Vertical and horizontal strain partitioning of the Central Tianshan (NW China): Evidence from structures and 40Ar/39Ar geochronology

As a major tectonic collage within the Central Asia Orogen Belt (CAOB), the Chinese Tianshan provides a natural laboratory for studying structures resulting from the amalgamation of different tectonic units. Detailed field and micro-tectonic observations along two orogen-perpendicular transects identify several structural domains with greatly variable structural styles that were generated at different structural levels but are now juxtaposed. Geological features combined with 40Ar/39Ar geochronologic data suggest the variable structural styles of the domains developed simultaneously during ca. 320-300 Ma. Kinematic reconstruction and deformation temperature estimation, taking 40Ar/39Ar thermochronological data into account, suggest both horizontal and vertical extrusion of material of the Central Tianshan magmatic arc assemblage (CTMA) crust, which is the expression of heterogeneous strain partitioning during orthogonal continental convergence. Such a kinematic scenario is quite compatible with, and satisfactorily interpreted in the light of the results of available analogue deformation experiments and numerical modeling. The structure styles with orthogonal symmetry of both the CTMA and the southern Tianshan fold-and-thrust belt revealed in this study, in conjunction with previously published paleomagnetic data from the Tarim Plate, suggest that the diachronous collision of the Illi-central Tianshan micro-continent with the Tarim Plate may have caused the Tarim block rotation.