Continental Rifts: Lithospheric Weakness and Strength Contrasts as Triggers for Necking Instabilities
Abstract:Rifted margin geometry is too complex and diverse to be explained by simple kinematic models. Instead, we consider the effects of strain localization and the growth of necking instabilities as they apply to rifted margins. The intrinsic layering of the lithosphere will affect the growth rate of necking instabilities, leading to depth-dependent extension. In addition, continents are far from homogeneous after multiple cycles of collision, strike-slip motion and rifting. The resulting inherited heterogeneities may serve to localize strain and initiate necking instabilities.
We use 2D finite element models containing embedded finite weak zones in the crust and/or mantle as well as a vertical lithospheric boundary across which lithospheric layering changes resulting in an overall strength contrast. We show that there are two controls on the style of rifting: Control 1, the stiff/pliable nature of the lithospheric layers and; Control 2, the distribution of the background strain rate in the lithosphere. Control 1 depends on the lithospheric rheology, such that necking instabilities grow faster in a stiff, dominantly plastic, layer than in equivalent layers with a pliable, mostly viscous, rheology. Control 2 is important where a strength contrast at a lithospheric boundary influences the distribution of the background strain rate. Necking is a mechanism that amplifies the background strain rate, which implies faster necking in parts of the lithosphere where background strain rates are highest. In a laterally homogeneous lithosphere, the background strain rate will be uniform in each layer and Control 1 will dominate giving necking in stiff layers. However, juxtaposed lithospheres with different strengths will distribute strain giving the weaker lithosphere the higher strain rate, implying the fastest necking may occur under Control 2 in pliable layers with the higher strain rate. An end-member case is where strong lithosphere acts as a rigid block. Here, no necking instability will develop, although inherited weaknesses may be present (Control 2). This has implications for the preservation of cratons, which are cold and strong, and probably stiff. Even though they contain inherited weak heterogeneities, they are protected by Control 2, provided they are surrounded by weakling lithospheres such as younger orogens.