THE EFFECT OF WATER CONTENT on THE STRENGTH OF GABBROIC ROCKS AT HIGH PRESSURE

The rheology of the lower crust controls many important processes important to our understanding of plate scale dynamics. These include, lower crustal seismicity, the long-term support of mountain belts, as well as coupling at the upper-lower crust and crust mantle interfaces. Diabase or plagioclase rheologic properties are commonly used to represent the strength of the lower continental and oceanic crust in geodynamic models. There is a wide range of strengths predicted by extrapolation of published diabase and plagioclase flow laws at continental and oceanic lower crustal P-T conditions dependent on the materials bulk composition (i.e. modal mineralogy) in the case of diabase experiments and on the use of either wet or dry flow laws for both materials. The implications of which flow law is used have a large effect on the resultant predicted strength of the lower crust and therefore the model interpretation.

We have conducted deformation experiments in a Griggs-type solid medium apparatus to investigate the effect of water content on the strength of Maryland diabase at high pressure. Our experiments involved samples that were dry, nominally anhydrous, as-recieved, and on water-added experiments. Our data show a strong dependence of the strength of diabase on very small amounts of water present in plagioclase. This relationship suggests that there is a very small threshold of plagioclase water content above which all experiments have the same strength as the ‘wet’ material.