PRELIMINARY ANALYSIS OF GRAVITY AND MAGNETIC DATA IN THE BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON

Mesozoic sedimentary and igneous rocks in the Blue Mountain province (BMP) of NE Oregon record a complex tectonic history of arc-related magmatism, basin formation, and terrane accretion. Recent synthesis of stratigraphic data provides evidence for Late Triassic collision of the Wallowa and Olds Ferry arcs, growth of a large Jurassic marine basin during collision of the BMP superterrane with North America, and Late Jurassic to Early Cretaceous deformation, metamorphism, and emplacement of 145-135 Ma plutons (Dorsey and LaMaskin, 2007, AJS, 307:1167-1193). Many questions remain regarding the crustal structure produced by Mesozoic terrane accretion and younger tectonic and magmatic processes. We seek to answer some of these questions through analysis of existing gravity and magnetic data in NE Oregon and surrounding areas, to be followed by a detailed gravity and magnetic analysis of new data that will be collected in the Izee area in summer 2008. Existing gravity data are widely spaced (~2 data points per 7.5' topographic quadrangle) and do not permit detailed analysis. Magnetic data are from the North American magnetic anomaly grid with line spacing of 2-3 km in NE Oregon. Existing data reveal: (1) the NE-trending Klamath-Blue Mtns lineament (KBL), which truncates Mesozoic terrane boundaries and is truncated by the Cascade volcanic arc; (2) correlation of a large Cretaceous marine basin with a gravity low adjacent to the KBL; (3) strong correlation of gravity lows with the Wallowa, Bald Mtn, and Idaho batholiths; (4) a large irregular gravity low east of John Day that overprints the faulted boundary between the Baker and Izee terranes, possibly indicating a subsurface J-K batholith in that area; and (5) weak correlation of gravity lows with older, Triassic-Jurassic sedimentary basins. Gravity lows associated with felsic intrusions appear to be stronger than and locally overprint lows related to Triassic-Jurassic basins. Gravity highs in the Baker terrane roughly correspond to mafic-ultramafic rocks. Magnetic and gravity data in the Izee area reveal a NNW-trending gravity/magnetic high that suggests a quasi-linear basin structure and/or dense material in underlying rocks of the Baker terrane. New data will be used to test these inferences and image the subsurface extent of the Mesozoic basin in the Izee area.