 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; August 2006; v. 34; no. 8; p. 669–672; doi: 10.1130/G22534.1; 3 figures; Data Repository item 2006136. 669 Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture Eric Tohver* Wilson Teixeira    Instituto de Geocie ˆ ncias, Universidade de Sa ˜ o Paulo, Rua do Lago, 562, 05508-080, Sa ˜ o Paulo, Brazil Ben van der Pluijm Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48104, USA Mauro C. Geraldes Instituto de Geocie ˆncias, Universidade Estadual, de Rio de Janeiro, Rio de Janeiro, 20550-900, Brazil Jorge S. Bettencourt Instituto de Geocie ˆ ncias, Universidade de Sa ˜ o Paulo, Rua do Lago, 562, 05508-080, Sa ˜ o Paulo, Brazil Gilmar Rizzotto Companhia de Pesquisas de Recursos Minerais (CPRM), Porto Velho, Rondo ˆ nia, Brazil ABSTRACT New 40 Ar/ 39 Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- topic age resetting at 1.18–1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasila ˆndia belt between these two cratons, new 40 Ar/ 39 Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40 Ar/ 39 Ar, and Rb/Sr thermochronologic data are used to establish post- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15–30 km) exposed in North America to shallower levels (5– 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- al thickness. This interpretation explains the widespread preservation of both pre- Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- genic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed. Keywords: Grenville Province, Amazon craton, thermochronology, orogenic structure, exhu- mation, asymmetry. Figure 1. Reconstruction ca. 1 Ga with po- sition of transects A-A’ and B-B’ across Grenvillian belts of Laurentia and Amazonia. SZ—shear zone; PC—Paragua craton. INTRODUCTION The Grenville Province of North America is widely recognized as the exhumed root of a segment of a major orogenic belt (Rivers et al., 1989). The overthickened crust that typi- fies orogenic zones has left imprints on the southern and eastern margin of Laurentia, where a long Mesoproterozoic history of ac- cretion of juvenile crust culminated in a con- tinental collision (Mosher, 1998; Rivers and Corrigan, 2000). Evidence for this ca. 1.2–1.0 Ga collision includes seismic images of im- bricated crust (e.g., Forsyth et al., 1994), a burial history marked by mid-crustal (800– 1000 MPa) mineral assemblages exposed at the surface (e.g., Streepey et al., 1997) with a residual 40–45 km of crust locally present, ex- tensive isotopic resetting following the relax- ation of crustal isotherms (e.g., Mezger et al., 1993), and a kinematic history reconstructed from study of mylonitic shear zones (e.g., Da- vidson, 1984). The development of crustal *Corresponding author e-mail: etohver@usp.br. overburden through thrust faulting eventually exceeded crustal strength, leading to the ex- tensional collapse of the Grenville Province (e.g., Culshaw et al., 1991). The prolonged postorogenic phase was marked by wide- spread motion on normal shear zones, with coupled exhumation and erosion reducing crustal thickness (e.g., Cosca et al., 1991). Observations of the exhumed Grenville Province are commonly cited in reference to deep-crustal processes active in younger col- lisional belts; the scale of this ancient belt in- vites comparison to the Himalayan-Tibetan system (e.g., Dewey and Burke, 1973; Wind- ley, 1986). Conversely, the kinematic frame- work of the Himalayan-Tibetan orogen, with its early accretionary history succeeded by a quasi-orthogonal continent-continent colli- sion, has influenced models of Grenville tec- tonics: the implication is that deformation along the 3000 km Grenville Province was caused by individual collisions with three sep- arate continents. These were identified by Dal- ziel (1991) and Hoffman (1991) as Kalahari, Amazonia, and Baltica, although recent evi- dence from Amazonia suggests that transpres- sive motion of the Amazon craton was re- sponsible for the all of the Grenvillian deformation of Laurentia (Tohver et al., 2002, 2004). All models place the Amazon against the central Grenville Province (i.e., Ontario, Quebec, New York) by 1.0 Ga (Fig. 1). The 1.0 Ga Grenvillian link between Ama- zonia and Laurentia permits us to reconstruct the tectonometamorphic history preserved on both plates of a major orogen in order to re- create the ancient belt’s principal structures. We report new 40 Ar/ 39 Ar analyses from a tran- sect of the southwest Amazon craton that are used with recently published data to establish the post-Grenvillian exhumation history for ancestral South America. These data are in- tegrated with the thermochronological record of the North American counterpart to build a crustal model for a restored Grenville belt, with implications for field studies of ancient orogens and evolutionary models of modern mountain belts (e.g., Beaumont et al., 2001).