Abstract The coupling of magnetic fabrics and magnetic remanences is critical in interpreting paleomagnetic data. To estimate whether primary magnetic fabrics imply primary magnetic remanences, and to assess the practicability of metamorphic rocks in magnetic study, we carried out petrographic, geochronological, rock magnetic investigations, and analyses in anisotropy of magnetic susceptibility and paleomagnetism on migmatites in the Central Tianshan, NW China. Petrological observations indicate no significant dynamic recrystallization post to the migmatization. In‐situ monazite U‐Pb dating suggests that the migmatization happened during ∼314–297 Ma. Rock magnetic results reveal that the magnetic properties of migmatites are dominated by biotites with minor titanomagnetites. Despite the structural and compositional complexities of migmatites, a simple magnetic fabric pattern is observed with concentrated magnetic foliations and dispersed magnetic lineations. The anisotropy degree and shape parameter significantly change from leucosomes, mesosomes to melanosomes, suggesting that the magnetic fabrics should have been acquired during the migmatization. Characteristic remanent magnetization directions were isolated from a quarter of samples with anomalous shallow magnetic inclination. Combined with available geochronological and paleomagnetic results from the Central Tianshan and neighboring blocks, the magnetic remanences preserved in the migmatites were suggested to be obtained at ∼314–303 Ma, later than the acquisition of magnetic fabrics, probably due to thermal remagnetization or resulted from long‐term progressive magnetization during tectonic exhumation of migmatites. This study provides an important yet rarely reported example to manifest the decoupling of magnetic fabrics from magnetic remanences. Meanwhile, migmatites are found to be operable materials for magnetic fabric and paleomagnetic studies.
The South Tianshan is located to the north of the Tarim block and defines the southern margin of the Paleozoic Central Asian Orogenic Belt (CAOB). This study presents new structural data, geochronological and geochemical results for the Wuwamen ophiolite mélange in the Chinese segment of the South Tianshan. In the south, the Wuwamen ophiolite mélange shows typical block-in-matrix fabrics and occurs in the footwall of a south-dipping thrust fault, hanging wall of which is composed of weakly metamorphosed and deformed Lower Paleozoic marine to deep marine sequences from the South Tianshan. In the north, a southdipping thrust fault juxtaposes the Wuwamen ophiolite mélange in its hanging wall against the high-grade and strongly deformed metasedimentary rocks from the Central Tianshan in its footwall.
Bogda Shan is a mountain belt located at the eastern extremity of the Chinese Tianshan and records a complex and debated exhumation history. Previous studies have reported a young Cenozoic thermal history for the exhumation of Bogda Shan, which is in conflict with the observation of preserved Mesozoic erosion surfaces in the area. This study re-evaluates the Meso-Cenozoic thermo-tectonic evolution of Bogda Shan using apatite fission track (AFT) thermochronology. Palaeozoic basement (meta-sandstone) samples collected from the northern and southwestern flanks of the mountain ranges reveal apparent Mesozoic AFT ages ranging from ~202 Ma to ~97 Ma. Inverse thermal history modelling results reveal slow to moderate basement cooling during the early Mesozoic, corresponding to relatively low levels of exhumation. This accounts for the preservation of low-relief Mesozoic peneplanation surfaces recognized at elevations of ~3500–4000 m. None of the presented AFT data and thermal history models show any evidence for significant deep Cenozoic exhumation. In the neighbouring Junggar Basin, a Middle Jurassic sandstone sample records partial resetting of the AFT system during the Cretaceous. This observation conflicts with previous data (from the same Jurassic strata) where complete resetting of the AFT clock during the Cenozoic was suggested. Furthermore, Lower Cretaceous and Palaeogene sediments from the Turpan-Hami Basin show non-reset detrital AFT age populations of ~197, ~135, and ~104 Ma, which are coincident with the main pulses of exhumation recorded in the Chinese North Tianshan. Based on a comprehensive summary of the published data, we argue for a Mesozoic building of the Bogda–Balikun–Harlik mountain chain in the eastern Chinese Tianshan. Subsequent Cenozoic exhumation must have been relatively modest at most (<2 km) as it was not recorded by AFT thermochronology.
The Narusongduo Pb-Zn deposit is located at the northern boundary of the Luobadui-Milashan fault zone (LMF) in central Tibet and is spatially associated with the Linzizong volcanic succession (LVS). Our study indicates that the regional structural setting was formed by two-stage tectonic events. The first stage, spanning from the late Mesozoic to early Paleocene, is characterized by significant N-S crustal shortening associated with the Cordilleran-type orogeny along the Gangdese arc. The region's Paleozoic-Mesozoic metasedimentary rocks were penetratively strained. Locally deformation was largely partitioned along the LMF. During the second stage (ca. 66–55 Ma), the area was affected by extensive multi-stage Linzizong volcanism, including caldera formation, as well as a coaxial N-S propagative deformation with a distinctive lower shortening rate. We recognized two types of mineralization, both were formed during the second stage. The first type of mineralization (orebody III) is governed by fractures within the extensively deformed Paleozoic carbonate rocks at the LMF's footwall. The overlying LVS, however, includes discrete but numerous mineralized sections. The terminal splays of the ore shoots typify the products of hydraulic fracturing. We propose that the propagative compressive deformation drained fluid reservoirs at depth to higher levels via the "Fault valve" effect. Episodic fluid influxes and mineral deposition formed the time-integrated mineralization. The second type of mineralization (orebody I) is hosted in the LVS in a number of breccia pipes and dykes that were controlled by the structural weaknesses generated by the intersection of the radial and ring fractures. Mineralization occurs as veinlets in the matrix and clasts inside the breccias, which are characterized by multi-stage brittle cracking, fluid injection and mineral precipitation. It is interpreted that the multi-stage magmatism (ca. 66–55 Ma) triggered repeated hydrothermal activities and incremental mineralization within the ore-bearing breccia bodies.
The Altai orogenic belt is a main constituent of the Central Asian Orogenic Belt, and serves as a crucial site for studying strain propagation from the Meso-Cenozoic plate margins to the Eurasian interior. The ranges of the Altai Mountains have undergone multiple reactivation events during the Mesozoic and Cenozoic, but the full extent of these events is not yet fully understood. To constrain the thermo-tectonic history of the southern Altai orogenic belt of Northwest China, apatite fission-track (AFT) and apatite (U-Th)/He (AHe) thermochronological methods were used to study 29 pre-Mesozoic basement rocks from several key localities, including the Altay, Xibodu, Fuyun, and Qinghe regions. Late Jurassic to Early Cretaceous AFT and AHe ages were found in the low-elevation Xibodu region, which has been characterized by slow-to-moderate rock cooling since the Jurassic. However, rock samples from all other regions investigated are characterized by comparable Late Cretaceous AFT and AHe ages. Inverse thermal history modeling results reveal moderate-to-rapid upper crustal cooling in the mid- to Late Cretaceous (ca. 110−70 Ma), which is interpreted to be related to distant plate-margin processes (e.g., the Cimmerian collisions). These findings, combined with previously published data, indicate that Late Cretaceous exhumation was widespread in the western Altai orogenic belt, including in the Chinese and Siberian (Russian) parts of the Altai region. As in many other areas of Central Asia, no Cenozoic low-temperature thermochronological signal was detected in this study. We propose that Cenozoic deformation indeed occurred in the southern Altai, but the magnitude of associated denudation was insufficient to have replaced the Cretaceous cooling signals.
The Tibetan Plateau geographically contains internal and external drainage areas based on the distributions of river flows and catchments.The internal and external drainage areas display similar highelevations, while their topographic reliefs are not comparable; the former shows a large low-relief surface, whereas the latter is characterized by relatively high relief.The eastern Lhasa terrane is a key tectonic component of the Tibetan Plateau.It is characterized by high topography and relief, but the thermal history of its basement remains relatively poorly constrained.In this study we report new apatite fission track data from the eastern part of the central Lhasa terrane to constrain the thermo-tectonic evolution of the external drainage area in the southern Tibetan Plateau.Twenty-one new AFT ages and associated thermal history models reveal that the basement underlying the external drainage area in southern Tibet experienced three main phases of rapid cooling in the Cenozoic.The Paleocene-early Eocene ($60-48 Ma) cooling was likely induced by crustal shortening and associated rock exhumation, due to accelerated northward subduction of the NeoTethys oceanic lithosphere.A subsequent cooling pulse lasted from the late Eocene to early Oligocene ($40-28 Ma), possibly due to the thickening and consequential erosion of the Lhasa lithosphere resulted from the continuous northward indentation of the India plate into Eurasia.The most recent rapid cooling event occurred in the middle Miocene-early Pliocene ($16-4 Ma), likely induced by accelerated incision of the Lhasa River and local thrust faulting.Our AFT ages and published low-temperature thermochronological data reveal that the external drainage area experienced younger cooling events compared with the internal drainage area, and that the associated differentiated topographic evolution initiated at ca. 30 Ma.The contributing factors for the formation of the high-relief topography mainly contain active surface uplift, fault activity, and the enhanced incision of the Yarlung River.
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