Thermal consequences of lithospheric extension: Pure and simple

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Abstract
Simple shear and pure shear extension of the lithosphere produce very different patterns of heat flow and topography. These differences are investigated using a numerical technique which solves for two‐dimensional conductive and advective heat transport through time. Simple shear extension of the lithosphere is modeled as occurring along a straight shear zone. Two parameters define the simple shear model: the dip of the shear zone and its width. Likewise, the pure shear model is defined by two variables: the initial width of a vertical zone of pure shear extension and the rate of change of its width. These pairs of parameters are varied between calculations, as is the overall rate of extension. Each model results in distinct patterns of crustal thinning, lithospheric thermal structure, heat flow, thermal uplift, crustal subsidence, and topography. For the simple shear model, extension results in asymmetric uplift across the rift, while the total volume of uplift is limited by the total amount of extension. The peak heat flow and thermal uplift are centered over the intersection of the shear zone with the surface. Isostatic response to simple shear extension results in successive, formerly active shear zones being rotated into listric faults which sole into a subhorizontal detachment. The pure shear results show that the surface heat flow is greater for smaller widths of the zone of extension. For the same overall extension rate, a pure shear model with a narrow zone of extension can result in pressure release melting of the mantle long before low angle simple shear models. These results are compared with topographic and heat flow data from the northern Red Sea rift, a Neogene continental rift which is close to initiating seafloor spreading. The long wavelength topographic asymmetry across the Red Sea, which has been cited as evidence for simple shear extension of the lithosphere, is not matched by any of the models. The observed high heat flow anomalies in the Red Sea require a large component of pure shear lithospheric extension centered under the region of maximum crustal extension. In contrast, at the plate separation rate of the northern Red Sea, simple shear extension of the lithosphere along a shallow (<30°) dip detachment is ineffective in reproducing the observed heat flow anomalies. Only a narrowing region of pure shear extension can satisfy the width of the rift, and the peak heat flow values and generate pressure release melting.