Surface Responses to Subducting Slab Detachment in Small Convergent Mountain Ranges

Alpine-type mountain ranges emerge from the collision of two continental plates. During the collision the subducting plate descends into the mantle. Given favourable thermo-physical conditions and time, the lower end of the subducting plate detaches from its upper section causing a dynamic surface uplift response over a geologically brief time period. This study investigates how, and if, such a process can be detected solely from the geomorphological record. Furthermore, it aims to identify minimum surface uplift conditions that would favour such observation in nature. The experimental set-up links typical kinematics for the lateral growth of a doubly-vergent orogens over 15 Myr with a surface processes model. This includes isostatic responses to erosion as well as buoyancy effects caused by crustal thickening. Two fundamental slab dynamics scenarios have been tested: the first scenario subjected the evolving orogen to a single surface uplift event representative of a slab break-off (Fig. 1). The orogen responds by immediate increases in mean elevation by ~10%, erosion rates by more than 10%, and river steepness by ~5% assuming a parabolic surface uplift of 1 mm/yr over 1 Myr across-strike the orogen. Notably, the orogen undergoes a prolonged decay period over ~1 Myr to reach conditions prior to the surface uplift event. The second scenario assumes an along-strike propagating surface uplift representing a slab tearing event. Geomorphological responses are similar to the first scenario but restricted to the location of highest surface uplift in space and time causing an asymmetric response along-strike the orogen. Both scenarios induce a two-step inversion of the foreland basins: firstly, as a result of the surface uplift event itself, and secondly, followed by the isostatic response to erosional unloading during the prolonged landscape decay. Hence, the study argues that the identification of geologically short-lived surface uplift events in Alpine-type orogens, caused, for example, by break-off or tearing of the subducting slab, require the observation of a coeval increase in erosion, local relief, river steepness and the inversion of the foreland basins during phases of surface uplift and erosional unloading

Deriving the exhumation history of the Alps with thermochronological data

Thermochronology is a unique tool to derive the exhumation history of rocks over millions of years. Exhumation in orogens is largely controlled by tectonic structures that formed during convergence. Therefore, thermochronological data can be used to reconstruct the geodynamic evolution of mountain ranges and, more precisely, the activity of large fault systems. The Alps are one of the best-studied mountain ranges, with several thousands of low-temperature thermochronological samples dated with a variety of methods (e.g. Herrman et al. 2013; Fox et al. 2015). In this study, we review the most recent thermochronological literature to summarise the exhumation history of the Alps and discuss their driving forces. The apatite (U-Th)/He system is sensitive to the most recent exhumation (closure temperature of ~60°C) and records in places the (over-)deepening and widening of valleys around the Alps (e.g., Valla et al. 2011; Glotzbach et al. 2011). The higher temperature systems, especially the ZFT system (closure temperature of ~240°C), reveal the location of deeper exhumation (>10 km) caused by large-scale fault activity (Fig. 1). While some parts of the Southern Alps and the northern part of the Western and Eastern Alps were not reset during the Alpine orogeny, most of the internal parts of the Alps reveal reset ZFT ages (Fig. 1). The timing of exhumation of these regions, however, varies significantly with distinct tectonic regions. The most recent ZFT ages are <15 Ma and located in the external crystalline massifs, the Lepontine Dome, and the Tauern Window. The latter two are exhumed by large-scale orogen-parallel extensional faulting and contemporaneous indentation. This event ceased in middle Miocene times when faulting and associated exhumation switched towards the Southern Alps (e.g. Eizenhöfer et al. 2021). Apatite fission-track ages (closure temperature of ~110°C) are the youngest (≤6 Ma) in the external crystalline massifs and record a long-lasting Miocene exhumation, whereas the early Miocene exhumation was caused by vertical tectonics related to rollback of the subducted European slab (e.g. Herwegh et al. 2017; 2019). Ongoing middle to late Miocene exhumation of the external crystalline massifs was instead related to in-sequence thrusting (Herwegh et al. 2019). The young thermochronological ages and related high post-Miocene exhumation in the western external crystalline massifs might be at least partly related to uplift caused by slab detachment (e.g. Fox et al. 2015). In the Eastern Alps, there is no evidence for comparable young (post-Miocene) exhumation ‘hotspots’, suggesting a rather stable geodynamic state and absence of large-scale changes in mantle processes

Constraining the near-surface response to lithospheric reorientation: Structural thermochronology along AlpArray geophysical transects

The shape of the present-day European Alps results from a complex tectonic and climatic history since the onset of convergence between the African and Eurasian plates. Low-temperature thermochronology data are a unique archive that can trace the cooling history of rocks back in time during exhumation from upper to middle crustal levels to Earth's surface. However, the precise mechanisms that led to cooling and exhumation are still debated. In this study, we investigated the potential for mantle processes, such as potential subducting slab break-off or slab reversal, to leave a fingerprint in the rock cooling record of the present-day surface along three key, north-south oriented geophysical transects: NFP-20E, TRANSALP and EASI. Along all transects, our zircon and apatite (U-Th)/He data reveal reset Neogene (and younger) cooling ages centred around core complexes such as the Lepontine Dome and the Tauern Window indicative of late exhumation during the Cenozoic Alpine orogeny. North and south of these complexes, the cooling ages become older, forming U-shaped age distributions around the reset centres. Thermal history reconstructions along TRANSALP confirm a conspicuous southward shift of cooling towards the Southern Alps approximately at the time of deep-seated exhumation of the Tauern Window driven by motion along the mid-crustal Tauern Ramp in the Mid-Miocene. Thermo-kinematic models along the transect confirm this southward shift of deformation and (i) reproduce the distribution of cooling ages and thermal history reconstructions, (ii) are consistent with the present-day structural geometry along the transect, (iii) and the observed surface heat flux. It is possible that rock cooling is primarily driven by rock displacement along active faults and less by climatic and/or mantle buoyancy forces, which are both not included in the applied modelling approach. Our comprehensive thermochronological analyses allow two interpretations concerning mantle processes: (i) Assuming a strong coupling between the subducting and overriding plate, hence, the applicability of doubly-vergent orogen kinematics, then the thermochronological data are most consistent with an ongoing reversal in continental subduction polarity. (ii) A high degree of decoupling would negate the possibility that mantle processes are archived in the thermochronological record

Thermo-Kinematic Evolution of the Eastern Alps along TRANSALP: Exploring the Transient Tectonic State towards Slab Reversal

The Eastern Alps are shaped by the indentation of Adria into Europe and exhibit a doubly-vergent lithospheric wedge geometry. Immediately after the subduction of the Penninic ocean, pro- and retro-wedges have been established in the European and Adriatic plates, respectively. Recent tomographic studies, depicting several detached slab fragments beneath the Alps, have been interpreted as evidence of continuous southward subduction, contrary to an often-invoked subduction polarity reversal. Systematic changes in orogen-scale exhumation, driven by rock displacement along active faults, should reflect such change in subduction polarity. Low temperature thermochronology can evaluate upper lithospheric cooling as a response to changes in tectonic and/or erosional boundary conditions. This study investigates whether a potential change in locations of the pro- and retro-wedges is reconcilable with observed crustal re-organisations, exhumation patterns and mantle tomography. A suite of thermo-kinematic forward models driven by a new 2D structural-kinematic reconstruction of continental collision along the TRANSALP profile in the Eastern Alps has been subject to systematic sensitivity analyses encompassing variations in shortening rates, thermophysical parameters and topographic evolution, supplemented by new apatite and zircon fission-track data. Results from the thermo-kinematic modelling reproduce: (i) the orogen-scale structural geometry, (ii) the distribution of low-temperature thermochronometer ages, (iii) independently determined time-temperature paths, and (vi) the present-day surface heat flux. We suggest that the observed thermochronologic record along the TRANSALP profile is primarily driven by cooling through rock displacement along active faults. Our thermo-kinematic reconstruction emphasises a systematic southward shift of deformation, in particular in the Southern Alps, since onset of motion along the Tauern Ramp. Interpreting both, the Tauern Ramp as a mega retro-thrust and the southward shift of deformation in the Southern Alps, as a response to new Coulomb-wedge criterions, then our results are consistent with a Mid-Miocene reversal of continental subduction polarity. This time frame is compatible with a detachment of the European slab and a tectonic re-organisation of the Eastern Alps since ~10-25 Ma

Thermo‐kinematic evolution of the Eastern European Alps along the TRANSALP transect

The eastern European Alps are shaped by the indentation of Adria into Europe. Recent tomography, depicting detached slab fragments, has been interpreted as evidence of continuous southward subduction of European lithosphere, contrary to an often-invoked subduction polarity reversal. Orogen-scale exhumation, driven by rock displacement along active faults, may reflect subduction polarity within the framework of doubly-vergent Coulomb wedge theory, provided the absence of rheological contrasts across the colliding plates. Low-temperature thermochronology can evaluate crustal cooling in response to changes in tectonic and erosional boundary conditions. This study investigates the consistency of observed crustal re-organization, exhumation, and mantle processes in the Eastern Alps. Thermo-kinematic forward models driven by reconstructions of crustal shortening along TRANSALP were subjected to variations in shortening rates, thermophysical parameters, and topographic evolution, supplemented by new fission-track data. The thermo-kinematic models reproduce: (i) the orogen-scale structural geometry, (ii) the distribution of thermochronometer ages, (iii) observed time-temperature paths, and (vi) the present-day surface heat flux. Results suggest that exhumation is driven by rock displacement along active faults without the need to involve mantle-driven buoyancy forces. Taken together, results identify two possible scenarios: if the Tauern Ramp is a retro-thrust and the southward shift of deformation in the Southern Alps is a response to new Coulomb-wedge conditions, then our results suggest a Mid-Miocene reversal of the subduction polarity. Alternatively, crustal deformation does not reflect mantle processes due to a high degree of inter-plate decoupling

Plume‐Induced Sinking of Intracontinental Lithospheric Mantle: An Overlooked Mechanism of Subduction Initiation?

Although many different mechanisms for subduction initiation have been proposed, only few of them are viable in terms of consistency with observations and reproducibility in numerical experiments. In particular, it has recently been demonstrated that intra-oceanic subduction triggered by an upwelling mantle plume could greatly contribute to the onset and operation of plate tectonics in the early and, to a lesser degree, modern Earth. On the contrary, the initiation of intra-continental subduction still remains underappreciated. Here we provide an overview of 1) observational evidence for upwelling of hot mantle material flanked by downgoing proto-slabs of sinking continental mantle lithosphere, and 2) previously published and new numerical models of plume-induced subduction initiation. Numerical modeling shows that under the condition of a sufficiently thick (>100 km) continental plate, incipient downthrusting at the level of the lowermost lithospheric mantle can be triggered by plume anomalies of moderate temperatures and without significant strain- and/or melt-related weakening of overlying rocks. This finding is in contrast with the requirements for plume-induced subduction initiation within oceanic or thinner continental lithosphere. As a result, plume-lithosphere interactions within continental interiors of Paleozoic-Proterozoic-(Archean) platforms are the least demanding (and thus potentially very common) mechanism for initiation of subduction-like foundering in the Phanerozoic Earth. Our findings are supported by a growing body of new geophysical data collected in various intra-continental areas. A better understanding of the role of intra-continental mantle downthrusting and foundering in global plate tectonics and, particularly, in the initiation of “classic” ocean-continent subduction will benefit from more detailed follow-up investigations

Plume‐Induced Sinking of Intracontinental Lithospheric Mantle: An Overlooked Mechanism of Subduction Initiation?

Although many different mechanisms for subduction initiation have been proposed, only few of them are viable in terms of consistency with observations and reproducibility in numerical experiments. In particular, it has recently been demonstrated that intra‐oceanic subduction triggered by an upwelling mantle plume could greatly contribute to the onset and operation of plate tectonics in the early and, to a lesser degree, modern Earth. On the contrary, the initiation of intra‐continental subduction still remains underappreciated. Here we provide an overview of 1) observational evidence for upwelling of hot mantle material flanked by downgoing proto‐slabs of sinking continental mantle lithosphere, and 2) previously published and new numerical models of plume‐induced subduction initiation. Numerical modeling shows that under the condition of a sufficiently thick (>100 km) continental plate, incipient downthrusting at the level of the lowermost lithospheric mantle can be triggered by plume anomalies of moderate temperatures and without significant strain‐ and/or melt‐related weakening of overlying rocks. This finding is in contrast with the requirements for plume‐induced subduction initiation within oceanic or thinner continental lithosphere. As a result, plume‐lithosphere interactions within continental interiors of Paleozoic‐Proterozoic‐(Archean) platforms are the least demanding (and thus potentially very common) mechanism for initiation of subduction‐like foundering in the Phanerozoic Earth. Our findings are supported by a growing body of new geophysical data collected in various intra‐continental areas. A better understanding of the role of intra‐continental mantle downthrusting and foundering in global plate tectonics and, particularly, in the initiation of “classic” ocean‐continent subduction will benefit from more detailed follow‐up investigations

Plume-Induced Sinking of the Intracontinental Lithosphere as a Fundamentally New Mechanism of Subduction Initiation

Although many different mechanisms for subduction initiation have been proposed, few of them are viable in terms of agreement with observations and reproducibility in numerical experiments. In particular, it has recently been demonstrated that intra-oceanic subduction triggered by an upwelling mantle plume could contribute greatly to the onset and functioning of plate tectonics in the early Earth and, to a lesser extent, in the modern Earth. In contrast, the onset of intracontinental subduction is still underestimated. Here we review 1) observations demonstrating the upwelling of hot mantle material flanked by sinking proto-slabs of the continental mantle lithosphere, and 2) previously published and new numerical models of plume-induced subduction initiation. Numerical modelling shows that under the condition of a sufficiently thick (&gt; 100 km) continental plate, incipient down thrusting at the level of the lowermost lithospheric mantle can be triggered by plume anomalies with moderate temperatures and without significant strain and/or melt-induced weakening of the overlying rocks. This finding is in contrast to the requirements for plume-induced subduction initiation in oceanic or thin continental lithosphere. Consequently, plume-lithosphere interactions in the continental interior of Paleozoic-Proterozoic (Archean) platforms are the least demanding (and therefore potentially very common) mechanism for triggering subduction-like foundering in Phanerozoic Earth. Our findings are supported by a growing body of new geophysical data collected in a variety of intracontinental settings. A better understanding of the role of intracontinental mantle downthrusting and foundering in global plate tectonics and, in particular, in triggering "classic" oceanic-continental subduction will benefit from further detailed follow-up studies

Solonker Suture in East Asia and its bearing on the final closure of the eastern segment of the Palaeo-Asian Ocean

The location and tectonic nature of the Solonker Suture in East Asia and hence the late Palaeozoic to early Mesozoic closure of the Palaeo-Asian Ocean have notoriously been enigmatic in the past decades due to limited rock exposure and the absence of unambiguous collision-related regional features. Several tectonic models have been proposed since, but in many cases these models were derived only from single key exposures or methodologies, often being over-simplified, or -interpreted, leading to questionable extrapolations on regional and global scales despite the complexity of the Palaeozoic accretionary tectonic framework. Now, the regionally consistent availability of geochronological, geochemical, stratigraphic and palaeo-geographic data enables us to integrate these into a highly detailed and coherent Palaeozoic tectonic review of the suture. The region across the Solonker Suture can generally be subdivided into three major Palaeozoic tectonic provinces, e.g., (i) a Sino-Cratonic Province that resembles the active Palaeozoic northern margin of the North China Craton to the south of the suture, (ii) a Mongolian Province that comprises the south-eastern margin of the Mongolian Terrane to the north of the suture, and (iii) an East Asian Pre-Pacific Province in north-east China that accreted with the Late Permian initiation of Pacific Plate subduction. The Sino-Cratonic Province experienced episodic tectonic activity that includes the accretion of the initially ensialic Bainaimiao Arc onto the passive northern margin of the North China Craton at ~ 437–453 Ma, subsequent southward directed subduction activity beneath the amalgamated margin until at least ~ 400 Ma, followed by a temporary cessation of magmatic activity accompanied by a switch from north- to southward directed continental drift, and finally concluded by renewed subduction activity until the final closure of the Palaeo-Asian Ocean. The Mongolian Province is suggested to have been more closely associated with the Siberian Craton before the presumably Late Devonian opening of the Mongol-Okhotsk Ocean. The East Asian Pre-Pacific Province, originally rifted away from the eastern margin of Gondwana at ~ 600–750 Ma, recorded early Palaeozoic (~ 500 Ma) orogenic processes along the then northern margin of the Siberian Craton. The Baolidao arc system successively developed along the present-day south-eastern margin of the Mongolian Province due to continued subduction activity throughout the Palaeozoic. After the Middle Carboniferous opening of the Hegenshan back-arc basin renewed subduction beneath a matured Baolidao Arc during Carboniferous and Permian times led to the obduction of the Hegenshan ophiolite not earlier than ~ 270 Ma. Shortly after the Palaeo-Asian Ocean diachronously closed from west to east along the Solonker Suture in the Late Permian to Early Triassic. The East Asian Pre-Pacific Province re-attached itself by the mid-Mesozoic, followed by a change in the regional stress regime controlled by Palaeo-Pacific plate subduction. A divergent double-sided subduction model for the closure of the Palaeo-Asian Ocean provides, based on the reviewed data, a tectonic model that is not only able to reasonably well explain the absence of typical collision-related features in the region but also the abundance of Mesozoic A-type granitic magmatism in north-east China