Subduction and Slab Detachment Under Moving Trenches During Ongoing India-Asia Convergence

The dynamics of slab detachment and associated geological fingerprints have been inferred from various numerical and analog models. These invariably use a setup with slab-pull-driven convergence in which a slab detaches below a mantle-stationary trench after the arrest of plate convergence due to arrival of continental lithosphere. In contrast, geological reconstructions show that post-detachment plate convergence is common and that trenches and sutures are rarely mantle-stationary during detachment. Here, we identify the more realistic kinematic context of slab detachment using the example of the India-Asia convergent system. We first show that only the India and Himalayas slabs (from India's northern margin) and the Carlsberg slab (from the western margin) unequivocally detached from Indian lithosphere. Several other slabs below the Indian Ocean do not require a Neotethyan origin and may be of Mesotethys and Paleotethys origin. Additionally, the still-connected slabs are being dragged together with the Indian plate forward (Hindu Kush) or sideways (Burma, Chaman) through the mantle. We show that Indian slab detachment occurred at moving trenches during ongoing plate convergence, providing more realistic geodynamic conditions for use in future numerical and analog experiments. We identify that the actively detaching Hindu Kush slab is a type-example of this setting, whilst a 25–13 Ma phase of shallow detachment of the Himalayas slab, here reconstructed from plate kinematics and tomography, agrees well with independent, published geological estimates from the Himalayas orogen of slab detachment. The Sulaiman Ranges of Pakistan may hold the geological signatures of detachment of the laterally dragged Carlsberg slab

Causes of Cretaceous subduction termination below South China and Borneo: Was the Proto-South China Sea underlain by an oceanic plateau?

The South China, Indochina, and Borneo margins surrounding the South China Sea contain long-lived arcs that became inactive at approximately 85 Ma, even though an embayment of oceanic crust (the ‘Proto-South China Sea’) remained in the intervening region. This oceanic crust eventually subducted in the Cenozoic below Borneo and the Cagayan arc, while the modern South China Sea opened in its wake. To investigate the enigmatic cessation of Mesozoic subduction below South China and Borneo, we studied a fragment of oceanic crust and overlying trench-fill sediments that accreted to NW Borneo during the final stages of Paleo-Pacific subduction. Based on radiolarian biostratigraphy of cherts overlying the pillow basalts and detrital zircon geochronology of the trench-fill, we constrained the minimum age of the oceanic crust during accretion to 40 Ma. This shows that subduction cessation was not related to ridge subduction. Geochemical analysis of pillow basalts revealed an enriched mid-ocean ridge basalt signature comparable to oceanic plateaus. Using paleomagnetism, we show that this fragment of oceanic crust was not part of the Izanagi Plate but was part of a plate (the ‘Pontus’ Plate) separated from the Izanagi Plate by a subduction zone. Based on the minimum 40 Ma age of the oceanic crust and its geochemistry, we suggest that Mesozoic subduction below South China and Borneo stopped when an oceanic plateau entered the trench, while the eastern plate margin with the Izanagi Plate remained active. We show how our findings offer opportunities to restore plate configurations of the Panthalassa-Tethys junction region

Tectonic evolution and paleogeography of the Kırşehir Block and the Central Anatolian Ophiolites, Turkey

In Central and Western Anatolia two continent-derived massifs simultaneously underthrusted an oceanic lithosphere in the Cretaceous and ended up with very contrasting metamorphic grades: high pressure, low temperature in the Tavsanli zone and the low pressure, high temperature in the Kirsehir Block. To assess why, we reconstruct the Cretaceous paleogeography and plate configuration of Central Anatolia using structural, metamorphic, and geochronological constraints and Africa-Europe plate reconstructions. We review and provide new Ar-40/Ar-39 and U/Pb ages from Central Anatolian metamorphic and magmatic rocks and ophiolites and show new paleomagnetic data on the paleo-ridge orientation in a Central Anatolian Ophiolite. Intraoceanic subduction that formed within the Neotethys around 100-90 Ma along connected N-S and E-W striking segments was followed by overriding oceanic plate extension. Already during suprasubduction zone ocean spreading, continental subduction started. We show that the complex geology of central and southern Turkey can at first order be explained by a foreland-propagating thrusting of upper crustal nappes derived from a downgoing, dominantly continental lithosphere: the Kirsehir Block and Tavsanli zone accreted around 85 Ma, the Afyon zone around 65 Ma, and Taurides accretion continued until after the middle Eocene. We find no argument for Late Cretaceous subduction initiation within a conceptual "Inner Tauride Ocean" between the Kirsehir Block and the Afyon zone as widely inferred. We propose that the major contrast in metamorphic grade between the Kirsehir Block and the Tavsanli zone primarily results from a major contrast in subduction obliquity and the associated burial rates, higher temperature being reached upon higher subduction obliquity.European Research Council ; Netherlands Organization for Scientific Research (NWO

A global apparent polar wander path for the last 320 Ma calculated from site-level paleomagnetic data

Apparent polar wander paths (APWPs) calculated from paleomagnetic data describe the motion of tectonic plates relative to the Earth's rotation axis through geological time, providing a quantitative paleogeographic framework for studying the evolution of Earth's interior, surface, and atmosphere. Previous APWPs were typically calculated from collections of paleomagnetic poles, with each pole computed from collections of paleomagnetic sites, and each site representing a spot reading of the paleomagnetic field. It was recently shown that the choice of how sites are distributed over poles strongly determines the confidence region around APWPs and possibly the APWP itself, and that the number of paleomagnetic data used to compute a single paleomagnetic pole varies widely and is essentially arbitrary. Here, we use a recently proposed method to overcome this problem and provide a new global APWP for the last 320 million years that is calculated from simulated site-level paleomagnetic data instead of from paleopoles, in which spatial and temporal uncertainties of the original datasets are incorporated. We provide an updated global paleomagnetic database scrutinized against quantitative, stringent quality criteria, and use an updated global plate motion model. The new global APWP follows the same trend as the most recent pole-based APWP but has smaller uncertainties. This demonstrates that the first-order geometry of the global APWP is robust and reproducible. Moreover, we find that previously identified peaks in APW rate disappear when calculating the APWP from site-level data and correcting for a temporal bias in the underlying data. Finally, we show that a higher-resolution global APWP frame may be determined for time intervals with high data density, but that this is not yet feasible for the entire 320–0 Ma time span. Calculating polar wander from site-level data provides opportunities to significantly improve the quality and resolution of the global APWP by collecting large and well-dated paleomagnetic datasets from stable plate interiors, which may contribute to solving detailed Earth scientific problems that rely on a paleomagnetic reference frame

Finding Argoland: Reconstructing a microcontinental archipelago from the SE Asian accretionary orogen

Based on the marine magnetic anomalies identified in the Argo Abyssal Plain offshore northwestern Australia, the conceptual continent of Argoland must have rifted off in the Late Jurassic (∼155 Ma) and drifted northward towards SE Asia. Intriguingly, in SE Asia there are no intact relics of a major continent such as India, but instead the region displays an intensely deformed, long-lived accretionary orogen that formed during more than 100 million years of oceanic and continental subduction. Within this orogen, there are continental fragments that may represent parts of Argoland. After accretion of these fragments, the orogen was further deformed. We compiled the orogenic architecture and the history of post-accretionary deformation of SE Asia, as well as the architecture and history of the NW Australian passive margin. We identified the Gondwana-derived blocks and mega-units of SW Borneo, Greater Paternoster, East Java, West Burma, and Mount Victoria Land as fragments that collectively may represent fragments of Argoland. These fragments are found between sutures with relics of Late Triassic to Middle Jurassic oceanic basins that all pre-date the break-up of Argoland. We systematically restore deformation within SE Asia in the upper plate system above the modern Sunda trench, use this to estimate where Gondwana-derived continental fragments accreted at the Sundaland (Eurasian) margin in the Cretaceous (∼110–85 Ma), and subsequently reconstruct their tectonic transport back to the Australian-Greater Indian margin. Our reconstruction shows that Argoland originated at the northern Australian margin between the Bird's Head in the east and Wallaby-Zenith Fracture Zone in the west, south of which it bordered Greater India. We show that the lithospheric fragment that broke off northwest Australia in the Late Jurassic consisted of multiple continental fragments and intervening Triassic to Middle Jurassic oceanic basins, which we here call Argopelago. Argoland broke up into Argopelago during the Late Triassic rifting of Lhasa from the northern margin of Gondwana, and consisted of multiple continental fragments that were surrounded by oceanic basins, similar to Zealandia offshore modern east Australia, and the reconstructed history of Greater Adria in the Mediterranean

Paleomagnetic Constraints From the Baoshan Area on the Deformation of the Qiangtang-Sibumasu Terrane Around the Eastern Himalayan Syntaxis

The Sibumasu Block in SE Asia represents the eastward continuation of the Qiangtang Block. Here we report a detailed rock magnetic and paleomagnetic study on the Middle Jurassic and Paleocene rocks from northern Sibumasu, to document the crustal deformation during the India-Asia collision since the Paleocene and reconstruct the overall strike of the Qiangtang/Sibumasu elements before the India-Asia collision. Although the fold test is inconclusive based solely on our data, a positive reversal test, a positive regional fold test with previous paleomagnetic results, and a detrital origin of hematite in the red beds as indicated by scanning electron microscopy suggest that the magnetizations obtained from the Jurassic and Paleocene rocks are most likely primary, showing an ~80° clockwise rotation since Paleocene. These results, together with previously published paleomagnetic data, suggest that the northern Sibumasu and northern Simao elements experienced a ~60-80° clockwise rotation since Paleocene. This large clockwise rotation is also consistent with the surface GPS velocity field and NE-SW fault networks, suggesting a rotational motion of crustal material from southeastern Tibet during late Cenozoic. We infer that the large clockwise rotation is a sum of rotation in the Eocene to Middle Miocene time associated with Indochina extrusion and rotation after the Middle Miocene associated with the E-W extension in central Tibet. This suggests that the eastward motion of Tibetan crustal material along the Xianshuihe-Xiaojiang fault after Middle Miocene is transmitted to the southwest toward Myanmar. Jurassic and Cretaceous paleomagnetic results suggest that the Qiangtang/northern Sibumasu was originally a curved structure with an orientation of N60°W in Tibet and changes to N10°W in southern Sibumasu

Paleomagnetic Constraints From the Baoshan Area on the Deformation of the Qiangtang-Sibumasu Terrane Around the Eastern Himalayan Syntaxis

The Sibumasu Block in SE Asia represents the eastward continuation of the Qiangtang Block. Here we report a detailed rock magnetic and paleomagnetic study on the Middle Jurassic and Paleocene rocks from northern Sibumasu, to document the crustal deformation during the India-Asia collision since the Paleocene and reconstruct the overall strike of the Qiangtang/Sibumasu elements before the India-Asia collision. Although the fold test is inconclusive based solely on our data, a positive reversal test, a positive regional fold test with previous paleomagnetic results, and a detrital origin of hematite in the red beds as indicated by scanning electron microscopy suggest that the magnetizations obtained from the Jurassic and Paleocene rocks are most likely primary, showing an ~80° clockwise rotation since Paleocene. These results, together with previously published paleomagnetic data, suggest that the northern Sibumasu and northern Simao elements experienced a ~60-80° clockwise rotation since Paleocene. This large clockwise rotation is also consistent with the surface GPS velocity field and NE-SW fault networks, suggesting a rotational motion of crustal material from southeastern Tibet during late Cenozoic. We infer that the large clockwise rotation is a sum of rotation in the Eocene to Middle Miocene time associated with Indochina extrusion and rotation after the Middle Miocene associated with the E-W extension in central Tibet. This suggests that the eastward motion of Tibetan crustal material along the Xianshuihe-Xiaojiang fault after Middle Miocene is transmitted to the southwest toward Myanmar. Jurassic and Cretaceous paleomagnetic results suggest that the Qiangtang/northern Sibumasu was originally a curved structure with an orientation of N60°W in Tibet and changes to N10°W in southern Sibumasu

Late Cretaceous extension and Palaeogene rotation-related contraction in Central Anatolia recorded in the Ayhan-Büyükkışla basin

The configuration and evolution of subduction zones in the Eastern Mediterranean region in Cretaceous time accommodating Africa–Europe convergence remain poorly quantitatively reconstructed, owing to a lack of kinematic constraints. A recent palaeomagnetic study suggested that the triangular Central Anatolian Crystalline Complex (CACC) consists of three blocks that once formed an ~N–S elongated continental body, underthrusted below ophiolites in Late Cretaceous time. After extensional exhumation and upon Palaeogene collision of the CACC with the Pontides of the southern Eurasian margin, the CACC broke into three fragments that rotated and converged relative to each other. Here, we date the extension and contraction history of the boundary between two of the rotating massifs of the CACC by studying the Upper Cretaceous–Palaeogene Ayhan–Büyükkışla basin. We report an 40Ar/39Ar age of an andesite at the base of the sequence to show that the deposition started in an E–W extensional basin around 72.11 ± 1.46. The basin developed contemporaneously with regional exhumation of the CACC metamorphics. The lower basin sedimentary rocks were unconformably covered by mid-Eocene limestones and redbeds, followed by intense folding and thrust faulting. Two balanced cross-sections in the study area yield a minimum of 17–27 km of post-mid-Eocene ~N–S shortening. We thus demonstrate the Cenozoic compressional nature of the Kırşehir–Niğde-Hırkadağ block boundary and show that the extensional exhumation of the CACC predates collision-related contraction. A plate kinematic scenario is required to explain these observations that involves two Late Cretaceous–Palaeogene subduction zones to the north and south of the CACC, for which we show a possible plate boundary configuration

Paleomagnetic constraints on the Mesozoic-Cenozoic paleolatitudinal and rotational history of Indochina and South China : Review and updated kinematic reconstruction

Paleomagnetic data have long been used to hypothesize that the Cenozoic extrusion of the Indochina Block along the left-lateral Ailao Shan-Red River fault, as a result of the India-Asia collision, may have been associated with a major southward paleolatitude shift of as much as 10–15°, and a vertical-axis rotation of as much as 25–40°. However, although numerous paleomagnetic studies have been conducted in the southeast margin of the Tibetan Plateau and in the Indochina region during the last few decades, the detailed rotation as well as the latitudinal displacement of the Indochina Block remain controversial because of apparently contradicting paleomagnetic results. Geological constraints also yield contrasting estimates on the amount of displacement along different segment of the Ailao Shan-Red River fault: 700 ± 200 km in the northwest, but only ~ 250 km in the southeast. In this paper, the available paleomagnetic data from the southeast margin of the Tibetan Plateau and Indochina, as well as the South China Block, from Jurassic and younger rocks are compiled and critically reviewed using the new paleomagnetic toolkit on Paleomagnetism.org. Our results show that (1) the South China Block has declinations that reveal no significant rotations relative to Eurasia since latest Jurassic. Inclinations are consistently shallower than expected, which is likely the result of inclination shallowing in sedimentary rocks; (2) there is no paleomagnetically resolvable southward motion of the Indochina Block with respect to Eurasia based on the paleomagnetic data. Paleomagnetic inclinations are in fact lower than expected, probably due to inclination shallowing in sediments; (3) paleomagnetic declinations reveal large, more or less coherently rotating blocks in the northern Indochina domain and the SE Tibetan margin that rotated up to 70° clockwise, much more than the ~ 10–15° rotation of the stable, SE part of the Indochina Block. These blocks are bounded by fold-thrust belts and strike-slip faults, which we interpret to have accommodated these block rotations during the Cenozoic. We designed a new tool on the online open-access portal Paleomagnetism.org that allows testing whether Euler rotations in a kinematic reconstruction fulfill paleomagnetic data. Using this tool, we built a first-order kinematic reconstruction of rotational deformation of northwest Indochina in Cenozoic. We show that the northwestern part of Indochina extruded 350 km more along the Ailao Shan-Red River fault than the southeastern part accommodated by internal northwest Indochina rotation and deformation. Estimates of 250 km of extrusion of the southeastern part of the Indochina then predicts ~ 600 km of left-lateral motion along the northwestern part of the Ailao Shan-Red River fault, which reconciles the small and large estimates that prevail in the literature of extrusion of Indochina from the Tibetan realm during the Cenozoic India-Asia collision