Analogue experiments of propagation of oblique rifts

We present the results of analogue models of oblique rifting, which were caried out using a isostatically compensated two-layered small-scale models. The experiments show that structures formed in the brittle layer under oblique extension comprise a series of lenticular, rifted basins, set en echelon along the axial zone of the experiment box. In time, the basins propagate lengthways to merge and form a unified composite rift. However, traces of the initial en echelon rifts, expressed in deeper axial basins separated by shallower threshold zones, are clearly discernible in the mature rifts. Both the basins and the threshold zones are located within the composite rift, so that the rift margins are abrupt and steep along the basins, and more diffuse along the threshold zones. The thinning in the brittle layer along the rift is enhanced by diapirs that ascend from the ductile layer and penetrate the rift floor. Model experiments of nearly orthogonal extension showed a single, unified rift, which differed conspicuously from the composite oblique rifts. The results of the experiments suggest that oblique extension forms en echelon rift segments, the axes of which are perpendicular to the extension direction, separated by threshold zones. Therefore, there is ground to presume that occurrence of a series of alternating basins and threshold zones inside the axial section of a major rift could suggest a regime of oblique tectonic extension. (C) 2000 Elsevier Science B.V. All rights reserved.