Remagnetization in Eastern San Juan Islands, WA, Suggests About $120\deg$ Clockwise Rotation or $35\deg$ Tilt Down to the Northwest Since Cretaceous Long Normal Superchron
Burmester, R (1), Engebretson, D (1), Blake, C (1)
(1) Western Washington Univ. Dept Geology, Bellingham, WA 98225-9080 United States
Abstract:
The Eastern San Juan Islands comprise an arc-related ophiolitic and one or more oceanic terranes. Rocks of the former, including red radiolarian tuffs and quartz-poor chert-rich clastics, have no penetrative fabric or high pressure minerals. Pillow basalts and ribbon cherts of the latter are highly fractured and the overlying quartz-rich metagraywacke is localy folded with a penetrative cleavage. All oceanic rocks have aragonite in veins, indicative of higher pressure conditions that experienced by the ophiolitic ones. Nevertheless, a downward magnetization is common to all, consistent with widespread remagnetization when the terranes were juxtaposed during the Cretaceous long normal superchron. Directions are similar between the two terrane types indicating little relative movement since remagnetization. This magnetization is carried by hematite, e.g., in the red cherts, and magnetite; fluid flow during or after amalgamation likely facilitated remagnetization at low temperature. The catch is that directions are moderately to steeply down to the southeast, far from directions expected with or without northward transport of Baja B.C. Observed and expected directions can be reconciled if the terranes had undergone about $120\deg$ clockwise rotation or $35\deg$ tilt down to the northwest since amalgamation. Structures responsible for terrane juxtaposition and their kinematic indicators need to be reoriented consistent with these results before their attitudes and orientations can be used to deduce geometry and history of terrane amalgamation.
