Paper No. 19
Presentation Time: 8:00 AM-5:00 PM
LITHOLOGIC RELATIONS AND THE STRUCTURAL ARCHITECTURE OF THE WHITE RIVER SHEAR ZONE, NORTH CASCADE MOUNTAINS, WA
The White River Shear Zone (WRSZ) (Magloughlin, 1986), located in the North Cascade Mountains, Washington, forms the contact between the Nason and Mad River terranes. The WRSZ is a dominantly NW-SE striking, NE dipping, tectonized zone of imbricated and folded schists and mylonites. Lineations plunge steeply to the NE implying dip slip movement. Near the WRSZ, the Nason terrane is composed of the Chiwaukum Schist whereas the Mad River Terrane is composed of the Napeequa Schist, metadiorite, tonalitic Tenpeak pluton, a mafic breccia, and ultrabasic bodies up to 2.8 km in length. Various lithologies exist within the WRSZ, some that might not be related to either terrane. Many lithologic contacts are imbricated and intermingled on various scales, especially those involving the Tenpeak pluton. There is also evidence for post-tectonic intrusion of Tenpeak phases, for there are contacts that are sharp and cross-cut the dominant foliation in adjacent rocks. Many contacts are metasomatized. The ultrabasic bodies are typically homogeneous and olivine-rich internally but retrogressed to talc-serpentine schist at the margins. They do not appear to be ophiolite related. Both coarse and fine grained hornblendite is present at or near the contact of the metadiorite and Tenpeak pluton. It is locally mylonitic and cross-cutting relations imply that it is older than the main Tenpeak intrusion. Folds are also present within the WRSZ. Some folds are SW vergent with NW-SE trending hinges. Other hinges plunge NE and could be the same age or younger. Late crenulations fold the dominant foliation. Preliminary conclusions are: 1) Movement along the zone resulted in deformation, imbrication and folding of numerous lithologies. 2) The Tenpeak-metadiorite contacts and local WRSZ trends appear to influence the foliation orientation. 3) The Tenpeak pluton appears to be broadly syn-kinematic, with some lower T deformation likely continuing after intrusion. 4) Hornblendite formed prior to the Tenpeak, either by intrusion or metasomatism at the metadiorite-ultrabasic contacts. 5) Ultrabasic bodies are likely mantle material brought up during movement on the WRSZ or emplaced prior to deformation. 6) Results thus far are consistent with the hypotheses that the WRSZ is a major crustal boundary.