Paper No. 110-13
Presentation Time: 11:35 AM
WHEN DID CONTINENTAL CRUST FORM AND WHEN WAS IT DESTROYED? THE BRAHMA/SHIVA RATIO AND THE GEOLOGIC RECORD
Continental crust (CC) is created by subduction and destroyed by subduction. We define the ratio of CC creation to destruction (CCC/CCD) as the “Brahma/Shiva ratio” (B/S) after the Hindu gods of creation and destruction. It is important to understand what is the B/S over multiple scales of Earth history. It appears that at least as much CC is destroyed as is created today, but it is unknown if this was always true. The oldest known CC is ~4.0 Ga and the cratonic cores of all continents contain large volumes of Archean CC. One robust proxy for CC production is the distribution of U-Pb zircon ages. Zircon is a strong, hard, dense, refractory, and nearly indestructible mineral. Zircons crystallize when granitic crust solidifies, which happens at all stages of CC formation. Over the ~500 Ma it takes to produce CC, granitic magmas progressively produce more zircon per unit volume (Zircon Fertility Factor of Dickinson, 2008 EPSL), introducing some bias but the abundance of zircons of different ages in global detrital sediments is nevertheless an outstanding indicator for important periods of CC production. U-Pb zircon age compilations (e.g. Condie & Aster 2010 Precam Res) identify major episodes of CC growth ~2.7 and 1.87 Ga and smaller peaks ~1000, 600, and 300 Ma). Condie & Aster conclude that ~33% of extant CC formed during the Archean, ~20% during the Paleoproterozoic, and only ~14% during the last 400 Ma. CC is mostly destroyed by subduction and collision, as a result of upper plate subduction erosion and when it is part of a subducting plate. How to constrain when CC was destroyed by these mechanisms? Ultra-high pressure (UHP) terranes demonstrate that CC descended to >100 km and returned to the surface and thus constrains CC destruction. UHP rocks are only known from the last ~650 Ma of Earth history. Another indicator of CC destruction is termination of continental margins and orogens, as is observed for W. North America during Permo-Triassic time and in E. Asia in Triassic time. The Brahma indicator zircon and two Shiva indicators suggest that CC was produced episodically throughout Earth history and that a viable interpretation of UHP occurrences in time is that this style of CC destruction by subduction only began at ~650 Ma. Lithoprobe profiles of crustal shortening and implicit CC destruction have to be reconciled with these indicators.