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A Conceptual Model of Deformation Near Tertiary Salt Welds

Abstract

Tertiary salt welds are curviplanar surfaces that form when allochthonous salt sheets are evacuated in response to overburden loading and minibasin subsidence. Depending on one’s scale of observation and the thickness and 3-D, lateral continuity of evaporites that remain after evacuation, a weld may be described as complete, incomplete, continuous, discontinuous and apparent. Regardless of the type of weld involved (i.e., primary, secondary, tertiary), the distribution of evaporites along a weld should exert a first-order control on the hydrological behavior and sealing capacity of a weld. Deformation near a weld is likely to exert an additional control, but there are few studies that document such deformation, particularly in association with tertiary welds. To facilitate the interpretation of deformation patterns near tertiary welds, we propose a three-phase conceptual model for weld formation that includes: (1) an early allochthonous salt emplacement phase, (2) an intermediate evacuation and welding phase, and (3) a final, post-welding/reactivation phase. Deformation in the early phase is highly asymmetric and spatially variable. Simple numerical models of the early phase suggest subsalt shear stresses do not exceed 4 MPa and are largely unaffected by subsalt relief. Subsalt normal and shear stress magnitudes increase as the salt advances and stress orientations are perturbed near the toe of the allochthonous sheet. Carapace deposited during this phase is passively transported with the advancing salt, but may locally undergo extension or slumping near the toe of the sheet. Contractional deformation may occur immediately in front of the toe of the sheet. Deformation during the evacuation and welding phase should be concentrated in the supra-salt strata and may exhibit spatial variability that correlates with subsidence patterns and complex, 3-D flow in the thinning salt sheet. Salt dissolution, compaction and the release of mineralizing fluids from the salt or the enveloping rocks may also occlude porosity or locally increase fluid pressure during this time. Subsequent to welding, changes in tectonic setting or stress state may induce slip along an existing weld and induce new deformation. The pattern of this deformation should be symmetric and less spatially heterogeneous than in the early phases of tertiary weld evolution. Any spatial heterogeneity is likely to correlate with the distribution of remnant evaporite and large clasts along the weld.