Unraveling the Brittle History of Cratonic Areas Reveals the Profound Mechanical Instability of “Stable” Shields
Abstract:Archean cratons are considered stable regions that have basically remained undeformed since the Precambrian, forming the ancient cores of the continents. While this is certainly true with respect to episodes of thoroughgoing ductile deformation, recent research indicates that shields are not nearly as mechanically stable within the field of environmental conditions leading to brittle deformation. Structural and illite K-Ar geochronological studies on fault gouges point to a significant mechanical instability, wherein large volumes of 'stable' rocks can become saturated with fractures and brittle faults soon after exhumation to below 300-350° C. Indeed, old crystalline basements present compelling evidence of long brittle deformation histories, often complex and thus challenging to unfold.
We use the Svecofennian Shield (SS) as an example of a supposedly 'stable' craton to show that it is possible to unravel the details of brittle histories spanning more than 1.5 Gyr. New structural and geochronological results from Finland are integrated with a review of existing data from Sweden to explore how the effects of far-field stresses are partitioned within a shield, which was growing progressively saturated with fractures as time passed from its initial consolidation. Comparison of time-constrained paleostress data from different locations of the shield shows a remarkably similar stress evolution through time, despite the different local geological boundary conditions. This suggests that the southern SS has behaved as a mechanically coherent block since the Late Mesoproterozoic, time when it had already reached structural maturity with respect to the saturation of brittle features. Structural reactivation rather than generation of new fractures is the key mechanism that has controlled the mechanical evolution of the shield and that will steer its future brittle evolution. Similar brittle histories within different domains of the shield also imply that far-field stresses can propagate over large distances and lead to similar deformational histories, with the local geology only playing a second-order role on the final brittle strain pattern recorded by the rock.