NEW LIDAR AND SEA FLOOR SONAR IMAGARY INVALIDATES THE SAN JUAN ISLANDS THRUST SYSTEM

For 40 years, the basic structural framework of the San Juan Islands has been considered to consist of five stacked, extensive, single–plane, low–angle thrust faults, known as the San Juan Thrust System (SJTS). Each thrust sheet was mapped as a single–plane, thrust fault extending over a wide area in the islands and was thought to bound rock terrains from sources hundreds of miles away. However, new LIDAR and seafloor sonar imagery now show that these thrust faults do not exist—the postulated thrust faults of the SJTS are actually segments of much younger, high–angle faults.

Recent LIDAR, sonar, and satellite imagery reveals remarkable geomorphological details not visible by any other means. Faults and other geologic structures are clearly visible on LIDAR and seafloor sonar images. Many near–vertical fault scarps are up to 1,000 feet high and together with the straightness of the faults across topographic relief, indicate that they are high–angle faults, not low–angle thrust sheets. Many of these high–angle faults follow faults mapped as part of the SJTS but depart from them and continue as straight scarps that truncate geologic structures on the seafloor.

Part of the fault mapped as the Rosario thrust fault extends along most of the western shore of San Juan Island and extends to the fault mapped as the Lopez thrust on Lopez Island. LIDAR and sonar imagery indicate that these are part of a long, high–angle, 1000 ft high fault scarp that extends from the NW end of San Juan Island to Deception Pass where it may connect to the Devil’s Mt fault. A high–angle fault on eastern Orcas Island follows the trace of the “Rosario thrust” for several miles before continuing the length of the coastline along a 1000–ft–high fault scarp that continues seaward where it truncates geologic structures.

All five of the faults mapped as the SJTS are actually high-angle faults, not thrust faults. That is not to say that thrust faulting and shearing have not occurred--it simply means that shearing has been distributed throughout the pre-late Cretaceous rocks rather than along postulated single–plane thrust sheets of large displacement. The new LIDAR and sonar imagery invalidates all of the five faults mapped as thrust sheets, which form the basis for the concept of the San Juan Thrust System, making it invalid.