We report new strain analyses of mechanically twinned calcite in veins hosted by Neogene (13.6-4.3 Ma) sedimentary and volcanic rocks recovered from the Terror Rift system in the southern Ross Sea, Antarctica, by the ANDRILL (ANtarctic geological DRILLing) McMurdo Ice Shelf (MIS) Project. Strain analyses of the ANDRILL MIS AND-1Bdrill core samples yield prolate and oblate ellipsoids with principal shortening and extension strains ranging from -7% to 9%, respectively. The majority of samples show ≤25% negative expected values, indicating homogeneous coaxial strain characterized predominantly by subvertical shortening. We attribute the subvertical shortening strains to mechanical twinning at relatively shallow depths in an Andersonian normal faulting stress regime induced by sedimentary and ice sheet loading of the stratigraphic sequence and characterized by low stress magnitudes. Oriented samples yield a northwest-southeast average extension direction that is subparallel to other indicators of Neogene extension. This northwest-southeast extension is consistent with strain predicted by Neogene orthogonal rifting in a north-northeast-trending rift segment, as well as models of right-lateral transtensional rifting. The overall paucity of a noncoaxial layer-parallel shortening signal in the AND-1B twin populations favors orthogonal extension in the Neogene Terror Rift system, but could also be due to spatial partitioning of strain in a transtensional rift regime.

Geochemical well logs were obtained through sediments at Sites 815, 817, 820, 822, and 823 of Leg 133. Corrections have been applied to the logs to account for variations in hole size, drilling-fluid composition, and drill-pipe attenuation. Oxide and calcium carbonate weight percentages have been calculated from the processed logs and are compared with the available carbonate measurements from core. Log-derived CaCO 3 measurements correlate well with shipboard CaCO 3 core-derived measurements in each of the logged open-hole intervals.

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