MAGMATIC AND STRUCTURAL EVOLUTION OF THE TORTUGA OPHIOLITE, SOUTHERN CHILE

The Tortuga ophiolite (TO) forms the southern portion of the Rocas Verdes ophiolites stretching from 50° S to Cape Horn, southern Chile. The TO represents an excellent locality to study the evolution of the boundary between the gabbroic lower oceanic crust and sheeted dikes of the upper oceanic crust. The TO includes a 600 m thick volcanic sequence that grades downward over a few tens of meters into a ~150 m-thick complex of 100% sheeted, aphanitic mafic dikes 0.5 to 1.0 m thick. Locally diabase and gabbro screens separate individual mafic dikes in the basal portion of the sheeted dikes. Down section a 200-500 m-thick unit of fine to coarse-grained, isotropic diabase occupies a transition zone between the overlying sheeted dikes and underlying, coarse-grained gabbros. The top of the diabase sequence includes sheets 1-10 m wide, sub-parallel to the WNW/steep orientation of the overlying sheeted dikes. Down section the diabase intrusions occur as more bulbous bodies (10-100 sq. m). Underlying the diabase sequence is 1500 m of texturally heterogeneous cumulate gabbros consisting of lower olivine gabbro, a middle gabbro, and upper anorthositic gabbro. Diabase and mafic dikes also cut the gabbroic intrusions, and olivine gabbro intrudes gabbro. Amphibolite grade, plastic shear zones cut the olivine gabbros and are in turn cut by mafic dikes with the same orientation as dikes in the overlying sheeted dike complex. WNW-trending faults, breccias and related debris flows within the sheeted dikes and volcanic sequence reflect on-axis tilting. These structures can be kinematically linked to plastic shear zones in the lower gabbros and indicate that amphibolite-grade deformation occurred on axis, before the last dikes were emplaced. We conclude: 1) diabase intrusions reflect individual "melt lenses" that fed the volcanic sequence via melt transfer through the sheeted dikes; and 2) the transition from bulbous to sheet-like geometry of the diabase intrusions may reflect a combination of drastic changes in host-rock rheology with depth (100's of m), changes in the deviatoric stresses at the ridge axis, the increasing influence of magmatic stresses on the bulk stress field in the oceanic middle crust, and/or eruption rate.