Lower Crustal Detachment in the Cordillera and Other Hot Backarcs

Abstract:

We summarize the evidence for lower crust horizontal detachment, and discuss where and when it occurs. Relative motion between the crust and upper mantle requires: temperatures hot enough for a very weak lower crust; a driving force, especially high elevation and continent or terrane collision; and a low resistance place for the upper crust to move into, especially a weak foreland thrust belt. It has recently been recognized that most subduction zones have wide well-defined backarc regions with uniformly high temperatures. The temperatures are high enough for a very weak lower crust and we conclude that backarcs are the main areas with the potential for detachment. For North America, the current or thermally-recent hot backarc includes most of the Cordillera. Detachment is common in backarcs based on: (1) Lower crust temperature estimates of 800-900C and laboratory data that indicate a very weak lower crust at such temperatures, 10x18-10x19 Pa s, even lower if some partial melt. In contrast, cratons are cold and strong, 400-500C at the Moho, greater than 10x21 Pa s. (2) Lower crust horizontal seismic reflectors interpreted to indicate current or past horizontal shear, (3) Structural and tectonic evidence that appears to require lower crustal detachment, especially long-distance strain transfer through the upper crust. (4) Outcrop sections of former lower crust that exhibit strong horizontal shearing fabric at over 800C. Large scale structural and tectonic interpretations need to allow for common upper crust transport and deformation in current and former backarcs, independent of the mantle.