NONCOAXIAL HORIZONTAL SHORTENING STRAINS PRESERVED IN TWINNED AMYGDULE CALCITE, DSDP HOLE 433, SUIKO SEAMOUNT, NORTHWEST PACIFIC PLATE

Core sections from the lower, basaltic portions of DSDP Hole 433 (tota l depth: 550.5 m) in Suiko seamount contain amygdule and vein fillings of calcite which contain mechanical twins. Analysis of the calcite tw ins reveals the presence of two horizontal shortening strains that are nearly orthogonal to one another; the azimuthal orientation of these strains is only known with respect to stratigraphic top and bottom as the cores are not oriented in any other manner. Maximum shortening str ain magnitudes for the best-developed, positive expected value (PEV) t win lamellae set is -1.7%. For the lesser-developed, negative expected value (NEV) split, the preserved horizontal strain magnitude is -4.3% . Inferred compressive palcostress magnitudes were on the order of 26 MPa. Horizontal differential stresses of this magnitude could be: (1) associated with hotspot plumes; and/or (2) are transmitted to the cent ral portions of thin oceanic plates from distant plate boundaries. Sui ko seamount is composed of Paleocene basalts overlain by lithified mid dle Paleocene limestones, and is part of the Emperor seamount chain on the Pacific plate. The absence of secondary vein calcite in the overl ying sediments suggests that the underlying amygdule calcite is Paleoc ene in age which indicates that the twinned calcite preserves a stress field reorganization (the NEV split) after the middle Paleocene that is oriented 64-degrees from the earlier Paleocene stress field (the PE V split).