architecture

This study presents a synthesis of the geomorphology, facies variability and depositional architecture of ice‐marginal deltas affected by rapid lake‐level change. The integration of digital elevation models, outcrop, borehole, ground‐penetrating radar and high‐resolution shear‐wave seismic data allows for a comprehensive analysis of these delta systems and provides information about the distinct types of deltaic facies and geometries generated under different lake‐level trends. The exposed delta sediments record mainly the phase of maximum lake level and subsequent lake drainage. The stair‐stepped profiles of the delta systems reflect the progressive basinward lobe deposition during forced regression when the lakes successively drained. Depending on the rate and magnitude of lake‐level fall, fan‐shaped, lobate or more digitate tongue‐like delta morphologies developed. Deposits of the stair‐stepped transgressive delta bodies are buried, downlapped and onlapped by the younger forced regressive deposits. The delta styles comprise both Gilbert‐type deltas and shoal‐water deltas. The sedimentary facies of the steep Gilbert‐type delta foresets include a wide range of gravity‐flow deposits. Delta deposits of the forced‐regressive phase are commonly dominated by coarse‐grained debrisflow deposits, indicating strong upslope erosion and cannibalization of older delta deposits. Deposits of supercritical turbidity currents are particularly common in sand‐rich Gilbert‐type deltas that formed during slow rises in lake level and during highstands. Foreset beds consist typically of laterally and vertically stacked deposits of antidunes and cyclic steps. The trigger mechanisms for these supercritical turbidity currents were both hyperpycnal meltwater flows and slope‐failure events. Shoal‐water deltas formed at low water depths during both low rates of lake‐level rise and forced regression. Deposition occurred from tractional flows. Transgressive mouthbars form laterally extensive sand‐rich delta bodies with a digitate, multi‐tongue morphology. In contrast, forced regressive gravelly shoal‐water deltas show a high dispersion of flow directions and form laterally overlapping delta lobes. Deformation structures in the forced‐regressive ice‐marginal deltas are mainly extensional features, including normal faults, small graben or half‐graben structures and shear‐deformation bands, which are related to gravitational delta tectonics, postglacial faulting during glacial‐isostatic adjustment, and crestal collapse above salt domes. A neotectonic component cannot be ruled out in some cases.

Indium Mineralization in the Xianghualing Sn-Polymetallic Orefield in Southern Hunan, Southern China

Although numerous W–Sn–Pb–Zn polymetallic deposits are located in southern Hunan, and In-bearing deposits are related to W–Sn–Pb–Zn polymetallic deposits, Indium mineralization in southern Hunan is poorly studied. In order to investigate the In mineralization of the Xianghualing orefield, which is a typical orefield in southern Hunan, ore bulk chemistry, microscopic observation, and electron-probe microanalysis of vein-type (type-I) and porphyry-type (type-II) Sn–Pb–Zn orebodies were studied. The In contents of the type-I orebodies varies from 0.79 to 1680 ppm (avg. 217 ppm, n = 29), and that of the type-II orebodies varies from 10 to 150 ppm (avg. 64 ppm, n = 10). Although chalcopyrite and stannite contain trace amounts of In, sphalerite is the most important In-rich mineral in the orefield. Sphalerite in type-I orebodies contains from <0.02 to 21.96 wt % In, and in type-II orebodies contains from <0.02 to 0.39 wt % In. Indium-rich chemical-zoned sphalerite contains 7 to 8 wt % In in its core and up to 21.96 wt % In in its rim. This sphalerite may be the highest In-bearing variety in Southern China. The Cd contents of the In-rich sphalerite ranges from 0.35 to 0.45 wt %, which places it in the the “Indium window” of the Cu–In–S phases. The geological and structural features of the Xianghualing orefield indicate that the In mineralization of the two types of In-bearing Sn–Pb–Zn orebodies is related to the volatile-rich, In-rich, A-type granites, and is controlled by the normal faults of magmatic-diapiric activity extensional features.

40Ar/39Ar geochronology reveals rapid change from plume‐assisted to stress‐dependent volcanism in the Newer Volcanic Province, SE Australia

AbstractHere we present 40Ar/39Ar ages of volcanic features in the Cenozoic intraplate Newer Volcanic Province in southeast Australia. The <5 Ma volcanic products in the Newer Volcanic Province can be subdivided into tholeiitic, valley‐filling Newer Plains basalts, and alkaline scoria cones, lava shields, and maars of the Newer Cones series. Plateau ages range from 3.76 ± 0.01 to 4.32 ± 0.03 Ma (2σ; all sources of uncertainties included) for the Newer Plains series, with production rates of volcanism decreasing post 4 Ma. We suggest that magmatism is related to the complex interplay of magma upwelling due to edge‐driven convection and the Cosgrove track mantle plume located in the northeast of the province at 6.5–5 Ma. Plateau ages range from 1290 ± 20 to 41.1 ± 2.2 ka (2σ) for the Newer Cones series, with a diffuse age progression in the onset of volcanism for these features from east to west. Analyses of the distribution and geomorphology of these volcanic features indicates a strong control of basement faults on volcanism, reflected in alignment of volcanic features along Paleozoic north‐south oriented basement faults in the east and Cretaceous northwest‐southeast oriented extensional features in the west. This age progression can be explained by a westerly migration of stress derived from the left‐lateral strike‐slip Tasman Fracture Zone. This suggests that the general mechanism of volcanism changed from upwelling due to plume‐assisted edge‐driven convection prior to ∼4 Ma to stress‐dependent upwelling at around 1.3 Ma.

Tectonics and Sedimentation of SW Sarawak Basin, Malaysia, NW Borneo

ABSTRACT Structurally SW Sarawak basin is a southward sloping basement characterized by passive margin tectonic that has undergone through varioius tectonic phases viz., Triassic extension, Cretaceous transpression and Oligo-Miocene compression. Rock types and sedimentation of deeper basin zone situated between Schwaner mountains block to the south and SW Sarawak basin to the north suggest progressive marine sedimentation. E-W trending Cretaceous carbonate platform (CCP) occurs in the SW Sarawak basin signify a shelf zone where shallow marine sedimentation progressed during Cretaceous transpression. Oligo- Miocene volcanics from subduction melts intercepted basin profusely forming northwest-southeast trending continental arc zone derived from partial melting of subducting slab underneath SW Sarawak basin. Back-arc extension prevailed during Oligo- Miocene and formed several extensional features. Oligo-Miocene subduction also resulted in closure and exhumation of Sri Aman marginal sea-basin to the east. SW Sarawak basin is further divided in two sub-basins viz., Senibong to the west and Kuching to the east separated by a northeast trending morphotectonic ridge that signify structural element formed due to shearing. Marine sedimentation progressed in these sub-basins mainly during Triassic–Jurassic while tidal and fluviatile sedimentation progressed during early to mid-Tertiary having total thickness of sediments about 9 km. Basin closure and exhumation is marked mainly by the formation of Cretaceous carbonate build-up that has been intruded and dissected by the Oligo-Miocene volcanics. Senibong and Kuching sub-basins are characterized by wide range of transpressive features, while, Sri Aman marginal sea-basin is characterized by oceanic assemblages, ophiolite, serpentinite and pillow basalt.

Transfer zones in an oblique back-arc basin setting: Insights from the Latium-Campania segmented margin (Tyrrhenian Sea)

New multichannel seismic reflection profiles were acquired to unravel the structure of a portion of the eastern margin of the Tyrrhenian basin. This extensional feature is part of an Oligocene to Present back-arc basin in the hangingwall of the west directed Apennines subduction system. The basin provides excellent conditions to investigate the early stage processes leading to the development of rifted passive margins and to the emplacement of oceanic crust in an oblique setting. The interpreted post-stack-migrated seismic profiles highlight the geometry and kinematics of the Pontine escarpment that connects the Latium-Campania continental margin to the Vavilov basin. The latter is the main feature of the area, related to the early Pliocene extension of the Tyrrhenian Sea. Several morphological variations are pointed out along strike, mirroring different structural settings of the margin itself: a steeper margin to the north corresponds to high-angle possibly transtensional faults, whereas a smooth slope in the southern portion corresponds to several more distributed listric faults. This work contributes to the understanding of the interplay between extensional and transtensional tectonics along the margins of an oblique back-arc basin.

The effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low-angle normal faults

To investigate the formation of core complexes and low-angle normal faults, we devise thermo-mechanical simulations on a simplified wedge-like orogenic hinterland that has initial topography, Moho relief, and a preexisting mid-crustal shear zone that can accommodate shear at very low angles (<20°). We mainly vary the strength of the lower crust and the frictional strength of the preexisting mid-crustal shear zone. We find that the strength of the lower crust, and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes. With increasing lower crustal strength, we recognize varying extensional features with decreasing exhumation rate: these are characterized by bivergent metamorphic massifs, classic Cordilleran metamorphic core complexes, multiple consecutive core complexes (or boudinage structures), and a flexural core complex underlined by a large subsurface low-angle detachment fault with a small convex curvature. Topographic loading and mantle buoyancy forces, together with divergent boundaries, drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading. The detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust. Though the regional stress is largely Andersonian, we find non-Andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient. Our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve.

Petrogenesis and provenance of distal volcanic tuffs from the Permian–Triassic Karoo Basin, South Africa: A window into a dissected magmatic province

We present zircon rare earth element (REE) compositions integrated with U-Pb ages of zircon and whole-rock geochemistry from 29 volcanic tuffs preserved in the Karoo Supergroup, South Africa, to investigate the history of magmatism in southern Gondwana. Whole-rock compositions suggest a subduction-driven magmatic arc source for early (before 270 Ma) to middle Permian (270–260 Ma) Karoo tuffs. After ca. 265 Ma, the magmatic source of the volcanic deposits transitioned toward intraplate shallow-sourced magmatism. Zircon U-Pb ages and REE chemistry suggest that early to middle Permian magmas were oxidizing, U- and heavy (H) REE–enriched, melts; middle Permian to Triassic zircons record HREE-depleted, more reduced magmatism. Middle Permian to Triassic tuffs contain increasingly large volumes of zircon cargo derived from assimilated crustal material; therefore magmas may have been zircon undersaturated, resulting in less zircon growth and increased inheritance in late Permian to Triassic Gondwanan volcanics. Zircon U-Pb ages and zircon REE chemistry suggest a shift from arc magmatism in the early Permian to extensional magmatism by the late Permian, which may be associated with development of a backarc magmatic system adjacent to western Antarctica that predates known extensional volcanism elsewhere in Gondwana. Opening of the Southern Ocean in the Jurassic–Cretaceous paralleled this extensional feature, which may be related to reactivation of this Permian–Triassic backarc. This study demonstrates the potential of zircon U-Pb age and REE compositions from volcanic tuffs preserved in sedimentary strata to provide a more complete record of magmatism, when the magmatic province has been largely lost to active tectonism.

The inverted Triassic rift of the Marrakech High Atlas: A reappraisal of basin geometries and faulting histories

The High Atlas of Morocco is an aborted rift developed during the Triassic–Jurassic and moderately inverted during the Cenozoic. The Marrakech High Atlas, with large exposures of basement and Triassic early syn-rift deposits, is ideal to investigate the geometries of the deepest parts of a rift, constituting a good analogue for pre-salt domains. It allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another. A detailed structural study of the main Triassic basins and basin-margin faults of the Marrakech High Atlas shows that only a few rift faults were reactivated during the Cenozoic compressional stage in contrast to previous interpretations, and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems. Preserved extensional features demonstrate a dominant dip-slip opening kinematics with strike-slip playing a minor role, at variance to models proposing a major strike-slip component along the main basin-bounding faults, including faults belonging to the Tizi n'Test fault zone. A new Middle Triassic paleogeographic reconstruction shows that the Marrakech High Atlas was a narrow and segmented orthogonal rift (sub-perpendicular to the main regional extension direction which was ~NW–SE), in contrast to the central and eastern segments of the Atlas rift which developed obliquely. This difference in orientation is attributed to the indented Ouzellarh Precambrian salient, part of the West African Craton, which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution. As for the Cenozoic inversion, total orogenic shortening is moderate (~16%) and appears accommodated by basement-involved large-scale folding, and by newly formed shortcut and by-pass thrusting, with rare left-lateral strike-slip indicators. Triassic faults commonly acted as buttresses.

Methods in Geoscience Field Instruction: University of Nebraska-Lincoln

Eight in-service teachers, one pre-service education student, three observers from other universities, and two instructors from the University of Nebraska-Lincoln engaged in an inquiry-based geology field course from June 13 to 28, 2015 through Wyoming, South Dakota, and Nebraska. This commnity of learners spent three days working in the Grand Teton National Park area. Geological features and history present in Grand Teton National Park are an important part of the course curriculum. Large-scale extensional features of the Teton Range and Jackson Hole, and the glacial geomorphology and related climate changes of this area are some of the unique features examined here.

Extensional salt tectonics in the partially inverted Cotiella post-rift basin (south-central Pyrenees): structure and evolution

The Cotiella Massif in the south-central Pyrenees hosts upper Cretaceous gravity-driven extensional faults which were developed in the Bay of Biscay–Pyrenean paleorift margin of the Atlantic Ocean. They accommodate up to 6 km of post-rift carbonates above relict upper Triassic salt. Subsequent Pyrenean contractional deformation preserved the main extensional features, but most of the upper Triassic salt was expulsed and then dissolved, leaving little indications of the original salt volume. Nonetheless, several distinctive salt-related features are still recognizable both at outcrop and at basin scale, providing an exposed analogue for salt-floored extensional basins developed on passive margins. Based on field research, we re-interpret the tectonic evolution of the area and suggest that passive diapirs were coeval with gravity-driven extension during the development of the Cotiella basin. The given interpretations are supported with detailed geological maps, original structural data, cross sections and outcrop photographs. The discovery of previously unknown post-rift salt structures in the Cotiella Massif is an extra element to consider in the paleogeographic reconstructions of the upper Cretaceous passive margin of the Bay of Biscay–Pyrenean realm and consequently helps in our understanding of the evolution of current Atlantic-type margins.

Plate tectonics on ice

Jupiter's icy moon Europa is criss-crossed by extensional features. A tectonic reconstruction suggests that Europa's extension is balanced by subduction — if so, Earth may not be the only planetary body with a plate tectonic system.