Cenozoic convergence between the Indian and Asian plates produced the archetypical continental collision zone comprising the Himalaya mountain belt and the Tibetan Plateau. How and where India–Asia ...convergence was accommodated after collision at or before 52 Ma remains a long-standing controversy. Since 52 Ma, the two plates have converged up to 3,600 ± 35 km, yet the upper crustal shortening documented from the geological record of Asia and the Himalaya is up to approximately 2,350-km less. Here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic Indian lithosphere within the Himalaya between approximately 50 and 25 Ma. Paleomagnetic data show that this extended continental and oceanic "Greater India" promontory resulted from 2,675 ± 700 km of North–South extension between 120 and 70 Ma, accommodated between the Tibetan Himalaya and cratonic India. We suggest that the approximately 50 Ma "India"–Asia collision was a collision of a Tibetan-Himalayan microcontinent with Asia, followed by subduction of the largely oceanic Greater India Basin along a subduction zone at the location of the Greater Himalaya. The "hard" India–Asia collision with thicker and contiguous Indian continental lithosphere occurred around 25–20 Ma. This hard collision is coincident with far-field deformation in central Asia and rapid exhumation of Greater Himalaya crystalline rocks, and may be linked to intensification of the Asian monsoon system. This two-stage collision between India and Asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography.
A long‐standing problem in the geological evolution of the India‐Asia collision zone is how and where convergence between India and Asia was accommodated since collision. Proposed collision ages vary ...from 65 to 35 Ma, although most data sets are consistent with collision being underway by 50 Ma. Plate reconstructions show that since 50 Ma ∼2400–3200 km (west to east) of India‐Asia convergence occurred, much more than the 450–900 km of documented Himalayan shortening. Current models therefore suggest that most post‐50 Ma convergence was accommodated north of the Indus‐Yarlung suture zone. We review kinematic data and construct an updated restoration of Cenozoic Asian deformation to test this assumption. We show that geologic studies have documented 600–750 km of N‐S Cenozoic shortening across, and north of, the Tibetan Plateau. The Pamir‐Hindu Kush region accommodated ∼1050 km of N‐S convergence. Geological evidence from Tibet is inconsistent with models that propose 750–1250 km of eastward extrusion of Indochina. Approximately 250 km of Indochinese extrusion from 30 to 20 Ma of Indochina suggested by SE Asia reconstructions can be reconciled by dextral transpression in eastern Tibet. We use our reconstruction to calculate the required size of Greater India as a function of collision age. Even with a 35 Ma collision age, the size of Greater India is 2–3 times larger than Himalayan shortening. For a 50 Ma collision, the size of Greater India from west to east is ∼1350–2600 km, consistent with robust paleomagnetic data from upper Cretaceous‐Paleocene Tethyan Himalayan strata. These estimates for the size of Greater India far exceed documented shortening in the Himalaya. We conclude that most of Greater India was consumed by subduction or underthrusting, without leaving a geological record that has been recognized at the surface.
Key Points
Post‐50 Ma Asian deformation north of India is 600–1050 km from east to west
The required size of Greater India is 1350–2600 km with a 50 Ma collision
Himalayan shortening greately underestimates Indian plate subduction since 50 Ma
Key in understanding the geodynamics governing subduction and orogeny is reconstructing the paleogeography of ‘Greater India’, the Indian plate lithosphere that subducted since Tethyan Himalayan ...continental collision with Asia. Here, we discuss this reconstruction from paleogeographic, kinematic, and geodynamic perspectives and isolate the evolution scenario that is consistent with all three. We follow recent constraints advocating a ~58 Ma initial collision and update a previous kinematic restoration of intra-Asian shortening with a recently proposed model that reconciles long-debated large and small estimates of Indochina extrusion. Our new reconstruction is tested against paleomagnetic data, and against seismic tomographic constraints on paleo-subduction zone locations. The resulting restoration shows ~1000–1200 km of post-collisional intra-Asian shortening, leaving a 2600–3400 km wide Greater India. From a paleogeographic, sediment provenance perspective, Eocene sediments in the Lesser Himalaya and on undeformed India may be derived from Tibet, suggesting that all Greater Indian lithosphere was continental, but may alternatively be sourced from the contemporaneous western Indian orogen unrelated to India-Asia collision. A quantitative kinematic, paleomagnetic perspective prefers major Cretaceous extension and a ‘Greater India Basin’ opening within Greater India, but data uncertainty may speculatively allow for minimal extension. Finally, from a geodynamic perspective, assuming a fully continental Greater India would require that subduction rates close to 20 cm/yr was driven by a down-going lithosphere-crust assemblage more buoyant than the mantle, which seems physically improbable. We conclude that the Greater India Basin scenario is the only sustainable one from all three perspectives. We infer that old pre-collisional lithosphere rapidly entered the lower mantle sustaining high subduction rates, whilst post-collisional continental and young Greater India basin lithosphere did not, inciting the rapid India-Asia convergence deceleration ~8 Myr after collision. Subsequent absolute northward slab migration and overturning caused flat slab subduction, Tibetan shortening, arc migration and arc volume decrease.
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•Following 58 Ma India-Asia collision, Asia shortening 1000-1200 km and 2600-3400 km of (Greater) India subducted•Combined paleogeographic, kinematic, and geodynamic constraints strongly suggest a Greater India Basin scenario•Post-58 Ma upper mantle slab buckling induced flat slab subduction causing India-Asia slowdown and Tibetan shortening
The surface uplift history of the Tibetan Plateau and Himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during Cenozoic time, its ...influence on monsoon intensity, and its reflection of the dynamics of continental plateaus. Models of plateau growth vary in time, from pre-India-Asia collision (e.g., almost equal to100 Ma ago) to gradual uplift after the India-Asia collision (e.g., almost equal to55 Ma ago) and to more recent abrupt uplift (<7 Ma ago), and vary in space, from northward stepwise growth of topography to simultaneous surface uplift across the plateau. Here, we improve that understanding by presenting geologic and geophysical data from north-central Tibet, including magnetostratigraphy, sedimentology, paleocurrent measurements, and ⁴⁰Ar/³⁹Ar and fission-track studies, to show that the central plateau was elevated by 40 Ma ago. Regions south and north of the central plateau gained elevation significantly later. During Eocene time, the northern boundary of the protoplateau was in the region of the Tanggula Shan. Elevation gain started in pre-Eocene time in the Lhasa and Qiangtang terranes and expanded throughout the Neogene toward its present southern and northern margins in the Himalaya and Qilian Shan.
The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative ...importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.
•Oldest relative paleointensity (RPI) record from marine sediments back to ∼50Ma.•RPI minima always at chron boundaries and large fluctuations during each chron.•First to show such characteristics ...persist at least since ∼49.3Ma.•Chron 18 stacked RPI curve from the northwest Atlantic and the equatorial Pacific.•Possible persistence of the RPI histogram skewed to the right.
Published relative paleointensity (RPI) records older than ∼3Ma are extremely limited in time and space. To develop a more robust understanding of RPI variations, we have conducted rock magnetic and paleomagnetic measurements on Eocene marine sediments recovered at Integrated Ocean Drilling Program (IODP) Sites U1403 and U1408 in the northwest Atlantic. A series of rock magnetic measurements indicate that the main remanence carrier is single domain biogenic magnetite. Paleomagnetic measurements yielded RPI records for Chrons C18–C21 and C22n, which correlates to ∼38.4–49.6Ma. The record is compromised in a few short intervals by inhomogeneous rock magnetic properties. RPI minima always occur at chron boundaries and RPI fluctuates between highs and lows within each chron. This record is the first to show that these characteristics persist at least since the onset of Chron C22n at ∼49.3Ma. We conclude that these are intrinsic and fundamental features of the geomagnetic field regardless of the polarity reversal rate. We produce a stacked RPI curve for Chron 18, named PIS-C18, on the basis of the RPI records obtained in this study and those from IODP Sites U1331 and U1332 in the equatorial Pacific that are matched by visual inspection. The PIS-C18 RPI of the stack is generally high with no prominent lows during Chron C18n.2n, whereas it is not as high and has several prominent lows almost equivalent to the RPI minima at the chron boundaries during Chrons C18n.1n and C18r. A histogram of RPI during Chron 18 is slightly skewed to the right, and the ratio of the standard deviation to the mean paleointensity is 0.38. These characteristics are similar to a histogram of the RPI stack for the last 1.5 million years. We interpret this to imply that character of the geodynamo for 104–106 years timescales has been unchanged since the Eocene.
Near-shore marine sediments deposited during the Paleocene-Eocene Thermal Maximum at Wilson Lake, NJ, contain abundant conventional and giant magnetofossils. We find that giant, needle-shaped ...magnetofossils from Wilson Lake produce distinct magnetic signatures in low-noise, high-resolution first-order reversal curve (FORC) measurements. These magnetic measurements on bulk sediment samples identify the presence of giant, needle-shaped magnetofossils. Our results are supported by micromagnetic simulations of giant needle morphologies measured from transmission electron micrographs of magnetic extracts from Wilson Lake sediments. These simulations underscore the single-domain characteristics and the large magnetic coercivity associated with the extreme crystal elongation of giant needles. Giant magnetofossils have so far only been identified in sediments deposited during global hyperthermal events and therefore may serve as magnetic biomarkers of environmental disturbances. Our results show that FORC measurements are a nondestructive method for identifying giant magnetofossil assemblages in bulk sediments, which will help test their ecology and significance with respect to environmental change.
Ongoing controversies on the timing and kinematics of the Indo–Asia collision can be solved by palaeomagnetically determined palaeolatitudes of terranes bounding the Indo–Asia suture zone. We show ...here, based on new palaeomagnetic data from the Linzizong volcanic rocks (54–47 Ma) near the city of Lhasa, that the latitude of the southern margin of Asia was 22.8 ± 4.2°N when these rocks were deposited. This result, combined with revised palaeomagnetic results from the northernmost sedimentary units of Greater India and with apparent polar wander paths of India and Eurasia, palaeomagnetically constrain the collision to have occurred at 46 ± 8 Ma (95 per cent confidence interval). These palaeomagnetic results are consistent with tomographic anomalies at 15–25°N that are interpreted to locate the Tethyan oceanic slab that detached following collision, and with independent 56–46 Ma collision age estimates inferred from the timing of slowing down of India, high pressure metamorphism, the end of marine sedimentation and the first occurrence of suture zone and arc detritus on the Greater Indian margin. When compared with apparent polar wander paths of India and Eurasia, the ∼46 Ma onset of collision at 22.8 ± 4.2°N implies 2900 ± 600 km subsequent latitudinal convergence between India and Asia divided into 1100 ± 500 km within Asia and 1800 ± 700 km within India.
To better constrain the Late Triassic paleolatitude of the Qiangtang block and the closure of the Paleo-Tethys Ocean, a combined paleomagnetic and zircon U/Pb geochronological study has been ...conducted on the Upper Triassic Jiapila Formation volcanic rocks on the northern edge of the Qiangtang block of Central Tibet (34.1°N, 92.4°E). These rocks are dated to 204–213 Ma. Progressive thermal or alternating field demagnetization successfully isolated stable characteristic remanent magnetizations (ChRM) that pass both the fold and reversal tests, consistent with a primary magnetization. These are the first volcanic-based paleomagnetic results from pre-Cretaceous rocks of the Qiangtang block that appear to average secular variation well enough to yield a reliable paleolatitude estimate. Based on our new paleomagnetic data from Upper Triassic lavas, we conclude that the Late Triassic pole of the Qiangtang block was located at 64.0°N, 174.7°E, with A95=6.6° (N=29). We compile published paleomagnetic data from the Qiangtang block to calculate a Late Triassic latitude for the Qiangtang block at 31.7 ± 3.0°N. The central Paleo-Tethys Ocean basin was located between the North China (NCB) and Tarim blocks to the north and the Qiangtang block to the south during Late Paleozoic–Early Mesozoic. A comparison of published Early Triassic paleopole from the Qiangtang block with the coeval paleopoles from the NCB and Tarim indicates that the Paleo-Tethys Ocean could not have closed during the Early Triassic and that its width was approximately ∼32–38° latitude (∼3500–4200 km). However, the comparison of our new combined Late Triassic paleomagnetic result with the Late Triassic poles of the NCB and Tarim, as well as numerous geological observations, indicates that the closure of the Paleo-Tethys Ocean at the longitude of the Qiangtang block most likely occurred during the Late Triassic.
•We provide the Late Triassic volcanic paleomagnetic results from Qiangtang block.•The Late Triassic latitude of the Qiangtang block was 31.7 ± 3.0°N.•The latitudinal width of Paleo-Tethys Ocean was ∼3600 km during Early Triassic.•The closure of Paleo-Tethys Ocean most likely occurred during Late Triassic.
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