9726256 Wolfe Normal faulting and horizontal crustal extension, not just erosion, ultimately destroy most major mountain belts. Because most such belts have been built by crustal shortening and thickening, some change in the regional geodynamics must occur to cause such a change from horizontal compression to extension. One possibility is that the lower part of the mantle lithosphere, thickened during the phase of compression, becomes unstable and sinks into the mantle beneath the range. When this happens, the range rises, giving it potential energy to drive normal faulting. To test such an idea requires that paleo-altitudes are measured before and after the normal faulting begins. The P.I.s plan to make such measurements using fossil leaf morphology, which has been calibrated to modern climates (and hence altitudes) with an extensive collection of such leaf morphology from modern forests. The Pacific Northwest is an ideal locality for such analysis because the region underwent crustal shortening in Mesozoic time and crustal extension in Cenozoic time and because there are numerous fossil leaf localities, both near sea level and inland where a high mountain belt once lay. ***
