KINEMATIC ANALYSIS OF FAULT-SLIP DATA FROM MINING DISTRICTS IN THE TERTIARY CASCADES, OR & WA

Data from slickensided fault surfaces near several hydrothermal mineral deposits in the Tertiary Cascades of Oregon and Washington are being collected to provide data on the evolution of the stress field during the last 40 m.y. of convergent margin magmatism. Such information should provide some understanding of the structural controls of the hydrothermal systems, as well as of their relationship to the emplacement of high-level magmas that typically provided the hydrothermal heat source.

In the North Cascades, plutonic complexes of two ages (33-38 Ma and 17-25 Ma) are each associated with porphyry Cu and related polymetallic vein systems. Sufficient data have been collected to characterize the younger system, including dikes at Cascade and Snoqualmie Passes, mineralized veins at Monte Cristo, Silver Creek, and Middle Fork Snoqualmie River, and slickensided fault surfaces at all sites. The fault-slip data show subhorizontal maximum compressive stress (sigma 1) at N20E, subvertical intermediate stress (sigma 2), and minimum compressive stress (sigma 3) at S70E. Dike and vein data indicate a similar sigma 3 but flipping of sigma 1 and sigma 2.

Fault-slip data from as yet undated mineralized systems in the Western Cascades of central Oregon (epithermal mineralization at Black and Hobart Buttes, and acid-sulfate alteration at Quartz Mtn. east of Roseburg; Rytuba et al, this meeting) indicate significantly different stress axes from that of the early Miocene of the North Cascades (sigma 1=subvertical; sigma 2=subhorizontal S55E; sigma 3=subhorizontal N35E). WNW-striking veins at Black Butte and in the nearby Bohemia district indicate similar stress axes. Bohemia vein mineralization is equivalent in age to the younger plutonic series in the North Cascades, suggesting that extension directions were nearly orthogonal in the two areas in early Miocene time, more nearly so when the roughly 25^o clockwise rotation of the Oregon Cascades indicated by paleomagnetic data is removed. Along-strike (N-S) extension in early Miocene Oregon Cascades contrasts with across-strike (E-W) extension throughout the Quaternary Oregon-Washington Cascade Range.