GRABENS GONE WILD: LATE CENOZOIC EXTENSION ON THE FISH LAKE PLATEAU, UTAH

The Fish Lake Plateau (FLP) forms a distinct topographic and structural block in the transition zone between the Colorado Plateau and Basin & Range of south-central Utah. The FLP is underlain by a thick sequence of Tertiary volcanic rocks that unconformably overlie Cretaceous to Lower Tertiary sedimentary units. Multiple generations of steeply-dipping normal faults cut bedrock units, define lineaments, and greatly influence the topographic character of the FLP. The regional dip of rocks on the FLP may be the result of progressive rotation caused by down-to-the-west slip on the plateau-bounding Thousand Lake and Paunsaguant fault zones. The Thousand Lake fault zone, to the SE of the FLP, is a major structure that juxtaposes mid-Tertiary volcanic rocks in the hanging wall against Jurassic strata, and has a total throw of ~1 km.

Numerous graben and half graben complexes segment the FLP. In the eastern FLP, a suite of en-echelon NNW-trending grabens becomes progressively younger towards the east. Grabens offset Pliocene trachybasalt and basin-fill deposits are overlain by 0.5 to 0.9 Ma boulder diamicts on Fremont River terraces. Fish Lake occupies a large graben complex produced by up to 500 m of displacement across a suite of NE-striking faults. The Fish Lake graben complex cuts older NNW-trending grabens and truncates an established SE-trending drainage network. At its southwestern end, the Fish Lake graben is cut by WNW-striking normal faults. The NE-trending Crater Lakes graben occupies a small, internally drained complex bound by transverse faults. The NE-trending Cedarless Flat graben truncated a pre-existing drainage network and is younger than the 0.5 to 0.9 Ma Fremont River terraces.

Normal faulting on the FLP accommodated 5-10% crustal extension. The variable orientation of grabens on the FLP records orthogonal extension of the region during the last 5 My and may be the manifestation of a uniaxial stress field generated by block-segmentation associated with uplift of the region.