Dallas H. Abbott, Walter D. Mooney and Jill A. VanTongeren
The character of the Moho and lower crust within Archean cratons and the tectonic implications (in Moho; 100 years after Andrija Mohorovicic )
Tectonophysics(December 2013), 609:690-705
Index Terms/Descriptors: Africa; Archean; Australasia; Australia; basement tectonics; body waves; Canadian Shield; continental crust; continental lithosphere; converted waves; cratons; crust; delamination; elastic waves; gneisses; granites; greenstone belts; igneous rocks; Kaapvaal Craton; lithosphere; lower crust; metamorphic belts; metamorphic rocks; models; Mohorovicic discontinuity; North America; Pilbara Craton; plutonic rocks; Precambrian; receiver functions; seismic waves; Southern Africa; tectonics; terranes; velocity; Western Australia; Yilgarn Craton; Zimbabwe Craton
Latitude & LongitudeN43°00'00'' - N79°00'00'' and W124°00'00'' - W56°00'00''S27°00'00'' - S18°00'00'' and E18°00'00'' - E30°00'00''S35°00'00'' - S14°00'00'' and E113°00'00'' - E129°00'00''S22°30'00'' - S17°00'00'' and E25°00'00'' - E33°00'00''
Undisturbed mid Archean crust (stabilized by 3.0-2.9 Ga) has several characteristics that distinguish it from post Archean crust. Undisturbed mid-Archean crust has a low proportion of internal seismic boundaries (as evidenced by converted phases in seismic receiver functions), lacks high seismic velocities in the lower crust and has a sharp, flat Moho. Most of the seismic data on mid-Archean crust comes from the undisturbed portions of the Kaapvaal and Zimbabwe (Tokwe segment) cratons. Around 67-74% of younger Archean crust (stabilized by 2.8-2.5 Ga) has a sharp, flat Moho. Much of the crust with a sharp, flat Moho also lacks strong internal seismic boundaries, but there is not a one to one correspondence. In cases where its age is known, basaltic lower crust in Archean terranes is often but not always the result of post Archean underplating. Undisturbed mid-Archean cratons are also characterized by lower crustal thicknesses (Archean median range=32-39 km vs. post-Archean average=41 km) and lower crustal seismic velocities. These observations are shown to be distinct from those observed in any modern-day tectonic environment. The data presented here are most consistent with a model in which Archean crust undergoes delamination of dense lithologies at the garnet-in isograd resulting in a flat, sharp Moho reflector and a thinner and more felsic-intermediate crust. We discuss the implications of this model for several outstanding paradoxes of Archean geology.