2005 GSA PRESIDENTIAL ADDRESS VOL. 16, No. 2 A PUBLICATION OF THE GEOLOGICAL SOCIETY OF AMERICA FEBRUARY 2006 TTectonicectonic iinheritancenheritance aatt a continentalcontinental marginmargin Inside: 2005 GSA Presidential Address: Tectonic inheritance at a continental margin, William A. Thomas, p. 4 2005 Medals, Awards, Honorary Fellows, and Division Awards, p. 15 Section Meetings Rocky Mountain, p. 18 VOLUME 16, NUMBER 2 FEBRUARY 2006 Cover: Map showing the record of tectonic inheritance through two complete Wilson cycles in eastern North America. See the GSA TODAY publishes news and information for more than 2005 GSA Presidential Address, “Tectonic 20,000 GSA members and subscribing libraries. GSA Today inheritance at a continental margin,” by lead science articles should present the results of exciting new William A. Thomas, p. 4–11. research or summarize and synthesize important problems or issues, and they must be understandable to all in the earth science community. 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Thomas, Department of Geological Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA, [email protected] INTRODUCTION The Grenville front, at the leading edge The pre-Rodinia (pre-Grenville) conti- Forty years ago, the eastern margin of the province, is mapped through gener- nental margin is unknown; however, the of North America inspired Tuzo Wilson ally broad curves along the outcrop in dextral bend of the Grenville front beneath (1966) to ask, “Did the Atlantic close and Canada and southward with decreasing the Gulf Coastal Plain suggests possible then re-open?” The Wilson cycle of clos- resolution in the subsurface, using drill inheritance from a dextral offset in the ing and opening of ocean basins incor- and geophysical data, approximately to older continental margin. Northwest-strik- porates the cyclic assembly and breakup central Tennessee; subsurface data define ing dikes (ca. 1350 Ma) in Oklahoma of supercontinents. Alternate processes of a separate segment in Texas (Figs. 1 and (Denison, 1982) parallel the trend of the extension and compression of continental 2). No data presently are available to locate dextral bend of the Grenville front and margins suggest an important potential for the Grenville front precisely beneath a suggest the possible orientation of the off- tectonic inheritance and overprinting. thick sedimentary cover in the Mississippi set of the older rifted continental margin Now, we recognize a succession of Embayment of the Mesozoic-Cenozoic (Fig. 2). Few other hints are available to two complete Wilson cycles in eastern Gulf Coastal Plain; however, the trace must suggest the trace of the pre-Rodinia rifted North America: closing of an ocean and accommodate a substantial dextral bend margin of cratonic proto-Laurentia. The assembly of the Rodinia supercontinent, between the mapped Grenville (Llano) Grenville front truncates internal tectonic breakup of Rodinia and opening of the front in Texas and the approximately fabrics within, and boundaries between, Iapetus Ocean, closing of Iapetus and located trace in the subsurface in central several older provinces, from the Archean assembly of the Pangaea supercontinent, Tennessee (Fig. 2). Superior province to the 1500–1300-Ma and breakup of Pangaea and opening of the Atlantic Ocean (Fig. 1). Precambrian rocks of cratonic North America indicate less well-defined, earlier cycles. Tectonic inheritance at a range of scales has been recognized in the successive continen- tal margins preserved within the crust of present eastern North America, posing several fundamental questions. Does each episode of supercontinent assembly and breakup adapt to the tectonic framework of a preexisting continental margin and, in turn, leave a mold for the next episode? Is tectonic inheritance through successive Wilson cycles a first-order constraint on the processes through which continental crust is accumulated and continental fab- rics evolve? Does tectonic inheritance in the shallow crustal structures reflect a per- vasive fabric of the deeper lithosphere? ASSEMBLY OF RODINIA (THE GRENVILLE OROGEN) Metamorphic and igneous rocks of the Grenville province, ranging in age from ca. 1350 to 1000 Ma, record closing of an ocean and assembly of the Rodinia superconti- nent (e.g., Hoffman, 1991). The long span of ages suggests multiple events during which multiple elements were swept up Figure 1. Map showing the record of tectonic inheritance through two complete Wilson cycles in and sutured successively to cratonic proto- eastern North America (compiled from Figs. 2–5). Assembly of Rodinia, opening of the Iapetus Laurentia (e.g., McLelland et al., 1996; Ocean, assembly of Pangaea, and opening of the Atlantic Ocean are color-coded on this map and Mosher, 1998). in Figures 2–5. GSA Today: v. 16, no. 2, doi: 10.1130/1052-5173(2006)016<4:TIAACM>2.0.CO;2 4 FEBRUARY 2006, GSA TODAY 2005 GSA PRESIDENTIAL ADDRESS Granite-Rhyolite