Conjugate rifted margins width and asymmetry: The interplay between lithospheric strength and thermomechanical processes

Abstract

Numerical experiments have been used to relate the range in the distribution and the style of deformation observed in rifted margins to localizing/delocalizing thermomechanical processes. The experiments give rise to four end‐members of margins for varying initial lithospheric strength and extension rates. The first two end‐members are narrow and asymmetric and narrow and near‐symmetric, conjugate margins. The third end‐member is asymmetric conjugate margins, wherein one side is <100 km wide and the other is >100–300 km wide. Lastly, we explore wide rift systems that may form very asymmetric conjugate margins with one narrow margin and a very wide conjugate, 200 km to > 350 km across. With initial and boundary conditions close to that inferred from the North and South Atlantic margins, we find that not all margins experience a polyphase rifting history of stretching‐thinning‐exhumation. Instead, the stretching mode can be very short or protracted, and the thinning or the exhumation modes can be incomplete or absent. The deformation localization of the thinning mode is in places associated with the formation of a keystone block or “block H.” A new mechanism for the formation of the unstable crustal root under block H is described, wherein the bounding border faults lead to differential thinning of the crust and mantle lithosphere. Nonuniform extension also occurs in both types of wide rift systems and is related to the sequential deformation migration outward of an initial graben, associated with effective lithospheric strengthening that occurs during crustal thinning and bending.

Thinning factor distributions viewed through numerical models of continental extension

“…Full explanation of the models can be found in Svartman Dias et al . []. We present a type example in Figure .…”

“…In Svartman Dias et al . [] we explored this effect by systematically changing initial crustal thickness, crustal strength, and the temperature field (thermal age) for numerous models employing the Fast Lagrangian Analysis of Continua method [ Cundall , ; Poliakov et al ., ]. The model results showed that the width and asymmetry of margins is highly dependent on initial conditions: (i) thicker/hotter original crust—with significant ductile lower crustal thickness—leads to wider asymmetric extensional systems and (ii) hotter and/or weaker mantle tends to enhance the asymmetry of these systems.…”

“…Here we revisit kinematic models of rifted margin evolution drawing on recently completed geodynamic numerical results [ Svartman Dias et al ., ]. A key aspect of the models of Svartman Dias et al .…”

“…A key aspect of the models of Svartman Dias et al . [] is that the main stress‐bearing phases, predominantly quartz and plagioclase, allow the ductile, temperature‐dependent portion of the shear strength envelope to intersect the brittle confining pressure‐dependent line for most normal geothermal gradients and a continental crustal thickness over 30 km [e.g., Steckler and ten Brink , ]. In Svartman Dias et al .…”

Structure and origin of the rifted margin of the northern Gulf of Mexico

“…Other factors that influence the style of rifting are the presence of preexisting weaknesses such as large faults or varying *Current address: BHP Billiton, 1500 Post Oak Boulevard, Houston, Texas 77056, USA amounts of decompression melting in the rising asthenosphere (Huismans and Beaumont, 2007;Armitage et al, 2009;Van Avendonk et al, 2009;Svartman Dias et al, 2015). We can improve our understanding of rift evolution if we compare the deep structure of mature rifted margins and consider the geological setting.…”