Calcite twinning strain constraints on the emplacement rate and kinematic pattern of the upper plate of the Heart Mountain Detachment

Two models that address the emplacement rate of the upper plate of the Heart Mountain Detachment (HMD) have been advanced. These are the catastrophic tectonic denudation model and the slow, incremental continuous allochthon model. We compared the two models by analyzing upper and lower plate rocks for a detachment-related overprint of the older, layer-parallel Laramide–Sevier calcite twinning strain fabric. Lower plate Pilgrim Limestone samples, even those collected a few meters below the detachment, show no evidence of a detachment-related twinning strain overprint (negative expected values, NEVs, avg. 8.3%) of the Laramide–Sevier layer-parallel fabric. The allochthonous upper plate carbonate rocks preserve the layer-parallel Laramide–Sevier shortening strain, but the shortening axis (e1) for each block occurs in different, non-EW and both subhorizontal and non-subhorizontal orientations. For the six upper plate blocks that preserve chaotic shortening axis orientations, no evidence of any detachment-related twinning strain overprint (NEVs avg. 7.5%) was found. In the absence of any HMD-related twinning overprint, the upper plate allochthon motion must have been rapid enough to be accommodated by fracturing of upper plate rocks without additionally twinning any calcite. We conclude that the emplacement of the upper plate of the HMD was not accompanied by an overprint of the Laramide–Sevier twinning fabric and that models that require catastrophic, rather than slow, incremental emplacement rates are best supported by these data.