Basal Thrust Research Articles

Seismicity and décollement geometry beneath the Sikkim - Darjeeling Himalaya using local earthquake tomography: Implications for seismotectonic and Seismogenesis

In this study, we attempted to establish the plausible depth of disposition of the basal décollement through 3-D seismic imaging using local earthquake tomography to understand the linkage between seismogenesis and collisional tectonics of the Sikkim-Darjeeling Himalayan region. We analysed 2105 events recorded by 33 broadband seismograph stations during 2005–2011. Detailed 3-D seismic imaging unravelled heterogeneous velocity structures (Vp and Vs) with distinct variations in Poisson's ratio (ϭ) at different depth ranges. A well-resolved low-velocity zone is found to persist consistently at a depth of about 20 km corresponding to the gentle north dipping décollement plane, exhibiting significant velocity perturbations across this interface. The geometry and depth of the décollement surface vary laterally, and its deepest disposition is observed at a depth of ∼30 km. The shallower low-velocity zone up to 10 km with higher-ϭ suggests the presence of the quaternary piedmont sediments with high fluid saturations. In contrast, the higher velocity perturbation with low ϭ at the mid-crustal layer is indicative of the competent parts of the crust, where seismogenesis is related to the occurrence of earthquakes of varying strengths beneath the Sikkim - Darjeeling Himalayan region. Our seismic imaging corroborates with the past moderate earthquakes that were confined to the vicinity of high and low velocities (Vp and Vs) and ϭ zones, whilst the strong size earthquakes (M > 6.0) occurred mostly below the décollement plane where high-velocity basement thrust exists along with Indian subducting plate. This study clearly expresses the well-defined disposition of the décollement zone that controls the nature and extent of seismogenesis.

A contribution to the quantification of crustal shortening and kinematics of deformation across the Western Andes ( ∼ 20–22° S)

Abstract. The Andes are an emblematic active Cordilleran orogen. Mountain building in the Central Andes (∼20∘ S) started by the Late Cretaceous to early Cenozoic along the subduction margin and propagated eastward. In general, the structures sustaining the uplift of the western flank of the Andes are dismissed, and their contribution to mountain building remains poorly constrained. Here, we focus on two sites along the Western Andes at ∼20–22∘ S in the Atacama desert, where structures are well exposed. We combine mapping from high-resolution satellite images with field observations and numerical trishear forward modeling to provide quantitative constraints on the kinematic evolution of the investigated field sites. When upscaling our local field interpretations to the regional scale, we identify two main structures: (1) the Andean Basement Thrust, a west-vergent thrust system placing Andean Paleozoic basement over Mesozoic strata, and (2) a series of west-vergent thrusts pertaining to the West Andean Thrust System, deforming primarily Mesozoic units. From our interpreted sections, we estimate that both structures together accommodate at least ∼6–9 km of shortening across the sole investigated ∼7–17 km wide field sites. This multi-kilometric shortening represents only a fraction of the total shortening accommodated across the whole Western Andes. The timing of the main deformation recorded in the folded Mesozoic series can be bracketed between ∼68 and ∼29 Ma – and possibly between ∼68 and ∼44 Ma – from dated deformed geological layers, with a subsequent significant slowing-down of shortening rates. Even though the structures forming the Western Andes only absorbed a small fraction of the total shortening across the whole orogen, their contribution was relatively significant at the earliest stages of Andean mountain building before deformation proceeded eastward.

Parametric mechanical analysis of thin- versus thick-skinned tectonics applied to the Jura belt

Field observations and seismic interpretations testify that the front of the Jura fold-and-thrust belt is still submitted to compressive deformation, but whether the basement is deforming (thick-skinned) or not (thin-skinned) is an active question. We propose a mechanical point of view using the Kinematic approach of the Limit Analysis theory (KLA). We first draw cross-sections containing a major shallow décollement level in the Triassic evaporites, including the Alps up to the topographic maximum and including the whole crust. We submit the cross-sections to a compressive force at their southern end, and the KLA determines the location and geometry of the incipient ruptures by optimisation of the associated compressive force, accounting for force balance and for the rock strength (Coulomb criterion). Five cross-sections span the whole Jura from west to east, allowing us to explore the lateral variations. From the analysis of 500 simulations (100 for each cross-section), varying the friction angles of the Triassic décollement and of the lower crust between 1° and 10°, we have identified five types of tectonics at the Jura front depending on the emergence of a basement thrust beyond the Jura front (type 1), at the Jura front (type 2) with simultaneous activation of the shallow décollement (type 3), or south of the Jura front (type 5), with activation of the shallow décollement at the Jura front (type 4). The analysis allows us to draw two conclusions. First, the transitions between the various tectonic styles occur abruptly upon continuous changes in the friction parameters, revealing a threshold behaviour that we interpret as an extension of the concept of wedge criticality in the Critical Coulomb Wedge theory: at criticality, several tectonic types may occur within a narrow, critical range of parameter values. Second, the critical range evolves systematically between cross-sections, in such a way that the front of the thick-skinned deformation crosses laterally the Jura belt. The two most western cross-sections exhibit only thin-skinned or no tectonics at the Jura front (types 1, 4 and 5), the central one hosts all five styles, and the two Eastern ones show thick-skinned solutions (types 1, 2 and 3), for all values tested. We also show that a thick-skinned tectonic style can be accompanied by a simultaneous activation of the shallow Triassic décollement (type 4), complicating the interpretation of apparent thin-skinned field structures. Overall, our simulations yield tectonic styles compatible with natural seismicity and GPS data for values of the lower-crust friction angle below 3 to 7°. Modifications of our cross-sections to explore the effect of a bumpy upper/lower crust interface, or of a major décollement at the upper/lower crust interface, or of higher cohesion values, show that the numerical outcomes are rather robust. They only slightly modify the critical ranges at which the tectonic changes occur. These findings may serve as guides, or first order questions, for more sophisticated mechanical approaches including temperature and rate-dependent rheologies and the three dimensions of space that are necessary to capture the competition between compressive and strike-slip tectonic modes.

Large thrusting and late faulting shape the Aiguilles Rouges crystalline massif (Western Alps), structural implications

Although the Helvetic nappes of the Western Alps have been widely studied, their style, amount, and timing of deformation during the Alpine orogeny remain a subject of controversy. The discussion partly arises from various interpretations of the external crystalline massifs geometry and of the cover/basement relationships. Here, we apply structural analysis based on new field data, microtectonics, and 3D geological modeling on the Aiguilles Rouges and Mont-Blanc massifs, to better quantify their tectonic evolution and their relationship with the nappe stacking. Our results show large thrusting of the Helvetic Mesozoic sedimentary cover above the Aiguilles Rouges basement towards the WNW. The basal contact of the Helvetic nappes, which we call the Helvetic Basal thrust (HBT), often corresponds to a décollement level localized above the Triassic unconformity, but that locally also affects the basement. We suggest that most of the subalpine ranges are klippes above the HBT, and estimate the shortening of the thrust from previously published cross-sections to be between 13 and 32 km. According to published ages, we argue that the HBT was active in the Oligo-Miocene, between ∼30 and ∼14 Ma. After ∼14 Ma and until ∼6 Ma, the HBT is offset by late steep out-of-sequence faults, including back-thrusts and the Mont-Blanc shear zone that was previously a ramp of the HBT. The transition of the major thrust system from the HBT rooting above (east) of the Mont-Blanc to the Alpine sole thrust rooting below (west) of the Aiguilles Rouges occurred between 18 and 14 Ma.

Lithospheric double shear zone unveiled by microseismicity in a region of slow deformation

The deformation style of the continental lithosphere is a relevant issue for geodynamics and seismic hazard perspectives. Here we show the first evidence of two well-distinct low-angle and SW-dipping individual reverse shear zones of the Italian Outer Thrust System in Central Italy. One corresponds to the down-dip prosecution of the Adriatic Basal Thrust with its major splay and the other to a hidden independent structure, illuminated at a depth between 25 and 60 km, for an along-strike extent of ~ 150 km. Combining geological information with high-quality seismological data, we unveil this novel configuration and reconstruct a detailed 3D geometric and kinematic fault model of the compressional system, active at upper crust to upper mantle depths. In addition, we report evidence of coexisting deformation volumes undergoing well-distinguished stress fields at different lithospheric depths. These results provide fundamental constraints for a forthcoming discussion on the Apennine fold-and-thrust system's geodynamic context as a shallow subduction zone or an intra-continental lithosphere shear zone.

The Importance of Rift Inheritance in Understanding the Early Collisional Evolution of the Western Alps

We reassess the architecture and tectonic history of the Western Alps based on recent knowledge developed at rifted margins. First, we replace the main Alpine units of our study area into a synthetic rifted margin template based on diagnostic petrologic, stratigraphic, and structural criteria. We find that some units previously attributed to the internal part of the thick-crusted Briançonnais domain may rather derive from the thin-crusted Prepiemonte hyperextended domain. We assert that the Briançonnais and Prepiemonte domains were separated by a mega-fault scarp. Second, we revisit the Paleogeography of the Alpine Tethys, suggesting that the Briançonnais was a ribbon of little thinned continental crust between two overstepping en-échelon rift basins, namely the Valais domain to the northwest and the Piemonte domain to the southeast. We affirm that this uneven-margin architecture can explain most of the Western Alps’ complexity. In our kinematic model, convergence between Adria and Europe was mainly accommodated by strike-slip movements in the Western Alps until the late Eocene. Orogeny began with the reactivation of the mega-fault scarp between the Briançonnais and Prepiemonte domains, which we name Prepiemonte Basal Thrust. Once hard collision started, the main shortening stepped inboard into the Valais/Subbriançonnais domain along the Penninic Basal Thrust.

Crustal focal mechanisms in the NW sierras de cordoba, and connections with mio-pliocene basement thrusting of the easternmost Sierras Pampeanas, south-Central Andes

The seismicity studied in this paper is located along the back arc region, between 30°35′ and 31°52’ S latitude, in the morphostructural unit known as Eastern Sierras Pampeanas. Seventeen crustal earthquakes could be identified and located, between June 2004 and March 2018, using seismic data recorded by the broadband seismological network of National Institute for Seismic Prevention (INPRES), combined with free seismic data from the Incorporated Research Institutions for Seismology (IRIS). Focal mechanisms were calculated from localized seismicity using the P-wave first motions and interpreted as a strain style on the western portion of Sierras de Córdoba. Overall, the seismicity in this region during the study period was constrained to the upper ∼25 km of the crust and is mostly concentrated along minor structures and not along the major range-bounding faults. Although there is evidence of quaternary activity along the larger structures, the relationship between these displacements and modern deformations is not well developed due to the lack of local seismic stations in the area and the long recurrence interval between earthquakes of significant magnitudes. However, very recent morphometric studies in the Sierras Pampeanas have analyzed the main topography controls and suggested relatively higher uplift rates in the central sectors of the Sierra de Pocho, and a balance between tectonic uplift and erosion rates. Nevertheless, three seismic events showed greater depths around 36 km according to previously proposed geological and geophysical models of crustal thickness and mohorovicic depth. Although focal mechanism solutions show varied results, distributed throughout all the region, it is possible to determine an inverse deformation with an oblique component assigned to regional faults and an area where a normal deformation with an oblique component predominates, near to the Ojo de Agua Lineament. Accordingly with seismological observations, we proposed a model in which the seismogenic structures seem to have detached from deep horizontal intra-crustal interfaces that affects the faults and brittle structures, which document that the Eastern Sierras Pampeanas are actively deforming in response to compression from the convergent margin.

Penetrative Strain and Partitioning of Convergence‐Related Shallow Crustal Shortening, Across Scales, in the Lesser‐ and Sub‐Himalayan Thrusts: Insights From the Eastern Himalaya, Sikkim

AbstractShortening estimates from regional balanced cross‐sections remain lower than the predicted shortening from paleomagnetism and area‐balancing methods in the Himalaya. To reconcile this shortening deficit, we evaluate the shortening contribution from the often‐ignored penetrative strain of quartz grains and integrate these results with two published regional balanced cross‐sections from Sikkim Himalaya. Penetrative strain progressively decreases from the internal to the external thrusts with changing deformation conditions. Strain increases toward the basal thrust zone within each thrust indicating strain localization. Strain ellipsoids are folded along with the thrusts indicating the strain initiated before thrusting and folding. The layer‐normal shortening (LNS) strain dominates the internal thrusts. A combination of LNS and layer‐parallel shortening (LPS) strain exist in the low‐strained external thrust sheets. Approximately 78–91 km of shortening is recorded as penetrative strain in the Lesser‐ and Sub‐Himalayan thrusts. Lateral structural variations in the thrusts are reflected in the thrust‐sheet scale shortening and in the strain ellipsoid orientations, but not in the grain‐scale shortening. Shortening is partitioned differently across scales; the duplex records the highest thrust‐sheet scale shortening, while the roof thrust toward the hinterland records the highest grain‐scale shortening. Due to the dominance of LNS strain, removing strain from thrusts will collectively decrease ∼54–59 km of base lengths, thereby reducing the total minimum shortening of the Sikkim Himalaya by ∼13%. Therefore, the penetrative strain does not contribute to the shortening deficit in the Sikkim Himalaya.

Active tectonics of the Thoubal‐Chandel Thrust, Manipur Hills, Indo‐Myanmar Range, northeast India: Insights into deformation pattern of adjacent thrust sheets in a growing orogen

The NE–SW trending and SE dipping Thoubal‐Chandel Thrust (TCT) in eastern Manipur Hills of the Indo‐Myanmar Range (IMR) places the Thoubal‐Chandel thrust sheet over the Churachandpur‐Mao thrust sheet. Despite being seismically active, the active tectonic aspects of the major structural discontinuities within the IMR, including TCT, are not yet studied in detail. The present study aims to examine the active tectonics of the TCT and other associated faults, with particular focus on deformation of the Churachandpur‐Mao, and Thoubal‐Chandel thrust sheets. Association and disposition of landforms, and statistical analyses of Mountain‐front Sinuosity Index (Smf), Stream Length‐Gradient Index (SL), Hypsometric Integral (HI), Drainage Basin Asymmetry Factor, and Transverse Topography Symmetry Factor (T) reveal that both of these adjacently lying thrust sheets are actively uplifting due to oblique‐slip movement on their basal thrusts, but with spatially variable rates. In general, higher SL (mode = 219) and HI (mode = 0.527) in large parts suggest faster uplift of the Thoubal‐Chandel thrust sheet than the Churachandpur‐Moa thrust sheet, which has comparatively lower SL (Mode = 140) and HI (mode = 0.416). However, differential along‐strike uplift rates have caused tilting of both the thrust sheets. Faster uplift of the northern part of Thoubal‐Chandel thrust sheet has caused its overall southward down‐tilting, whereas faster uplift of the southern part of Churachandpur‐Mao thrust sheet has caused its overall northward down‐tilting. The study brings out that the deformation pattern of even the adjacent thrust sheets of growing orogens could be at stark variance albeit the same stress regime.

New insights into the Rhône–Simplon fault system (Swiss Alps) from a consistent earthquake catalogue covering 35 yr

SUMMARYSeismotectonic interpretations in regions characterized by low to moderate seismicity require consistent earthquake catalogues covering periods of several decades. Inevitable changes in network configuration and analysing procedures, however, introduce significant bias to the hypocentre parameters and uncertainty estimates reported in such catalogues. To overcome these limitations, we developed a procedure using coupled hypocentre-velocity inversions to compute consistent hypocentre locations covering time periods of several decades while accounting for changes in network geometry. We apply these procedures to 35 yr of instrumentally recorded seismicity along the Rhône–Simplon fault system in southwest Switzerland, which is at the transition between the Central and Western Alps. The entire catalogue is relocated using a probabilistic location algorithm in combination with the derived minimum 1-D velocity models. A combination of location parameters is used to define consistent location-quality classes allowing for reliable interpretation of epicentres and focal depths. The relocated catalogue is interpreted together with a recent 3-D P-wave tomographic model and available 2-D reflection seismic profiles. The relocated hypocentres indicate that the major band of seismicity north of the Rhône valley is associated with a 30–40 km long, steeply north-dipping shear zone, which roots in the crystalline basement of the Aar Massif and extends to the shallowest levels of the sedimentary cover of the Helvetic nappes in the Rawil Depression. Seismicity towards the southwest indicates the existence of a similar shear zone within the Aiguille Rouge Massif. This zone possibly extends to the northeast and joins the Rawil fault zone. To the south of the Rhône valley, seismicity is scattered within the Penninic nappes, but limited to the hanging wall of the Pennine Basal Thrust (PBT). The Penninic nappes are characterized by a relatively higher VP of about 5 per cent compared to the Aar Massif, indicating differences in composition or metamorphic grade across the PBT.

The chronology and rotational kinematics in the South-Eastern Jaca Basin (Southern Pyrenees): Las Bellostas section

Despite the large number of magnetostratigraphic studies in the South Pyrenean Basin aiming to calibrate the basin chronostratigraphy and the biostratigraphic scales, the South Eastern Jaca Basin remains unexplored from this perspective, and its relation with the Ainsa Basin is not fully understood. In this work we contribute with new magnetostratigraphic data from the 950m thick Las Bellostas section, located in the northern hinge of the Balzes anticline. Well-proven primary signal (positive fold test and two pseudo-antiparallel polarities) supported by numerous primary data in the surroundings allow us building a reliable local polarity sequence of eight magnetozones (from R1 to N4). Additionally, seven new biostratigraphic samples (Nummulites and Assilina) in the lower part of the section (marine environment) allows tightening the section to the Geomagnetic Polarity Time Scale (GPTS) and proposing a refined age model for the southeastern Jaca Basin. The section starts in the Boltaña Fm., of Cuisian age (Shallow Benthic Zone, SBZ11), is followed by a sedimentary gap from C22n to C20n as witnessed by biostratigraphic data (SBZ11 underneath the hiatus and SBZ16 just atop). The deltaic Sobrarbe Fm. can be tracked until the C19n (Late Lutetian). From this point (200m) until the top of the section (950m), at least, the entire C18n chron can be recognized within the molassic Campodarbe Fm. (C18n.2n-C18n.1r and C18n.1n) (Bartonian) equivalent to the West to the prodeltaic Arguis marls Fm. The Middle Cusian (SBZ11) to Middle Lutetian (SBZ15) stratigraphic hiatus is, in part, enhanced by the structural position at the hinge of the Balzes anticline. These new chronostratigraphic constraints help refining the W-E and N-S stratigraphic relationships in the eastern Jaca Basin and in the Ainsa Basin. This section also allows us to accurately refine the kinematics of the rotational activity in the eastern External Sierras. The important difference in magnetic declination along the section and neighboring paleomagnetic data from the Balzes anticline (from ≈70º clockwise at the base of the stratigraphic section to non-significant at the top) together with the new age model for the Eastern Jaca Basin help characterizing the rotational activity of the Balzes thrust sheet. The rotation took place between chrons C20r (Middle Lutetian; 45Ma) and C17 (Lower Priabonian 37-38Ma) in agreement to nearby structures (Boltaña, Picodel Aguila anticlines) but clearly diachronic to western ones (Santo Domingo anticline). Besides, the rotational activity seems to follow a linear and continuous pattern (velocity 9-11º/Ma, R: 0.83-0.96) in contrast to closer structures that show two distinct rotational velocities (i.e. Boltaña). These new data still let open the debate on the rotational kinematics along the South Pyrenean basal thrust.

Assessing the Internal Uppermost Crustal Structure of the Central Pyrenees by Gravity‐Constrained Cross Sections

AbstractThe Pyrenees constitutes an exceptional example of an Alpine orogenic belt characterized by basement thrust sheets involving Paleozoic rocks and Mesozoic and Cenozoic cover units detached on the Triassic evaporites, the main décollement level. This work is located in the Central Pyrenees, where gravity data help to better constrain the internal architecture of the upper crust of the southern half of the Axial Zone and the northern part of the South Pyrenean Zone, a key area to understand the orogenic evolution of the chain. Previous and new gravity, petrophysical and geological data have been used to obtain the Bouguer and residual anomaly maps of the study area and six serial gravity‐constrained cross sections perpendicular to the main structural trend. The residual anomaly map shows a good correlation between basement units involved in thrust sheets of the study area and gravity highs whereas negative anomalies are interpreted to correspond with Mesozoic/Cenozoic basins, Triassic evaporites, Late Variscan igneous bodies, and Ordovician gneisses. The six gravity‐constrained cross sections highlight strong along‐strike variations on the gravity signal due to lateral differences of the superficial and subsurface occurrence of Triassic evaporites, different geometry at depth of the Late Variscan igneous bodies outcropping in the study area, and geometric lateral variations of the basement thrust sheets and their relationship with the Mesozoic‐Cenozoic units.

Ship-board determination of whole-rock (ultra-)trace element concentrations by laser ablation-inductively coupled plasma mass spectrometry analysis of pressed powder pellets aboard the D/V <i>Chikyu</i>

Abstract. The Oman Drilling Project (OmanDP), performed under the International Continental Scientific Drilling Program (ICDP), is an international scientific research project that undertook drilling at a range of sites in the Semail ophiolite (Oman) to collect core samples spanning the stratigraphy of the ophiolite, from the upper oceanic crust down to the basal thrust. The cores were logged to International Ocean Discovery Program (IODP) standards aboard the D/V Chikyu. During ChikyuOman2018 Leg 3 (July–August 2018), participants described cores from the crust–mantle transition (CM) sites. The main rock types recovered at these sites were gabbros, dunites and harzburgites, rocks typically forming the base of the oceanic crust and the shallow mantle beneath present-day spreading centres. In addition to the core description, selected samples were analysed by X-ray fluorescence spectrometry (XRF) for their chemical compositions, including major, minor and some trace elements. To complement these standard procedures, we developed new approaches to measure ultra-trace element concentrations using a procedure adapted from previous works to prepare fine-grained pressed powder pellets coupled with laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis using instrumentation aboard the D/V Chikyu. First, three (ultra)mafic reference materials were investigated to test and validate our procedure (BHVO-2, BIR-1a and JP-1), and then the procedure was applied to a selection of gabbro and dunite samples from the CM cores to explore the limitations of the method in its current stage of development. The obtained results are in good agreement with preferred values for the reference materials and with subsequent solution replicate analyses of the same samples performed in shore-based laboratories following Leg 3 for the CM samples. We describe this procedure for the determination of 37 minor and (ultra-)trace elements (transition elements and Ga, Li and Large-Ion Lithophile Elements (LILE), Rare Earth Elements (REE), High-Field-Strength Elements (HFSE), U, Th, and Pb) in mafic and ultramafic rocks. The presented method has the major advantage that it allows the determination at sea of the (ultra-)trace element concentrations in a “dry”, safe way, without using acid reagents. Our new approach could be extended for other elements of interest and/or be improved to be adapted to other rock materials during future ocean drilling operations aboard the D/V Chikyu and other platforms.

Listvenite Formation During Mass Transfer into the Leading Edge of the Mantle Wedge: Initial Results from Oman Drilling Project Hole BT1B

AbstractThis paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO2‐rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid‐rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front.

Tectonic control on the palaeogeographical evolution of the Miocene Seaway along the Western Alpine foreland basin

Abstract The Miocene of the Western Alpine foreland basin were deposited in a north–south seaway along the active alpine orogenic front. In the subalpine massifs and the southern Jura mountains, the revised Miocene stratigraphy documents a detailed chronology of thrust propagation at the western alpine front, where tectonic activity had a primary influence on seaway palaeogeographical evolution. Here we propose nine palaeogeographical maps during the Miocene, the first of which depicts the initial Miocene transgression at c. 21.0 Ma. Between c. 18.05 and c. 12.0 Ma, a westward retreat of the Miocene Sea occurred in response to activation of the basal thrust of the Belledonne massif, which in turn triggered successive fault zones from east to west. At c. 10.0 Ma, a major uplift phase intervened and induced a rapid southward retreat of the Miocene Sea. The reconstructed palaeogeographical maps outline the main controls on the foreland basin seaway evolution: (1) the timing of the main thrusts; (2) the inherited palaeotopography; and (3) eustatic sea-level changes during the Miocene. These reconstructions are integrated at the basin scale, highlighting the southward- to westward-directed seaway migration in response to the Belledonne thrust activity that deeply shaped the palaeogeographical evolution during the early to middle Miocene.

Deep Sourced Fluids for Peridotite Carbonation in the Shallow Mantle Wedge of a Fossil Subduction Zone: Sr and C Isotope Profiles of OmanDP Hole BT1B

AbstractCompletely carbonated peridotites represent a window to study reactions of carbon‐rich fluids with mantle rocks. Here, we present details on the carbonation history of listvenites close to the basal thrust in the Samail ophiolite. We use samples from Oman Drilling Project Hole BT1B, which provides a continuous record of lithologic transitions, as well as outcrop samples from listvenites, metasediments, and metamafics below the basal thrust of the ophiolite. 87Sr/86Sr of listvenites and serpentinites, ranging from 0.7090 to 0.7145, are significantly more radiogenic than mantle values, Cretaceous seawater, and other peridotite hosted carbonates in Oman. The Hawasina sediments that underlie the ophiolite, on the other hand, show higher 87Sr/86Sr values of up to 0.7241. δ13C values of total carbon in the listvenites and serpentinites range from −10.6‰ to 1.92‰. We also identified a small organic carbon component with δ13C as low as −27‰. Based on these results, we propose that during subduction at temperatures above >400°C, carbon‐rich fluids derived from decarbonation of the underlying sediments migrated updip and generated the radiogenic 87Sr/86Sr signature and the fractionated δ13C values of the serpentinites and listvenites in core BT1B.

A regional scale Cretaceous transform fault zone at the northern Austroalpine margin: Geology of the western Ammergau Alps, Bavaria

Abstract We reinvestigated parts of the northern Austroalpine margin and provided structural and kinematic field data in order to interpret the kinematic relationship between the Cenoman-Randschuppe (CRS) marginal slice, Falkensteinzug (FSZ), Tannheim- and Karwendel thrust sheets occurring in a narrow strip at the northern front of the northwestern Northern Calcareous Alps (NCA). As a consequence, we propose a revised model for the tectonic evolution of the northern Austroalpine margin. As thrusting propagates from SSE to NNW (Cretaceous orogeny), the Karwendel thrust sheet (including its frontal part, the FSZ) was emplaced onto the Tannheim thrust sheet in the Albian, deduced from (i) upper-footwall deposits, the youngest sediments below the Karwendel thrust (Tannheim- and Losenstein Fms.), and (ii) thrust-sheet-top deposits unconformably overlying the deeply eroded northern Karwendel thrust sheet (Branderfleck Fm.). The future CRS marginal slice was, at that time, part of the foreland of this Early Cretaceous Alpine orogenic wedge. Pervasive overprint by sinistral shear within the CRS marginal slice and northern Tannheim thrust sheet suggests sinistral W-E striking transform faults cutting across this foreland, decoupling CRS marginal slice and FSZ from the main body of the NCA and enabling an independent evolution of the CRS marginal slice from the Early Cretaceous onwards. Subsequent Late Cretaceous and younger shortening leads to successive incorporation of Arosa zone, Rhenodanubian Flysch (RDF) and Helvetic units into the Alpine nappe stack; the Tannheim thrust representing the basal thrust of the NCA. Growth strata within thrust-sheet-top deposits (Branderfleck-Fm.) give evidence for refolding of thrust sheet boundaries. In a typical thin-skinned fold-and-thrust belt, deformation should cease towards the thrust front, whereas within the NCA it increases. An Austroalpine thrust front controlled by E-trending transform faults could cause an increase in deformation towards the most external NCA and explain the absence of the Arosa zone between Allgäu and Vienna. Such faults would most probably also cut out Lower Austroalpine units. Therefore, RDF and CRS marginal slice are juxtaposed; the latter found in the tectonic position of the Arosa zone. The presence of transform faults underlines the strong imprint of the opening of the North Atlantic Ocean on the depositional setting and tectonic evolution of the NCA.

Chronology of thrust propagation from an updated tectono-sedimentary framework of the Miocene molasse (western Alps)

Abstract. After more than a century of research, the chronology of the deformation of the external part of the western Alpine belt (France) is still controversial for the Miocene epoch. In particular, the poor dating of the foreland basin sedimentary succession hampers a comprehensive understanding of the deformation kinematics. Here we focus on the Miocene molasse deposits of the northern subalpine massifs, southern Jura, Royans, Bas-Dauphiné, Crest, and La Bresse sedimentary basins through a multidisciplinary approach to build a basin-wide tectono-stratigraphic framework. Based on sequence stratigraphy constrained by biostratigraphical, chemostratigraphical (Sr isotopes), and magnetostratigraphical data between the late Aquitanian (∼ 21 Ma) and the Tortonian (∼ 8.2 Ma), the Miocene molasse chronostratigraphy is revised with a precision of ∼ 0.5 Ma. The Miocene molasse sediments encompass four different paleogeographical domains: (i) the oriental domain outlined by depositional sequences S1a to S3 (∼ 21 to ∼ 15 Ma), (ii) the median domain characterized by sequences S2 to S5 (∼ 17.8 to ∼ 12 Ma), (iii) the occidental domain in which sequences S2a to S8 (∼ 17.8 to ∼ 8.2 Ma) were deposited, and (iv) the Bressan domain where sedimentation is restricted to sequences S6 to S8 (∼ 12 to ∼ 8.2 Ma). A structural and tectono-sedimentary study is conducted based on new field observations and the reappraisal of regional seismic profiles, thereby allowing the identification of five major faults zones (FZs). The oriental, median, and occidental paleogeographical domains are clearly separated by FZ1, FZ2, and FZ3, suggesting strong interactions between tectonics and sedimentation during the Miocene. The evolution in time and space of the paleogeographical domains within a well-constrained structural framework reveals syntectonic deposits and a westward migration of the depocenters, allowing for proposing the succession of three deformation phases at the western Alpine front. (i) The first is a compressive phase (P1) corresponding to thrusting above the Chartreuse oriental thrust (FZ1), which was likely initiated during the Oligocene and rooted east of Belledonne. This tectonic phase generated reliefs that limited the Miocene transgression to the east. (ii) The second is a ∼ WNW–ESE-directed compressive phase (P2) lasting between 18.05 ± 0.25 Ma and ∼ 12 Ma, with thrusts rooted in the Belledonne basal thrust. Thrusts were activated from east to west: the Salève (SAL) and Gros Foug (GF) thrusts and then successively FZ2, FZ3, FZ4, and FZ5. Along two WNW–ESE balanced cross sections the amount of horizontal shortening is of ∼ 6.3 to 6.7 km, corresponding to average shortening rates of ∼ 1.2 km Myr−1 and migration of the deformation toward the west at a rate of ∼ 2.9 km Myr−1. During ∼ 6 Myr, the Miocene Sea was forced to regress rapidly westwards in response to westward migration of the active thrusts and exhumation of piggyback basins atop the fault zones. Phase P2 thus deeply shaped the Miocene paleogeographical evolution of the area and appears as a prominent compressive phase at the scale of the western Alps from the Swiss molasse basin to the Rhodano–Provencal one. (iii) The third is a ∼ 300 m phase of uplift in the Bas-Dauphiné (P3) of probable Tortonian age (∼ 10 Ma), which would have induced southward sea retreat and been coeval with the folding of the Jura in the north and possibly with back-thrusting east of the Chartreuse massif.

Basement and cover architecture in the Central Pyrenees constrained by gravity data

A new gravity survey (1164 gravity stations and 180 samples for density analysis) combined with two new geological cross sections has been carried out in a sector of the Central Pyrenees in order to improve the characterization of basement and cover architecture. From North to South, the study area comprises the southern half of the Axial Zone and the northernmost part of the South-Pyrenean Zone. New gravity data were combined with previous existing databases to obtain the Bouguer and residual anomaly maps of the study area. The two cross sections, oriented NNE–SSW, were built from field data and previous surficial and subsurface data and cross the La Maladeta plutonic complex. The residual anomaly map shows values ranging from −18 to 16 mGal and anomalies mainly oriented N120E. The two 2.5D modelled cross sections show similar observed gravity curves coinciding with similar interpreted structural architecture. Data show a gravity high oriented N120E coinciding with the Orri basement thrust sheet and an important gravity depression, with the same orientation, coinciding with the leading edge at depth of the Rialp basement thrust sheet and interpreted as linked to a large subsurface accumulation of Triassic evaporites. The volume at depth of the La Maladeta and Arties granites has been constrained through gravity modelling. This work highlights that the combination of structural geology and gravity modelling can help to determine the structural architecture of an orogen and localize accumulations of evaporites at depth.

Geochemical Profiles Across the Listvenite‐Metamorphic Transition in the Basal Megathrust of the Semail Ophiolite: Results From Drilling at OmanDP Hole BT1B

AbstractThe transition from the Semail ophiolite mantle to the underlying metamorphic sole was drilled at ICDP OmanDP Hole BT1B. We analyzed the bulk major, volatile and trace element compositions of the mantle‐derived listvenite series and metamorphic rocks, with the aim to constrain chemical transfers associated with peridotite carbonation along the ophiolite basal thrust. The listvenite series comprise variously carbonated serpentinites and (fuchsite‐bearing) listvenites. They have high CO2 (up to 43 wt.%) and variable H2O (0–12 wt.%). Yet, they have compositions close to that of the basal banded peridotites for most major and lithophile trace elements, with fuchsite‐bearing listvenites overlapping in composition with amphibole‐bearing basal lherzolites (e.g., Al2O3 = 0.1–2.2 wt.%; Yb = 0.05–1 x CI‐chondrite). The protolith of the listvenite series was likely similar in structure and composition to serpentinized banded peridotites which immediately overlie the metamorphic sole elsewhere in Oman. The listvenite series are enriched in fluid mobile elements (FME) compared to Semail peridotites (up to ∼103–104 x Primitive Mantle), with concentrations similar to the underthrusted metabasalts and/or metasediments for Cs, Sr and Ca and sometimes even higher for Pb, Li, As, and Sb (e.g., Li up to 130 μg/g; As up to 170 μg/g). We also observe a decoupling between Sr‐Ca enrichments and other FME, indicating interactions with several batches of deep CO2‐rich fluids transported along the basal thrust. These results suggest that peridotite carbonation could represent one of the major trap‐and‐release mechanisms for carbon, water and FME along convergent margins.