Integrated multidisciplinary approach to constrain range fronts structure, a case study of the northern Tianshan piedmont (NW China)

Mountain fronts are key areas where to study the deformation mechanisms and the geodynamic evolution of orogenic belts. Different approaches based on either geological or geophysical data analysis have been proposed. However, in spite of recent theoretical and technical developments, these often remain within a single disciplinary framework and diverging views and models sometimes arise. The front of the northern Tianshan intracontinental collision range is thus quite exemplary for the variety of the tectonic models that have been proposed to explain its development. This paper introduces a multidisciplinary approach combining field geological/structural observations, reflection seismic profile interpretation, borehole results and a gravity study performed in the study area. This approach was conducted in the case study of a representative cross-section within the North Tianshan area, along the Jingou river. An extensive geological/structural survey across the fold and thrust belt was first carried out providing surface constraints on the shallowest parts of the section. Deep structure within the Junggar basin was constrained through processing and interpretation of reflection seismic data together with available borehole results. As it is often the case, the available seismic profiles do not extend to the frontal basement contact zone, or quality of the data was not good enough to allow interpretation. Consequently, a gravity survey and modelling finally allowed interpreting the deep structure of the cover/basement contact. By integrating all the data, the resulting regional-scale cross-section shows a new comprehensive image of the upper crustal structure in this area. In particular, results imply that the mountain basement thrust northwards onto the sedimentary layers with development of a second order fold. Besides, this study further suggests thick-skinned type deformations below the fold and thrust belt. From the example of the Jingou river section, this study illustrates the interest of incorporating gravity data analysis together with – more classical – seismic profiles interpretation and structural analyses for studying orogenic belt frontal areas.