THE RELATIVE ROLES OF HEAT AND STRAIN RATE DURING CONTINENTAL RUPTURE: INSIGHTS FROM NUMERICAL MODELS

The evolution of strain during continental rupture can be quite complex as it responds to the naturally evolving strength profile of the lithosphere and/or to changes in far-field stresses and thermal conditions. Here we use a 2-d finite element model to examine the relative impact of strain rate versus the initial thermal structure of the lithosphere on the evolution of rift systems. We begin with the geometry of a paleo-convergent zone with a thick crustal welt (42 km thick crust) surrounded by lithosphere with a “normal” crustal thickness (30 km).

Model results indicate that the initial thermal structure of the lithosphere has first-order control on both the geometry and evolution of rifting and subsequent rupture, while stretching rate places a second-order control on the geometry and evolution of rifting. In general, we recognize two end-members of rift geometry, and two end members of rift evolution. The two geometries are rifts those that rupture in the center of the crustal welt, and those that rupture on the margin of the crustal welt. The two rift evolutions are those that display a narrow zone of extension and rupture quickly, and those that display a wide zone of extension, and accommodate significant extension prior to rupture.

We find that cooler upper mantle temperatures (<~ 700°C) are associated with rifts that rupture in the center of the crustal welt, while warmer temperatures in the upper mantle result in rifts that rupture near the margins of the crustal welt. Rifts with thicker initial lithosphere accommodate extension across a narrow region and after minimal amount of extension, while rifts with thin initial lithosphere accommodate extension across a wide region, and accommodate significant extension prior to rupture.

Detailed tracking of the modeled rifts provides key insights to the importance of the interaction between the evolving thermal structure, strength profile, and rift geometry. For example, the West Antarctic Rift System displayed an early stage of wide rifting, and then transitioned to rifting across a narrow region. Numerical simulations of the region suggest that this transition in rifting style was the natural result of the evolving thermal/strength structure of the lithosphere, and no change in plate motions nor impingement of a thermal plume is necessary to explain the strain evolution.