Zhaojie Guo

Discordant paleomagnetic direction in Miocene rocks from the central Tarim Basin: evidence for local deformation and inclination shallowing

From exposures at the southeastern end of the Maza Tagh range in the central Tarim Basin (latitude: 38.5‡N; longitude: 80.5‡E), 55 paleomagnetic sites were collected from red mudstones and sandstones of the Miocene Wuqia Formation. Thermal demagnetization revealed a high unblocking temperature characteristic remanent magnetization (ChRM). Five sites collected across a kink fold yield a positive fold test at 99% confidence level. The mean directions computed from normal and reversed polarity sites are antipodal suggesting a primary origin for the ChRM. In stratigraphic coordinates, the final set of 30 site-mean ChRM directions yields a section-mean direction: inclination (I) = 29.4‡; declination (D) = 24.7‡; K95 = 6.2‡. When compared to the Miocene expected direction (at 20 Ma), the observed direction indicates 30.8 < 5.5‡ flattening of inclination and 15.3 < 6.7‡ clockwise vertical-axis rotation. Anisotropy of magnetic susceptibility measurements on 155 samples show a strong foliation of 1.092 with a subvertical minimum susceptibility axis. These observations indicate a rock-magnetic (depositional or compaction shallowed) origin for the inclination flattening. The clockwise deflection of the observed declination can be interpreted as either: (1) 15.3 < 6.7‡ clockwise rotation of the entire Tarim Basin since the Miocene; or (2) a local km-scale structural deformation. It is not a simple matter to discard the interpretation of 15.3 < 6.7‡ clockwise rotation of the Tarim Basin because the fastest rates of rotation determined from global positioning system and slip-rate studies of Quaternary faults could produce such a rotation if extrapolated to 20 Ma. Nevertheless, we argue that local deformation is the preferred interpretation because the map pattern of local structures shows V20‡ clockwise deflection toward the southeastern end of the Maza Tagh range where the paleomagnetic samples were collected. ? 2002 Elsevier Science B.V. All rights reserved.

Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India-Asia collision?

Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U-Pb dates indicate an age of ~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end-member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India-Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° (35.7°, 40.5°) (95% significance) and a secondary remanence with inclination of 42.9° (41.5°,44.4°) (95% significance). The paleolatitude defined by the modeled primary remanence—21°N (19.8°N, 23.1°N)—is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya-Asia collision was situated at ~20°N and took place by ~50 Ma.

What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou Basin, Tibet)

The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou Basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-Pb zircon dates of ~52 Ma for the upper Linzizong, and 40Ar/39Ar dates (~51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of ~5°N, compared to the upper part (~20°N) and to underlying Cretaceous strata (~20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by ~52 Ma at ~20°N.

Paleomagnetic tests of tectonic reconstructions of the India-Asia collision zone

Several solutions have been proposed to explain the long-standing kinematic observation that postcollisional upper crustal shortening within the Himalaya and Asia is much less than the magnitude of India-Asia convergence. Here we implement these hypotheses in global plate reconstructions and test paleolatitudes predicted by the global apparent polar wander path against independent, and the most robust paleomagnetic data. Our tests demonstrate that (1) reconstructed 600–750 km postcollisional intra-Asian shortening is a minimum value; (2) a 52 Ma collision age is only consistent with paleomagnetic data if intra-Asian shortening was ~900 km; a ~56–58 Ma collision age requires greater intra-Asian shortening; (3) collision ages of 34 or 65 Ma incorrectly predict Late Cretaceous and Paleogene paleolatitudes of the Tibetan Himalaya (TH); and (4) Cretaceous counterclockwise rotation of India cannot explain the paleolatitudinal divergence between the TH and India. All hypotheses, regardless of collision age, require major Cretaceous extension within Greater India.

Ophiolitic mélanges in crustal-scale fault zones: Implications for the Late Palaeozoic tectonic evolution in West Junggar, China

The Baijiantan and Darbut ophiolites in West Junggar are exposed in steep fault zones (>70°) containing serpentinite melange, in contact on either side with regionally distributed Upper Devonian-Lower Carboniferous ocean floor peperitic basalts and overlying sedimentary successions. The ophiolitic melanges show classic structural features created by strike-slip faulting and consistent shear sense indicators of left-slip kinematics. Sandstone blocks within the melanges resemble the surrounding sediments in lithology and age, indicating that the ophiolitic melanges consist of locally derived rocks. The ophiolitic melanges therefore originated from left-slip fault zones within a remnant basin and are not plate boundaries nor subduction suture zones. Sandstone is the youngest lithology involved in the melange and provides a maximum age for the melange of 322 Ma, whereas stitching plutons are younger than 302 Ma. Multiple clusters in zircon ages from single gabbro blocks in the melange at ~375, ~360, ~354, and ~340 Ma are inconsistent with accretionary incorporation of subducting ocean crust but rather suggest that episodic movement of the faults provided pathways for magma from the mantle into magma chambers. Late Paleozoic tectonic evolution of West Junggar involved Late Devonian to Carboniferous relative motion between the Junggar block and West Junggar ocean basin, which triggered the left-slip fault zones within a remnant ocean basin, along which the oceanic crust was disrupted to form linear ophiolitic melanges. Final filling of this remnant ocean basin and its dismemberment by strike-slip faulting occurred in the late Carboniferous, followed by crustal thickening by juvenile granites at the Carboniferous-Permian boundary.

Anisotropy of magnetic susceptibility of Eocene and Miocene sediments in the Qaidam Basin, Northwest China: Implication for Cenozoic tectonic transition and depocenter migration

The Cenozoic evolution of the Qaidam basin, especially its paleostress field, can provide a better understanding of the dynamistic process of the northern Tibetan Plateau. Under certain conditions, Anisotropy of Magnetic Susceptibility (AMS) holds great potential for investigating early tectonic events, even where macroscopic and microscopic evidence of deformation is invisible. A basin-scale AMS study of the middle to late Eocene Xiaganchaigou Formation and the early to middle Miocene Xiayoushashan Formation from seven locations was conducted, covering most outcrops of these two formations within the Qaidam basin. In the western Qaidam basin, principal stress directions inferred from AMS ellipsoids consist with those inferred from fold axial traces, while at Eboliang and in the northern Qaidam basin, most principal stress directions reflected by AMS ellipsoids are different from those reflected by fold axial traces. Two epochs of compressive strain have been identified: an early N-S strain no later than Oligocene and a late NE-SW strain since Miocene. The early N-S compression is more intense in the northern Qaidam basin than that in the western Qaidam basin, while the late NE-SW compression, which dominates the modern NW-SE trending fold axial traces, is more intense in the western Qaidam basin than that in the northern Qaidam basin. The stress transfer provides a reasonable explanation for the southeastward migration of the deposition center in the Qaidam basin during Cenozoic. Moreover, the appearance of E-component compression may be in close relationship with the beginning of the left-lateral strike-slip Kunlun Fault or the eastward channel flow to the south of the Kunlun Fault.

Source to sink relation between the Eastern Kunlun Range and the Qaidam Basin, northern Tibetan Plateau, during the Cenozoic

Understanding the source to sink relationship through time between the Eastern Kunlun Range, one of the major mountain belts in the northern Tibetan Plateau, and the actively deforming Qaidam Basin to the north has important implications for unravelling the growth history of the Tibetan Plateau. In this study, U-Pb dating (laser-ablation–inductively coupled plasma–mass spectrometry) of detrital zircons from 22 sandstone samples (Paleocene to Holocene) collected from four sections within the southwestern Qaidam Basin is combined with provenance analysis and new seismic profile interpretation to investigate the mountain building of the Eastern Kunlun Range and its effects on the development of the Qaidam Basin. The U-Pb age distributions of detrital grains from Paleocene strata are characterized by a major component of Paleozoic to late Proterozoic ages. Furthermore, carbonate debris containing foraminifera have been recognized in the Paleocene conglomerate sequences. We thus suggest that the Eastern Kunlun Range was already exhumed prior to the Paleocene. The southward onlaps of Paleocene to Oligocene strata observed on seismic profiles and the appearance of a Mesozoic component in the detrital zircon age spectra of Eocene to Oligocene strata indicate that the Qaidam Basin was widening southward during that early Cenozoic period. Well-developed post-Oligocene growth strata and the increasing proportion of Mesozoic and Paleozoic U-Pb ages in detrital zircon grains from late Neogene strata demonstrate that the relief of the Eastern Kunlun and Altyn Tagh Ranges increased, leading to isolation and narrowing of the Qaidam Basin, from Miocene to the present. The inferred pulsed deformation in the Eastern Kunlun Range highlights the complex growth history of the Tibetan Plateau.

Initial rupture and displacement on the Altyn Tagh fault, northern Tibetan Plateau: Constraints based on residual Mesozoic to Cenozoic strata in the western Qaidam Basin

The Altyn Tagh fault, located in the northern Tibetan Plateau, is a large left-lateral strike-slip fault heavily responsible for the growth and formation of the plateau during Cenozoic time. Despite its significance, the initial timing and kinematic patterns of movement along the Altyn Tagh fault remain highly debated. Here, we present a detailed analysis of the stratigraphy and geochronology of three key lithologic sections (Tula, Anxi, and Caishiling) along the Altyn Tagh fault to better understand this kinematic history. By correlating stratigraphic contacts and lithology with the U-Pb age spectra of Mesozoic samples within the western Qaidam Basin, we find the Altyn Tagh fault has experienced a total of ∼360 km of displacement during the Cenozoic. By combining seismic profile data with geologic observations, we divide the activity along this fault into two distinct stages of motion: (1) an initial stage, which occurred between early Eocene (ca. 49 Ma) and mid-Miocene time (ca. 15 Ma) and resulted in ∼170 km of offset, and (2) an early stage, which began in the late Miocene Epoch and continues into the present, resulting in ∼190 km of offset along the fault. We identify the Tula and Anxi sections as piercing points along the western segment of the Altyn Tagh fault and define these regions as residual parts of the original Qaidam Basin. These estimates suggest that motion along the Altyn Tagh fault has accelerated from an average left-lateral strike-slip rate of ∼5.0 mm/yr during initial stage faulting to a rate of ∼12.6 mm/yr between the late Miocene Epoch and present day.

Oligocene clockwise rotations along the eastern Pamir: Tectonic and paleogeographic implications

Despite the importance of the Pamir range in controlling Asian paleoenvironments and land-sea paleogeography, its tectonic evolution remains poorly constrained in time and space, hindering its potential for understanding deep to surface processes. We provide here new constraints on vertical-axis tectonic rotations from the southwest Tarim Basin along the eastern flank of the Pamir arcuate range based on paleomagnetic results. Two well-dated Eocene to Oligocene sections, previously analyzed using biostratigraphy and magnetostratigraphy, yield consistently clockwise rotations of 21.6 ± 4.2° in 41 to 36 Ma strata then 17.1 ± 6.5° in 33 to 28 Ma strata at the Aertashi section and 14.2 ± 11.5° in 41 to 40 Ma strata at the Kezi section. Combined with a regional review of existing paleomagnetic studies, these results indicate that most of the clockwise rotations along the eastern Pamir occurred during Oligocene times and did not extend systematically and regionally into the Tarim Basin. In contrast, on the western flank of the Pamir tectonic rotations in Cretaceous to Neogene strata are regionally extensive and systematically counterclockwise throughout the Afghan-Tajik Basin. This timing and pattern of rotations is consistent with paleogeographic reconstructions of the regional sea retreat out of Central Asia and supports a two-stage kinematic model: (1) symmetric rotations of either flanks of the Pamir arcuate range until Oligocene times followed by (2) continued rotations on its western flank associated with radial thrusting and, along the eastern flank, no further rotations due to decoupled transfer slip starting in the Early Miocene.

Paleomagnetism of Eocene and Miocene sediments from the Qaidam basin: Implication for no integral rotation since the Eocene and a rigid Qaidam block

The Qaidam basin is the largest topographic depression inside the Tibetan Plateau and it is a key factor to understanding the Cenozoic evolution of the northern Tibetan Plateau. Paleomagnetic data was obtained from the middle to late Eocene Xiaganchaigou Formation and the early to middle Miocene Xiayoushashan Formation from seven localities. The paleomagnetic results indicate that the Qaidam basin has not undergone obvious basin-scale vertical axis rotation with respect to the Eurasia Plate since the Eocene. Local clockwise rotation took place only at a few special locations along the northern margin of the Qaidam basin. The uniform paleomagnetic results at different localities support that the Qaidam basin is a relatively rigid block. Regional paleomagnetic and geodetic observations also suggest that crust south of the Kunlun fault moves eastward faster than crust north of the Kunlun fault.