Influence of zones of pre-existing crustal weakness on strain localization and partitioning during rifting: Insights from analogue modeling using high resolution 3D digital image correlation

The factors controlling the selective reactivation of pre-existing crustal structures and strain localization process in natural rifts have been studied for decades but remain poorly understood. We present the results of surface strain analysis of a series of analogue rifting experiments designed to test the influence of the size, orientation, depth, and geometry of pre-existing crustal weak zones on strain localization and partitioning. We apply distributed basal extension to crustal-scale models that consist of a silicone weak zone embedded in a quartz sand layer. We vary the size and orientation (θ-angle) of the weak zone with respect to the extension direction, reduce the thickness of the sand layer to simulate a shallow weak zone, and vary the geometry of the weak zone to reflect a range of anticlinal, either linear or curvilinear natural weak zone geometries. Our results show that at higher θ-angle (≤ 60o) both small- and large-scale weak zones localize strain into graben-bounding (oblique-) normal faults. At lower θ-angle (≤ 45o), small-scale weak zones do not localize strain effectively, unless they are shallow. We observe diffuse, second-order strike-slip internal graben structures, which are conjugate and antithetic under orthogonal and oblique extension, respectively. In general, the changing nature of the rift faults (from discrete fault planes to diffuse fault zones, from normal to oblique and strike-slip) highlights the sensitivity of rift architecture to the orientation, size, depth, and geometry of pre-existing weak zones. Our generic models are comparable to observations from many natural rift systems like the northern North Sea and East Africa, and thus have implications for understanding the role of structural inheritance in rift basins globally.

Constitutive Gradients and Pore Tolerance in Two Weldments

The mechanical constitutive behavior of welded components is difficult to characterize at any size scale, but particularly challenging in the case of sub-millimeter welds such as produced by laser welding. Recently emerging digital image correlation (DIC) techniques provide a means for extracting local constitutive response. The tensile stress-strain response of laser welded austenitic 304L is evaluated in the present study. Weld fusion zones were found to have slightly higher yield strengths than the corresponding basemetal, and retained significant ductility (~45%). In this ductile, flaw tolerant stainless steel, the local weld ductility was essentially unaffected by the presence of weld-root porosity which was induced only by pulsed laser welding. In contrast, in a parallel study on the local constitutive properties of a welded Ni-Gd-Mo-Gd alloy with much less macroscopic ductility, the presence of weld root porosity was found to localize strain only in the vicinity of the largest pores, causing premature failure at low macroscopic strains.