ABSTRACT

Surface evidence suggests that the Trench near Cranbrook is generally synclinal with major, west-dipping, normal faults along its east side. The Trench did not form by strike-slip faulting, because Paleozoic and Mesozoic structures trend across the Trench without offset.

Gravity data reveal sediment filled basins extending below sea level beneath the Trench. Because processes of erosion are not known to produce bedrock basins of such depth (1,200 to 5,000 feet), the basins must have a structural origin. Block faulting within the Trench explains the gravity data and the surface data.

Geologic history in the region, deformed rocks of Miocene age within the Trench, seismic activity beneath the Trench, and the evidence of light fill within the Trench all indicate a Cenozoic age for the Trench.

Cenozoic block faulting contrasts with strike-slip faulting and thrust faulting reported for other segments of the Trench. Influence by the margin of an ancient (Precambrian) continent may explain topographic continuity of the Trench (for over 1,000 miles), even though structures in various segments of the Trench differ.