A document by An Yin. Click on the document to view its contents.

HiRISE-based mapping reveals five landform assemblages in western Jezero crater, each defined by a landform association interpretable using Earth-based landsystem models and well-understood Earth analogues. 1) The northwestern assemblage hosts boulder hills rimmed by lobate ridges, mounds and mesas on a valley floor, and valley-bounding ridges superposed by striations (= parallel boulder-bearing ridges and grooves). 2) A trough zone hosts variously shaped depressions, intra-trough islands, linear and curvilinear boulder ridges, and highland strips topped by striated surfaces and rimmed by boulder-bearing ridges. 3) The steep-sided fan-shaped plateau (“western Jezero delta”) hosts mesas, highland-rim boulder ridges, depressions, linear and curvilinear ridges, and a plain superposed by radially trending striations. 4) The crater-margin assemblage hosts a steep-sided ridged and pitted hummocky terrain, a terrace-like capping surface, and mounds surrounded by radially trending boulder ridges. 5) The crater-floor assemblage hosts a polished and striated terrain that displays fold-like and streamlined ridges, hummocky landforms dominated by quasi-circular depressions with raised rims, mesas exposing fold-thrust strata, flat-topped steep-sided ridges with U-shaped map traces, polygonal-grooved plains, and unconsolidated boulder mounds and ridges. Although any aforementioned landform unit could be explained by multiple formative mechanisms, the spatiotemporal relationships mapped in this study within and among the assemblages place stringent constraints for any self-consistent interpretation. A model capable of explaining the mapping results involves northeast-flowing glaciation, ice-sheet collapse with ice-fracture patterns controlling the formation of polygonal grooves via crevasse filling and ice pressing, and minor aeolian modification. In the model, the plateau and crater-margin assemblages were formed by ice-walled subglacial deposition, the trough zone by subglacial flooding, the northwestern and basin-floor assemblages by glacial deformation and deposition, circular depressions with raised rims by melt out and down pressing of spherical dead-ice blocks (i.e., thermal karsts and kettle holes), mesas by kame formation, and striations by glacial fluting.

Current geologic observations support two contrasting views on the evolution of the northeastern Tibet north of the Qaidam basin: (1) crustal shortening started at the onset of the India-Asia collision about 50-60 Ma, and (2) crustal shortening did not start until after 20-15 Ma. To reconcile these two seemingly contradicting views supported both by observations, we perform a series of 2-D thermo-mechanical simulations with the goal of assessing the role of the lithospheric structure and strength of the Qaidam basin in controlling the Tibet deformation history. Our simulations yield three end-member scenarios for the first-order Tibetan lithospheric deformaiton: (1) mantle-lithosphere de-lamination in central Tibet accompanied by a deformation-free northern Tibet margin; (2) northward motion and deformation of southern Tibet accompanied by the southward subduction of the Asian lithosphere below northern Tibet, and (3) northward motion and deformation of southern Tibet accompanied by a deformation-free northern Tibet margin. Due to strong crust-mantle coupling of the Qiadam lithosphere, we suggest that the Tibetan lithospheric deformation is accorded with the third scenario. Our model results also show that the pre-existing weaknesses in northeastern Tibet is activated shortly after the onset of the India-Asia collision. This deformation field stays stable until after the removal of the mantle lithosphere in central Tibet, during which a second wave of northward propagating shortening sweeps across northern Tibet north of the Qaidam basin. This result is consistent with the existing data and reconcile the two end-member views on the tectonic history of the northern Tibet.