LIDAR AND SONAR IMAGERY OF THE SAN JUAN ISLANDS SPELLS THE DEMISE OF THE SAN JUAN THRUST (NAPPE) SYSTEM

New LIDAR and sonar imagery has revealed remarkable geomorphic details never before seen and not visible by any other means. Numerous faults and other geologic structures are clearly visible on LIDAR and sonar images, including many large fault scarps along the margins of the larger islands and many faults that criss-cross the islands. Sea floor morphology, visible on sonar imagery, shows many submerged faults as long linear scarps with relief up to 300m (1000 ft), some of which visibly truncate geologic structures. The San Juan‒Lopez fault, the largest fault in the islands, extends for at least 65 km (40 mi) from Stuart Island to Rosario strait with a scarp up to 330m (1000 ft) high.

Since 1975, the basic structural framework of the San Juan Islands has been considered to consist of five discreet, stacked, thrust faults, the Rosario, Orcas, Haro, Lopez, and Buck Bay faults, shuffling together far distant terranes constituting the San Juan Thrust (Nappe) System. However, the new LIDAR and sonar imagery shows that most of the mapped extent of these postulated faults are actually segments of high‒angle dipslip faults and are not thrust faults at all.

The age of these faults is not accurately known and more than one period of high‒angle faulting may have occurred. Faults shown on LIDAR images of the surface of the islands appear as visible gashes, etched out by erosion of fault zones with few fault scarps. However, the sea floor faults have bold relief and high scarps. A late Pleistocene moraine lies undisturbed across the San Juan‒Lopez fault.

Thus, the San Juan Thrust (Nappe) System does not exist.