STRAIN REGIME PARTITIONING IN THE CONTINENTAL CRUST: LESSONS FROM NUMERICAL EXPERIMENTS

To unravel the tectono-thermal evolution of a particular region, or that unfolding during a given tectonic process, geologists rely on a combination of geological and geophysical mapping, finite strain and kinematic analyses, metamorphic petrology and geochronology. This heterogeneous, often patchy, dataset is then integrated into conceptual models, usually presented as cartoonistic 2D or 3D time sequences. Physics is the crucial missing ingredient in these conceptual models. A new generation of open-source, community driven, numerical code, provides geologists with the capability to construct physically robust models taking into account diffusion of heat as well as advection of matter under the complex interplay between temperature, strain rates and stress.

A coupled thermal-mechanical experiment applied to the formation of metamorphic core complex is discussed here. This simple numerical experiment documents an example of depth-dependent strain regime in which a strong, crustal scale, contractional strain regime develops in an overall extensional setting. Localization of extension in the upper crust triggers, in the deep hot crust, oppositely verging horizontal flows that converge beneath the extended region. Upon viscous collision, both flowing regions rotate upward to form two upright domes of foliation (double domes) separated by a steep median high-strain zone. Dome material follows a complex depth-dependent strain history, from shearing in the deep crustal channel, to contraction upon viscous collision in the median high-strain zone, to extension upon advection into the shallow crust. This experiment tells us that contractional and extensional fabrics can develop at the same time at different levels in the crust. The spatial partitioning of strain regime is certainly far more common than hitherto recognized.