Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

Abstract

Abstract. During lithospheric extension, localization of\ndeformation often occurs along structural weaknesses inherited from previous\ntectonic phases. Such weaknesses may occur in both the crust and mantle, but\nthe combined effects of these weaknesses on rift evolution remain poorly\nunderstood. Here we present a series of 3D brittle–viscous analogue models\nto test the interaction between differently oriented weaknesses located in\nthe brittle upper crust and/or upper mantle. We find that crustal weaknesses\nusually express first at the surface, with the formation of grabens parallel\nto their orientation; then, structures parallel to the mantle weakness\noverprint them and often become dominant. Furthermore, the direction of\nextension exerts minimal control on rift trends when inherited weaknesses\nare present, which implies that present-day rift orientations are not always\nindicative of past extension directions. We also suggest that multiphase\nextension is not required to explain different structural orientations in\nnatural rift systems. The degree of coupling between the mantle and upper\ncrust affects the relative influence of the crustal and mantle weaknesses:\nlow coupling enhances the influence of crustal weaknesses, whereas high\ncoupling enhances the influence of mantle weaknesses. Such coupling may vary\nover time due to progressive thinning of the lower crustal layer, as well as\ndue to variations in extension velocity. These findings provide a strong\nincentive to reassess the tectonic history of various natural examples.\n