Is past deformation of continental interiors a key to understanding modern intraplate seismicity?
Abstract:A fundamental problem for research into modern intraplate seismicity is the short time span of available observations. The geological record holds many examples of large (>106 km2) intraplate areas affected by distributed extension or contraction of low magnitude. Typically, the deformation has remained active over a few tens of millions of years and bulk strains accommodated are a few tens of kilometers. Judging from probable, active counterparts, this deformation was very likely associated with seismicity.
We discuss the case of Late Cretaceous and Palaeogene contractional deformation in Europe that created a variety of structures from the British Isles to Poland. Many of these structures are due to inversion of basins and grabens that originated in an earlier phase of distributed extension, but they coexist with large basement-cored uplifts and fault-propagation folds resembling the Laramide structures of Western North America and the modern Sierras Pampeanas in Northern Argentina. The entire fault system is strongly segmented and comprises NW-striking faults linked by N-striking transfer structures. Continuous fault strands rarely exceed a few tens of km in length. Shortening rates are 0.5 to 1.5 mm/yr both for order-of-magnitude estimates of the entire belt and well-constrained local structures, possibly suggesting an episodic, clustered deformation pattern of these structures similar to modern intraplate seismicity. The main phase of intraplate shortening between 90 and 70 Ma can be correlated with a relatively uniform regional stress field reflecting the convergence of Africa, Iberia and Europe. The close juxtaposition of deforming and non-deforming areas thus suggests a control by localized mechanical weakness rather than particularly high stress. On a local scale, this inference is borne out by the frequent reactivation of normal faults.
Our Mesozoic-Early Paleogene example is probably a suitable model for some, but not all current seismogenic intraplate deformation. The prevalence of large structures in this case, in the Laramides and the Sierras Pampeanas raises the challenging question if regions of active intraplate seismicity that lack such structures are similar phenomena at an immature stage or represent a different phenomenon altogether.