Rheological and structural inheritance : key parameters for intra-plate deformation. A study based on analogue models

Mountain ranges are impressive tectonic features that characterize the Earth’s surface. Their formation is often associated with regions where two tectonic plates, making up the Earth surface, collide, as in the case of the Himalaya. While the surface is forced to uplift, the displacement of rocks along faults can generate big earthquakes. However, mountain ranges, and the earthquakes associated with their formation, are found also at great distances from plate boundaries, like the Tian Shan Mountains in central Asia. It is important to study these Intraplate mountains to better understand why earthquakes occur is specific regions of plate interiors and their potential hazard. The lithosphere (the rigid Earth’s outer shell) of a continental plate is made of layers consisting of different rock types. Each layer can behave differently (they can fracture or they can flow), depending on its composition, depth and temperature in the lithosphere. The strength of this “layer-cake” structure of the lithosphere is variable in space, with the final results that weak and strong lithospheric domains are found next to each other within a single plate. As a consequence, when the plate is squeezed, because of the interaction with other plates, the boundaries between weak and strong domains are preferred sites for localization of deformation, formation of Intraplate mountain ranges and occurrence of related earthquakes. This research studies the initiation of Intraplate mountains at the boundaries between weak and strong lithospheric domain making use of tectonic analogue models. Analogue models are physical models, where the different layers of the lithosphere are simulated with different materials, like sand and silicon putties. The experiments are built inside a box, and are squeezed by a moving wall, to simulate the contraction of a tectonic plate. The results of the experiments show that when the interior of a continental plate characterized by the presence of a strong region, Intraplate mountains are created around this strong rigid block. Thus, it is more likely to have earthquakes around a strong domain of the lithosphere. An example of this process can be found north of the Himalaya, where the Tian Shan and Tibetan Plateau mountains border the Tarim Basin, a flat region, stronger than the surroundings. Another important aspect of these Intraplate mountains is that they show very different geometries, asymmetric or symmetric in vertical cross sections, linear (like the Pyrenees in northern Spain) or curved (like the Ouachita in southern U.S and Mexico) in plan-view. Different geometries are associated with different arrangements in space of the faults along which slivers of crust are displaced one over another, and as a consequence with a different earthquake hazard distribution. The analogue experiments used in this research show that the shape of Intraplate mountains depends on the rheological stratification of the lithosphere, on the vertical distribution of weak and strong layers and on the obliqueness of strong regions with respect to the direction of shortening.