Indentation of a continent with a built-in thickness change: experiment and nature

Orogens oblique to the direction of plate convergence are currently attribu ted to obliquity between the margins of one or both of the sutured continen ts to their direction convergence. We use a single analogue experiment and natural examples to illustrate a potential additional factor: variations in strength of the indented continent at a high angle to the convergence dire ction. The wavelengths of structures in laterally shortened lithosphere dep end on the strength of the most competent layers. Lateral variations in cru stal thickness must therefore lead to structures oblique to any applied lat eral compression. An analogue experiment was performed to explore this phenomenon. A two-laye r 'indented continent' was modelled by a brittle upper crust of sand above a lower crust of high-viscosity polymer floating on a single layer of low-v iscosity syrup representing the mantle. The well-known strike-slip structur es allowing lateral escape to distant weak boundaries were hindered by late ral boundaries in front of the indenter. This allowed us to focus on the ef fects of a thickness change built into the 'indented continent' along a zon e parallel to the direction in which a vertical rigid wall advancing at a s teady rate represented the indenter. Vertical escape led to an 'orogenic be lt' oblique to the advancing wall; this obliquity influences subsequent lat eral escape. Model scaling and interpretations are based on Extended Thin S heet Approximation (ETSA) and standard theories of faulting. Four sectors of the Alpine-Himalayan orogen (Iran, Tunisia, the Eastern Alp s and the Himalaya) are oblique to the continental convergence direction, a nd we point to thickness changes at high angles to the suture that may acco unt for this geometry. As crustal thicknesses north of oblique sectors of t he Himalayas are not yet known, we speculate on them. We infer from the main difference between our experiment and all our exampl es chosen from nature that vertical orogenic escape was oblique to our mode l suture but can be parallel to natural sutures. (C) 2000 Elsevier Science B.V. All rights reserved.