PHASE-CHANGE AND THERMAL SUBSIDENCE OF THE WILLISTON BASIN

A mechanism of subsidence for the Williston basin is proposed and test ed. The mechanism combines cooling and thermal contraction of the lith osphere with a phase transformation in the lower crust. The cooling an d the phase boundary movement following a sudden change in the thermal boundary condition at the base of the lithosphere are calculated; iso static adjustments and sea-level variations are included to determine subsidence and sediment accumulation at the basin's centre, which are compared with the data. The subsidence record could not be explained b y temperature boundary conditions. The calculations show that: (1) the heat-flow boundary condition explains the long duration (more than 35 0 Myrs) of subsidence of the Williston basin and (2) the delay of the phase-change subsidence (about 40 Myrs) explains an acceleration of su bsidence in the early stages of the basin's evolution. When isostatic adjustments and the effect of sea-level variations are included, the c alculated sediment accumulation history fits well the sediment record of the Williston basin.