QUANTIFYING NEOGENE EXTENSION ACROSS THE OWYHEE PLATEAU OF SW IDAHO: WHERE DID THE BASIN AND RANGE PROVINCE GO?

The Owyhee Plateau of southwestern Idaho appears to be remarkably unextended compared to all adjacent areas of the Great Basin and western and eastern Snake River Plain. It is notable for (1) being the only area along the Yellowstone-Snake River Plain hotspot track that is not apparently extended, and (2) located at the intersection between the distinctive West Snake River Plain graben and the surrounding Basin and Range extensional province. Adjacent parts of the hotspot track are characterized by prominent half-graben filled by silicic and mafic volcanic rocks erupted proximally when the loci of time-transgressive extension and magmatism were coincident - but there is no evidence of this in the Owyhee Plateau in spite of the presence of voluminous silicic volcanism c. 16 - 10 Ma. Our question: Do the Miocene volcanic successions of the Owyhee Plateau simply bury and obscure "classic" Basin and Range structural architecture that is evident in the surrounding region?

Our investigation is focused on documenting the occurrence, lengths, and orientations of extensional faults and putative extensional fracture systems (initially mapped as lineaments) on the Owyhee Plateau using the latest high-resolution NAIP airphoto imagery and NED 1/3 minute DEMs. Then, using the same methodology, we will map adjacent more-extended domains in the Snake River Plain and northern Great Basin to quantify geometric and fault length-throw relationships. Preliminary data suggest that the Owyhee Plateau has not extended homogeneously, and rather its margins transition into the adjacent extensional regime; moreover, some areas display two extensional fabrics and we expect that cross-cutting relationships will be identifiable, thereby providing relative dating of extension outside the Plateau.