RECONSTRUCTING CALIFORNIA AT 90 Ma USING FINITE-ELEMENT PALEOGEOLOGIC MAPS

Finite-element modeling of the kinematic reconstruction of southwestern North America provides an objective basis for paleogeologic maps back to 90 Ma. This modelling is very successful in restoring slip along major strike-slip faults, such as the San Andreas, Hayward, San Gregorio-Hosgri and Nacimiento fault systems. It tends to underfit data in restoration of vertical-axis rotations, detachment faults and thrust faults. This modelling also tends to distort faults and outcrops in non-physical ways. Therefore, manual modifications are needed to provide more realistic paleogeologic maps. Starting with the 6 Ma paleogeologic map, Adobe Illustrator was used to manually modify underfit and distorted map components. Each modification was tabulated for each map to ensure transparency and consistency. Changes to younger maps propagated to all the older maps; examination of older maps occasionally forced reconsideration of changes to younger maps, which thereby had to be iteratively redone. The result is realistic paleogeologic maps at 6 Ma intervals back to 90 Ma. Individual rock masses that have been moved hundreds of kilometers by 90 Ma can be identified by tracing them forward through each map back to 0Ma, and vice versa. These systematically produced paleogeologic maps provide objective criteria for the testing of paleotectonic and paleogeographic models.