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The late Mesozoic tectonic evolution of the eastern North China craton was intimately related with the subduction of the paleo-Pacific plate. This study sheds light on Late Jurassic-Early Cretaceous basin-mountain coupling during paleo-Pacific subduction based on low-temperature thermochronology and geochemical analyses from the easternmost Yanshan fold belt, located at the northern edge of the craton. We performed apatite U-Pb and fission-track double dating and trace element analyses of basement and sedimentary rock samples, integrated with zircon and apatite (U-Th)/He analyses on bedrock samples. Our results revealed that the easternmost Yanshan fold belt experienced two stages of rapid cooling and exhumation in the Middle Jurassic-earliest Cretaceous (ca. 170-140 Ma) and in the Early Cretaceous (ca. 140-90 Ma). The Middle Jurassic-earliest Cretaceous exhumation (ca. 170-140 Ma) is mainly recorded by bedrock along the margins of intermontane basins of the eastern Yanshan fold belt. This exhumation event is consistent with compressional Yanshan movement during the Middle Jurassic-earliest Cretaceous coeval with paleo-Pacific flat-slab subduction. Subsequent Early Cretaceous rapid cooling is ascribed to the progressive evolution of the metamorphic core complexes and associated tectonic exhumation of major plutons of northeastern Asia, which were likely controlled by rollback of the paleo-Pacific slab. A middle Cretaceous (ca. 110-90 Ma) tectonic exhumation event is revealed by very low lag-time values in middle Cretaceous strata. Our findings illustrate the potential of a thermochronology approach that combines single-grain double dating and trace element analyses in synmagmatic orogenic systems, which may find application to other orogenic settings worldwide. [ABSTRACT FROM AUTHOR]

The anisotropy of seismic velocities in the mantle, when integrated with high-resolution tomographic models and geologic information, can be used to detect active mantle flows in complex plate boundary areas, providing new insights on the impact of mantle processes on the topography of mountain belts. Here we use a densely spaced array of temporary broadband seismic stations to analyze the seismic anisotropy pattern of the western Alpine region, at the boundary between the Alpine and Apenninic slabs. Our results are supportive of a polyphase development of anisotropic mantle fabrics, possibly starting from the Jurassic to present. Geophysical data presented in this work, and geologic evidence taken from the literature, indicate that: (i) fossil fabrics formed during Tethyan rifting may be still preserved within the Alpine and Apenninic slabs; (ii) mantle deformation during Apenninic slab rollback is not compensated by a complete toroidal flow around the northern tip of the retreating slab; (iii) the previously observed continuous trend of anisotropy fast axes near-parallel to the western Alpine arc is confirmed. We observe that this arc-parallel trend of fast axes is located in correspondence to a low velocity anomaly in the European upper mantle, beneath regions of the Western and Ligurian Alps showing the highest uplift rates. We propose that the progressive rollback of the Apenninic slab, in the absence of a counterclockwise toroidal flow at its northern tip, induced a suction effect at the scale of the supraslab mantle. The resulting mantle flow pattern was characterized by an asthenospheric counterflow at the rear of the unbroken Western Alps slab and around its southern tip, and by an asthenospheric upwelling, mirrored by low P wave velocities, that would have favored the topographic uplift of the Alpine belt from the Mont Blanc to the Mediterranean sea. [ABSTRACT FROM AUTHOR]

Detrital thermochronology is often employed to assess the evolutionary stage of an entire orogenic belt using the lag-time approach, i.e., the difference between the cooling and depositional ages of detrital mineral grains preserved in a stratigraphic succession. The impact of different eroding sources to the final sediment sink is controlled by several factors, including the short-term erosion rate and the mineral fertility of eroded bedrock. Here, we use apatite fertility data and cosmogenic-derived erosion rates in the Po river catchment (Alps–Apennines) to calculate the expected percentage of apatite grains supplied to the modern Po delta from the major Alpine and Apenninic eroding sources. We test these predictions by using a cutting-edge dataset of trace-element and Nd-isotope signatures on 871 apatite grains from 14 modern sand samples, and we use apatite fission-track data to validate our geochemical approach to provenance discrimination. We found that apatite grains shed from different sources are geochemically distinct. Apatites from the Lepontine dome in the Central Alps show relative HREE enrichment, lower concentrations in Ce and U, and higher 147 Sm/ 144 Nd ratios compared to apatites derived from the External Massifs. Derived provenance budgets point to a dominant apatite contribution to the Po delta from the high-fertility Lepontine dome, consistent with the range independently predicted from cosmonuclide and mineral-fertility data. Our results demonstrate that the single-mineral record in the final sediment sink can be largely determined by high-fertility source rocks exposed in rapidly eroding areas within the drainage. This implies that the detrital thermochronology record may reflect processes affecting relatively small parts of the orogenic system under consideration. A reliable approach to lag-time analysis would thus benefit from an independent provenance discrimination of dated mineral grains, which may allow to proficiently reconsider many previous interpretations of detrital thermochronology datasets in terms of orogenic-wide steady state. [ABSTRACT FROM AUTHOR]

The Southern Variscan Front in the Tinerhir area involves Palaeozoic allochthonous units (Ouaklim and Tilouine units) thrust onto the northern edge of the West African Craton during late Carboniferous time. Illite crystallinity data highlight an anchizonal grade for the Ouaklim Unit, and a diagenesis-anchizone transition for the Tilouine Unit during deformation phase D1. The tectonic stack is crosscut by major dextral reverse faults bounding E–W trending domains of dominant shortening deformation (central domain) and strike-slip deformation (northern and southern domains), later segmented by a network of post-Variscan faults. This complex deformation pattern is the result of kinematic partitioning of dextral transpression along the Southern Variscan Front, coeval with the Neovariscan (300–290 Ma) oblique convergence observed at the scale of the whole Moroccan Variscides. Partitioning of dextral transpression described in the Tinerhir area is consistent with dextral wrench faulting along the Tizi n’ Test Fault, and with Appalachian-style south-directed thrusting in the Tinerhir and Bechar-Bou Arfa areas. [ABSTRACT FROM AUTHOR]

Collision orogens such as the Alps or the Himalayas are generated by plate convergence, culminating in attempted subduction of a thinned continental margin. Massive amounts of metamorphic rocks displaying high-pressure parageneses are produced during such relatively brief tectonic events and then rapidly exhumed to form the axial backbone of the new orogen. Sediment composition provides a fundamental key to identify past events of continental subduction, although coupled detrital-geochronology techniques are needed to discriminate neometamorphic and paleometamorphic sources of detritus. Within the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments and mostly produces lithic to quartzolithic sedimentaclastic sands yielding very poor heavy mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements and sheds quartzolithic to feldspatholithoquartzose metamorphiclastic sands yielding moderately rich epidote-amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent-ocean transitions and supplies feldspatholithoquartzose/feldspathoquartzose high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely rich suites dominated by garnet, hornblende, or epidote, depending on protoliths (continental vs. oceanic) and pressure/temperature paths during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the heavy mineral concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulic-sorting effects. Rather than the pressure peak reached at depth, the metamorphic index (MI) and hornblende color index (HCI) reflect peak temperatures reached at later stages, when subduction is throttled by arrival of thicker continental crust and geothermal gradients increase, as documented in detritus derived from the Tauern window and Lepontine Dome. Experience gained from modern sediments provides fundamental help to decrypt the information stored in the sedimentary record and thus to identify and reconstruct subduction events of the past. [ABSTRACT FROM AUTHOR]

The High Atlas belt of Morocco is a doubly vergent intracontinental belt formed during Cenozoic convergence between the African and Eurasian plates. This belt is characterized by the simultaneous occurrence of high topographic elevation, minor crustal thickening and weak tectonic shortening, which are commonly explained by models of inversion tectonics and mantle upwelling. This paper aims to test the consistence of this type of model, presenting the results of a multi-scale tectonic study, including mapping of major faults and fault-slip data inversion, from key areas homogeneously distributed through the entire High Atlas. Our data highlight kinematic components parallel to the orogen trend, supporting a transpressional structuring. Integration between new tectonic data and available geophysical and thermochronologic constraints has allowed to extend to depth the main faults detected at the surface, resulting in the overall geometry of a positive flower structure. This scenario is the result of a long history of transpressional and transtensional reactivations of major lithospheric faults since the break-up of Pangea. Lithospheric-scale faults isolate independently-behaving lithospheric blocks characterized by different exhumation rates. In this framework, the compressive component of the transpressional geodynamic regime can produce the high topographic elevation of the High Atlas belt without crustal thickening. Image 1 [ABSTRACT FROM AUTHOR]

Detrital thermochronology studies based on the lag-time approach are increasingly employed to investigate the erosional evolution of mountain belts and perform paleotectonic reconstructions starting from the analysis of sedimentary rocks. However, simple predictions of lag-time conceptual models are often in conflict with observations in sedimentary basins. In this review article, we discuss the major assumptions of the lag-time approach, and present conceptual models to illustrate the main factors that may influence the final complexity of the detrital thermochronologic record in a sedimentary basin. These factors include: (i) the original complexity of the thermochronologic age structure in the source region; (ii) mixing of detritus from multiple source regions characterized by different geologic evolution; (iii) modifications of the original thermochronologic fingerprint from source to sink; (iv) potential post-depositional annealing due to thick sedimentary burial. Based on this synthesis and discussion, we present a list of interpretive guidelines and fundamental criteria for the geologic interpretation of the detrital thermochronologic record derived from the erosion of single and mixed sources. These interpretive guidelines are then applied to published detrital thermochronology datasets from the European Alps, Taiwan and the Himalayas in order to illustrate benefits and pitfalls of the geologic interpretation of thermochronologic age trends through a stratigraphic succession. Results provide evidence for (i) non-steady-state exhumation of the European Alps, (ii) late Miocene onset of arc-continent collision in Taiwan, and (iii) late Miocene morphogenic phase of mountain building in the Himalaya. Concepts presented in this article reinforce existing approaches and provide new perspectives for the application of the detrital thermochronologic approach to tectonic settings where the geologic evolution may be poorly understood. As such, they are expected to guide geologists towards interpretations that are both consistent with evidence provided by different thermochronologic systems, and geologic evidence provided by the rock record. Unlabelled Image • Review of detrital thermochronology and guidelines for the interpretation of data. • Synthesis of detrital thermochronology data sets from the Alps, Taiwan and Himalaya. • Data sets reveal a late Miocene morphogenic phase of mountain building in Himalaya. • Detrital ZFT data point to a late Miocene onset of arc-continent collision in Taiwan. • Detrital thermochronology data indicate non steady-state exhumation in the Alps. [ABSTRACT FROM AUTHOR]

Detrital thermochronology is generally used to reconstruct tectonic and erosional processes at the scale of the entire orogen (Garver et al., 1999). Comparison of apatite-fertility maps and thermochronological data from the European Alps (Malusà et al., 2016) and Taiwan orogens indicate instead that the detrital signal in these settings only reflects processes limited to surprisingly small areas.Detrital signals, in fact, are determined by various controlling factors, including short-term erosion rate and fertility of the target mineral in eroded bedrock. Moreover, different grains of the same mineral may have different chemical composition, hence different durability to diagenesis and metamorphism, and - last but not least - propensity to be dated by radiometric methods (Malusà and Fitzgerald, 2019).In the Po Plain sedimentary sink south of the Alps, the combined effect of apatite fertility and erosion rates determines a virtually-exclusive contribution of apatites with young cooling ages from the high-fertility Lepontine dome (Malusà et al., 2017).In Taiwan, the same approach indicates that apatite contribution to river sediments is dominated by only a small area along the backbone of the orogen. Fast exhumation and low U concentration result in a great abundance of zero-track grains that, if ignored, lead to a misinterpretation of the location of erosional foci and to an underestimation of long-term erosion rates.CITED REFERENCESGarver, J. I., Brandon, M. T., Roden-Tice, M., & Kamp, P. J. (1999). Exhumation history of orogenic highlands determined by detrital fission-track thermochronology. Geological Society, London, Special Publications, 154(1), 283-304.Malusà, M. G., and Fitzgerald, P. G., eds (2019). Fission-Track Thermochronology and its Application to Geology (393 pp). Springer.Malusà, M. G., Resentini, A., & Garzanti, E. (2016). Hydraulic sorting and mineral fertility bias in detrital geochronology. Gondwana Research, 31, 1-19.Malusà, M. G., Wang, J., Garzanti, E., Liu, Z. C., Villa, I. M., & Wittmann, H. (2017). Trace-element and Nd-isotope systematics in detrital apatite of the Po river catchment: Implications for provenance discrimination and the lag-time approach to detrital thermochronology. Lithos, 290, 48-59. [ABSTRACT FROM AUTHOR]

High-resolution images of Earth’s interior provide pivotal information for the understanding of a range of geodynamic processes, including continental subduction and exhumation of ultrahigh-pressure (UHP) metamorphic rocks. Here we present a synthesis of available global seismic observations on continental subduction zones, and selected examples of seismic probing from the European Alps, the Himalaya-Tibet and the Qinling-Dabie orogenic belts. Our synthesis and examples show that slabs recognized beneath exhumed continental UHP terranes generally have shallow dip angles (<45°) at depths <100 km, to become much steeper at depths >100 km. Slabs underlined by a clear high velocity anomaly from Earth’s surface to the mantle are generally Cenozoic in age. Some of these slabs are continuous, whereas other continental subduction zones are located above discontinuous high velocity anomalies possibly suggesting slab breakoff. The density of seismic stations and the quality of recordings are of primary importance to get high-resolution images of the upper mantle to be used as a starting point to provide reliable geodynamic interpretations. In some cases, areas previously indicated as possible site of slab breakoff, such as the European Alps, have been later proven to be located above a continuous slab by using higher quality travel time data from denser seismic arrays. Discriminating between oceanic and continental slabs can be challenging, but valuable information can be provided by combining teleseismic tomography and receiver function analysis. The upper mantle beneath most continental UHP terranes generally shows complex seismic anisotropy patterns that are potentially preserved even in pre-Cenozoic subduction zones. These patterns can be used to provide information on continental slabs that are no longer highlighted by a clear high-velocity anomaly. [ABSTRACT FROM AUTHOR]

The first discovery of ultrahigh-pressure coesite in the European Alps 30 years ago led to the inference that a positively buoyant continental crust can be subducted to mantle depth; this had been considered impossible since the advent of the plate tectonics concepts. Although continental subduction is now widely accepted, there remains debate because there is little direct (geophysical) evidence of a link between exhumed coesite at the surface and subducted continental crust at depth. Here we provide the first seismic evidence for continental crust at 75 km depth that is clearly connected with the European crust exactly along the transect where coesite was found at the surface. Our data also provide evidence for a thick suture zone with downward-decreasing seismic velocities, demonstrating that the European lower crust underthrusts the Adriatic mantle. These findings, from one of the best-preserved and long-studied ultrahigh-pressure orogens worldwide, shed decisive new light on geodynamic processes along convergent continental margins. [ABSTRACT FROM AUTHOR]

• 24904 RFs from 1182 events were collected by 307 stations to study MTZ. • Effect of velocity of upper mantle was studied via IASP91 and a 3D velocity model. • MTZ thickening was seen under the Western Alps due to uplift of discontinuities. • Localized arc-shaped thinning of MTZ was located along the alpine orogenic belt. • Temperature rise by mantle upwelling promoted topographic uplift of orogenic belt. To better constrain the deep structure and dynamics of the Western Alps, we studied the mantle transition zone (MTZ) structure using P-wave receiver functions (RFs). We obtained a total of 24904 RFs from 1182 events collected by 307 stations in the Western Alps. To illustrate the influence of the heterogeneity on the upper mantle velocity, we used both IASP91 and three-dimensional (3-D) velocity models to perform RF time-to-depth migration. We documented an MTZ thickening of about 40 km under the Western Alps and most of the Po Plain due to the uplift associated with the 410-km discontinuity and the depression associated with the 660-km discontinuity. Based upon the close spatial connection between the thickened MTZ and the location of the subducted slabs, we proposed that the thick MTZ was due to the subduction of the Alpine slab through the upper MTZ and the presence of remnants of subducted oceanic lithosphere in the MTZ. The uplift associated with the 410-km discontinuity provided independent evidence of the subduction depth of the Western Alps slab. In the Alpine foreland in eastern France, we observed localized arc-shaped thinning of the MTZ caused by a 12 km depression of the 410-km discontinuity, which has not been previously reported. This depression indicated a temperature increase of 120 K in the upper MTZ, and we proposed that it was caused by a small-scale mantle upwelling. Hardly any uplift of the 660 km discontinuity was observed, suggesting that the thermal anomaly was unlikely to be the result of a mantle plume. We observed that the thinning area of the MTZ corresponded to the area with the highest uplift rate in the Western Alps, which may have indicated that the temperature increase caused by the mantle upwelling contributed to the topographic uplift. [ABSTRACT FROM AUTHOR]