Understanding the dynamics of the uplift of the Tibetan blocks requires constraints on the timing and magnitude of the crustal shortening accommodated by the different blocks. However, the estimates ...of the magnitude of post-collisional intra-Asian convergence range from >3000 km to a few hundred kilometers. Here we present new paleomagnetic and geochronological results from the Meisu Formation lavas on the southwestern margin of the western Qiangtang block, western Tibet. Zircon U/Pb data reveal that these volcanic rocks erupted during the late Eocene (∼40 Ma). Following progressive thermal demagnetization, stable characteristic remanent magnetizations (ChRMs) were successively isolated from 28 sites. These ChRMs passed the fold and reversal tests, consistent with a primary remanence. The paleopole at 50.2°N, 163.2°E with A95 = 5.9° yields a paleolatitude of ∼29.5 ± 5.9°N at ca. 40 Ma for the southern margin of the western Qiangtang block (33.2°N, 80.9°E). A comparison of the Eocene latitudes between the western Qiangtang and Tarim block indicates a ∼ 700 km of post-40 Ma latitudinal crustal shortening between them at the longitude of ∼80°E. Our review of published Cretaceous-Paleogene paleopoles from western Tibet suggests that the western Qiangtang block was positioned at a stable latitude during ∼116–30 Ma. Our paleomagnetic compilation also indicate a discrepancy in the latitude (∼9.7 ± 4.3°) of western Lhasa and Qiangtang during the interval of 132 and 67 Ma, which vanished at ca. 30 Ma. We suggest a crustal shortening model to interpret this 1076 ± 477 km discrepancy, which was accommodated by the interior of the Lhasa block, the Bangong-Nujiang suture zone, and/or an eastward extrusion of Indochina from an original position between the Lhasa and Qiangtang blocks during ∼67–30 Ma. We conclude that the crustal shortening caused by the ongoing India-Asia collision spread progressively northwards into Asia. We emphasize that reliable paleomagnetic data from Paleocene and lower Eocene units in the western Qiangtang and Lhasa blocks have the potential to further refine these crustal shortening estimates.
•We provide late Eocene paleomagnetic results from western Qiangtang.•The western Qiangtang block had a paleolatitude of 29.5 ± 5.9°N at ca. 40 Ma.•∼700 km of post-40 Ma N-S crustal shortening between western Qiangtang and Tarim.•∼1100 km of N-S crustal shortening between western Lhasa and Qiangtang during ∼67–30 Ma.•Crustal shortening spread progressively northwards into Asia.
The strong present-day Asian monsoons are thought to have originated between 25 and 22 million years (Myr) ago, driven by Tibetan-Himalayan uplift. However, the existence of older Asian monsoons and ...their response to enhanced greenhouse conditions such as those in the Eocene period (55-34 Myr ago) are unknown because of the paucity of well-dated records. Here we show late Eocene climate records revealing marked monsoon-like patterns in rainfall and wind south and north of the Tibetan-Himalayan orogen. This is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from Myanmar, and by aeolian dust deposition in northwest China. Our climate simulations support modern-like Eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Tibetan relief on precipitation. These strong monsoons later weakened with the global shift to icehouse conditions 34 Myr ago.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The MBT–CBT proxy for the reconstruction of paleotemperatures and past soil pH is based on the distribution of branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids. The Methylation ...of Branched Tetraether (MBT) and the Cyclisation of Branched Tetraether (CBT) indices were developed to quantify these distributions, and significant empirical relations between these indices and annual mean air temperature (MAT) and/or soil pH were found in a large data set of soils. In this study, we extended this soil dataset to 278 globally distributed surface soils. Of these soils, 26% contains all nine brGDGTs, while in 63% of the soils the seven most common brGDGTs were detected, and the latter were selected for calibration purposes. This resulted in new transfer functions for the reconstruction of pH based on the CBT index: pH=7.90–1.97×CBT (r2=0.70; RMSE=0.8; n=176), as well as for MAT based on the CBT index and methylation index based on the seven most abundant GDGTs (defined as MBT′): MAT=0.81–5.67×CBT+31.0×MBT′ (r2=0.59; RMSE=5.0°C; n=176). The new transfer function for MAT has a substantially lower correlation coefficient than the original equation (r2=0.77). To investigate possible improvement of the correlation, we used our extended global surface soil dataset to statistically derive the indices that best describe the relations of brGDGT composition with MAT and soil pH. These new indices, however, resulted in only a relatively minor increase in correlation coefficients, while they cannot be explained straightforwardly by physiological mechanisms. The large scatter in the calibration cannot be fully explained by local factors or by seasonality, but MAT for soils from arid regions are generally substantially (up to 20°C) underestimated, suggesting that absolute brGDGT-based temperature records for these areas should be interpreted with caution.
The applicability of the new MBT′–CBT calibration function was tested using previously published MBT–CBT-derived paleotemperature records covering the last deglaciation in Central Africa and East Asia, the Eocene–Oligocene boundary and the Paleocene–Eocene thermal maximum. The results show that trends remain similar in all records, but that absolute temperature estimates and the amplitude of temperature changes are lower for most records, and generally in better agreement with independent proxy data.
Back-arc basins and the related subduction systems have been extensively studied, however back-arc basins exhibiting episodic jumps of extension centers by the subduction of different ocean slabs are ...poorly known. The East China Sea Shelf Basin (ECSSB) comprises two eastward-younging subparallel depression zones, and its formation and evolution is accompanied by a complicated subduction history and regional tectonic events offer an excellent opportunity to decipher the various geological features of back-arc basins. Here, we review the tectonic and sedimentary features of the subbasins in the ECSSB and its adjacent regions and then reconstruct the evolution of the sedimentary paleogeography. The west depression zone (WDZ) of the ECSSB experienced rifting in the Late Cretaceous during the subduction of the Izanagi Plate at the eastern continental boundary. In the northern WDZ, the Changjiang Depression was mainly dominated by fluvial to lacustrine deposits. This depression experienced strong compression characterized by folds and erosion during the transition from a synrift to postrift setting at the end of the Paleocene. Then another compression event occurred that resulted in extensive folds and uplift accompanied by a large-scale alluvial fan in the eastern Changjiang Depression, coinciding with the subparallel subduction of the Izanagi-Pacific ridge in the early Eocene. However, the Lishui-Jiaojiang Sag characterized by diffuse extension rather than focused rifting in its late rifting period. This sag was mainly covered by paralic to marine deposits and experienced extensive erosion caused by tilted sequences from the late Eocene to Oligocene, which shared the same characteristics as basins around the Taiwan region. It is widely accepted that the formation and evolution of basins around Taiwan were controlled by the evolving of the South China Sea (SCS) and the related regional tectonics. Considering the eastward extent of SCS, it would seem that the southern ECSSB was part of the passive margin of SCS. Accordingly, the southern ECSSB might be significantly influenced by the SCS. The east depression zone (EDZ) commenced rifting in approximately the middle Eocene as inferred from the absence of the compressional event that occurred in the early Eocene and the existence of a thick layer reflection below the Pinghu Fm. (late Eocene) due to the subduction of the Pacific Plate. The depocenters of the ECSSB and the Okinawa Trough jumped eastward successively triggered by the three oceanic slabs of the Izanagi Plate at ~72 Ma, the Pacific Plate at ~50 Ma and the Philippine Sea Plate at ~15 Ma. Thus, a new model for the formation and evolution of the ECSSB is suggested in which a back-arc basin of episodic extension with jumping depocenters in respond to subduction of three oceanic slabs, meanwhile its differences between northern and southern part are caused by the dominant subduction and opening of SCS, respectively. The ECSSB and Okinawa Trough might formed mainly by rollback of the oceanic slab, possibly, due to slab stagnation and toroidal flow related to the subduction, however the effects of large-scale mantle flow cannot be exclusively identified in this research. In addition, further research in this region will give new insights into both the formation and evolution of the SCS and the continental breakup process.
Lake hydrological change and its driving mechanisms in the Eocene warmhouse climate are important to evaluating environmental and ecological changes induced by global warming. Our limited ...understanding of lake hydrological changes during the Eocene is in part due to the scarcity of well-dated and continuous terrestrial records. Here we refine the middle-late Eocene cyclostratigraphy and decipher high-resolution lake hydrology changes using sedimentary noise modeling and proxy data at four terrestrial basins (Dongying Depression, Nanxiang Basin, Jianghan Basin, and Fushun Basin) in mid-latitudes of East Asia. Our new astrochronology provides a robust timescale for the period of 48.5-38.5 Ma, and our tuned magnetostratigraphy is at least comparable with other timescales between Chrons C18n.1r and C20n. The new astrochronology reveals synchronous lake-level changes in the four basins in East Asia following 1.2 Myr obliquity and 2.4 Myr eccentricity cycles, indicating astronomical forcing on lake-level changes. The 2.4 Myr cycles of lake-levels are generally in phase with 2.4 Myr cycles of global sea-level changes, suggesting that East Asian lake hydrology was modulated by global sea-level variations in the Eocene warmhouse. Therefore, global sea-level variations may be an important driver of East Asia hydroclimate and freshwater resources.
•New terrestrial astrochronology bridges timescale gap for middle-late Eocene.•Lake-level changes in four basins in East Asia are synchronous.•Lake-level changes show astronomical cycles of 1.2 Myr and 2.4 Myr in East Asia.•Sea-level variation is an important driver of East Asian lake hydrology.
The Upper Eocene rocks (Qasr El-Sagha Formation) at northwestern Qarun Lake, in Fayum depression, revealed numerous abraded and bio-eroded Kerunia cornuta specimens, with calcified hydractinian ...encrustation. The symbiotic relationship between the encrusted genus Hydractinia and the host gastropod shell is discussed through thin section examination. A report of in situ hermit crab in the host gastropod shell in Kerunia cornuta is documented here for the first time from the Eocene rocks of Egypt. The studied bioerosion structures in Kerunia are attributed to three ichnospecies: Gastrochaenolites lapidicus Kelly and Bromley1984; Gastrochaenolites torpedo Kelly and Bromley1984 and Gastrochaenolites isp. These ichnospecies are related to Trypanites ichnofacies in an offshore environment under storm wave base. The taphonomic processes that affected the Kerunia cornuta specimens are also discussed in this work. The present study indicates that the paleoenvironment prevailed during the deposition of the Kerunia cornuta specimens reflects shallow agitated and bioturbated environments.
•Kerunia cournuta collected from of Upper Eocene of Egypt.•Kerunia is encrusted with Hydractinia and affected by bioerosion.•Hermit crab is recorded in Keunia for the first time in Egypt.
The timing of formation of the low‐gradient, internally drained landscape of the Tibetan Plateau is fundamental to understanding the evolution of the plateau as a whole. Well‐dated sedimentary ...records of internal drainage of rivers into lakes are used to reveal the timing of this evolution. Here we redate the youngest continental sedimentary successions of central Tibet in the Lunpola Basin and propose a new age range of ca. 35 to 9 Ma, significantly younger than previously thought. We demonstrate long‐standing internal drainage in central Tibet since the late Eocene and stable sedimentary environments, source regions, and low topographic relief since at least the early Miocene. We suggest that sediment aggradation of internal drainage and reduction of hillslope gradients by erosion dominate the formation of low‐relief landscapes and that the late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere.
Plain Language Summary
Internal drainage of rivers into lakes is a characteristic of the high plateaus of the world and, most notably, the Tibetan Plateau. Internal drainage generates local perched base levels for Tibetan rivers, enabling geomorphic isolation from the rapidly incising rivers of the Himalaya and surrounding regions. However, the question of when the low‐relief plateau topography was initiated has been largely ignored, and its formation mechanism is controversial. Here we report a detailed investigation in the Lunpola Basin of central Tibet and propose a new depositional age range of ca. 35–9 Ma. We demonstrate that the internal drainage kept eroding the mountain ranges and filling the surrounding lowlands since at least the late Eocene. By no later than the early Miocene, a gentle landscape formed in central Tibet. The late Cenozoic basins in central Tibet developed in response to deep crustal or mantle flow and associated upper crustal deformation.
Key Points
Robust age constraints of the youngest continental stratigraphic unit from the Lunpola Basin in central Tibet are reported
Aggradation and erosion of internal drainage dominated the formation of low‐relief topography in central Tibet by the early Miocene times
Late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere
We present a new stratigraphy of the Jianchuan basin, one of the largest Cenozoic sedimentary basins in southeastern Tibet. This basin was regarded as recording sedimentation from the Eocene up to ...the Pliocene, and as such has been the focus of several studies aiming at constraining the environmental, tectonic and topographic evolution of the area. Within the Shuanghe and Jianchuan formations thirteen new zircon U/Pb ages and one biotite 40Ar/39Ar age of interbedded and cross-cutting ultrapotassic magmatic rocks show that a brief magmatic event occurred from ~35.7 to ~34.5Ma (35.2±0.4Ma on average). The uppermost formation (Jianchuan Fm), supposedly Pliocene in age, is related to this magmatic event and is 35.4±0.8Ma old. All sedimentary formations are thus Eocene in age, with neither Oligocene nor Miocene sediments. The coal-bearing Shuanghe Formation yields a fossil of a large amynodontid typical of the Upper Eocene Ergilian interval (37.2 to 33.9Ma). Sedimentation of the Shuanghe Formation took place in a short time interval at ~35.9±0.9Ma, after a large-scale drainage reorganization that induced the abandonment of a large braided-river system. This reorganization was possibly linked with the initiation of the left-lateral Ailao-Shan Red River fault and/or to widespread magmatism in the Jianchuan basin. Previous high paleoaltitude estimates for the Jianchuan basin are thoroughly re-evaluated and yield a value of 1200±1200m.a.s.l. for the Upper Eocene.
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•New stratigraphy for the Jianchuan basin with all deposits younger than Oligocene•The Jianchuan Fm is ~35.4±0.8Ma old and the Shuanghe Fm is 35.9±0.9Ma old.•Transition from braided-fluvial to lacustrine-palustrine prior to ~36Ma•UltraK magmatic event (35.2±0.4Ma) coeval with sedimentation and E-W shortening•Previous high paleoaltitude estimates for the basin are re-evaluated to 1200±1200m
About 34 million years ago, Earth's climate cooled and an ice sheet formed on Antarctica as atmospheric carbon dioxide (CO₂) fell below ~750 parts per million (ppm). Sedimentary cycles from a drill ...core in the western Ross Sea provide direct evidence of orbitally controlled glacial cycles between 34 million and 31 million years ago. Initially, under atmospheric CO₂ levels of ≥600 ppm, a smaller Antarctic Ice Sheet (AIS), restricted to the terrestrial continent, was highly responsive to local insolation forcing. A more stable, continental-scale ice sheet calving at the coastline did not form until ~32.8 million years ago, coincident with the earliest time that atmospheric CO₂ levels fell below ~600 ppm. Our results provide insight into the potential of the AIS for threshold behavior and have implications for its sensitivity to atmospheric CO₂ concentrations above present-day levels.
The middle to late Eocene marked a key transition from greenhouse to coolhouse. However, a lack of high-resolution and temporally well-constrained continental records hinders our understanding of the ...driving mechanisms of climate change during this period. In this study, we combined magnetostratigraphy and cyclostratigraphy to establish a high-resolution astronomical time scale spanning the middle to late Eocene (∼42.4 Ma to ∼35.3 Ma) through a 214-m succession of the continental Honghe Formation in the Weihe Basin, North China. Our results show that variability in the hydroclimate of the Weihe Basin at this time was dominated by eccentricity (∼405 kyr and ∼100 kyr) and obliquity (∼1.15 Myr and ∼173 kyr) forcing. Meridional insolation gradient variations controlled by obliquity, and low-latitude summer insolation via eccentricity modulation of precession jointly controlled paleolake evolution in the basin during the ∼42.4 Ma to ∼35.3 Ma interval, rather than being controlled by eccentricity or obliquity as previously inferred. Our study provides new insights into the driving mechanisms of orbital-scale hydroclimate evolution in continental lake basins during the ice-free middle to late Eocene.
•Astronomical time scale for the middle to lower Honghe Formation of the Weihe Basin, North China.•Obliquity and Eccentricity jointly controlled paleolake evolution in the Weihe basin during middle to late Eocene.
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