Middle Proterozoic Belt Basin Syndepositional Faults and Their Influence on Phanerozoic Thrusting and Extension: ABSTRACT

Abstract

During the middle Proterozoic, continental crust of the Belt region was cut by nearly east-west and northwest-striking faults that produced a mosaic of large basement blocks. Blocks that subsided formed the Belt basin and were surrounded mostly by uplifted blocks. The Dillon block bounded the basin on the south along the Perry line, and, together with blocks to the south and west, furnished most of the sediment that filled the basin. Great alluvial aprons sloped basinward from the uplifted blocks down to extensive flats that bordered the Belt intracratonic sea. Sediments were deposited in the deeper parts of the sea by underflows and interflows. The graben blocks, including the Helena embayment and the diagonal block to the northwest, received the thickest sediments.Figure Cretaceous to Paleocene compression thrust the Belt rocks and Phanerozoic cover rocks eastward and northeastward, forming first a western, and then an eastern thrust belt. Thrusts on the blocks formed long sheets that deflected and tore along the block boundaries, where depth to basement and tectonic transport distances changed. Where thrusts crossed northwest-trending basement faults, they ramped locally. Eocene extension produced fault patterns that change from block to block. Differential extension formed right-lateral strike-slip faults across block boundaries. Proterozoic faults that cut the continental crust, not only formed the framework of the Belt basin, but affected patterns of later compression and extension. End_of_Article - Last_Page 1361------------