Decoupled Upper-Crustal Deformation and Foreland Growth in Intracontinental Orogens: A Thermo-Kinematic Perspective from the South Tianshan, Central Asia

Fold-and-thrust belts commonly exhibit along-strike variability in shortening, rock uplift, and exhumation. It often remains unclear how strongly these reflect lower lithospheric processes. The South Tianshan is an active intracontinental orogen and characterised by southward propagation of deformation into the northern Tarim foreland. Along the South Tianshan–Tarim Basin interface, foreland deformation, cooling histories, and drainage (re-)organization vary strongly along-strike and geophysical constraints indicate the underthrusting of Tarim block northward beneath the Tianshan. This suggests weak lithospheric coupling. To validate this prediction, we examine four transects along the northern margin of the Tarim Basin, by using forward thermo-kinematic modelling of sequentially restored and balanced cross-sections.

Our models quantify the timing, geometry, and magnitude of fault-driven rock uplift across the four transects and we use this to evaluate the role of crustal deformation in driving foreland exhumation, surface uplift and subsequent drainage organization. Existing low-temperature thermochronological datasets, including apatite fission-track (AFT) and apatite (U–Th)/He (AHe) ages, are used to validate model predictions of regional cooling histories.

Best-fit models reproduce the thermochronological data across four transects, demonstrating that fault-driven deformation exerts first-order control on exhumation. At Kuqa, young, reset AHe cooling ages (~25–5 Ma) in the foreland are linked to rock uplift along structural ramps at ~10 km depth as part of subsurface blind duplex formation. The delayed activation (~5–0 Ma) of shallower ramps at 3–7 km depth generated the rock uplift required for the formation of frontal anticlines (Qiulitage at Kuqa and Atushi–Kashi at Kashi) that partially reset the AFT system. Such foreland deformation is largely accommodated above décollements at multiple depths, and the slip is partitioned along thick (~2–4 km) Paleogene (Kuqa) and Miocene (Kashi) evaporites at a minimum of 8–10 km depth. At Keping, motion along a shallower décollement at ~5 km depth, and an additional ~12 km of out-of-sequence thrusting on the Kekebukesansha Fault at ~10 Ma is required to fit observed ages (~11.5 Ma). Replicating older cooling ages preserved in the hinterland (~30 Ma Kashi and ~40 Ma Kuqa) is achieved by ~7–10 km of displacement along the South Tianshan Thrust System (STS) at shortening rates of ~0.5–1.2 mm/yr. This reflects a diachronous initiation of the STS at ~25 Ma and ~36 Ma. The shortening rate at Keping is consistently higher (~4 mm/yr), Kuqa and Kashi exhibit ~0.5mm/yr that increases to ~1.5–2.5 mm/yr. This acceleration is associated with growth of the foreland anticlines at Kuqa and thrusting of the Keketamu structure at Kashi during ~15–10 Ma.

Our results suggest that the along-strike variability in thermochronological signals along the South Tianshan is primarily linked to crustal deformation, without invoking lower lithospheric involvement. Our modelled surface uplift scenarios, furthermore, provide a tectonic framework to explain the transition from transverse to longitudinal drainage patterns, highlighting how fold–and–thrust belt evolution drives drainage reorganization. These findings are consistent with a flat-lying Moho beneath the Tarim, seismicity largely within ~30 km depth, and a stiff lithosphere rheology. Together, these observations support a largely decoupled upper and lower lithosphere along the South Tianshan-Tarim Basin interface.