Supercontinents, superplumes and continental growth: the Neoproterozoic record

Between 1300 and 51X)Ma the Ncopmtcrozoic supercontincnl Rodinia aggrcgalcd (1300—950Ma). broke up (850—6tNThla) and a new supcrcontincnt, Pannotia—Goodwana. formed (680—550Ma). Only c. 11% of the preserved continental crust was produccd during this 800Ma time interval and most of this cnjst formed as arcs, chiefly continental margin arcs. At least 50% of juvenile continental crust produced between 750 and 5SOMa is in the Arabian—Nubiun Shield and in otherterranes that formed alone the northern bnrjcrof Amazonia and WestAfrica. An additional 20% occurs in Pan-African orogens within Amazonia. and c. 16% in (he Adamastor and West African orogens. The growth rate of continental crust between 1350 and S(X)Ma was similar or less than the average rale of continental growth during the Phanerozoic of I km’/a, and this low rate characterizes both formation and breakup stages of the supercontincots. The low rates of continental growth during the Neoproterozoic may be due to (he absence of a superplume event associated with either Rodinia or Paonotia—Gondwana. If supercontinent bre-up is required to produce a supcrptume event, perhaps by initiating catastrophic collapse of lithospheric slabs at the 660km seismic discontinuity, the absence of a Meso-proterozoic— Neoproterozoic superplume event may mean that a Palaeoproterozoic supereontiocot did not fully breakup prior to aggregation of Rodinia. Although it is now recognized that continental crust has grown rapidly at several periods in the geologic past, the relationship olproduction rate of continen tal crust to the supercontinent cycle is not well understood (Condie 1998, 2000). Is there a change in the growth rate of continental crust during the supercontinent cycle? For instance, do continents grow more rapidly during the formation stage of a supercontinent than they do during the breakup stage? During the aggregation of supercontinents, juvenile ernst, such as arcs, oceanic plateaus and ophiolites, can be trapped in collisional orogens, thus adding mass to the continents. However, during the breakup stage of supercontinents. oceanic ernst forms more rapidly in response to new ocean ridges, and increased mantle plume activity may also produce more oceanic plateaus and flood basalts. This leads to the question of where and in what tec tonic setting does most continental crust originate. Although an unknown but probably small amount of continental crust is added by underplating of mafic magmas from plume sources, it would appear that the bulk of continental growth occurs when juvenile crust is trapped in collisional orogens between col liding cratons, or in peripheral orogens, as oceanic terranes collide with continental margins. Are periods of rapid continental growth associated with superpiume events in the last I Ga as suggested by Condie (1998, 2000) for earlier periods, and if so, what, if any, relationship exists between superplume evenls and supercontinents? Using the Neoproterozoic—Early Phanerozoic supercontinents Rodinia and Gondwana as exam ples, in this chapter questions related to the growth of continental crust and the relationship, if any. between cmstal growth and the supercontinent cycle will be addressed. The relationship of continental growth lo the early rifting and breakup stages of Rodinia (I l00—750Ma) and to the later collisional events that led to formation of the younger supercon tinents, Gondwana and Pannotia (750—550Ma), will also be examined. First of all, it is important to review the configuration of plates in Rodinia, discuss uncertainties in plate location and summar ize the tectonic historyof Neoproterozoic supercon tinents. The Sr isotopic record of seawaterduring the Neoproterozoic as a proxy for supercontinent evolu tion will also be reviewed. Rodinia configuration Although the configuration of plates in Rodinia is becoming better constrained with more precise iso topic ages, the connection between East Gondwana (Antarctica, Australia, India) and Laurentia is still uncertain, as is the position of many of the smaller plates such as North China and Malaysia. The SW US—East Antarctic (SWEAT) reconstruction of East Gondwana proposed by Moores (1991) and Dalziel (1991) places Antarctica adjacent to southwestern Laurentia and Australia adjacent to western Canada. Karlstrom ci at. (1999. 2001) and Burrett & Berry (2000) suggested that Australia satisfies more From YnstttnA, M., Wtsot.Lv, B. F. & DAssuins, S. (eds) 2003. Pn,iemcotc Ecu Gonthtwto: Suprrcvitrinett: Asseothh’ wtdB;rakop. Geological Society, London, Special Publications, 206, 1—21. 0305—8719/03/$15 OThe Geological Society of London.