Tectonic Escape Research Articles

Eocene contractional deformation in the NW corner of the Arabian plate and its relation to Arabia-Eurasia collision in SE Türkiye

ABSTRACT Field evidence indicates c. 40 Ma (Early-Middle Eocene) contractional deformation in the S Amanos Mountains. Following latest Cretaceous southward ophiolite emplacement, an intra-platform basin infilled with shallow to deeper marine, dominantly carbonate sediments. Localized thrusting, reverse faulting, and folding were followed by subaerial erosion and non-marine clastic deposition, sealed by an Early Miocene marine transgression. Eocene emergence elsewhere in SE Türkiye probably also resulted from regional crustal shortening. Alternative hypotheses for collision in SE Türkiye propose latest Cretaceous, Mid-Late Eocene, Early Miocene, or Late Miocene suturing of S Neotethys. Much geological evidence in SE Türkiye and NW Iran supports Mid-Late Eocene initial continent-continent collision. Plate reconstructions suggest S Neotethys survived in this region until the Mid-Late Eocene. Biogeographical evidence points to minor Eocene mammal migration across S Neotethys, followed by Early Miocene large-scale terrestrial migrations. Seawater isotopic data suggest a major decrease in inter-Indian Ocean-Mediterranean Sea current flow during the Early Miocene. Instrumental geophysics and both numerical and analogue modelling suggest that the northern Arabian platform underthrust Eurasia (>200 km in S central Iran). In SE Türkiye, Eocene initial ‘soft’ collision was followed by Early Miocene ‘hard collision’, with crustal thickening and exhumation continuing until the Late Miocene, followed by westward ‘tectonic escape’.

Tectonic evolution of high grade metamorphic tectonites of the Meelpaeg structure near Port aux Basques, southwestern Newfoundland during the Silurian Salinic and Early to Middle Devonian Acadian orogenies

The Meelpaeg structure in southwestern Newfoundland comprises allochthonous tectonites formed during the Salinic (D1-2) and Acadian (D3) orogenies. D1-2 occurred between 451 and 417 Ma and culminated in Barrovian metamorphism during terminal collision of the Gander margin with composite Laurentia. Collision was followed by tectonic escape of the deeply buried rocks between 417 and 412 Ma. Rocks of the Victoria arc and Exploits backarc preserved in the Port aux Basques and Grand Bay complexes were emplaced during D1 by the Grandys River shear zone, which is outlined by a narrow band of the ca. 451 Ma Port aux Basques granite, towards the southeast above the Harbour le Cou Group. The contrasting histories displayed across the Grandys River shear zone are typical of the Dog Bay Line further northeast. Salinic structures were overprinted by faults and folds formed during Acadian D3 transpression (≤ 412 Ma), which produced the bi-vergent Meelpaeg structure and emplacement of amphibolite facies tectonites above greenschist facies rocks along its bounding shear zones. F3 folding progressively steepened the faults, which in turn led to progressive localisation of dextral strike-slip in narrow fault zones. The High grade of metamorphism in the Meelpaeg structure is attributed to protracted underthrusting of the Cabot promontory of the Gander margin beneath composite Laurentia. Salinic convergence was sinistral but became dextral during the Early Devonian, diachronous Acadian orogeny. The kinematic switch is proposed as a tool to separate Salinic from Acadian structures in the central part of the northern Appalachians.

Partitioning Anatolian Kinematics into Tectonic Escape and Slab Rollback Dominated Domains

Partitioning Anatolian Kinematics into Tectonic Escape and Slab Rollback Dominated Domains

Formation of the Tibetan Plateau during the India-Eurasia Convergence: Insight from 3-D Multi-Terrane Thermomechanical Modeling

Various models exist to explain the formation of the Tibetan Plateau, including “tectonic escape”, “pure shear thickening”, “convective removal of the lithospheric mantle”, and “lower crustal flow” model. The first two models are primarily constructed on pure mechanical models but are unable to reasonably explain the tension and shear phenomena inside the plateau. The latter two are rheological dynamic models based on deep geophysical observations. However, the spatial range of the lower crustal flow and its role in the plateau formation/uplift remain controversial. Five multi-terrane viscoplastic thermomechanical models were constructed to simulate the uplift and lithospheric structure change of the Tibetan Plateau during the post-collision stage (since 35 Ma) under the convergence of the Indian Plate. Results show that the plateau’s formation begins with crustal thickening, blocked by strong terranes at the northern plateau, and expanded laterally to the east. The lithosphere thickens gradually and experiences delamination at its base, elevating temperature within the crust and forming partial melting layers in the central plateau. As convergence persists on the southern side, the northern plateau’s lithosphere bends downward and undergoes delamination, further heating the crust and promoting the northward and eastward flow of partial melting layers, leading to secondary uplift around the plateau.

Tectono‐Stratigraphic Insights on the Dynamics of a Complex Subduction Zone, Northern Peruvian Forearc

AbstractTwo main types of subduction are recognized around the world: accretionary and erosive. The northern Peruvian margin is a well‐known example of a margin subjected to subduction erosion, but to date the along‐margin variability and temporal changes in subduction process and forearc basin evolution have not been characterized in detail. Interpretation of regional seismic lines and integration of oil‐industry wells and seafloor data captures the erosive nature of subduction underneath the forearc with only a minor accretionary component to the north. Episodes of uplift driven by plate coupling were followed by normal faulting/extensional collapse due to plate decoupling. This mechanism explains the dominance of normal faulting across the forearc until the Oligocene with a slight reactivation within the Miocene. The subduction history is complex and includes a reduction in plate convergence rate related to forearc crustal shortening, represented by large‐scale structures including the Peru fault (reactivated) and the Illescas fault‐propagation anticlines of the Northwest Peru transpressional system. This crustal deformation started in the Miocene. Integration with magnetic anomaly data indicates that activity of the present‐day transpressional system driven by tectonic escape of the Nazca Sliver toward the northeast, may explain the seismicity gap in southern Ecuador and northern Peru. An evolutionary model of the northern Peruvian margin shows how subduction zone geodynamics left its erosive fingerprint in the forearc basin configuration.

Revisiting the geodynamics of the Middle East region from an integrated geophysical perspective

A long-standing question in geodynamics is whether mantle flow is driven by the plate motion alone, or mantle upwelling makes a significant contribution to it. Subducting slabs and lateral variations of the continental lithosphere can further influence the asthenospheric flow and control its direction. The Middle East region (MER) is a complex continental setting where different processes such as rifting, break-up, plate collision, and tectonic escape kinematically interact with each other. In this context, the role that lithospheric structure, mantle flow, and active upwellings may play is debated. Tomographic images provide a snapshot of the current thermal conditions of a region and seismic anisotropy can also help resolve mantle convection. Here, we synthesize shear-wave splitting observations together with up-to-date tomography models of the mantle structure beneath the MER and other geophysical data. Low-velocity anomalies are seen at asthenospheric depths beneath W Arabia, NW Iran, and Anatolia, suggesting a spreading zone of warm mantle. Two deep low-velocity bodies in Afar and Levant –interpreted as hot mantle plumes– are the sources of this shallower mantle flow. Where low velocities are imaged, we observe predominantly NE–SW oriented anisotropy, anomalously high topography, and abundant basaltic volcanism. The integrated analysis suggests that a horizontal component associated with active upwelling is present in the upper-mantle flow field. The large-scale circulation flow fed by the Afar and Levant Plumes, aided by the subduction-induced forces, facilitates the lateral motion of the Anatolian microplate and affects the dynamic evolution of the Zagros orogen. The proposed scenario demonstrates that the interplay between plate-tectonic events and mantle dynamics controls the kinematics of the region and can explain the general patterns of deformation observed at the surface.

Distribution of Active Faults and Lithospheric Discontinuities in the Himalayan-Tibetan Orogenic Zone Identified by Multiscale Gravity Analysis

The lithospheric structure of the Tibetan Plateau and its adjacent area is a hot topic in geodynamic research. It is important to reveal the mechanism of crustal deformation and tectonic evolution of the study area. In this study, the techniques of wavelet multiscale decomposition and field edge detection were used to study the discontinuities of the lithosphere revealed by multilevel Bouguer gravity anomalies. Specifically, we evaluated the depth characteristics of the major active faults in the study area and identified 15 deep major faults that cut through the lithosphere. They are Chaman fault, Shyok suture zone, Altyn-Tagh fault, Karakash fault, Karakoram fault, Talas-Fergana fault, Kashgarr-Yeshgar transfer system, Rushan-Pshart suture zone, Sangri-Nacuo fault, Main Frontal thrust, Burmese fold belt, Yadong-Gulu fault, Gaoligong fault, Sagaing fault and Nujiang fault. We have also elucidated the tectonic mechanisms of two famous geodynamic phenomena in the Pamir Plateau. The first is the intense intermediate depth seismicity beneath Pamir-Hindukush. It cannot simply be described as the rupture of a subducted residual plate, which could be divided into two distinct tectonic units. One belongs to the Indian plate, the other to the Eurasian plate. Secondly, the mechanism of intense seismicity confined to the western upper crust of the Pamir Plateau could be explained as significant fragmentation of crustal material. Finally, and most importantly, we summarized the coupling mechanism between deep geodynamics and horizontal deformation as observed by modern geodetic techniques. In the upper mantle, the leading edge of the subducting Indian plate reached the SW boundary of Tarim basin and forms a closed structure in western Himalaya. Then, the Tibetan Plateau underwent a tectonic escape towards the east under the continuous compression between the Indian and Eurasian plates. During the process of tectonic escape, the role of the N–S direction normal faults in the Himalayan tectonic zone is limited.

Phases of Enhanced Exhumation During the Cretaceous and Cenozoic Orogenies in the Eastern European Alps: New Insights From Thermochronological Data and Thermokinematic Modeling

AbstractAustroalpine nappes in the Eastern European Alps have preserved the record of orogenies in the Cretaceous and Cenozoic but their cooling and exhumation history remains poorly constrained. Here we use low‐temperature thermochronology and thermokinematic modeling to unravel the exhumation history of the Austroalpine nappes in the Gurktal Alps. Our data reveal marked differences between the exhumation of units located at different positions within the nappe stack and relative to the Adriatic indenter. Units located at a high structural level and farther away from the indenter cooled through the zircon fission track closure temperature in the Late Cretaceous and have resided at depths of ≤5–6 km since the Oligocene, as indicated by apatite fission track ages of 35–30 Ma. Thermokinematic modeling constrained that these units experienced enhanced exhumation (∼0.60 km/Ma) between ∼99 and ∼83 Ma due to syn‐ to late‐orogenic Late Cretaceous extension. After a phase of slow exhumation (∼0.02 km/Ma), the exhumation rate increased to ∼0.16 km/Ma at ∼34 Ma due to the onset of the Europe‐Adria collision. In contrast, zircon fission track ages from units at a lower structural level and near the indenter indicate cooling during the Eocene; apatite fission track ages cluster at ∼15 Ma. These units were rapidly exhumed (∼0.76 km/Ma) from ∼44 to ∼39 Ma during an Eocene phase of shortening prior to the Europe‐Adria collision. After slow exhumation (∼0.13 km/Ma) between ∼39 and ∼18 Ma, the exhumation rate increased to ∼0.27 km/Ma in the wake of Miocene escape tectonics in the Eastern Alps.

Breaking plates: Creation of the East Anatolian fault, the Anatolian plate, and a tectonic escape system

Abstract Lateral movement of lithospheric fragments along strike-slip faults in response to collision (escape tectonics) has characterized convergent settings since the onset of plate tectonics and is a mechanism for the formation of new plates. The Anatolian plate was created by the sequential connection of strike-slip faults following ≥10 m.y. of distributed deformation that ultimately localized into plate-bounding faults. Thermochronology data and seismic images of lithosphere structure near the East Anatolian fault zone (EAFZ) provide insights into the development of the new plate and escape system. Low-temperature thermochronology ages of rocks in and near the EAFZ are significantly younger than in other fault zones in the region, e.g., apatite (U-Th)/He: 11–1 Ma versus 27–13 Ma. Young apatite (U-Th)/He ages and thermal history modeling record thermal resetting along the EAFZ over the past ~5 m.y. and are interpreted to indicate thermal activity triggered by strike-slip faulting in the EAFZ as it formed as a through-going, lithosphere-scale structure. The mechanism for EAFZ formation may be discerned from S-wave velocity images from the Continental Dynamics–Central Anatolian Tectonics (CD-CAT) seismic experiment. These images indicate that thin but strong Arabian lithospheric mantle extends ~50–150 km north beneath Anatolian crust and would have been located near the present surficial location of the Bitlis-Zagros suture zone (co-located with the EAFZ in our study area) at ca. 5 Ma. Underthrusting of strong Arabian lithosphere facilitated localization of the EAFZ and thus was a fundamental control on the formation of the Anatolian plate and escape system.

Geochemistry of the Upper Cretaceous - Miocene strata in the Tekman-Karayazı Basin (Turkey): Tectonic processes, sedimentation patterns, and the source rock quality along the northern branch of the Neo-Tethys Basins and northeastern Arabian plate

Tekman-Karayazı Basin (TKB) is one of the Eastern Anatolia basins which is located in the south of the Izmir-Ankara-Erzincan Suture Zone and is characterized by sedimentary units ranging in age from Upper Cretaceous to Pliocene. The basin began to form in Upper Cretaceous (Maastrichtian) in a tectonic depression, which over time evolved into a collisional foreland basin. While the Jurassic and Cretaceous Oceanic Anoxic Event (OAE) rocks were preserved in the NE of the Arabian Plate (SW of Iran), they are represented by an ophiolitic mélange which was formed as a result of the closure of the northern branch in the TKB and other basins on the Anatolian microplate. Four tectono-sedimentary phases are recognized over the ophiolitic mélange: Phase I (Maastrichtian-Paleocene (?), after obduction) is comprised of submarine fan deposits. Phase II (Paleocene - Lower Eocene, obduction phase) is characterized by major changes in gradually depositional environments in the TKB (clastics-nummulitic limestone), and continental to ramp carbonate to slope facies in others (e.g., Yakacık-Orhaniye, Haymana-Polatlı). Phase III (Middle – Upper Eocene, ongoing collision phase) shows the development of turbiditic and continental facies show that both TKB and/or the coeval basins have poor source rock potential. Phase IV (Oligo-Miocene, ending collision) represents from eastern to central in eastern Anatolian basins alluvial-fluvial to lacustrine and marine units during Oligocene – Miocene, respectively. The geochemical correlation between the bitumen extracted from sandstone sample and the Ağçakoca shales suggests that this formation can be considered as the corresponding source rock unit; however, according to the Tmax values (364–444 °C, mean: 428 °C), this formation is mostly immature, a scenario supported by the dry wells in the region. The regional volcanic activities triggered in Miocene by continent-continent collision seems to have provided the heat which, in turn, caused the thermal cracking of kerogen in Ağcakoca shales and locally induced petroleum expulsion. Mixing with the underground water on its way to reach a reservoir rock, the generated oil formed the oil seep. Therefore, we propose that a speculative Mescitli-Haneşdüzü (?) petroleum system may exist in this basin. We suggest that the combination of factors such as the asymmetric closure of the Tethys Ocean and the escape tectonics of the Anatolian Plate gave rise to the deposition of Paleocene to Miocene organic matter-rich shales in the NE of the Arabian plate, whereas uplift, antecedent topography, and the global paleo-climatic events disturbed the favorable conditions and caused the studied basins to lack the aforementioned generative source units.

Escape Mechanism with Shallow Ramp and Décollements in Southwest Taiwan

The escape structure in southwest Taiwan has long been discussed. The counterclockwise rotation in GPS ground motion is argued to be evidence of tectonic escape. However, tectonic escape events worldwide are always bounded by lithosphere-cutting strike-slip faults at the boundary of the lithospheric-scale rotating block, and these have not observed in Taiwan. In this study, we propose that the escape structure in southwest Taiwan is a thin-skinned deformation due to the open boundary in the continental slope, the ramp, and the shallow décollements. We tested this shallow escape hypothesis using a 3D numerical simulation with elastoplastic rheology. We found that a conjugate pair of forethrust and backthrust developed above the ramp. The strike and location of the forethrust mainly follow the ramp. However, the strike of the forethrust rotates perpendicular to the open boundary when in proximity. From north to south, the strike of the forethrust transfers from NNE to NE, and the deformation transfers from thrust to dextral thrust. This leads to a counterclockwise rotation in ground motion, which matches the GPS observation. This research provides a different explanation of the mechanism for the escape structure in southwest Taiwan.

The role of V-shaped oceans and ribbon continents in the Brasiliano/PanAfrican assembly of western Gondwana

Western Gondwana amalgamated by collision of continental blocks that did not form prior conjugated margins (extroversion), and by typical Wilson cycles, when continental blocks that rifted away giving birth to new oceans were subsequently re-joined in approximately the same position (introversion). The introverted systems are characterized by the opening of V-shaped basins through rifting and hyperextension of various continental pieces (micro- and ribbon continents) from a former Central African Block. These continental fragments lost substantial parts of their mantle lithosphere and became decratonized while drifting towards the external Goiás-Pharusian ocean. Protracted seafloor spreading and consumption through subduction of the internal and external oceans, respectively, ultimately led to multiple, diachronous collisions with other continental blocks detached from Rodinia (Amazonian, West Africa, Embu, etc.). These collisions pushed the ribbon continents back and closed the introverted basins, squeezing and incorporating the reworked basement tracts between the main colliding blocks and the rigid remainder of the Central African Block (the São Francisco-Congo craton). Continental extrusion and lateral escape tectonics ensued, generating thousands-of-km long networks of anastomosing directional shear zones (keirogens), as a consequence of both the accretionary systems developed between the involved blocks and the highly deformable nature of the decratonized ribbon continents.

Geophysical and morphological constraints on active tectonics in the Eastern Pontides (Turkey)

The Eastern Pontides in northern Anatolia (Turkey) offers an exceptional record of significant tectonic evolution from subduction to crustal thickening and Anatolian escape tectonics. Here, we provide ground magnetic measurements at 435 points integrated with aeromagnetic data, remote sensing, and surface geological features from the literature reveal all these tectonic deformation effects on the Eastern Pontides. The region, which is tectonically separated into three sub-zones, clearly revealed from both aero and ground magnetic data. Lineament analysis was carried out to reveal the deformation styles of the region and to compare different data sets. According to the lineament analyzes obtained from the aero&ground magnetic data and all the other data sets, similar results were obtained for the three sub-zones. Although slight differences were obtained results between the used methods and zones, the main lineament orientation is calculated in the E-W direction. Thus, the obtained results emphasize that tectonic deformation effects resulting from long-term ∼N-S convergence and crustal shortening (subduction, collision, obduction) may still dominate the region. In addition to the lineament results, high (low) magnetic zones in the E-W orientation are also evident. Considering the resolution of the data used, these magnetic changes can be associated with large scale geological structures rather than surface lithological changes. Despite the existence of an active dextral the North Anatolian Fault Zone, the zone is not evident from the magnetic results so the transcurrent tectonic regime probably still not dominant in the entire region.

Refined Tectonic Evolution of the Betic‐Rif Orogen Through Integrated 3‐D Microstructural Analysis and Sm‐Nd Dating of Garnet Porphyroblasts

AbstractHigh‐resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm‐Nd garnet geochronology has provided new insight into the tectonic history of the Betic‐Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion‐trails. Inclusion‐trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk‐garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride‐Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado‐Filabride complex (NFC) allow to deduce NNE‐SSW directed shortening in the Late Eocene changing to NW‐SE shortening in the early Oligocene, alternating with suborthogonal NE‐SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa‐Iberia plate‐motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P‐T‐t paths, the new garnet ages support a common tectono‐metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re‐buried under the first.

The 23 February 2020 Qotur-Ravian earthquake doublet at the Iranian-Turkish border: Seismological and InSAR evidence for escape tectonics

We present the source mechanisms and rupture processes for the damaging 23 February 2020 earthquake doublet of Mw 5.8 and Mw 5.9 that occurred near the Turkish-Iranian border regions of Qotur-Goharan-Mir'Omar-Ravian (NW Iran), extending towards Saray and Başkale (Eastern Turkey), as obtained from seismological waveform analysis and space geodesy imaging. Seismotectonic characteristics of the sequence highlight the role of indentation tectonics developed within regional-scale compressional environment where the Arabian microplate collides with the Eurasian plate. Here we report optimal finite-fault slip distribution patterns of the 2020 Qotur-Ravian earthquake doublet revealing complex co-seismic rupture propagation along the fault planes with maximum displacements ranging from 20 to 50 cm, stretching from the hypocentre to the surface. Analysis of aftershocks based on 3.5 months-long seismicity confirms distributed deformation. This energetic earthquake sequence demonstrates the distinct rupture characteristics illuminating differences in seismogenic properties and seismic hazard. Coulomb stress transfer modelling predicts triggering of the second event of Mw 5.9 by the first event of Mw 5.8. The zone of Coulomb stress changes attributed to varying pore pressure linking to geothermal water resources in the region as a driving force, may have an impact on the nucleation of triggered faulting. Evaluation of Interferometric Synthetic Aperture Radar (InSAR) data reveals the activated faults with evident co-seismic slip. Specifically, (1) we detected a rare case in earthquake-induced ground deformation where there is overlapping surface deformation due to sequential shallow events located closely in the crust, (2) the initial event ruptured a normal fault located towards W-NW of the latter inferred strike-slip fault and (3) the conjugate system of faults is closely placed at a few km apart. The frequent Sentinel-1 interferograms enhanced our imaging abilities of geometry and kinematics of shallow moderate-size M < 6.0 earthquakes and to trace seismogenic structures in remote and mountainous earthquake prone regions.

The Campo de Dalias GNSS Network Unveils the Interaction between Roll-Back and Indentation Tectonics in the Gibraltar Arc.

The Gibraltar Arc includes the Betic and Rif Cordilleras surrounding the Alboran Sea; it is formed at the northwest–southeast Eurasia–Nubia convergent plate boundary in the westernmost Mediterranean. Since 2006, the Campo de Dalias GNSS network has monitored active tectonic deformation of the most seismically active area on the north coast of the Alboran Sea. Our results show that the residual deformation rates with respect to Eurasia range from 1.7 to 3.0 mm/year; roughly homogenous west-southwestward displacements of the northern sites occur, while the southern sites evidence irregular displacements towards the west and northwest. This deformation pattern supports simultaneous east-northeast–west-southwest extension, accommodated by normal and oblique faults, and north-northwest–south-southeast shortening that develops east-northeast–west-southwest folds. Moreover, the GNSS results point to dextral creep of the main northwest–southeast Balanegra Fault. These GNNS results thus reveal, for the first time, present-day interaction of the roll-back tectonics of the Rif–Gibraltar–Betic slab in the western part of the Gibraltar Arc with the indentation tectonics affecting the eastern and southern areas, providing new insights for improving tectonic models of arcuate orogens.

Comment on “The Chongón/Colonche orocline transrotational bending and the coeval opening of the Progreso basin in southwestern Ecuador” by Alemán et al

• Lack of evidence to support a counterclockwise rotation of the Piñon block and the coeval opening of the Progreso Basin. • Forearc basin controlled by the complex interplay of oblique subduction, rise of an outer forearc high, and tectonic escape. • Post-accretion strain partitioning across an outer wedge with a varying trench-parallel oriented deformation.

Electrical resistivity structure across the Tethyan tectonic belt in western Yunnan, SW China

The Tethyan tectonic belt in western Yunnan, SW China, has witnessed a complex, multistage Tethyan evolution since the Paleozoic, including a late stage of continental collision between the Indian and Eurasian plates in the Cenozoic. Consequently, this belt has become a collage of Gondwana-derived microcontinental blocks and arc terranes wedged between two continents, including the Tengchong block, Baoshan block, Lincang block, and Simao block from west to east. To better understand the structure of these ancient continental blocks and the dynamic mechanism of the Indian-Eurasian continental collision, we investigate the electrical resistivity structure of the crust and uppermost mantle in the region. Magnetotelluric data were collected along a NW-SE striking transect crossing multiple tectonic terranes and were then modeled with a 3-D approach. The final resistivity model shows conductors beneath the Tengchong volcanic area, which may represent magma chambers that feed the active volcanism in the area. The electrical structure between the Tengchong area and the Wanding fault consists of layer-like lower crustal conductors along the transect, which may indicate mechanically weak layers where the tectonic escape of the Tibetan Plateau may occur via lower crustal flow. The Lincang block shows higher resistivity characteristics than the surrounding crust and demonstrates a trend of eastwardly napping resistors onto the Simao block, making up a crustal boundary of high-resistivity and low-resistivity bodies. This observation suggests that the Changning-Menglian suture between the Baoshan block and the Lincang block may be the boundary between Gondwana and Cathaysia.

Neoproterozoic U‐Th‐REE‐bearing Pegmatites in Wadi Ibib, South Eastern Desert, Egypt: Structural and Geochemical Measures for a Syn‐Tectonic Anatectic Model of Formation

AbstractThe Wadi Ibib area is situated in the northern part of the Neoproterozoic Hamisana Shear Zone (HSZ), which is a high strain zone evolved during the late stages of the Pan‐African orogeny, likely as a tectonic escape structure. Amphibolite facies pelitic metasedimentary windows crop out in the axial parts of the HSZ and are noticeably associated with numerous N‐trending pegmatite dikes. Whole‐rock geochemistry of the pegmatites reveals a peraluminous (S‐type) affinity, with low K/Rb ratios and elevated concentrations of U, Th, REE, Rb, Li, Cs, Y, Nb and Ta. Structurally, the pegmatite sets intrude along the shear plane of the HSZ, corresponding to the regional N‐trending tectonic fabrics, such as axial planar foliation and dextral‐shearing in the metasedimentary host rock. Field relationships, including structural context, coupled with geochemical characteristics of the Wadi Ibib pegmatites, do not support their formation as a complementary part of evolved granitic magmas. Space‐localized decompression‐induced partial melting of peraluminous garnet‐bearing metapelites was alternatively the underlying process for formation of these pegmatites. Such decompression was associated with regional escape tectonics and stress axes permutations during the late deformation stage (D3) in the evolution of the south Eastern Desert terrane, due to end‐orogeny system pressure‐release.

Syn-kinematic emplacement of granitic batholith and leucogranite along the extensional detachment shear zone system of the Tin Begane area, Laouni terrane (LATEA metacraton, Central Hoggar, Algeria)

The vast Tin Begane batholith (5000 km2) is located in the Laouni terrane, LATEA metacraton, Central Hoggar and emplaced in the middle crust (c. 4.2 kbar). High-K calc-alkaline, metaluminous and magnesian as a whole, it comprises two distinct groups: i/ a porphyritic amphibole-biotite monzogranite (598 ± 3 Ma, U–Pb zircon) display a LREE enriched pattern and a negative Eu anomaly while ii/ a porphyritic biotite monzogranite (596 ± 4, U–Pb zircon) and a fine-grained monzogranite (591 ± 6 Ma, U–Pb zircon) show a highly fractionated REE patterns and slight Eu anomalies. Both groups enclosed numerous microgranular magmatic enclaves with the same contrasted chemical characteristics. In the center of the batholith, a basement sheet bounded by faults comprises migmatitic country rocks and leucogranites (595 ± 6 Ma, U–Pb zircon) and a garnet-muscovite dyke swarm. Geochemical compositions of the Tin Begane magmatic rocks, including magmatic enclaves, and inherited zircons point to a Paleoproterozoic composite lower crustal metasedimentary source with a psammitic garnet-free component and a garnet-bearing greywacke melted at c. 900 °C with a minor basaltic mantle component. Leucogranites originated from the same sources but at a lower temperature (c. 780 °C) not far from the emplacement of the Tin Begane batholith.A detailed structural study shows that the Tin Begane batholith is post-collisional, syn-kinematic, bounded and controlled by large scale detachment shear zones, which restricted the flow by channeling the magma from depth to the middle crust and accommodated the magma emplacement. Late, when the batholith was largely crystallized, the subhorizontal magmatic fabric was progressively replaced by pronounced subvertical planar fabric when approaching the shear zones. The Tin Begane area shows a local but major high-temperature NNE-SSW (subvertical) extensional deformation, especially marked by a large NW-SE detachment, within the WNW-ESE regional compression (subhorizontal shortening) induced by the global transpressive system linked to the tectonic escape of the Tuareg terranes toward the north. This contrasted behavior is favored by the metacratonic character of LATEA.