MAJOR CRUSTAL GROWTH EPISODES: MANTLE PLUMES OR CONCENTRATION OF TERRANES BY PLATE TECTONICS?

In Precambrian continental evolution, there are a number of well-documented instances of extremely rapid continental growth within restricted regions and time intervals. Examples are the ~1.9-1.7 Ga crust of the northern hemisphere and the ~0.8-0.6 Ga Arabian-Nubian Shield. There is also a widely-held view that major crustal growth is a globally episodic process. Because of the apparent conundrum of "too much crust, in too short a time, in too small a region", mantle plumes have often been invoked as the ultimate cause of such major episodes of continent generation. On the other hand, the potential of normal plate-tectonic processes to generate the appearance of intense crustal growth has not been properly explored. The central problem is localization of crustal growth into small regions. This can be achieved by transport of approximately coeval terranes parallel to the continental edge in orogenic zones. We deduce the frequent occurrence of this process from an analysis of the proportion of present-day continental margins that are dominated by strike-slip motion, together with the proportion of subduction zones showing obliquity greater than 30°. There is a 16% probability of margin-parallel terrane transport on a scale greater than 400 km, and a few margins show transport on a scale above 1000 km. We contend that along-margin transport and sheafing together of terranes, whether at a continental margin, or at an intra-oceanic arc, can be a very important component of all orogenic events where juvenile crust is generated. The results suggest that concentration, by normal plate-tectonic means, of juvenile arc materials into restricted locations can explain both the apparent episodicity and rapid genesis of Precambrian juvenile provinces.