2005 GSA PRESIDENTIAL ADDRESS Tectonic inheritance at a continental margin William A. 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 province (Fig. 2), which (760–650 Ma), pervasive rifting (620–545 and terranes will define additional pierc- have distinct ages and tectonic origins. Ma) along most of the Laurentian mar- ing points for reconstruction of conjugate Fabrics from older cycles of tectonic accre- gin, and late-stage rifting (540–530 Ma) of margins within the assembly of Rodinia. tion may hold clues to the trace of the microcontinents. Later sedimentary burial Contrasts in crustal structure and tec- pre-Rodinia margin, perhaps extending and deformation have obscured the trace tonic history distinguish transform, upper- tectonic inheritance during cyclic assem- of the Iapetan rifted margin, segments of plate-rift, and lower-plate-rift segments bly and breakup of supercontinents back which are dispersed in the Appalachian along the Iapetan rifted margin of Lauren- in time to the Archean cratons. allochthon and Ouachita footwall. Data tia, consistent with a low-angle-detach- Sedimentary deposits in the Grenville from outcrop geology, deep wells, and ment simple-shear mechanism (Thomas, foreland in Ohio and Kentucky (Fig. 2) geophysical surveys provide for palinspas- 1993, Figure 2 therein). Narrow (~25 km) are interpreted to record filling of an tic reconstruction of the rift (e.g., Thomas, zones of transitional crust characterize intracratonic rift system (Drahovzal et 1977, 1991; Cawood et al., 2001). transform faults, which function as verti- al., 1992) or, alternatively, a synorogenic The palinspastically restored Iapetan cal fracture zones to offset the rift and to foreland basin (Santos et al., 2002), possi- rifted margin follows an orthogonally bound domains of opposite dip of the bly a broken foreland. Further resolution zigzag trace defined by northeast-striking detachment (e.g., Lister et al., 1986). A of the structure of the Grenville foreland rift segments offset by northwest-striking wide zone (>200 km) of extended transi- may help to constrain the shape of the transform faults (Figs. 1 and 3). Intersec- tional crust with rotated basement gra- pre-Rodinia rifted margin by analogy tions of rift segments and transform faults ben blocks and thick (>10 km) synrift with younger foreland basins that have frame promontories (convex oceanward) sediment on a lower-plate rifted margin distinct tectonic inheritance from the and embayments1 (concave oceanward) contrasts with a more narrow zone of preceding rifted margin. of the rifted continental margin. The trace transitional crust, general lack of pre- The range of ages and compositions of of the Iapetan Alabama-Oklahoma trans- served synrift sediment, and a post-rift Grenville rocks, variations in Pb isotopic form corresponds to the probable loca- residual thermal uplift on an upper-plate ratios, and tectonic fabrics indicate intra- tion of the large-scale dextral bend in the margin (e.g., Thomas, 1993; Thomas and Grenville sutures, consistent with multiple Grenville front, suggesting tectonic inheri- Astini, 1999). Despite the important con- accreted terranes and conjugate continen- tance from the shape of the Grenville oro- trasts in crustal structure, no clear asso- tal margins within the assembly of Rodinia gen, as well as from the possible dextral ciation with fabrics of Grenville and older (e.g., McLelland et al., 1996; Hatcher et al., offset in the pre-Rodinia rifted margin of rocks indicates tectonic inheritance of 2004). One possible intra-Grenville suture, proto-Laurentia and the northwest-strik- the internal style of the Iapetan rift; how- the New York–Alabama magnetic linea- ing 1350-Ma dike fabric (Fig. 3). Excepting ever, these structures did set the stage for ment (King and Zietz, 1978), has a nearly the Alabama-Oklahoma transform, no tec- pervasive inheritance during the assem- straight trace that may reflect either accre- tonic inheritance from Grenville margins, bly of Pangaea. tion along a straight segment of the pre- sutures, or other fabrics has been recog- Rift-parallel graben systems (Mississippi Grenville rifted margin of proto-Laurentia, nized in the Iapetan rift system. Valley, Birmingham, and Rome; Fig. 3) accretion at a margin already smoothed The trace of the Iapetan rifted margin is indicate Early to early Late Cambrian, late by accreted terranes, or orogen-parallel almost entirely within rocks of the Gren- synrift extension inboard from the rifted slip cutting across the shape of the mar- ville province, leaving a belt of Grenville margin (Thomas, 1991). A dextral offset gin. A Grenville age and Pb isotopic ratios rocks along the eastern margin of Lauren- from the Mississippi Valley graben to the link the basement rocks of the Argentine tia (Fig. 3). Along the Iapetan Alabama- Rome trough, including the transverse Precordillera terrane to Laurentia (Kay et Oklahoma transform, however, granite Rough Creek graben, suggests a transform al., 1996), indicating that rifting during boulders in Ordovician slope deposits offset of the intracratonic graben systems breakup of Rodinia cut across intra-Gren- have ages (Bowring, 1984) that corre- (Thomas, 1993). Any possible relationship ville sutures. spond to the Granite-Rhyolite province, between the intracratonic rift-parallel gra- suggesting that the rift and transform in ben systems and older fabrics is obscure BREAKUP OF RODINIA AND the corner of the Ouachita embayment and presently unrecognized; however, like OPENING OF THE IAPETUS OCEAN cut across the Grenville front (Fig. 2). The the structures of the rifted margin, these Synrift sedimentary and igneous rocks, Iapetan rift evidently cut across intra-Gren- faults provide a mold for tectonic inheri- a post-rift unconformity, and early post-rift ville sutures, for example, leaving some tance by later structures. The Southern sedimentary strata document the breakup isotopically distinct, possibly “non-Lau- Oklahoma fault system parallels the Ala- of Rodinia, the opening of the Iapetus rentian Grenville” rocks of the Blue Ridge bama-Oklahoma transform and extends Ocean, and the isolation of Laurentia by attached to “Laurentian Grenville” rocks into the Laurentian craton from the Oua- Cambrian time (ca. 530 Ma). A range of (Loewy et al., 2002; Hatcher et al., 2004) chita embayment in the rifted margin (Fig.