Tarr Inlet Suture Zone

The Tarr Inlet suture zone is the boundary that lies between the Chugach terrane to the west and the Alexander terrane to the east. Regionally, it is part of the Border Ranges fault system, which can be traced for about 1,200 miles from Baranof Island to offshore of Kodiak. The fault zone is partially made up of deformed faulted pieces of the Wrangellia terrane. As the rocks in this zone were exposed to changes in heat and pressure induced by plate motion the changes were recorded as metamorphic fabrics in the rocks.

Geologist have a poor understanding of processes that shape arc-forearc boundaries (the volcanic arc created by subduction, the basin and sediments that fill that basin associated with the subduction zone and the sediment that gets scraped off and accreted during subduction) partially because there are very few on-land exposures. In modern convergent margins this boundary is under water and/or buried beneath sediments. This Border Ranges fault system/Tarr Inlet suture zone is one of the best exposures of an arc-forearc boundary in on earth. The best exposure along its entire length is in the Tarr and John Hopkins inlets. The Border Ranges fault was reactivated during the late Mesozoic and Cenozoic time as a strike slip fault, playing a major role in reshaping Alaska. Although this boundary played an important role in moving the pieces that make up today's Alaska into their present configuration, large segments of the fault are poorly understood.