Anisotropy of magnetic susceptibility of the Jingou River section: Implications for late Cenozoic uplift of the Tian Shan

Abstract

[1] Better constraints on the uplift history of Tian Shan will increase our understanding of both mountain building processes and deformation patterns related to the India-Asian collision. Anisotropy of magnetic susceptibility (AMS) holds great potential to track variations in tectonic strain, which is ultimately responsible for mountain uplift. Here we present an AMS record, spanning the interval of 28-4 Ma, from the fluviolucastrine Jingou River section in the Tian Shan foreland. Rock magnetic measurements show that the AMS are dominated by antiferromagnetic hematite and some paramagnetic minerals, both of which have a crystallographic preferred orientation, and therefore the AMS sequence may be applied to reconstruct tectonic strain. Although the principal minimum (K min ) direction is nearly perpendicular to the bedding plane, indicative of a predominantly sedimentary fabric for the section, the tightly grouped principal maximum (K max ) parallel to the fold axis suggests that the section was subject to an embryonic deformation at least since the Late Oligocene. The K max direction is most likely associated with the N-S strain caused by the India-Asian collision. During the intervals of 23.3-20.0 Ma and 16.5-14.0 Ma, the strain markedly increased as evidenced by more tightly grouped K max directions and K min largely distributed within a clear N-S girdle. These two intervals are accompanied by changes in sedimentary facies (higher energy conditions and doubled sedimentation rates), and are interpreted as the periods when the Tian Shan experienced significant uplift. A conceptual model is tentatively proposed to reconcile various timings for the Tian Shan uplift.

Oligocene‐Miocene magnetostratigraphy and magnetic anisotropy of the Baxbulak section from the Pamir‐Tian Shan convergence zone

“…A plot of magnetostratigraphic age versus stratigraphic level (Figure ) suggests mean sediment accumulation rate (SAR) in the Baxbulak section of 8.4 cm/ka, comparable to those of the surrounding sites from the same period [e.g., Huang et al ., ; Charreau et al ., ; Tang et al ., ]. Moreover, the plot shows an increase in SAR, from ∼8.0 cm/ka to 10.2 cm/ka, occurred at 22.5 Ma, roughly coincident with the appearance of highly frequent sandstones within the study section.…”

“…The tilt‐corrected K max are grouped in the direction parallel to the fold axis with a low‐mean inclination (mean declination = 105.5°, mean inclination = 15.0°), while the tilt‐corrected K min show a slight girdle distribution perpendicular to the fold axis. These distribution patterns of the principal axes are also observed in deformed sediments from neighboring foreland basins [e.g., Huang et al ., ; Charreau et al ., ; Tang et al ., ; Lu et al ., ; Yu et al ., ].…”

“…On the study section, for example, the AMS‐inferred strengthening of strain occurred at 26 Ma, ∼4 Ma prior to the increase in sedimentary accumulation rate (Figure ). While this discrepancy could be explained by uncertainties in the dating methods, we highlight the differences in mountain building processes that are recorded by these methods [ Tang et al ., ]. The AMS sequence tracks regional strain history that essentially induces crustal deformation, while thermochronologic evidence generally correlates to rapid exhumation of uplifted mountains.…”

“…On the north of the Pamir indentor, the Tian Shan was reactivated during the Cenozoic as a result of the Indo‐Asian collision [e.g., Tapponnier and Molnar , ; Tang et al ., ] with little or no northward convergence relative to Eurasia since ∼60 Ma [ Huang et al ., ]. South of the western Tian Shan is the Alai Basin, the last vestige of the Mesozoic to early Cenozoic sedimentary basin that formerly connected the Tajik and Tarim Basins [ Şengör , ; Burtman , ].…”

Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China

“…Although the AMS parameters (P j , T, and q) were widely used to reflect changes of tectonic stress in central Asia (e.g., Gilder et al, 2001;Charreau et al, 2005Charreau et al, , 2009Huang et al, 2006;Tang et al, 2012), it is worth to stress that care must be taken when using AMS as tectonic parameters because they are related to the provenance changes, rather than tectonic deformation on our section.…”