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Constraints on the timing of the India‐Asia collision and unroofing history of the Himalayan orogen using detrital zircon U‐Pb‐Hf and whole‐rock Sr‐Nd isotopes in Cretaceous‐Miocene Lesser Himalayan sedimentary rocks
Abstract
Cretaceous‐Miocene sedimentary rocks in the Nepalese Lesser Himalaya provide an opportunity to decipher the timing of India‐Asia collision and unroofing history of the Himalayan orogen, which are significant for understanding the growth processes of the Himalayan‐Tibetan orogen. Our new data indicate that detrital zircon ages and whole‐rock Sr‐Nd isotopes in Cretaceous‐Miocene Lesser Himalayan sedimentary rocks underwent two significant changes. First, from the Upper Cretaceous‐Paleocene Amile Formation to the Eocene Bhainskati Formation, the proportion of late Proterozoic‐early Paleozoic zircons (quantified by an index of 500‐1200 Ma/1600‐2800 Ma) increased from nearly 0 to 0.7‐1.4, and the percentage of Mesozoic zircons decreased from ~14% to 5‐12%. The whole‐rock 87Sr/86Sr and εNd(t=0) values changed markedly from 0.732139 and ‐17.2 for the Amile Formation to 0.718106 and ‐11.4 for the Bhainskati Formation. Second, from the Bhainskati Formation to the lower‐middle Miocene Dumri Formation, the index of 500‐1200 Ma/1600‐2800 Ma increased to 2.2‐3.7, and the percentage of Mesozoic zircons abruptly decreased to nearly 0. The whole‐rock 87Sr/86Sr and εNd(t=0) values changed significantly to 0.750124 and ‐15.8 for the Dumri Formation. The εHf(t) values of Early Cretaceous zircons in the Taltung Formation and Amile Formation plot in the U‐Pb‐εHf(t) field of Indian derivation, whereas εHf(t) values of Triassic‐Paleocene zircons in the Bhainskati Formation demonstrate the arrival of Asian‐derived detritus in the Himalayan foreland basin in the Eocene based on available datasets. Our data indicate that (1) the timing of terminal India‐Asia collision was no later than the early‐middle Eocene in the central Himalaya, and (2) the Greater Himalaya served as a source for the Himalayan foreland basin by the early Miocene. When coupled with previous Paleocene‐early Eocene provenance records of the Tethyan Himalaya, our new data challenge dual‐stage India‐Asia collision models, such as the Greater India Basin hypothesis and its variants and the arc‐continent collision model.
... 49 Ma; Mathur et al., 2009;this study). Southward, the Lesser Himalaya started to enter into the forebulge depozone, while the majority of the Lesser and sub-Himalaya still stayed in the backbulge (DeCelles et al., 1998(DeCelles et al., , 2004Feng et al., 2023). During this period, seawater retreated from the northern Tethyan Himalaya but still occupied the southern Tethyan, Lesser, and sub-Himalaya (Fig. 14B). ...
... Parts of the Lesser Himalaya lay in foredeep and forebulge depozones. However, the Bhainskati and Subathu Formations in the Lesser and sub-Himalaya still represented the backbulge, but started to receive materials from the Asian continent and the uplifted Tethyan Himalaya (DeCelles et al., 1998;Najman and Garzanti, 2000;Jain et al., 2009;Colleps et al., 2020;Feng et al., 2023). At ca. 49 Ma, seawater completely retreated from the entire Tethyan Himalaya, which took place quasi-simultaneously in the west and in the east Nicora et al., 1987;Fuchs and Willems, 1990;Zhu et al., 2005;Green et al., 2008;Mathur et al., 2009;Najman et al., 2010;Zhang et al., 2012). ...
Timing of seawater retreat from the Tethyan Himalaya is of great importance, as it provides a minimum age control on the initial India-Asia collision. In south Tibet, however, it is still in dispute, with suggested ages ranging from >50 Ma to ca. 34 Ma. Here we present data of (1) larger benthic foraminifera from the topmost Zongpu Formation; (2) planktonic foraminifera; and (3) detrital zircon U-Pb ages from the Youxia and Shenkeza Formations at Guru, Tingri, and Gamba, with a special emphasis on the poorly studied Guru area. At Guru, existence of the Shallow Benthic Zonation (SBZ) 7 Zone index fossils of Alveolina moussoulensis Hottinger and Al. laxa Hottinger constrains the initial deposition of the Youxia Formation at ca. 54 Ma. Coexistence of the youngest planktonic foraminifera Acarinina boudreauxi Fleisher, Ac. coalingensis (Cushman and Hanna), and Ac. bullbrooki (Bolli) from the Youxia and Shenkeza Formations indicates their final deposition at ca. 49 Ma (E 7 Zone, where E stands for Eocene Planktonic foraminiferal Zone). Zircon U-Pb data are generally similar, with the youngest age peak of ca. 68−50 Ma resulting from input of the Gangdese Arc volcanic detritus. The complete lack of <49 Ma data among >1000 accepted ages implies that marine deposition probably ceased at ca. 49 Ma, consistent with the planktonic foraminiferal result. Integrating with previous results, we conclude that seawater retreated from both south Tibet and Zanskar at ca. 49 Ma. The initial collision happened quasi-simultaneously in the western and eastern Tethyan Himalaya, before ca. 49 Ma. Our findings do not support either a younger collisional age or a diachronous collisional process from the west to the east.
... The Neo-Tethys oceanic crust began to subduct northward under the Asian Block in the early Mesozoic (Parsons et al., 2020). Various studies do not support the existence of the "Greater India Basin" between the LHS and THS in the Mesozoic (Colleps et al., 2020;Feng et al., 2023) (Figure 2a). During the Palaeocene (65-55 Ma), continental-continental collision occurred between the India and Lhasa blocks . ...
A large concentration of ore deposits (i.e., tungsten, tin, lithium, beryllium, lead, zinc, silver, antimony and gold) related to metamorphism and hydrothermal activity of granite developed in the Himalayas and are located in the southern part of the Qinghai‐Tibet Plateau. As one of the newest and largest examples in the world of continental–continental collisional orogenic belts, the Himalayas are optimal for studying the coupling of continental collisions with metamorphic, tectonic, magmatic events and ore resources. Although considerable research has been conducted in the Himalayas, the timing of various geological processes remains debated, which limits the understanding of the genesis of ore deposits. To better understand the influence of orogeny on the metallogenic process, this study comprehensively presents the time of activity of the above‐mentioned four Cenozoic geological processes in the Himalayas. This contribution focuses on the context of the timeline to determine the internal connections and discusses the interrelationships of the geological processes. This study argues that Cenozoic lower crust–mantle interactions deep within the Indian continental plate below the Himalayas controlled the tectonics, metamorphism, magmatism and mineralization in the middle–upper crust. Cenozoic magmatic rocks in the upper crust are the direct results of the main‐collisional and post‐collisional stages of the Indian and Eurasian continental blocks. Intense collisional shearing and metamorphism during the Eocene formed the Himalayan fault–fold Belt and orogenic gold deposits. The break‐off, detachment and slab tearing of the Indian lithosphere during the Miocene led to the extensional structure of the middle–upper crust. There is a close coupling relationship between the exhumation of the middle–lower crust and the formation of large‐scale leucogranites during the Miocene in the upper crust, which provides the geological context for the main metallogenic period during which tungsten, tin, lithium, beryllium, lead, zinc, silver, antimony and gold ore widely developed. With technological advancements, the analytical accuracy of geological ages has gradually improved. In the future, research on the temporal constraints of important geological activities should be strengthened, and then a comprehensive evolutionary model of multiple spheres of geological processes in the Himalayas should be established.
Understanding the nature of the Kohistan-Ladakh arc before the India-Asia collision is crucial for reconstructing the Tethyan paleogeography, deciphering geodynamic process of the India-Asia collision, and restoring Asia’s history of intracontinental deformation. However, there are two opposing views dealing with nature of the Kohistan-Ladakh arc: the continental marginal arc and intra-oceanic island arc. Based on the accumulated published paleomagnetic data for the Kohistan-Ladakh arc and its surrounding terranes during the Cretaceous–Paleocene, we reconstruct paleogeography of the Kohistan-Ladakh arc and its surrounding terranes, and explore the nature of the Kohistan-Ladakh arc. The Karakoram terrane was situated on the southern edge of Asia at a paleolatitude consistent with that of the western Lhasa terrane in the late Cretaceous. The Kohistan-Ladakh arc was situated in the equatorial Neo-Tethys Ocean (southern hemisphere) in the early Late Cretaceous (ca. 90 Ma) before drifting northward to reach the low-latitude northern hemisphere in the early Paleocene (ca. 64 Ma). The central and eastern parts of the Tethyan Himalaya terrane, as the northern margin of India prior to the collision, were located at paleolatitudes of 16.7S and 19.4S at ca. 75 Ma, respectively. The Kohistan-Ladakh arc was separated from the western Lhasa terrane by an ocean basin with a north-south extent of ~900 km at ca. 64 Ma. The above analyses suggest that the Kohistan-Ladakh arc demonstrated intra-oceanic characteristics prior to the India-Asia collision. Therefore, the Kohistan-Ladakh arc should be considered in the India-Asian collision system. Recently published paleomagnetic data obtained from the Tethyan Himalaya terrane, data obtained from the Kohistan-Ladakh arc, and the apparent polar wander path of India inspired us to propose a triple-stage India-Asia collision model. The first collision occurred at ca. 64 Ma between the Tibetan Himalaya terrane and the Kohistan-Ladakh arc. The second collision occurred at ca. 61 Ma between the Tibetan Himalaya terrane (including the accreted Trans-Tethyan subduction zone) and Asia. The final collision occurred during 53–47 Ma between India and the Tibetan Himalaya terrane, leading to diachronous suturing of the North India Sea from west to east. The proposed collision model reconciles multiple lines of geologic, geophysical, geochemical, and paleontologic evidence. Especially, we highlight that the new scenario well matches the history of the India-Asia convergence rates, which were documented by the relative plate motions between India-Africa and India-Antarctica. In addition, this new model provides important constraints for intracontinental deformation in Asia. For example, the second collision between the Tibetan Himalaya terrane and Asia occurred along the Shyok-Yarlung Tsangpo suture zone, rather than the Indus-Yarlung Tsangpo suture zone. The Shyok and Yarlung Zangbo suture zones have ever been connected, implying a total offset greater than 400 km for the Karakoram fault.
New zircon U-Pb dating results from the Zonggei Formation volcanics indicate that the volcanic rocks formed at ∼114–110 Ma. Paleomagnetic data, petrography, and rock magnetism confirm the primary nature of isolated characteristic remanent magnetizations carried by titanomagnetite and hematite. A statistical analysis of the combined results from the Zonggei and Duoni formations reveals a group-mean direction of D±ΔD = 0.4° ± 6.0°, I±ΔI = 22.2° ± 5.6°, α95 = 5.6°, k = 35.2 after bedding correction based on 20 group-mean directions. The corresponding paleopole was calculated to be λp = 70.3°N, φp = 270.5°E with A95 = 5.2°. The interpretation of our data alongside the Cenozoic data from the Tethyan Himalaya indicates that the India–Asia collision initiated by 61.7 ± 3.0 Ma at 13.0° ± 1.8°N, assuming a single-collision model. Intracontinental crustal shortening totaling 1,770 ± 470 km took place on the Asian side since the onset of India–Asia collision. Furthermore, the data show that the Neo-Tethys Ocean reached its maximum N-S width of 7,100 ± 530 km at ∼132 Ma and shrank to 6,400 ± 550 km by ∼115 ± 5 Ma. This is consistent with previous estimates based on the geophysical images of the subducted Neo-Tethyan slab beneath Eurasia.
The India-Asia collision zone is the archetype to calibrate geological responses of continent-continent collision, but hosts a paradox: there is no orogen-wide geological record of oceanic subduction after initial collision around 60-55 Ma, yet thousands of kilometers of post-collisional subduction occurred before arrival of unsubductable continental lithosphere that currently horizontally underlies Tibet. Kinematically restoring incipient horizontal underthrusting accurately predicts geologically estimated diachronous slab break-off, unlocking the Miocene of Himalaya-Tibet as natural laboratory for unsubductable lithosphere convergence. Additionally, three end-member paleogeographic scenarios exist with different predictions for the nature of post-collisional subducting lithosphere but each is defended and challenged based on similar data types. This paper attempts at breaking through this impasse by identifying how the three paleogeographic scenario each challenge paradigms in geodynamics, orogenesis, magmatism, or paleogeographic reconstruction and identify opportunities for methodological advances in paleomagnetism, sediment provenance analysis, and seismology to conclusively constrain Greater Indian paleogeography.
The enigmatic geodynamic processes involved in the India-Asia collision shape our understanding of uplift and deformation of the Tibetan Plateau and the subsequent changes in the land-sea distribution pattern, monsoon-arid environments and biotic evolution in Central Asia. However, the current geodynamic models dealing with the India-Asia collision history exhibit significant discrepancies in how, when and where exactly the collision of India with Asia occurred. Recently, we proposed a new hypothesis that favored the opening and closure of a North India Sea during the late Cretaceous to Paleogene. Here we present new additional paleomagnetic data from the red siliceous shales and cherts of the Sangdanlin Formation, which were deposited on the most distal continental margin of the Tethyan Himalaya terrane, i.e. the southern margin of the Neo-Tethys Ocean, during the middle Paleocene. The combination of our new and previously reported paleomagnetic data shows that the Tethyan Himalaya was at a paleolatitude of 14.1° 1.9°N during the interval 62.5-59.2 Ma and refines the maximum size of the North India Sea to about 2200 500 km. Combining our geodynamic model with recently published paleomagnetic data from the Kohistan-Ladakh arc, we conclude that the India-Asia collision was a triple-stage process, which involved an arc-continent collision followed by a two-stage continent-continent collision. The arc-continent collision occurred at ca. 64 Ma between the Tibetan Himalaya (Tethyan Himalaya plus Greater Himalaya) and the Trans-Tethyan subduction zone at a paleolatitude of ~8°N. The initial continent-continent collision took place at ca. 61 Ma between the Tibetan Himalaya (plus the accreted Trans-Tethyan subduction zone) and Lhasa terranes at a paleolatitude of ~14°N. The final continent-continent collision occurred during 5347 Ma between India and the Tibetan Himalaya, diachronously suturing the North India Sea from west to east. The triple-stage collision scenario is in agreement with the history of India-Asia convergence rates, and is reflected by the presence of three slab remnants below Tibet and India as documented by seismic tomographic imaging. Our updated collision scenario reconciles multidisciplinary evidence for the India-Asia collision and provides better constraints on the resulting paleoclimatic and paleoenvironmental changes in Central-Eastern Asia.
The India-Asia collision zone is the archetype to calibrate geological responses of continent-continent collision, but hosts a paradox: there is no orogen-wide geological record of oceanic subduction after initial collision around 60-55 Ma, yet thousands of kilometers of post-collisional subduction occurred before arrival of unsubductable continental lithosphere that currently horizontally underlies Tibet. Kinematically restoring incipient horizontal underthrusting accurately predicts geologically estimated diachronous slab break-off, unlocking the Miocene of Himalaya-Tibet as natural laboratory for unsubductable lithosphere convergence. Additionally, three end-member paleogeographic scenarios exist with different predictions for the nature of post-collisional subducting lithosphere but each is defended and challenged based on similar data types. This paper attempts at breaking through this impasse by identifying how the three paleogeographic scenario each challenge paradigms in geodynamics, orogenesis, magmatism, or paleogeographic reconstruction and identify opportunities for methodological advances in paleomagnetism, sediment provenance analysis, and seismology to conclusively constrain Greater Indian paleogeography.
Unconformities in foreland basins may be generated by tectonic processes that operate in the basin, adjacent fold-thrust belt, and broader convergent margin. Foreland basin unconformities represent shifts from high accommodation to nondepositional or erosional conditions in which the interruption of subsidence precludes net sediment accumulation. This study explores the genesis of long duration (>1–20 Myr) unconformities and condensed stratigraphic sections by considering modern and ancient examples from the Andes. These cases highlight potential geodynamic mechanisms of accommodation reduction and hiatus development in Andean-type retroarc foreland settings, including: (a) shortening-induced uplift in the frontal thrust belt and proximal foreland; (b) growth and advance of a broad, low-relief flexural forebulge; (c) uplift of intraforeland basement blocks; (d) tectonic quiescence with regional isostatic rebound; (e) cessation of thrust loading and flexural subsidence during oblique convergence; (f) diminished accommodation or sediment supply due to changes in sea level, climate, erosion, or transport; (g) basinwide uplift during flat-slab subduction; and (h) dynamic uplift associated with slab window formation, slab breakoff, elevated intraplate (in-plane) stress, or related mantle process. These contrasting mechanisms can be distinguished on the basis of the spatial distribution, structural context, stratigraphic position, paleoenvironmental conditions, and duration of unconformities and condensed sections.
Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts
The India-Asia collision is the most spectacular, recent, and still active tectonic event of the Earth’s history, leading to the uplift of the Himalayan-Tibetan orogen, which has been explained through several hypothetical models. Still, controversy remains, such as how and when it occurred. Here we report a paleomagnetic study of Cretaceous-Tertiary marine sediments from the Tethyan Himalaya (TH) in the Hazara area, north Pakistan, which aims to constrain timing for the onset of the India-Asia collision and to confirm the validity of already proposed models, particularly in western Himalaya’s perspective. Our results suggest that the TH was located at a paleolatitude of 8.5°S ± 3.8° and 13.1°N ± 3.8° during the interval of ca. 84−79 Ma and 59−56 Ma, respectively. A comparison between paleopoles obtained from the current study and coeval ones of the India Plate indicates that the TH rifted from Greater India before the Late Cretaceous, generating the Tethys Himalaya Basin (THB). Our findings support a model for a multi-stage collision involving at least two major subduction systems. A collision of the TH with the Trans-Tethyan subduction system (TTSS) began first in Late Cretaceous-Early Paleocene times (ca. 65 Ma), followed by a later collision with Asia at 55−52 Ma. The onset of the collision between the TH (plus TTSS) and Asia could not have occurred earlier than 59−56 Ma in the western Himalaya. Subsequently, the India craton collided with the TH, resulting in the diachronous closure of the THB between ca. 50 and ca. 40 Ma from west to east. These findings are consistent with geological and geochemical evidence and have a broad implication for plate reconfigurations, global climate, and biodiversity of collisional processes.
The timing and location India–Asia collision is crucial to understanding the evolution of the Himalayan–Tibetan orogen and global climate change; however, estimates of the timing of the India–Asia collision and the size of Greater India remain highly controversial. We report here the first reliable early Eocene paleomagnetic results from the Zhepure Formation limestone in the Tethyan Himalaya (TH). Both positive fold and reversal tests indicate a primary origin. The tilt‐corrected site‐mean direction is Ds = 332.6°, Is = 20.1°, ks = 90.9, α95 = 2.8° (N = 30). The site‐mean inclination increased from 20.1° to 27.5° after inclination shallowing correction, yielding a paleopole at 61.2°N, 337.3°E, A95 = 2.6° and a corresponding paleolatitude of 15.2° ± 2.6°N for the reference point (29.5°N, 91.0°E). Our new paleomagnetic results, together with the reliable Cretaceous to Paleogene paleomagnetic results from the TH, Lhasa terrane and Indian craton, suggest that there was ∼900–1,140 km north‐south crustal shortening occurred between the TH and the Indian craton, and that India collided with Asia at no later than ∼51–49.5 Ma.
Plain Language Summary
The locus of the initial India‐Asia collision can be constrained using paleomagnetic studies on the Paleocene volcanics from the Linzhou Basin, South Tibet. However, the primary nature of the magnetic signature previously reported from the Dianzhong Formation was questioned. This study carries out an intraformational conglomerate test collected from the middle part of the Dianzhong Formation. The stable remanences isolated from a layer of intercalated lava cobbles yield a random distribution in contrast to the well‐grouped directions obtained from the over‐ and underlying lava layers resulting in a positive conglomerate test. We therefore argue for a primary nature for the characteristic remanence recorded by the Dianzhong lavas from the Linzhou Basin. Our study confirms a low latitude of ∼7°N, that is, within the equatorial humid belt, for the southern margin of Asia during ∼64–60 Ma. An initial low‐latitude collision between India and Asia is critical for understanding the tectonic and climatic significance of the India‐Asia collision.
The timing of the India-Asia collision onset, essential to understanding the evolution of the Himalayan-Tibetan orogen, has been widely investigated through multidisciplinary approaches. Among these, the India to Asia provenance reversal (IAPR) documented in the Indian passive margin successions has proved to be most effective. We present integrated stratigraphic, sedimentological, and provenance
data on Upper Cretaceous-Paleogene strata from the newly investigated Mubala section exposed south of the Yarlung-Zangbo suture zone (YZSZ) in southern Tibet, which preserves continuous deep-marine turbiditic and biogenic sedimentation on the distal Indian passive margin. Sandstone petrography, heavy minerals, detrital zircon geochronology and Hf isotopes, and detrital Cr-spinel geochemistry constrain the IAPR to later than 62.7 Ma (youngest zircon ages from the earliest Asian-derived sandstone) and by 61.0 ± 0.3 Ma (SIMS age of a tuffaceous layer ∼30 m above this bed). The onset of intercontinental collision along the YZSZ began by 61 Ma.
Significance
We present paleomagnetic constraints on the latitude of an intraoceanic subduction system that is now sutured between India and Eurasia in the western Himalaya. Our results demonstrate that the India–Eurasia collision was a multistage process involving at least two subduction systems rather than a single-stage event. This resolves the discrepancy between the amount of convergence and the observed crustal shortening in the India–Eurasia collision system, as well as the 10–15 Ma time lag between collision onset in India and the initiation of collision-related deformation and metamorphism in Eurasia. The presence of an additional subduction system in the Neotethys ocean explains the rapid India–Eurasia convergence rates in the Cretaceous and global climate variations in the Cenozoic.
Knowledge of the topographic evolution of the Tibetan Plateau is essential for understanding its construction and its influences on climate, environment, and biodiversity. Previous elevations estimated from stable isotope records from the Lunpola Basin in central Tibet, which indicate a high plateau since at least 35 Ma, are challenged by recent discoveries of low-elevation tropical fossils apparently deposited at 25.5 Ma. Here, we use magnetostratigraphic and radiochronologic dating to revise the chronology of elevation estimates from the Lunpola Basin. The updated ages reconcile previous results and indicate that the elevations of central Tibet were generally low (<2.3 km) at 39.5 Ma and high (3.5 to 4.5 km) at ~26 Ma. This supports the existence in the Eocene of low-elevation longitudinally oriented narrow regions until their uplift in the early Miocene, with potential implications for the growth mechanisms of the Tibetan Plateau, Asian atmospheric circulation, surface processes, and biotic evolution.
Significance
The ancient topography of the Tibetan Plateau and its role in biotic evolution are still poorly understood, mostly due to a lack of fossil evidence. Our discovery of ∼47-Mya plant fossils from a present elevation of 4,850 m in central Tibet, diminishes, significantly, that lack of knowledge. The fossils represent a humid subtropical vegetation and some of the 70 different plant forms show affinity to Early-Middle Eocene floras in both North America and Europe. Using leaf architecture, we calculate that the forest grew at ∼1,500-m elevation within an east–west trending valley under a monsoonal climate. Our findings highlight the complexity of Tibet’s ancient landscape and emphasize the importance of Tibet in the history of global biodiversity.
The Early Cretaceous palaeogeography prior to the India‐Asia collision is still controversial. There are two end‐member models – an integrated passive margin versus an archipelago. Here, we report zircon U–Pb ages, Hf, and whole‐rock Nd isotopes of newly defined Cretaceous mica schists from the Main Central Thrust to constrain the Early Cretaceous palaeogeography of the Himalaya. The mica schists have Early Cretaceous zircons with εHf(t) values of −21.1 to −9.2. The mica schists are characterized by TDMC values of 1.8–2.5 Ga and whole‐rock εNd(0) value of −24.0. These features imply that these zircons generated from partial melting of lower crust as a result of an extension in some older continental crust, which were transported and deposted to form the protolith of the Cretaceous mica schists. Age pattern comparisons indicate that the age populations of the Cretaceous mica schists are significantly distinct from those of other Cretaceous strata in the Himalaya. It is obvious that the protolith of the Cretaceous mica schists were formed in an independent terrane (Arun) in the Neo‐Tethys with significant tectonic separations proventing any inputs from the Tibetan Himalaya, Greater Himalaya, or Indian Craton since 120 Ma. Combined with published U–Pb ages and other geological data, our data supports the archipelago palaeogeography with rifted terranes in the Neo‐Tethys Ocean, shedding light on the multiple amalgamation events along the India‐Asia collision zone.
The India-Asia collision is an outstanding smoking gun in the study of continental collision dynamics. How and when the continental collision occurred remains a long-standing controversy. Here we present two new paleomagnetic data sets from rocks deposited on the distal part of the Indian passive margin, which indicate that the Tethyan Himalaya terrane was situated at a paleolatitude of ~19.4°S at ~75 Ma and moved rapidly northward to reach a paleolatitude of ~13.7°N at ~61 Ma. This implies that the Tethyan Himalaya terrane rifted from India after ~75 Ma, generating the North India Sea. We document a new two-stage continental collision, first at ~61 Ma between the Lhasa and Tethyan Himalaya terranes, subsequently at ~53-48 Ma between the Tethyan Himalaya terrane and India, diachronously closing the North India Sea from west to east. Our scenario matches the history of India-Asia convergence rates and reconciles multiple lines of geologic evidence for the collision.
During the Cretaceous, the Indian plate moved towards Eurasia at the fastest rates ever recorded. The details of this journey are preserved in the Indian Ocean seafloor, which document two distinct pulses of fast motion, separated by a noticeable slowdown. The nature of this rapid acceleration, followed by a rapid slowdown and then succeeded by a second speedup, is puzzling to explain. Using an extensive observation dataset and numerical models of subduction, we show that the arrival of the Reunion mantle plume started a sequence of events that can explain this history of plate motion. The forces applied by the plume initiate an intra-oceanic subduction zone, which eventually adds enough additional force to drive the plates at the anomalously fast speeds. The two-stage closure of a double subduction system, including accretion of an island arc at 50 million years ago, may help reconcile geological evidence for a protracted India-Eurasia collision.
In this study, we integrate bedrock datasets, observations of subducted slabs in the mantle, and plate kinematic constraints, to constrain models for the India-Asia collision and the central Tethys oceans. To accomplish this, we review: (1) the post-Triassic bedrock record of subduction in Tibet; (2) seismic tomographic imaging of subducted slabs in the mantle; (3) timing of the India-Asia collision; and (4) the pre-collisional size of Greater India. Following the assumption that slabs sink vertically through the mantle, their positions and geometries determined from seismic tomography constrain the locations and kinematics of paleo-subduction zones. Integrating this with bedrock constraints allows us to constrain post-Triassic subduction zone configurations for the central Tethys oceans. Neotethys was consumed by at least two subduction zones since the Jurassic. At the onset of the India-Asia collision at 59 ± 1 Ma, one subduction zone was active along the southern Asian continental margin at ~20°N. At that time, a second may have been active at subequatorial latitudes, but support for this from a bedrock perspective is lacking. This subduction zone configuration allows for three reconstructions for Greater India: The (1) minimum-area; (2) enlarged-area; and (3) Greater India Basin reconstructions. We integrate these reconstructions and subduction zone configurations in a plate kinematic framework to test their validity for the India-Asia collision. No single model is entirely satisfactory and each invokes assumptions that challenge accepted concepts. These include our understanding of suture zones, and the limits of continental subduction. We explore these challenges and their implications for our understanding of the India-Asia collision and continental collisions in general.
The precollisional paleolatitude of the Tethyan Himalaya is essential to constrain the India‐Asia collision, the size of Greater India, and the Neotethyan paleogeography. However, reliable Late Cretaceous‐Paleocene paleomagnetic datasets are still scarce because of serious remagnetization. Here we report the first redbed‐based paleomagnetic results from the Paleocene (60‐58 Ma) redbeds in the Saga area of the northern Tethyan Himalaya. The tilt‐corrected site‐mean direction obtained from 36 paleomagnetic sites is D=178.7°, I=9.5° with ɑ95=5.4°, corresponding to a paleopole at 55.9°N, 267.6°E with dp/dm=2.8°/5.5° and a paleolatitude of 4.8°±2.8°S for the study area (29.3°N, 85.3°E). The site‐mean inclination increased from 9.5 to 14.9° after the anisotropy‐based inclination shallowing correction, leading to corresponding paleolatitudes increasing from 4.8 to 7.6°S. This reliable paleomagnetic dataset passes positive fold tests and supports that the northern Tethyan Himalaya was located at 6.3±4.3°S during 60‐58 Ma. Comparison with the coeval paleolatitudes expected from the Indian craton indicates that a north‐south crustal shortening of 3.1±5.5° (340±610 km) occurred between the Indian craton and the northern Tethyan Himalaya after 59 Ma, and no wide ocean extended between them after the Latest Jurassic. Our new Paleocene results together with reliable Cretaceous paleomagnetic results from the Lhasa terrane show that the India‐Asia collision occurred at 47.1±4.5 Ma.
Convergence between the Indian and Asian plates has reshaped large parts of Asia, changing regional climate and biodiversity, yet geodynamic models fundamentally diverge on how convergence was accommodated since the India–Asia collision. Here we report palaeomagnetic data from the Burma Terrane, which is at the eastern edge of the collision zone and is famous for its Cretaceous amber biota, to better determine the evolution of the India–Asia collision. The Burma Terrane was part of a Trans-Tethyan island arc and stood at a near-equatorial southern latitude at ~95 Ma, suggesting island endemism for the Burmese amber biota. The Burma Terrane underwent significant clockwise rotation between ~80 and 50 Ma, causing its subduction margin to become hyper-oblique. Subsequently, it was translated northward on the Indian Plate by an exceptional distance of at least 2,000 km along a dextral strike-slip fault system in the east. Our reconstructions are only compatible with geodynamic models involving an initial collision of India with a near-equatorial Trans-Tethyan subduction system at ~60 Ma, followed by a later collision with the Asian margin.
New data from the lower Miocene Dumri Formation of western Nepal document exhumation of the Himalayan fold‐thrust belt and provenance of the Neogene foreland basin system. We employ U‐Pb zircon, Th‐Pb monazite, ⁴⁰Ar/³⁹Ar white mica, and zircon fission track chronometers to detrital minerals to constrain provenance, timing, and rate of exhumation of Himalayan source regions. Clusters of Proterozoic–early Paleozoic (900–400 Ma) Th‐Pb monazite and ⁴⁰Ar/³⁹Ar white mica detrital ages provide evidence for erosion of a Greater Himalayan sequence protolith unaffected by high‐grade Eohimalayan metamorphism. A small population of ~40 Ma cooling ages in detrital white mica grains shows exhumation of low‐grade metamorphic Tethyan Himalayan sequence through the ~350 °C closure temperature along the Tethyan Frontal thrust (proto‐South Tibetan detachment) during the late Eocene. Dumri Formation detritus shows a ~12 Myr time difference between cooling of its source rocks through the ~350 and ~240 °C closure temperatures as recorded by ~40–38 Ma youngest peak cooling ages in ⁴⁰Ar/³⁹Ar detrital white mica and ~28–24 Ma youngest populations in detrital zircon fission track. Exhumation between circa 40 and 28 Ma is consistent with slip and exhumation along the Main Central Thrust. Combined with similar data from northwestern India, our study suggests west‐to‐east spatially variable exhumation rates along strike of the Main Central Thrust. Our data also show an increase in exhumation during middle Miocene–Pliocene time, which is consistent with growth of the Lesser Himalaya duplex.
The Himalayan-Tibetan orogen culminated during the Cenozoic India – Asia collision, but its geological framework and initial growth were fundamentally the result of multiple, previous ocean closure and intercontinental suturing events. As such, the Himalayan-Tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general, and how the Earth’s highest and largest orogenic feature formed in specific. This paper synthesizes the Triassic through Cenozoic geology of the central Himalayan-Tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere-scale cross-sections and paleogeographic maps. We suggest that north-dipping subducting slabs beneath Asian continental terranes associated with closure of the Paleo-, Meso-, and Neo-Tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing. These trench retreat events created ophiolites in forearc extensional settings and/or a backarc oceanic basins between rifted segments of upper-plate continental margin arcs. This process may have occurred at least three times along the southern Asian margin during northward subduction of Neo-Tethys oceanic lithosphere: from ~174 to 156 Ma; 132 to 120 Ma; and 90 to 70 Ma. At most other times, the Tibetan terranes underwent Cordilleran-style or collisional contractional deformation. Geological records indicate that most of northern and central Tibet (the Hoh-Xil and Qiangtang terranes, respectively) were uplifted above sea level by Jurassic time, and southern Tibet (the Lhasa terrane) north of its forearc region has been above sea level since ~100 Ma. Stratigraphic evidence indicates that the northern Himalayan margin of India collided with an Asian-affinity subduction complex – forearc – arc system beginning at ~60 Ma. Both the Himalaya (composed of Indian crust) and Tibet show continuous geological records of orogenesis since ~60 Ma. As no evidence exists in the rock record for a younger suture, the simplest interpretation of the geology is that India – Asia collision initiated at ~60 Ma. Plate circuit, paleomagnetic, and structural reconstructions, however, suggest that the southern margin of Asia was too far north of India to have collided with it at that time. Seismic tomographic images are also suggestive of a second, more southerly Neo-Tethyan oceanic slab in the lower mantle where the northernmost margin of India may have been located at ~60 Ma. The geology of Tibet and the India – Asia suture zone permits an alternative collision scenario in which the continental margin arc along southern Asia (the Gangdese arc) was split by extension beginning at ~90 Ma, and along with its forearc to the south (the Xigaze forearc), rifted southward and opened a backarc ocean basin. The rifted arc collided with India at ~60 Ma whereas the hypothetical backarc ocean basin may not have been consumed until ~45 Ma. A compilation of igneous age data from Tibet shows that the most recent phase of Gangdese arc magmatism in the southern Lhasa terrane initiated at ~70 Ma, peaked at ~51 Ma, and terminated at ~38 Ma. Cenozoic potassic-adakitic magmatism initiated at ~45 Ma within a ~200-km-wide elliptical area within the northern Qiangtang terrane, after which it swept westward and southward with time across central Tibet until ~26 Ma. At 26-23 Ma, potassic-adakitic magmatism swept southward across the Lhasa terrane, a narrow (~20 km width), orogen-parallel basin developed at low elevation along the axis of the India – Asia suture zone (the Kailas basin), and Greater Himalayan Sequence rocks began extruding southward between the South Tibetan Detachment and Main Central Thrust. The Kailas basin was then uplifted to >4 km elevation by ~20 Ma, after which parts of the India – Asia suture zone and Gangdese arc experienced >6 km of exhumation (between ~20 and 16 Ma). Between ~16 and 12 Ma, slip along the South Tibetan Detachment terminated and east-west extension initiated in the northern Himalaya and Tibet. Potassic-adakitic magmatism in the Lhasa terrane shows a northward younging trend in the age of its termination, beginning at 20-18 Ma until volcanism ended at 8 Ma. We interpret the post-45 Ma geological evolution in the context of the subduction dynamics of Indian continental lithosphere and its interplay with delamination of Asian mantle lithosphere.
Greater India comprises a part of the Indian plate that subducted under Asia to help form the Tibetan Plateau. Defining the size of the Greater India is thus a key constraint to model the India-Asia collision, growth of the plateau, and the tectonic evolution of the Neo-Tethyan realm. We report Early Cretaceous paleomagnetic data from the central and eastern Tethyan Himalaya that yield paleolatitudes consistent with previous Early Cretaceous paleogeographic reconstructions. These data suggest Greater India extended at least 2,675 ± 720 and 1,950 ± 970 km farther north from the present northern margin of India at 83.6°E and 92.4°E, respectively. An area of lithosphere ≥4.7 × 10 ⁶ km ² was consumed through subduction, thereby placing a strict limit on the minimum amount of Indian lithosphere consumed since the breakup of Gondwanaland.
Shifts in global seawater 187Os/188Os and 87Sr/86Sr are often utilized as proxies to track global weathering processes responsible CO2 fluctuations in Earth history, particularly climatic cooling during the Cenozoic. It has been proposed, however, that these isotopic records instead reflect the weathering of chemically distinctive Himalayan lithologies exposed at the surface. We present new zircon (U-Th)/He thermochronometric and detrital zircon U-Pb geochronologic evidence from the Himalaya of northwest India to explore these contrasting interpretations concerning the driving mechanisms responsible for these seawater records. Our data demonstrate in-sequence southward thrust propagation with rapid exhumation of Lesser Himalayan strata enriched in labile 187Os and relatively less in radiogenic 87Sr at ∼16 Ma, which directly corresponds with coeval shifts in seawater 187Os/188Os and 87Sr/86Sr. Results presented here provide substantial evidence that the onset of exhumation of 187Os-enriched Lesser Himalayan strata could have significantly impacted the marine 187Os/188Os record at 16 Ma. These results support the hypothesis that regional weathering of isotopically unique source rocks can drive seawater records independently from shifts in global-scale weathering rates, hindering the utility of these records as reliable proxies to track global weathering processes and climate in deep geologic time.
Integration of regional stratigraphic relationships with data on sediment accumulation, provenance, paleodrainage, and deformation timing enables a reconstruction of Mesozoic-Cenozoic subduction-related mountain building along the western margin of South America. Sedimentary basins evolved in a wide range of structural settings on both flanks of the Andean magmatic arc, with strong signatures of retroarc crustal shortening, flexure, and rapid accumulation in long-lived foreland and hinterland basins. Extensional basins also formed during pre-Andean backarc extension and locally in selected forearc, arc, and retroarc zones during Late Cretaceous-Cenozoic Andean orogenesis. Major transitions in topography and sediment routing are recovered through provenance studies, particularly detrital zircon U-Pb geochronological applications, which distinguish three principal sediment source regions—the South American craton, Andean magmatic arc, and retroarc fold-thrust belt. Following the cessation of Late Triassic–Early Cretaceous extensional and/or postextensional neutral-stress conditions, a Late Cretaceous-early Paleocene inception of Andean shortening was chronicled in retroarc regions along the western margin by rapid flexural subsidence, a wholesale reversal in drainage patterns, and provenance switch from eastern cratonic sources to Andean sources. An enigmatic Paleogene hiatus in the Andean foreland succession recorded diminished accumulation and/or regional unconformity development, contemporaneous with a phase of limited shortening or neutral to locally extensional conditions. Seemingly contradictory temporal fluctuations in tectonic regimes, defined by contrasting (possibly cyclical) phases of shortening, neutral, and extensional conditions, can be linked to the degree of mechanical coupling along the subduction plate boundary. Along-strike variations in Late Cretaceous-Cenozoic deformation and crustal thickening demonstrate contrasting high-shortening versus low-shortening modes of Andean orogenesis, in which the central Andes are distinguished by large-magnitude east-west shortening (> 150–300 km) and corresponding cratonward advance of the fold-thrust belt and foreland basin system, several times that of the northern and southern Andes. These temporal and spatial changes in shortening and overall tectonic regime can be related to variable plate coupling during first-order shifts in plate convergence, second-order cycles of slab shallowing and steepening, and second-order cycles of shortening, lithospheric removal and local partial extensional collapse in highly shortened and thickened segments of the orogen.
The Himalayan orogen is a type example of continent–continent collision. Knowledge of the timing of India–Asia collision is critical to the calculation of the amount of convergence that must have been accommodated and thus to models of crustal deformation. Sedimentary rocks on the Indian plate near the suture zone can be used to constrain the time of collision by determining first evidence of Asian-derived material deposited on the Indian plate. However, in the Himalaya, for this approach to be applied successfully, it is necessary to be able to distinguish between Asian detritus and detritus from oceanic island arcs that may have collided with India prior to India–Asia collision. Zircons from the Indian plate, Asian plate and Kohistan–Ladakh Island arc can be distinguished based on their U–Pb ages combined with Hf signatures. We undertook a provenance study of the youngest detrital sedimentary rocks of the Tethyan Himalaya of the Indian plate, in the Western Himalaya. We show that zircons of Asian affinity were deposited on the Indian plate at 54 Ma. We thus constrain terminal India–Asia collision, when both sutures north and south of the Kohistan–Ladakh Island arc were closed, to have occurred in the Western Himalaya by 54 Ma.
The northernmost exposures of sub-Himalayan Cenozoic strata in the Hazara–Kashmir syntaxial region of north Pakistan comprises the Paleocene–Eocene marine strata in the lower part and Oligocene–Miocene nonmarine strata in the upper part. This study provides the detrital zircon U–Pb geochronology of the Cenozoic strata in this area. The strong resemblance of U–Pb age spectra of Paleocene Hangu, Lockhart and Patala formations with those of Himalayan strata indicate an Indian plate provenance. The first appearance of <100Ma detrital zircon U–Pb ages within the lower most part of the Early EoceneMargalla Hill Limestone indicates a shift from an Indian to Asian provenance. Geologic mapping shows the existence of a disconformity between the lower and upper most part of the Patala Formation, which is interpreted to have been formed by the migration of a flexural forebulge through this region. We consider the upper most part of the Patala Formation to have been deposited within the distal foredeep of the foreland basin. The Indian to Asian provenance shift and the presence of a possible foreland basin forebulge provide strong evidence that India–Asia collision was underway in northern Pakistan at ca. 56–55Ma.
Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth's surface today - represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
The timing of the initial India–Asia collision and the mechanisms that led to the eventual formation of the high (>5 km) Tibetan Plateau remain enigmatic. In this Review, we describe the spatio-temporal distribution and geodynamic mechanisms of surface uplift in the Tibetan Plateau, based on geologic and palaeo-altimetric constraints. Localized mountain building was initiated during a Cretaceous microcontinent collision event in central Tibet and ocean–continent convergence in southern Tibet. Geological data indicate that India began colliding with Asian-affinity rocks 65–60 million years ago (Ma). High-elevation (>4 km) east–west mountain belts were established in southern and central Tibet by ~55 Ma and ~45 Ma, respectively. These mountain belts were separated by ≤2 km elevation basins centred on the microcontinent suture in central Tibet, until the basins were uplifted further between ~38 and 29 Ma. Basin uplift to ≥4 km elevation was delayed along the India–Asia suture zone until ~20 Ma, along with that in northern Tibet. Delamination and break-off of the subducted Indian and Asian lithosphere were the dominant mechanisms of surface uplift, with spatial variations controlled by inherited lithospheric heterogeneities. Future research should explore why surface uplift along suture zones — the loci of the initial collision — was substantially delayed compared with the time of initial collision. The geodynamic mechanisms and timing of Tibetan Plateau formation are debated, but are critical to understanding tectonic–climatic links. This Review discusses the stages of Tibetan Plateau evolution, and highlights that inherited weaknesses from pre-Cenozoic tectonic events influenced its variable surface uplift history. The Tibetan Plateau did not get uplifted as a large entity or grow systematically outward from the India–Asia suture (IAS), because lithospheric heterogeneities in Asia imparted by pre-Cenozoic tectonic events created relatively weak and strong zones that deformed differently during collision.Cretaceous tectonic events built embryonic mountains belts and weakened the lithosphere in southern and central Tibet.Continental Asian detritus appeared in Indian continental margin sedimentary rocks by 65–60 million years ago (Ma). The most conservative interpretation based on available geologic constraints is that these sediments mark the initiation of India–Asia collision.The quest to further quantify the history of surface elevation change across Tibet spurred the field of quantitative palaeo-altimetry, such as measurement of oxygen and hydrogen isotopes in palaeo-water proxies, carbonate clumped isotope thermometry and fossil leaf physiognomy.Quantitative palaeo-altimetry suggests that high (≥4 km) elevations were obtained in southern Tibet by ~55 Ma and in central Tibet by ~45 Ma, whereas an intervening valley remained at <2 km elevation until between ~38 and 29 Ma. The IAS zone and Himalaya Mountains were rapidly uplifted from <3 km to near-modern elevations at ~20 Ma.Subcrustal processes such as subduction, delamination and break-off of Indian and Asian continental lithosphere were important tectonic events during the formation of the Tibetan Plateau. The Tibetan Plateau did not get uplifted as a large entity or grow systematically outward from the India–Asia suture (IAS), because lithospheric heterogeneities in Asia imparted by pre-Cenozoic tectonic events created relatively weak and strong zones that deformed differently during collision. Cretaceous tectonic events built embryonic mountains belts and weakened the lithosphere in southern and central Tibet. Continental Asian detritus appeared in Indian continental margin sedimentary rocks by 65–60 million years ago (Ma). The most conservative interpretation based on available geologic constraints is that these sediments mark the initiation of India–Asia collision. The quest to further quantify the history of surface elevation change across Tibet spurred the field of quantitative palaeo-altimetry, such as measurement of oxygen and hydrogen isotopes in palaeo-water proxies, carbonate clumped isotope thermometry and fossil leaf physiognomy. Quantitative palaeo-altimetry suggests that high (≥4 km) elevations were obtained in southern Tibet by ~55 Ma and in central Tibet by ~45 Ma, whereas an intervening valley remained at <2 km elevation until between ~38 and 29 Ma. The IAS zone and Himalaya Mountains were rapidly uplifted from <3 km to near-modern elevations at ~20 Ma. Subcrustal processes such as subduction, delamination and break-off of Indian and Asian continental lithosphere were important tectonic events during the formation of the Tibetan Plateau.
The India-Asia collision processes are under current debate, mainly due to the large variations of the pre-collision position of the southern margin of Asia, in the range of 10–30°N. To resolve this uncertainty, we report new paleomagnetic data of the Xigaze forearc basin sediments from the Zhongba Cuojiangding area. Characteristic remanent magnetizations (ChRMs) were obtained from the upper Cretaceous Qubeiya Formation (Fm) and the early Eocene Jialazi Fm. Both mean directions passed the fold test and also passed the reversal test. An elongation-inclination analysis for all the Jialazi Fm ChRM directions yielded an unflattened mean inclination of 41.6°, with 95% confidence limits between 36.0° and 49.0° (f = 0.76), consistent with the Qubeiya Fm results. The overall mean direction for the Qubeiya Fm is D = 334.8°, I = 42.7°, k = 11.8, α95 = 6.7°, n = 42 in stratigraphic coordinates, corresponding to a paleolatitude of 24.8°N and a paleopole at 67.1°N, 347.5°E with A95 = 7.1°. The tilt-corrected mean direction, combined with published data, for the Jialazi Fm is D = 340.5°, I = 41.6°, k = 8.7, α95 = 5.2°, n = 87, corresponding to a paleolatitude of 23.9°N and a paleopole at 71.6°N, 340.0°E with A95 = 5.0°. Our updated results indicate a paleolatitude of ~24°N for the Xigaze forearc basin during 70–50 Ma, suggesting that a significant Greater Asia is very unlikely, and strongly supporting a two-stage model for the India-Asia collision with a ~ 55 Ma initial collision of India with an equatorial intra-oceanic arc in the northern hemisphere low latitude followed by the collision between India-plus-arc and Asia in late Eocene.
Significance
The Himalayan mountain belt results from continuing convergence between the Indian Plate and Asia. Damaging earthquakes occur on major thrust faults north of the Main Frontal Thrust (MFT). To the south, the Ganga foreland basin is typically described as undeformed. We show that active thrust and strike-slip faults, with accumulated slip up to ∼100 m, pass under the trace of the MFT into the foreland basin in eastern Nepal, leading to propagation of deformation at least ∼37 km into the foreland basin beneath the densely populated Ganga plain. The development of these faults at the active thrust front helps to explain structures preserved in higher thrust sheets of the Himalaya, and in ancient mountain belts elsewhere.
Knowing the original size of Greater India is a fundamental parameter to quantify the amount of continental lithosphere that was subducted to help form the Tibetan Plateau and to constrain the tectonic evolution of the India-Asia collision. Here, we report paleomagnetic data from Upper Cretaceous rocks of the western Tethyan Himalaya that are consistent with a model that Greater India extended ∼2700 km farther north from its present northern margin at the longitude of 79.6°E before collision with Asia. Our result further suggests that the Indian plate, together with Greater India, acted as a single entity since at least the Early Cretaceous. The pre-collision geometry of Greater India's leading margin helped shape the India-Asia plate boundary. The proposed configuration produced right lateral shear east of the indenter, thereby accounting for the clockwise vertical axis block rotations observed there.
Whereas the timing for India–Asia collision remains debated, contrasting collisional models provide testable predictions in terms of sediment source contributions during the accumulation of Paleocene to middle Eocene deposits now exposed in the frontal Himalayan system. Within the Lesser Himalaya and frontal thrust system of northwest India, discontinuous exposures of the Cretaceous Singtali Formation and upper Paleocene–middle Eocene Subathu Formation yield a record of these early collisional stages. To test competing collisional models, we analyze the provenance of these deposits with new detrital zircon U-Pb and Hf isotopic data which can distinguish among Indian plate, Asian plate, Kohistan-Ladahk arc, and various Himalayan sources. Detrital zircon age distributions for the Singtali Formation are dominated by Paleoproterozoic zircons with a distinct Cretaceous age component, whereas age data from the Subathu Formation record (1) a marked increase in the relative abundance of Cambrian–Neoproterozoic grains, (2) a decrease in the proportion of Paleoproterozoic grains, and (3) distinct Permian and Late Cretaceous–Paleocene age components. All Cretaceous grains from the Singtali Formation yielded crustal Hf isotopic signatures, indicating a distinctive pre-collisional source of Cretaceous grains of Indian affinity. Zircon Hf isotopic signatures from <320 Ma grains in the Subathu Formation show a significant lower to middle Eocene increase in source diversity, including juvenile grains most likely originating from the Asian plate and Kohistan-Ladakh arc. This requires inception of India-Asia collision by ∼44–50 Ma—the
depositional age of the uppermost Subathu Formation. This major provenance shift is similar to that observed for pre- and post-collisional Tethyan Himalayan strata in the north, which is suggestive of a single contiguous basin linking the Lesser Himalaya by ∼44–50 Ma and contests the notion that the Lesser and Tethyan Himalaya were separated by a “Greater India Basin” during the Eocene. When coupled with the well documented Paleocene–early Eocene provenance record of the Tethyan Himalaya, these new data provide support for a collisional model in which Asian detritus reached the northernmost edge of India by ∼59 Ma with terminal closure of both the Shyok and Indus-Yarlung suture zones by ∼54 Ma.
To reconstruct the early tectonic history of the Himalayan orogen before final India-Asia collision, we carried out geochemical and geochronological studies on the Early Cretaceous Aulis Trachyte of the Lesser Himalaya. The trace-element geochemistry of the trachytic lava flows suggests formation in a rift setting, and zircon U-Pb ages indicate that volcanism occurred in Early Cretaceous time. The felsic volcanics show enrichment of more incompatible elements and rare earth elements, a pattern that is identical to the trachyte from the East African Rift (Kenya rift), with conspicuous negative anomalies of Nb, P, and Ti. Although much of the zircon age data are discordant, they strongly suggest an Early Cretaceous eruption age, which is in agreement with the fossil age of intravolcanic siltstones. The Aulis Trachyte provides the first corroboration of Cretaceous rifting in the Lesser Himalaya as suggested by paleomagnetic data associated with the concept that the northern margin of India separated as a microcontinent and drifted north in the Neo-Tethys before terminal collision of India with Asia.
Reconstructing the stratigraphic architecture of deposits prior to Cenozoic Himalayan uplift is critical for unravelling the structural, metamorphic, depositional and erosional history of the orogen. The nature and distribution of Proterozoic and lower Paleozoic strata have helped elucidate the relationship between lithotectonic zones, as well as the geometries of major bounding faults. Stratigraphic and geochronological work has revealed a uniform and widespread pattern of Paleoproterozoic strata >1.6 Ga that are unconformably overlain by <1.1 Ga rocks. The overlying Neoproterozoic strata record marine sedimentation, including a Cryogenian diamictite, a well-developed carbonate platform succession and condensed fossiliferous Precambrian–Cambrian boundary strata. Palaeontological study of Cambrian units permits correlation from the Indian craton through three Himalayan lithotectonic zones to a precision of within a few million years. Detailed sedimentological and stratigraphic analysis shows the differentiation of a proximal realm of relatively condensed, nearshore, evaporite-rich units to the south and a distal realm of thick, deltaic deposits to the north. Thus, Neoproterozoic and Cambrian strata blanketed the northern Indian craton with an extensive, northward-deepening, succession. Today, these rocks are absent from parts of the inner Lesser Himalaya, and the uplift and erosion of these proximal facies explains a marked change in global seawater isotopic chemistry at 16 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 would have been 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 ACCEPTED MANUSCRIPT
The Late Cretaceous location of the Lhasa Terrane is important for constraining the onset of India-Eurasia collision. However, the Late Cretaceous paleolatitude of the Lhasa Terrane is controversial. A primary magnetic component was isolated between 580 °C and 695 °C from Upper Cretaceous Jingzhushan Formation red-beds in the Dingqing area, in the northeastern edge of the Lhasa Terrane, Tibetan Plateau. The tilt-corrected site-mean direction is Ds/Is = 0.9°/24.3°, k = 46.8, α95 = 5.6°, corresponding to a pole of Plat./Plon. = 71.4°/273.1°, with A95 = 5.2°. The anisotropy-based inclination shallowing test of Hodych and Buchan (1994) demonstrates that inclination bias is not present in the Jingzhushan Formation. The Cretaceous and Paleogene poles of the Lhasa Terrane were filtered strictly based on the inclination shallowing test of red-beds and potential remagnetization of volcanic rocks. The summarized poles show that the Lhasa Terrane was situated at a paleolatitude of 13.2° ± 8.6°N in the Early Cretaceous, 10.8° ± 6.7°N in the Late Cretaceous and 15.2° ± 5.0°N in the Paleogene (reference point: 29.0°N, 87.5°E). The Late Cretaceous paleolatitude of the Lhasa Terrane (10.8° ± 6.7°N) represented the southern margin of Eurasia prior to the collision of India-Eurasia. Comparisons with the Late Cretaceous to Paleogene poles of the Tethyan Himalaya, and the 60 Ma reference pole of East Asia indicate that the initial collision of India-Eurasia occurred at the paleolatitude of 10.8° ± 6.7°N, since 60.5 ± 1.5 Ma (reference point: 29.0°N, 87.5°E), and subsequently ~ 1300 ± 910 km post-collision latitudinal crustal convergence occurred across the Tibet. The vast majority of post-collision crustal convergence was accommodated by the Cenozoic folding and thrust faulting across south Eurasia.
Understanding the geological history of the Lhasa Terrane prior to the India-Asia collision (~ 55 ± 10 Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene (~ 18–10 Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183–174 Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr-Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I-S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a site for juvenile crustal accretion in the southern Lhasa Terrane.
This paper deals with the possible provenance of the middle Miocene to early Pleistocene fluvial sediments of the Siwalik Group along with the Karnali River section (Western Nepal) and adds new insights into the formation and evolution of the Himalayan orogeny by means of detrital zircon U-Pb dating by LA-ICP-MS supplemented with sandstone petrography. The modal composition of the dated sandstone samples shows a ‘recycled orogen’ field in QFL diagram, indicating significant reworking and recycling of the detrital sediments during the mountain building process. The detrital zircon U-Pb ages from the Lower Siwalik cluster around ∼490-600 Ma and ∼750-1300 Ma with major peaks at 560 Ma, 927 Ma, and 983 Ma. The result shows that the Tethys Himalaya, Higher Himalaya and possibly the Lesser Himalaya were the predominant sources during the deposition of the Lower Siwalik Group. Contrarily, the detrital zircon U-Pb ages in the Middle Siwalik rocks contains clusters around ∼299-727 Ma, ∼750-1200 Ma, ∼1650-1900 Ma with major peaks at 469 Ma and 905 Ma. However, the increased input of mid-Proterozoic detritus (∼1600Ma) points to the possibility of denudation of the lower Lesser Himalaya following the deposition of the lower part of Middle Siwalik (i.e. since ∼10 Ma).
Most workers regard the Main Central Thrust (MCT) as one of the key high strain zones in the Himalaya because it accommodated at least 90 km of shortening, because that shortening exhumed and buried hanging wall and footwall rocks, and due to geometric and kinematic connections between the Main Central Thrust and the structurally overlying South Tibet Detachment. Geologists currently employ three unrelated definitions of the MCT: metamorphic-rheological, age of motion-structural, or protolith boundary-structural. These disparate definitions generate map and cross-section MCT positions that vary by up to 5 km of structural distance. The lack of consensus and consequent shifting locations impede advances in our understanding of the tectonic development of the orogen. Here, I review pros and cons of the three MCT definitions in current use. None of these definitions is flawless. The metamorphic-rheological and age of motion-structural definitions routinely fail throughout the orogen, whereas the protolith boundary-structural definition may fail only in rare cases, all limited to sectors of the eastern Himalaya. Accordingly, a definition based on high strain zone geometry and kinematics combined with identification of a protolith boundary is the best working definition of the MCT.
A newly identified and dated segment of the South Tibetan detachment in the Karnali klippe, western Nepal Himalaya, constrains initiation of mid-crustal tectonically driven exhumation to the early Oligocene. The folded top-to-the-northeast high-temperature (~600 °C) shear zone separates amphibolite-facies rocks with a ca. 36-30 Ma prograde metamorphic history in the footwall from weakly to non-metamorphosed upper crustal rocks in the hanging wall. In situ dating of syn-kinematic-post-metamorphic peak monazite indicates that the base of the shear zone was active from ca. 30-29 to <24 Ma, and a post-deformation muscovite cooling age implies that ductile shearing had ceased by ca. 19 Ma. Deformation along the South Tibetan detachment in western Nepal was thus synchronous with thrustsense shearing along the lower boundary of a zone of migmatitic rocks, compatible with tectonic models involving mid-crustal channelized flow during the Oligocene. Along with other published data from the Himalayan range, this suggests that the South Tibetan detachment actively exhumed the middle crust for almost 20 m.y.
The Upper Triassic Langjiexue Group, exposed south of the Yarlung-Zangbo suture zone in south Tibet, shows sedimentary features different from typical Tethyan Himalayan successions, and its origin is controversial. In this article we combine field observations with paleocurrent, petrologic, geochronological and isotopic data to determine the provenance of Langjiexue sandstones. These middle to distal deep-sea-fan turbidites are crosscut by Lower Cretaceous diabase sills and dikes generated during rifting of India from Gondwana, indicating that the Langjiexue Group was originally deposited along or adjacent to the northern passive continental margin of India. Flute casts at the base of turbidite beds indicate mostly WNW-ward paleocurrents, pointing to provenance from a source located east of the depositional area. Common volcanic fragments and plagioclase grains together with a cluster of 400–200-Ma-aged magmatic zircons with uniform εHf(t) values from − 5 to + 10 are incompatible with any nearby sources, including the Qiantang Block, the Lhasa Block or the India subcontinent, and indicate instead supply from a long-lived magmatic-arc terrane. Considering what is known about Late Triassic paleogeography, a plausible source for Langjiexue sediments is represented by the Gondwanide Orogen, generated during subduction of the pan-Pacific oceanic lithosphere beneath southeastern Gondwana. This scenario is supported by the age range and Hf isotopic signatures of Late Paleozoic–Early Mesozoic zircons contained in Langjiexue turbidites as in coeval turbidites exposed in western Myanmar. New data are needed to confirm/falsify the existence of a thousand-km-long sediment-routing system similar to the modern Amazon, which – sourced in a cordillera-type orogen rising along the southeastern margin of Gondwana – crossed an entire continent to feed turbiditic fans now exposed from western Myanmar to the northern Tethys Himalaya.
The plume head composition of the newly recognized Comei Large Igneous Province (LIP) in southeastern Tibet has not yet been well constrained. This paper first reports zircon Hf-isotope and trace elemental data of OIB-type mafic samples from Jibutang, Dalong, Comei and Chigu Tso located within the Comei LIP. These data, combined with the whole-rock geochemical data, are used to explore this issue. The OIB-type mafic rocks reported in this paper include alkaline (Group I) and sub-alkaline (Group II) gabbros and diabases, which outcropped as dikes and sills. These mafic rocks have high TiO2 (2. 61% similar to 4. 07%) and high P2O5 (0. 32% similar to 0. 51%) contents, and are enriched in light rare earth elements (LREEs) and high field strength elements (HFSEs), similar geochemically to Ocean Island basalt (OIB). Geochemical signatures of whole-rock trace element indicate little or no appreciable crustal contamination for Group I samples and significant crustal contamination for Group II samples, respectively. One sample in Group I (JBT03-1) display a wide range of zircon epsilon(Hf)(t) values (-4. 8 similar to + 5. 3), indicating the involvement of component from crust and/or sub-continental lithospheric mantle. This result suggests that zircon Hf-isotope data can be used as an effective geochemical fingerprint superior to whole-rock geochemistry to determine whether mafic rocks contain components derived from crust and/or subcontinental lithospheric mantle or not. The OIB-type mafic samples in the Comei LIP (Group I and Group II) display distinct zircon rare earth element patterns and Ti-in-zircon temperature from those of mafic rocks from subduction zone and oceanic crust. Such difference is most likely controlled by the different temperature and composition of their magma source regions. Considering the whole-rock geochemical and zircon Hf-isotope signatures, this paper proposes that the Dalong gabbroic pluton (represented by sample DL01 in Group I) without contributions from crust and/or sub-continental lithospheric mantle likely represents the pure compositions of plume head of the Comei LIP [namely: Sr-87/Sr-86((t)) = 0. 7047, epsilon(Nd)(t) = + 1. 5, zircon epsilon(Hf)(t) = + 2. 1 similar to + 5. 7]. These compositions are very similar to those of the basalts from Site 1138 and Bunbury Casuarina in SW Australia that were interpreted to represent the Cretaceous Kerguelen plume head materials. This similarity indicates that the OIB-type mafic rocks in the Comei LIP may be the product of decompression melting of the Kerguelen plume head materials.
There is ongoing debate as to the subduction direction of the Bangong-Nujiang Ocean during the Mesozoic (northward, southward or bidirectional subduction). Arc-related intermediate to felsic intrusions could mark the location of the subduction zone and, more importantly, elucidate the dominant geodynamic processes. We report whole rock geochemical and zircon U–Pb and Hf isotopic data for granitoids from the west central Lhasa subterrane (E80° to E86°). All rocks show metaluminous to peraluminous, calc-alkaline signatures, with strong depletion of Nb, Ta and Ti, enrichment of large ion lithophile elements (e.g., Cs, Rb, K), a negative correlation between SiO2 and P2O5, and a positive correlation between Rb and Th. All these features are indicative of I-type arc magmatism. New zircon U–Pb results, together with data from the literature, indicate continuous magmatism from the Late Jurassic to the Early Cretaceous (160 to 130 Ma). Zircon U–Pb ages for samples from the northern part of the west central Lhasa subterrane (E80° to E82°30′) yielded formation ages of 165 to 150 Ma, whereas ages of 142 to 130 Ma were obtained on samples from the south. This suggests flat or low-angle subduction of the Bangong-Nujiang Ocean, consistent with a slight southward decrease in zircon εHf(t) values for Late Jurassic rocks. Considering the crustal shortening, the distance from the Bangong-Nujiang suture zone, and a typical subduction zone melting depth of ~ 100 km, the subduction angle was less than 14° for Late Jurassic magmatism in the central Lhasa interior, consistent with flat or low-angle subduction. Compared with Late Jurassic rocks (main εHf(t) values of − 16 to − 7), Early Cretaceous rocks (145 to 130 Ma) show markedly higher εHf(t) values (mainly − 8 to 0), possibly indicating slab roll-back, likely caused by slab foundering or break-off. Combined with previously published works on arc magmatism in the central Lhasa and west part of the southern Qiangtang subterranes, our results support the bidirectional subduction of the Bangong-Nujiang Ocean along the Bangong-Nujiang Suture Zone, and indicates flat or low-angle southward subduction (165 to 145 Ma) followed by slab roll-back (145 to 130 Ma).
In order to improve estimation of the paleolatitude of the southern margin of Asia during the Late Cretaceous, as well as the constraints on the intracontinental convergence within Asia due to the India–Asia collision, we performed a combined geochronological and paleomagnetic investigation of Late Cretaceous red bed sequences and intercalated volcanic rocks from the Linzhou Basin in the Lhasa terrane (29.9°N/91.1°E). The results of potassium–argon (K–Ar) dating indicated that the basalt sequences are of Late Cretaceous age (68–75 Ma). Stepwise thermal demagnetization successfully isolated high unblocking-temperature characteristic directions from 10 red bed sites and 11 lava flow sites. Positive fold and reversal tests indicate a primary origin for the characteristic remanence of the red beds and basalt sites. The consistent inclination recorded in the red beds and basalt lava flows indicates that no significant compaction-related inclination shallowing has occurred in the red beds from the studied section. The tilt-corrected mean direction is D/I = 0.5°/20.2° with α95 = 6.4° and N = 21 sites, corresponding to a paleopole at 70.5°N, 269.6°E with A95 = 4.9°. The paleomagnetic results yield a paleolatitude of 10.4 ± 4.9°N for the southern margin of Asia in the Late Cretaceous. Compared with expected paleomagnetic directions from the Tethyan Himalaya and Asia, significant crustal shortening of 17.7 ± 5.5° (1960 ± 610 km) and 9.4 ± 4.7° (1040 ± 520 km), respectively, may have occurred between the southern margin of Asia and the Tethyan Himalaya, and within Asia, since the Late Cretaceous. Based on previous Cretaceous and Paleocene data from the Lhasa terrane, the latitude of the southern margin of Asia was located at a stable paleolatitudinal position (10°–15°N) during the Cretaceous and Paleocene.