DYNAMICS OF THE EMPLACEMENT OF THE HEART MOUNTAIN ALLOCHTHON AT WHITE MOUNTAIN: CONSTRAINTS FROM CALCITE TWIN STRAIN, ANHYSTERETIC MANETIC SUSCEPTIBILITY, AND THERMODYNAMIC CALCULATIONS

White Mountain is so named because the allochthonous Paleozoic rocks are marbles that rest upon the bedding plane portion of the Heart Mountain Detachment (HMD). These are the only widespread thermally metamorphosed rocks in the HMD upper plate, and the detachment exposed at White Mountain preserves among the thickest carbonate "pseudotachylite" (3 meters) anywhere in the HMD system. Thermal metamorphism of upper plate rocks occurred before the emplacement of the upper plate.

Calcite twin analysis of lower plate limestones preserve a layer-parallel, E-W pre-detachment shortening strain with no HMD twinning strain overprint. Allochthonous upper plate marbles preserve a layer-parallel shortening strain (203°) that requires a minimum 113° counterclockwise rotation during emplacement. Basal detachment striations here trend 154°. The upper plate marbles preserve an unusual strain overprint (high NEVs) that is a vertical shortening fabric, presumably of lithostatic origin. Footwall detachment-parallel calcite veins record a similar vertical shortening strain. Continuous samples through the 3 meters of pseudotachylite were analyzed using AMS techniques (n=35) as a proxy for flow during emplacement of the upper plate. The contoured Kmax maxima is in the plane of the detachment striations (154°) at an angle of 40° to the detachment. The pseudotachylite also was demagnetized using AF and thermal techniques, and both methods record a paleo-pole that is westerly and downward (290°, 30°), not southerly and upward (0°, 45°) as would be expected for this portion of the Eocene.

Thermodynamic and mechanical calculations based on the frictional melting of calcite and subsequent generation of pseudotachylite, and the geometrical characteristics of White Mountain zone indicates an upper plate emplacement rate of ~280 m/sec at White Mountain. The duration of the emplacement event was less than four minutes, which was too short of time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. During this time, White Mountain was rotated, and translated to its present position. Following emplacement. The vertical calcite twin strain overprint was developed owing to burial by volcanic rocks following emplacement of the upper plate.