Crust-mantle Decoupling Research Articles

Hot Cordilleran hinterland promoted lower crust mobility and decoupling of Laramide deformation

The Late Cretaceous to Paleogene Laramide orogen in the North American Cordillera involved deformation >1,000 km from the plate margin that has been attributed to either plate-boundary end loading or basal traction exerted on the upper plate from the subducted Farallon flat slab. Prevailing tectonic models fail to explain the relative absence of Laramide-aged (ca. 90–60 Ma) contractional deformation within the Cordillera hinterland. Based on Raman spectroscopy of carbonaceous material thermometry and literature data from the restored upper 15–20 km of the Cordilleran crust we reconstruct the Late Cretaceous thermal architecture of the hinterland. Interpolation of compiled temperature data (n = 200) through a vertical crustal column reveals that the hinterland experienced a continuous but regionally elevated, upper-crustal geothermal gradient of >40 °C/km during Laramide orogenesis, consistent with peak metamorphic conditions and synchronous peraluminous granitic plutonism. The hot and partially melted hinterland promoted lower crust mobility and crust-mantle decoupling during flat-slab traction.

The coherence function and lithospheric elastic thickness of the Zagros fold and thrust belt

SUMMARY This study derives the spatial variation of the elastic thickness (Te) and its implications for understanding the structure, geodynamic and seismicity of the lithosphere for the Zagros fold and thrust belt region of the Arabia–Eurasia collision zone. Te is calculated using the coherence function in the fan wavelet domain based on recent terrestrial Bouguer gravity and topography data as input signals. Utilizing the load deconvolution method and Brent's method of 1-D minimization, the final Te for the survey region is estimated for each grid node of the studied area. To illustrate the mass distribution in the studied area, the subsurface loading fraction (F) is calculated simultaneously with Te in the inversion. The crust thickness and density from three different global crustal models are tested and the results obtained for these input models do not yield substantially different Te patterns. The final results are in accord with the global Te models as well as previous rheological, geodynamical and flexural studies, however, this study establishes much more detailed regional information. The calculations yield a mean value of Te of 61 km for the Zagros, with a mean estimated error of about 5 km. The high-Te values (>70 km) are observed in the southeast of the studied area (some parts of the Sanandaj–Sirjan zone, Urumieh–Dokhtar magmatic arc and most of the Central Iranian blocks); while over most of the northwest of the studied area, the value of Te is about 58 km. The Te results are consistent with the lithospheric structure of the study area and also support the idea of the crust–mantle decoupling. Further, there is a positive and negative correlation between the surface wave velocity and surface heat flow, respectively. The mean value estimated for the internal loading friction (F) of 0.4 means in most of the studied areas we may consider that the surface loading is dominant, or at least the ratio of the surface and subsurface loading can be assumed equal. Based on earthquake distribution in the period 1900–2020, seismicity is more likely to occur in areas with a relatively low value of Te.

Intraplate lithospheric extension revealed by seismic reflection profiling of South China

How lithospheric extension evolves in an intraplate setting remains uncertain due to the lack of reliable constraints on the lithospheric architecture. Here we present seismic reflection data across the Cretaceous extensional system of South China. Our results show that extension in magma-poor conditions was accommodated by localized normal faulting in the upper crust and distributed ductile stretching in the lower crust, followed by localized crustal necking and Moho uplift associated with mantle shear-zone formation. These vertically spaced crustal and mantle features appear to be kinematically linked as follows. First, lower-crustal stretching was accompanied by normal faulting and localized exhumation of extensional domes along low-angle detachments in the upper crust. Second, sub-horizontal lower-crustal stretching tends to compensate for upper-crustal heterogeneous thinning and distribute crustal strain evenly, enabling the crust to maintain an overall smooth and flat Moho. Third, mantle shear zones likely affected lithospheric extension by controlling localized Moho uplift and crustal necking. Our compilation of seismic observations suggests that the extensional modes vary laterally from magma-poor to magma-rich conditions, reflected in increased crustal melting, decreased crust-mantle decoupling, and the replacement of a two-layer (high-strength vs. low-strength) lithospheric mantle by a single-layer, low-strength lithospheric mantle. These findings reveal a first-order configuration of depth-dependent extension over ∼800 km, with vertical and lateral variations as a function of lithospheric strength, rheology, and temperature. This extension mechanism provides a basis for assessing modes of lithospheric extension in other tectonic settings.

Lithosphere architecture characterized by crust-mantle decoupling controls the formation of orogenic gold deposits.

This study, via combined analysis of geophysical and geochemical data, reveals a lithospheric architecture characterized by crust-mantle decoupling and vertical heat-flow conduits that control orogenic gold mineralization in the Ailaoshan gold belt on the southeastern margin of Tibet. The mantle seismic tomography indicates that the crust-mantle decoupled deformation, defined from previous seismic anisotropy analysis, was formed by upwelling and lateral flow of the asthenosphere, driven by deep subduction of the Indian continent. Our magnetotelluric and seismic images show both a vertical conductor across the Moho and high Vp/Vs anomalies both in the uppermost mantle and lowest crust, suggesting that crust-mantle decoupling promotes ponding of mantle-derived basic melts at the base of the crust via a heat-flow conduit. Noble gas isotope and halogen ratios of gold-related ore minerals indicate a mantle source of ore fluid. A rapid decrease in Cl/F ratios of lamprophyres under conditions of 1.2 GPa and 1050°C suggests that the ore fluid was derived from degassing of the basic melts. Similar lithospheric architecture is recognized in other orogenic gold provinces, implying analogous formational controls.

Constraints on the process and mode of the Paleo-Asian Ocean closure from the lithospheric conductivity structure of the south-eastern Central Asian Orogenic Belt

The Paleo-Asian Ocean eventually closed along the Solonker Suture Zone during the Late Permian and Early Triassic, before the collision between the Siberian Craton and the North China Craton (NCC) formed the eastern Central Asian Orogenic Belt (CAOB). In this paper, Magnetotelluric (MT) profile data across the south-eastern CAOB and the northern margin of the NCC are explored to image the two-dimensional resistivity structure of the region. According to the resistivity model, a north-dipping conductive layer to the north of the Xilinhot fault and a south-dipping conductive layer to the south of the Linxi fault are revealed. The two conductors are interpreted as evidence of the late-stage low-angle bidirectional subduction of the Paleo-Asian Ocean, which constrains the northern and southern deep boundaries of the Solonker Suture Zone. Another couple of mantle conductors with opposite dipping angles are revealed under the Solonker Suture Zone and Bainaimiao Belt, which are interpreted as evidence of the early-stage high-angle bidirectional subduction of the Paleo-Asian Ocean. Compared with the results from previously reported MT profiles in this area, the subduction angle becomes more gentle from west to east. The difference of convergence rate between the NCC and the Siberian Craton is considered to be the major cause of the closure difference along the strike. The northern margin of the NCC has a conductive lower crust, which may result from the magma underplating or crust-mantle decoupling, as it is imaged to be connected with the south-dipping conductive layer beneath the Solonker Suture Zone.

Crustal flow and fluids affected the 2021 M7.4 Maduo earthquake in Northeast Tibet

• Fluids exist in the Maduo source zone and affected the rupture nucleation. • Depth-varying seismic anisotropy occurs and reflects lower-crustal flow. • The ductile flow in the mid-lower crust affected the seismotectonics. We present detailed 3-D tomographic images of P and S wave velocity (Vp, Vs), Poisson's ratio (ν) and Vp azimuthal anisotropy of the crust and uppermost mantle beneath the source area of the 21 May 2021 Maduo earthquake (M 7.4) in the NE Tibetan plateau. The images are obtained by inverting a large number of P and S wave arrival-time data of 11,235 local earthquakes and Maduo aftershocks recorded at 67 seismic stations. Our results show that the 2021 Maduo mainshock occurred in a low-Vs and high-ν anomaly, probably reflecting crustal fluids that affected the rupture nucleation. Our Vp anisotropy results show that at 40 km depth under the southern part of the study region, the fast-velocity direction (FVD) is NW-SE, which is mainly controlled by the India-Eurasia collision. At 60 km depth under the study region and at 40 km depth under the northern part of the region, the FVDs are NE-SW to N-S, reflecting lower crustal flow. The FVDs are roughly E-W at 60 km depth beneath the Qilian mountain range, which reflect the lower crustal flow that is blocked by a rigid terrane in the north. The lower crustal flow may lead to intra-crustal and crust-mantle decoupling and affect seismotectonics in NE Tibet.

Advances in the deep tectonics and seismic anisotropy of the Lijiang-Xiaojinhe fault zone in the Sichuan-Yunnan Block, Southwestern China

The Sichuan-Yunnan Block (SYB) is located at the SE margin of the Qinghai-Tibetan Plateau (TP). Under the influence of the southeastward movement of material originated from the TP, intense crustal deformation, frequent seismic activity, and complex geological structures are observed in the SYB. The Lijiang-Xiaojinhe fault (LXF) goes through the central part of the SYB, dividing it into two blocks from north to south, and forming an intersecting fault system with the surrounding faults. This paper firstly introduces the morphology and the nature of the LXF, the distribution of the regional surface displacements and the focal mechanisms, and then analyzes the medium deformation and the effects of faults. Moreover, according to the regional tectonics and geophysical patterns, the paper discusses the characteristics of the north-south blocks of the SYB and the abrupt change of deep structure along the LXF zone. Since seismic anisotropy is an essential property for detecting crustal stress, deep structures and dynamical mechanisms, this paper is dedicated to the advances in seismic anisotropy at different depths and different scales in the study area. There are noteworthy differences in the anisotropic features between the north part and the south part of the SYB, possibly associated with a clear boundary adjacent to the LXF. Such phenomenon suggests some close correlation between anisotropic zoning boundary and the LXF, although this boundary is not consistent with the LXF in strike. The results from the deformation of the crust and the upper mantle elucidate the distribution patterns of the crust-mantle coupling in the north part and the crust-mantle decoupling in the south part, even though this conclusion needs to be further verified by more studies. Presently, the scientific understanding of the deep tectonics and the media deformation around the “generalized” LXF i.e. the LXF with the Jinpingshan fault on its eastern side, is still insufficient, and related equivocal topics deserve more in-depth studies.

Difference of shortening strain for the crustal deformation model in the eastern and western Qaidam Basin and their implications, northern Tibetan Plateau, China

To recognize the differences in the west–east crustal deformation model of the Qaidam Basin, we presented the shortening amount of each fold and fault along 12 sections. According to the eastward weakening trend of shortening strain in the southwestern part of the basin and the non‐continuous trend in the northeastern part of the basin, the Qaidam Basin can be divided by a new line into the western Qaidam Basin (WQB) and the eastern Qaidam Basin (EQB). The WQB is characterized by a maximum shortening strain of 25%, transpressional structures, and crust–mantle decoupling, which is possibly weakened by the eastward extension of the crust–mantle mix zone caused by upwelling material along the Altyn Tagh Fault (ATF). In contrast, the EQB is characterized by a minimum shortening strain of 2%, pure thrust boundary faults, and gentle folds, which is defined as a rigid craton basin. The crust–mantle mix zone beneath ATF and the low‐velocity zone beneath the eastern Kunlun Shan show the limited impact of the lithospheric weakening on the EQB. The Miocene reorganization of the ATF is the key tectonic event that accounts for the differences in the east–west crustal deformation model of the Qaidam Basin, as it weakened the lithosphere and facilitated the transpressional structures of the WQB. The WQB was originally a rigid craton basin like the EQB, but it became a destabilized craton after the a forementioned tectonic event.

Differences in east-west thermo-rheological structure and lithospheric deformation in the Qaidam Basin, Northern Tibetan Plateau, China

ABSTRACT To investigate the differences in east-west lithospheric deformation in the Qaidam Basin, we present thermo-rheological models of two profiles across the western Qaidam Basin (WQB) and eastern Qaidam Basin (EQB). The differences in east-west geodynamic deformation styles are also described, involving GPS motions, focal mechanisms (P axes), seismic anisotropy (SKS-wave splitting), and low-velocity zones (LVZs). The WQB is characterized by a warm destabilized cratonic basin with a weak lower crust, and the rheological structure changes from a rather weak crème brûlée-1 regime into a strong jelly sandwich-1 regime, from the Altyn-Qaidam boundary to the northeastern corner of the WQB. Combined with the limited distribution of the LVZs and the strong crust-mantle decoupling revealed by the unmatched pattern between P axes (N20ºE) and SKS-wave splitting (N110°E), the crust-mantle mixing related to the under-thrusting of the Tarim Basin along the Altyn Tagh Fault is suggested as the primary tectonic dynamic inducement of the destabilized WQB craton, which weakens the lithospheric strength greatly and contributes to the shallow brittle deformation. However, the EQB performs as a typical cold and rigid cratonic basin characterized by a jelly sandwich-2 rheological regime, which is sufficiently strong to maintain crust-mantle coupling. The LVZs beneath the EQB revealed by recent wide-angle seismic profiles have only a small effect on the lithospheric strength drop. The EQB could be regarded as a solid basin, anchored in the NE Tibetan Plateau, which shows a strong resistance to NE extrusion of the weak plateau material, leading to clockwise rotation.

Differential crustal rotation and its control on giant ore clusters along the eastern margin of Tibet

AbstractControls on the formation and distribution of mineralization in continental collisional settings remain unclear. However, our synthesis of diverse geophysical data sets from the eastern margin of Tibet revealed that differential crustal rotation played a key role in the production of a variety of mineralization types. Due to Cenozoic continental collision between India and Eurasia, the elongated continental blocks in the eastern margin of Tibet were extruded and reoriented. Prior to block extrusion in the Eocene, two giant porphyry-skarn ore clusters formed at the boundaries between the central segment and both the northern and southern segments of the Jinshajiang-Ailaoshan suture zone. These crustal segment boundaries displayed counterclockwise rotation, due to clockwise rotation of the central segment relative to both the essentially immobile northern and southern segments, combined with crust-mantle decoupling. This is considered to have induced crustal friction and resultant generation of fertile magmas that formed the porphyry-skarn Cu-Au deposits. During Oligocene–Miocene block extrusion, differential rotation of upper crust occurred on the western and eastern sides of the north-northwest–trending Central Axis fault in the Lanping-Simao basin. Two Oligocene–Miocene Mississippi Valley–type ore clusters occur on fault segments with anomalous differential rotation of 70° to 80°, suggesting that this differential rotation resulted in local extension with consequent ore-fluid influx.

Testing oceanic crust–mantle decoupling by Sr–Nd–Hf–Os isotopes of Neo-Tethyan ophiolites

Oceanic crust and the mantle are thought to have identical Sr–Nd–Hf–Os isotopic compositions. However, there is increasing evidence of isotopic differences between the two units, suggesting the existence of oceanic crust–mantle decoupling. Here we present updated whole-rock and mineral Sr–Nd–Hf–Os isotope data for the Kızıldağ (southeast Turkey) and Xigaze (south Tibet, China) Neo-Tethyan ophiolites to test if oceanic crust–mantle decoupling characterizes these ophiolites. Mantle peridotites from the Kızıldağ ophiolite have large variations in clinopyroxene and orthopyroxene Hf isotopic compositions (εHf (t) = −27.7 to +20.6) and unradiogenic Os isotope ratios (187Os/188Os = 0.11853–0.12720). These mantle peridotites have experienced ancient (up to ~1.6 Ga) melt depletion events, as evidenced by their old Re depletion ages (TRD = 1632–357 Ma), although Hf isotopes have been locally reset by recent melt re-fertilization, which likely occurred in a forearc basin setting. Mantle peridotites from the Xigaze ophiolite mainly have DMM (depleted mid-ocean ridge basalt mantle)-like to ultra-depleted clinopyroxene Hf isotopic compositions (εHf (t) = +9.1 to +29.8 for 17 out of 19 analyses), which were extensively overprinted by silicate melt percolation. However, the Os isotopic compositions of the Xigaze mantle peridotites are similar to those of the Kızıldağ ophiolite, again suggestive of ancient melt extraction long before the Early Cretaceous formation of this oceanic crust. These ancient and ultra-depleted mantle domains are not uncommon in modern oceanic lithosphere, forearc basins and ophiolites, and are less dense than the fertile mantle, and also are too refractory to contribute to the recent generation of oceanic crust. Sampling bias is unavoidable when comparing the oceanic crust and its underlying lithospheric mantle. As such, the oceanic crust alone cannot be used to estimate the isotopic composition of its entire mantle source.

Quaternary deformation at the southeastern margin of the Ordos Block, North China: Inferred from structural analysis of basin-bounding faults

The southeastern margin of the Ordos Block (SEMOB) is located at the juncture of the North China Block, the Ordos Block, and the Qinling Orogen. A series of faulted basins has developed in this area since the Cenozoic. The latest focal mechanism solution and stress field analyses indicate that the SEMOB is located at the adjacent regions of present different stress field, although the structural activity and internal mechanisms of this changing remain unclear. In this paper we describe the results of a study into the fault systems and Quaternary deformation between the Linfen and Weihe basins on the SEMOB. Based on the deformation sequences, kinematic analyses and activity timing, we found that the strain field in SEMOB underwent three evolutionary stages during the Quaternary: NW–SE extension in the early Pleistocene, NE–SW extension in the middle Pleistocene, and ca. N–S oblique extension from the late Pleistocene to the Holocene. This evolution of the strain field affected both the geomorphic patterns and the activity of paleo-earthquake events along the SEMOB during the Quaternary. Quaternary extensional deformation in the upper crust along the SEMOB possibly resulted from crust-mantle decoupling, which may be related to eastward expansion of the Tibetan Plateau and subduction of the Pacific and Philippine Sea plates.

Inherited terrane properties explain enigmatic post-collisional Himalayan-Tibetan evolution

AbstractObservations highlight the complex tectonic, magmatic, and geodynamic phases of the Cenozoic post-collisional evolution of the Himalayan-Tibetan orogen and show that these phases migrate erratically among terranes accreted to Asia prior to the Indian collision. This behavior contrasts sharply with the expected evolution of large, hot orogens formed by collision of lithospheres with laterally uniform properties. Motivated by this problem, we use two-dimensional numerical geodynamical model experiments to show that the enigmatic behavior of the Himalayan-Tibetan orogeny can result from crust-mantle decoupling, transport of crust relative to the mantle lithosphere, and diverse styles of lithospheric mantle delamination, which emerge self-consistently as phases in the evolution of the system. These model styles are explained by contrasting inherited mantle lithosphere properties of the Asian upper-plate accreted terranes. Deformation and lithospheric delamination preferentially localize in terranes with the most dense and weak mantle lithosphere, first in the Qiangtang and then in the Lhasa mantle lithospheres. The model results are shown to be consistent with 11 observed complexities in the evolution of the Himalayan-Tibetan orogen. The broad implication is that all large orogens containing previously accreted terranes are expected to have an idiosyncratic evolution determined by the properties of these terranes, and will be shown to deviate from predictions of uniform lithosphere models.

Deep‐seated lithospheric geometry in revealing collapse of the Tibetan Plateau

Abstract The Tibetan Plateau's enigmatic collapse, which was indicated based on extensional faulting in the interior of the plateau and large eastward-directed strike-slip faults, has inspired intensive geologic inquiry and debate. Interaction of the subducting Indian plate with the overlying Asian lithosphere is one factor that may be affecting the plateau. A more detailed image of the subducting Indian plate can help constrain its role, but regional high-resolution seismic data have not been widely available at a relevant scale. Here we present an integrated interpretation based on two types of seismic datasets: (1) two N-S oriented deep seismic-reflection profiles that cross the Yarlung-Zangbo suture, and (2) two E-W receiver function profiles that cross the Xainza-Dingjye and Nyima-Tingri rifts, respectively. These images reveal different crustal-scale structures in the dominant collision zone between the western and central regions, as well as distinctly offset Moho discontinuities across the N-S trending rift grabens in southern Tibet. These crustal variations, combined with previous tomographic studies in Tibet, outline an easterly tilt of the subducting Indian slab, along which crust-mantle decoupling occurred. Together with the spatio-temporal distribution of synchronous potassium-rich volcanics exposed within the grabens in southern Tibet, this offset Moho structure beneath the grabens lends support to the hypothesis that eastward steepening of the subducting Indian slab was accompanied by slab tearing beneath each north-south striking rift. This overall crustal geometric variation suggests a stepwise flattening of the torn Indian slab from west to the east, a process that brought easterly migration of gravitational instability to the overriding Tibetan crust that drove collapse of the Tibetan Plateau once gravitational instability reached a critical level to the east.

Varied thermo-rheological structure, mechanical anisotropy and lithospheric deformation of the southeastern Tibetan Plateau

To investigate the lithospheric structure and deformation of the Tibetan Plateau, we present thermo-rheological models for two transects across the southeastern Tibetan Plateau (SE Tibet), showing strong lateral heterogeneity. The geodynamic implications are also investigated in relation to GPS motions, seismic anisotropy, seismicity distribution and crustal isostatic state. The unmatched pattern between Pms and SKS splittings which even intersect at right angles to the south of 26°N, suggests the crust-mantle decoupling. Constrained by a brittle load-bearing layer at the uppermost mantle beneath the Indochina block, the decoupling may occur below the uppermost mantle. The strong crust beneath the South China plate and Indochina block implied by the seismicity distribution, has two thick brittle load-bearing layers, indicating a coupled system. The crust beneath the Emeishan large igneous province (ELIP) also has two brittle load-bearing layers, but the brittle deformation is restricted to the topmost 10 km of the upper and lower crust. Only one brittle load-bearing layer resides in the upper crust in the other part of the Chuandian block. Combining with GPS and Pms splitting, we can infer that the weak crust with seismicity restricted into the depth of <30 km beneath the Chuandian block becomes decoupled. The heterogeneous rheological behavior under SE Tibet is further supported by different crustal isostatic states among the blocks. Through the comparison with observations in the central-northern Tibetan Plateau and Chuandian block, we suggest that the layers with aqueous fluids and (or) partial melting are distributed only in some place/channels of the weak middle-lower crust.

Construction and destruction of the North China Craton with implications for metallogeny: Magnetotelluric evidence from the Hengshan–Wutai–Fuping region within Trans-North China Orogen

The Hengshan–Wutai–Fuping region within the Trans-North China Orogen is an important corridor that preserves the records of Precambrian evolutionary history and Phanerozoic magmatic and metallogenic events in the North China Craton. Here we present 2-D and 3-D inversion models of magnetotelluric data collected from the Hengshan–Wutai–Fuping region and surrounding regions. Our data reveal nearly horizontal high-conductivity layer at lower crustal and upper mantle depths of ~35–45km beneath the Hengshan and Wutai mountains, providing direct evidence for crust–mantle decoupling and magmatic underplating. In the Mesozoic Yixingzhai gold deposits and Boqiang Cu–Mo–Au deposits, two nearly upright conductors cut through the crust and connect the lower crustal high-conductivity layer suggesting frozen channels of fluids and magmas. A west-dipping slab-shaped conductor within the high-resistivity lithosphere beneath the Wutai Complex appears to be the deep extension of the exposed Longquanguan ductile shear zone. These resistivity anomalies can provide new constraints on the subduction–collision events during Precambrian tectonic evolution of the Trans-North China Orogen as well as on the Phanerozoic tectonic–magmatic and metallogenic events in the Hengshan–Wutai–Fuping region. The west-dipping slab-shaped conductor may indicate a westward oceanic subduction between the Wutai and Fuping Complexes with subsequent collision associated with the formation of the Trans-North China Orogen in the Paleoproterozoic. We interpret the crustal and upper mantle conductors to be part of an interconnected fluid/melt transport channel, which probably played an important role for the Mesozoic tectonic–magmatic and metallogenic events as well as the Cenozoic basaltic volcanism in the Hengshan–Wutai–Fuping region.

Geoscience Data Integration: Insights into Mapping Lithospheric Architecture

SUMMARY In order to develop a 4D understanding of the architecture of the entire lithosphere, it is necessary to embrace integration of multi-disciplinary, multi-scale data in a GIS environment. An holistic understanding has evolved whereby geologic, geochemical and geophysical signals are consistent with a subcontinental lithospheric mantle (SCLM) dominated by a mosaic of domains of Archean ancestry, variably overprinted by subsequent tectonothermal events. Pristine Archean SCLM is mostly highly depleted (high Mg#), low density, high velocity and highly resistive, and preserves intact Archean crust. There is a first order relationship between changes to these signals and the degree of tectonothermal overprint (by melts, fluids). Continental crust is comprised largely of reconstituted Archean components, variably diluted by juvenile addition, symptomatic of the various overprinting events. These events impart crustal fabrics and patterns dictated by SCLM architecture, influenced by the free surface and crust-mantle decoupling.

Reconstructing the Alps–Carpathians–Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion

Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust–mantle decoupling during Adria–Europe collision. In particular, the switch from south-directed European subduction to north-directed “wrong-way” Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench–trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps–Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust–mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.

Coupled mantle dripping and lateral dragging controlling the lithosphere structure of the NW-Moroccan margin and the Atlas Mountains: A numerical experiment

Recent studies integrating gravity, geoid, surface heat flow, elevation and seismic data indicate a prominent lithospheric mantle thickening beneath the NW-Moroccan margin (LAB >200km-depth) followed by thinning beneath the Atlas Domain (LAB about 80km-depth). Such unusual configuration has been explained by the combination of mantle underthrusting due to oblique Africa–Eurasia convergence together with viscous dripping fed by asymmetric lateral mantle dragging, requiring a strong crust–mantle decoupling. In the present work we examine the physical conditions under which the proposed asymmetric mantle drip and drag mechanism can reproduce this lithospheric configuration. We also analyse the influence of varying the kinematic boundary conditions as well as the mantle viscosity and the initial lithosphere geometry. Results indicate that the proposed drip–drag mechanism is dynamically feasible and only requires a lateral variation of the lithospheric strength. The further evolution of the gravitational instability can become either in convective removal of the lithospheric mantle, mantle delamination, or subduction initiation. The model reproduces the main trends of the present-day lithospheric geometry across the NW-Moroccan margin and the Atlas Mountains, the characteristic time of the observed vertical movements, the amplitude and rates of uplift in the Atlas Mountains and offers an explanation to the Miocene to Pliocene volcanism. An abnormal constant tectonic subsidence rate in the margin is predicted.

Metallogeny in response to lithospheric thinning and craton destruction: Geochemistry and U–Pb zircon chronology of the Yixingzhai gold deposit, central North China Craton

Magmatism and associated metallogeny provide important constraints on geodynamic processes. The Hengshan terrain of northern Taihang Mountains located within the central North China Craton is well-known for several Mesozoic gold deposits. Here we investigate the Yixingzhai quartz vein-hosted gold deposit associated with the Sunzhuang quartz monzodiorite intrusive complex. We present U–Pb data on zircon grains from the monzodiorite which constrain the timing of emplacement of the pluton as ca.134Ma, coeval with the formation of Yixingzhai quartz vein gold deposit (at ca. 131Ma). The magmatism and metallogenesis during early Cretaceous in the Hengshan terrain are part of a major regional magmatic and metallogenic event in the Taihang Mountains and elsewhere in the North China Craton at ca. 130Ma ago. We characterize the zircon grains from the Sunzhuang quartz monzodiorite through morphology, internal structures and chemistry, and also present data from analyses of the chemistry (Co and Ni), and S–Pb–D–O isotopes of the minerals from the Yixingzhai gold deposit and Sunzhuang quartz monzodiorite. Our results reveal a lower crustal signature with mantle input for the source magma of the Sunzhuang Pluton. The magma was enriched in water and alkalis and shows a wide range of crystallization temperature (850–550°C, and mainly 650 to 700°C) under high oxygen fugacity. The ore minerals and the mineralizing fluids in the Yixingzhai gold deposit were derived from magma generated in the lower crust with additional input of mantle components. Our study links the gold metallogenesis with magmatism in the Hengshan terrain. Based on the new results reported in this study, and in conjunction with our interpretation of the ultra-broadband high-precision magnetotelluric sounding profile from the region, we suggest that the lithosphere beneath the Hengshan terrain was strongly thinned and decoupled between the crust and mantle in the early Cretaceous, and that the partly destructed lithosphere was largely preserved through the Cenozoic to present. The metallogenesis in the Hengshan terrain was closely related to the lithosphere thinning and crust–mantle decoupling in the North China Craton.