NEW UNEQUIVOCAL LIDAR AND SONAR EVIDENCE THAT THE FAULTS OF THE SAN JUAN ISLANDS THRUST SYSTEM ARE HIGH ANGLE, DIPSLIP FAULTS, NOT THRUST SHEETS

For more than 40 years, the basic structural framework of the San Juan Islands has been considered to consist of discreet, single-plane, stacked, thrust faults bringing together far−distant terranes. However, recent LIDAR and SONAR imagery, which reveals remarkable morphological details not visible by any other means, clearly show that all the faults making up the San Juan Island Thrust System are actually high−angle dipslip faults, not thrust faults. The evidence is:

1. LIDAR shows that these faults are straight (or only slightly curved) for many miles and are unaffected by topographic relief, showing that the faults must dip at a high angle.

2. Thrust faults are low−angle faults whose topographic expression is irregular without high scarps, but all the so-called thrust faults have high, nearly vertical, fault scarps, some more than 1000’ high.

3. The faults extend hundreds of feet below sea level where subaerial or shoreline processes are inoperative, so the scarps cannot have been produced by erosion. They must be tectonic fault scarps

4. The ‘thrust faults’ were buried by thousands of feet of Nanaimo sandstone so there is no way they could have been uncovered by erosion as 1000’ scarps.

5. Part of the Haro ‘thrust’ on Orcas Island cuts across Nanaimo sandstone so the scarp must be younger than the age of thrusting.

6. One of the ‘Cretaceous thrust faults’ truncates Chuckanut sandstone so must be post−Eocene in age.

7. SONAR imagery shows a prominent submarine landslide off the Haro ‘thrust’, indicating a post−Pleistocene age of the scarp.

8. Fault scarps at right angles to the direction of Pleistocene ice flow show no evidence of glacial erosion, suggesting that the scarps are post−Pleistocene.

This evidence proves that the faults of the San Juan Island Thrust System cannot be thrust faults and that the concept of discreet, single-plane, stacked, thrust faults bringing together far−distant terranes can no longer be considered valid. The mechanism for emplacing various terranes in the islands needs to be reconsidered.