Restored Transect Across the Exhumed Grenville Orogen of Laurentia and Amazonia, with Implications for Crustal Architecture

Home , Laurentia

Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture

Eric Tohver* ⎤ ⎥ Instituto de Geocieˆncias, Universidade de Saõ Paulo, Rua do Lago, 562, 05508-080, Saõ Paulo, Brazil Wilson Teixeira ⎦ Ben van der Pluijm Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48104, USA Mauro C. Geraldes Instituto de Geocieˆncias, Universidade Estadual, de Rio de Janeiro, Rio de Janeiro, 20550-900, Brazil Jorge S. Bettencourt Instituto de Geocieˆncias, Universidade de Saõ Paulo, Rua do Lago, 562, 05508-080, Saõ Paulo, Brazil Gilmar Rizzotto Companhia de Pesquisas de Recursos Minerais (CPRM), Porto Velho, Rondoˆnia, Brazil

ABSTRACT ziel (1991) and Hoffman (1991) as Kalahari, New 40Ar/39Ar analyses from a transect across the major tectonic units of the southwest Amazonia, and Baltica, although recent evi- Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision dence from Amazonia suggests that transpres- with the Grenville margin of North America. Basement rocks of the Amazon and adjacent sive motion of the Amazon craton was re- Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- sponsible for the all of the Grenvillian topic age resetting at 1.18–1.12 Ga is caused by Grenvillian activation of widespread, deformation of Laurentia (Tohver et al., 2002, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilaˆndia belt 2004). All models place the Amazon against between these two cratons, new 40Ar/39Ar data record cooling through 920 Ma after the the central Grenville Province (i.e., Ontario, granulite facies deformation of this suture zone. Regional cooling rates calculated from Quebec, New York) by 1.0 Ga (Fig. 1). compiled U/Pb, 40Ar/39Ar, and Rb/Sr thermochronologic data are used to establish post- The 1.0 Ga Grenvillian link between Ama- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. zonia and Laurentia permits us to reconstruct Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary the tectonometamorphic history preserved on from uniformly deep levels (15–30 km) exposed in North America to shallower levels (5– both plates of a major orogen in order to re- 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a create the ancient belt’s principal structures. change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus We report new 40Ar/39Ar analyses from a tran- strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- sect of the southwest Amazon craton that are al thickness. This interpretation explains the widespread preservation of both pre- used with recently published data to establish Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more the post-Grenvillian exhumation history for homogeneous array of cooling ages from the North American Grenville Province marking ancestral South America. These data are in- the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- tegrated with the thermochronological record genic architecture from the reconstructed Grenville belt mirrors cross sections proposed of the North American counterpart to build a for modern orogenic belts where deep-crustal rocks are not yet exposed. crustal model for a restored Grenville belt, with implications for field studies of ancient Keywords: Grenville Province, Amazon craton, thermochronology, orogenic structure, exhu- orogens and evolutionary models of modern mation, asymmetry. mountain belts (e.g., Beaumont et al., 2001). INTRODUCTION overburden through thrust faulting eventually The Grenville Province of North America exceeded crustal strength, leading to the ex- is widely recognized as the exhumed root of tensional collapse of the Grenville Province a segment of a major orogenic belt (Rivers et (e.g., Culshaw et al., 1991). The prolonged al., 1989). The overthickened crust that typi- postorogenic phase was marked by wide- fies orogenic zones has left imprints on the spread motion on normal shear zones, with southern and eastern margin of Laurentia, coupled exhumation and erosion reducing where a long Mesoproterozoic history of ac- crustal thickness (e.g., Cosca et al., 1991). cretion of juvenile crust culminated in a con- Observations of the exhumed Grenville tinental collision (Mosher, 1998; Rivers and Province are commonly cited in reference to Corrigan, 2000). Evidence for this ca. 1.2–1.0 deep-crustal processes active in younger col- Ga collision includes seismic images of im- lisional belts; the scale of this ancient belt in- bricated crust (e.g., Forsyth et al., 1994), a vites comparison to the Himalayan-Tibetan burial history marked by mid-crustal (800– system (e.g., Dewey and Burke, 1973; Wind- 1000 MPa) mineral assemblages exposed at ley, 1986). Conversely, the kinematic frame- the surface (e.g., Streepey et al., 1997) with a work of the Himalayan-Tibetan orogen, with residual 40–45 km of crust locally present, ex- its early accretionary history succeeded by a tensive isotopic resetting following the relax- quasi-orthogonal continent-continent colli- ation of crustal isotherms (e.g., Mezger et al., 1993), and a kinematic history reconstructed sion, has influenced models of Grenville tec- from study of mylonitic shear zones (e.g., Da- tonics: the implication is that deformation along the ϳ3000 km Grenville Province was Figure 1. Reconstruction ca. 1 Ga with po- vidson, 1984). The development of crustal sition of transects A-A’ and B-B’ across caused by individual collisions with three sep- Grenvillian belts of Laurentia and Amazonia. *Corresponding author e-mail: [email protected]. arate continents. These were identified by Dal- SZ—shear zone; PC—Paragua craton.

᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; August 2006; v. 34; no. 8; p. 669–672; doi: 10.1130/G22534.1; 3 figures; Data Repository item 2006136. 669 gin of the PC was first described by Litherland et al. (1989), and was marked by calc-alkaline, granitic magmatism emplaced into fold belts bordered by large strike-slip shear zones (San- ta Catalina SZ and San Diablo SZ) with sinistral offsets active ca. 1.08–1.0 Ga (Lither- land et al., 1989; Boger et al., 2005). This record contrasts with the sparse igneous rocks and localized low-grade deformation of the in- tracratonic AB. Aguapeı´ deformation occurred ca. 950 Ma (Geraldes et al., 1997), well after the cessation of deformation in the Sunsas belt. The northern boundary of the PC is de- fined by the ϳ2000 km, east-west–trending NBMB, where upper amphibolite to granulite metamorphism of intercalated marine sediments and mafic rocks record the transpressive suturing of the AC and PC ca. 1.09 Ga (Toh- ver et al., 2004). North of the NBMB suture, the AC basement rocks are chiefly metaig- neous suites that record a protracted history of arc construction and remobilization throughout the Mesoproterozoic (Bettencourt et al., 1999; Payolla et al., 2002). Here, Grenvillian deformation occurred at 1.2–1.12 Ga, result- ing in the JPSZ network of sinistral strike-slip shear zones that extends over hundreds of kilometers.

40Ar/39Ar GEOCHRONOLOGY OF SOUTHWEST AMAZON CRATON Sampling in western Brazil was carried out along a 750 km transect across the major tectonic units exposed in the states of Rondoˆnia and Mato Grosso. Three domains were sampled: the Paleoproterozoic-Mesoproterozoic basement of the AC, including mylonitic rocks of the 1.2–1.15 Ga JPSZ; metasediments of the western exposures of the NBMB; and PC basement rocks, including the AB. Sample locations, argon age spectra, and a complete reference list for Grenville thermochronology data are included in Data Repository Appen- dix A.1 The results of the 40Ar/39Ar analyses from 34 samples can be grouped into three age brackets: (1) pre-Grenvillian ages (1.3–1.55 Ga) observed in the AC and PC basement rocks, including biotite samples in the latter; (2) 1.18–1.12 Ga ages in samples from JPSZ mylonites that crosscut the AC north of the NBMB; and (3) 1.0–0.9 Ga ages from Figure 2. Summary of compiled thermochronologic age data for (A) southwest Amazon amphibolite-grade metabasites intercalated craton (AC; PC is Paragua craton) and (B) Laurentia (data sources in Appendix A; see with NBMB (Fig. 2). While widespread 1.3 footnote). NBMB—Nova Brasilaˆndia metasedimentary belt; NPt.—Neoproterozoic; Pz— Paleozoic, Kt—Cretaceous. Ga hornblende ages in the AC largely reﬂect metamorphism subsequent to igneous crystal- lization, AC biotite ages are all younger than GRENVILLIAN OROGEN OF THE (2) the ca. 0.95 Ga Aguapeı´ belt (AB) internal ca. 1.1 Ga. Both biotite and hornblende ages AMAZON CRATON to the PC in western Brazil; (3) the 1.1 Ga The Grenvillian deformation of the south- Nova Brasilaˆndia metasedimentary belt 1GSA Data Repository item 2006136, Appendix west Amazon craton is observed in four sep- (NBMB) that sutures the Amazon craton (AC) A, sample locations, argon age spectra, and a com- arate features (from south to north): (1) the ca. and PC; and (4) the 1.2–1.15 Ga Ji-Parana´ plete reference list for Grenville thermochronology data, is available online at www.geosociety.org/ 1.08 Ga Sunsas belt and adjacent regions shear zone (JPSZ) network that affected the pubs/ft2006.htm, or on request from editing@ along the southwest margin of the subconti- Amazon basement north of the NBMB (Fig. geosociety.org or Documents Secretary, GSA, P.O. nental Paragua craton (PC) of eastern Bolivia; 1). The Sunsas belt along the southwest mar- Box 9140, Boulder, CO 80301, USA.

670 GEOLOGY, August 2006 from the PC samples correspond closely to published U-Pb zircon ages (Geraldes et al., 2001), suggestive of undisturbed magmatic ages. The 1.18Ϫ1.12 Ga hornblende ages from the JPSZ mylonites record the timing of deformation, not regional cooling, an interpretation supported by feldspar thermometry on recrystallized grains and hornblende ages older than 1.3 Ga from undeformed samples outside the shear zones (Tohver et al., 2005). The youngest ages (ca. 1.0–0.92 Ga) come from the NBMB, exhumed from mid-crustal depths (ϳ800 ЊC at 800 MPa) following the suturing of the Amazon and Paragua cratons ca. 1.09 Figure 3. Top: Calculated paleodepths for 1.0 Ga from observed cooling rates with 30 ؇C/km Ga (Tohver et al., 2004). geotherm. Dashed gray line (moving average) is depth of modern surface at 1.0 Ga (10؋ The ϳ600 m.y. variation in ages observed vertical exaggeration [v.e.]), e.g., rock cooling at 2 ؇C/m.y. through hornblende blocking ؇ at the modern surface of the southwest Ama- temperature of 500 C at 1.1 Ga would be ~10 km from surface by 1.0 Ga. Bottom: Paleo- -depth line superimposed on structure inferred from seismic and field studies (5؋ v.e.) No zon indicates the heterogeneity of Grenvillian tice correlation of paleodepth and thrusting in Laurentia vs. shallow exhumation of south- tectonothermal effects. For example, the inte- west Amazonia tied to predominance of strike-slip faults. NBMB—Nova Brasilaˆndia rior of the PC was at high crustal levels (i.e., metasedimentary belt; AB—Aguapeı´ belt; PC—Paragua craton; AHL—Adirondack High- cooler than the closure temperature of biotite) lands; FLL—Frontenac-Adirondack Lowlands; CMB—Central Metasedimentary Belt; CGB— throughout the late Mesoproterozoic, with de- Central Gneiss Belt. formation confined to the marginal Sunsas and Nova Brasilaˆndia belts. In the AC basement, depths at 1.0 Ga circumvents the problem of bounded domains has been documented as extensive deformation was accommodated by decreasing cooling and/or exhumation rates, part of the extensional collapse of overthick- strike-slip shear zones active at temperatures especially below 300 ЊC (e.g., Streepey et al., ened crust in a late-orogenic to postorogenic of ϳ450–550 ЊC, but rocks (and their pre- 2001). Scatter in the calculated paleodepths period (e.g., Culshaw et al., 1991; Cosca et Grenville geochronological record) outside of using different minerals may reflect variation al., 1991; Mezger et al., 1993; Busch et al., shear zones were unaffected. In contrast, gran- in the actual Tc for individual grains, localized 1997), the contrast with the southwest Ama- ulite facies deformation in the ϳ100-km-wide changes in cooling rates, or deformation after zon data is relevant here, where Grenvillian NBMB signifies deeper exhumation from cooling. However, this scatter should not sig- crustal thickening was limited in both degree mid-crustal levels, with isotopic resetting nificantly affect the relative paleodepth values and geographic occurrence. upon cooling. between tectonic domains. One complication From the uneven exhumation of the re- is presented by nine older Amazon samples stored Grenville orogen emerges a picture of DISCUSSION AND CONCLUSIONS (i.e., age ca. 1.5 Ga or older), where assumed an asymmetric orogenic structure, with a thick Thermochronologic data from U/Pb dating long-lived exhumation results in negative cal- crustal stack in the thrust-dominated foreland of garnet, monazite, allanite, titanite, and ru- culated paleodepths, i.e., above the modern of Laurentia and localized crustal thickening tile; 40Ar/39Ar dating of hornblende, phlogo- surface. In these cases, the paleodepth is nor- in the strike-slip dominated hinterland of the pite, muscovite, and biotite; and Rb-Sr dating malized to zero. southwest Amazon craton. The widespread of biotite from studies of the central Grenville Paleobaths calculated in this manner for the preservation of pre-Grenvillian metamorphic Province and southwest Amazonia were com- southwest Amazon at 1.0 Ga are remarkably ages in the southwest Amazon contrasts with piled with our new results (Fig. 2). The dif- consistent for individual tectonic domains the uniform age resetting ca. 1.1–1.0 Ga of ferent closure temperatures (Tc) of these min- across the sampled transect (Fig. 3). For ex- metamorphic chronometers from the North erals were used to calculate regional cooling ample, the NBMB exhibits the most deeply American Grenville Province. This difference rates for the major tectonic units across the exhumed crust of the southwest Amazon, sup- is interpreted as a result of orogenic structure; transects (Fig. 1; Appendix A [see footnote porting the contention that this belt represents whereas regionally uniform isotherms in the 1]). In the case of deformation ages recovered an exhumed suture zone. The PC and AC thrust-imbricated Laurentian crust reset all from mylonites, the deformation temperature basement rocks on either side of this suture thermochronometers, the effects of predomi- (ϳ450 ЊC; Tohver et al., 2005) is used instead zone were much closer to the surface by 1.0 nantly strike-slip deformation in southwest of the thermal blocking temperature. Using Ga, as evidenced by the absence of thermal Amazonia were more localized. Thus, the a geotherm of 30 ЊC/km from calculated resetting at this time. A single hornblende Laurentian belt records cooling in the post- pressure-temperature (P-T) conditions (Stree- sample from the PC records an age of 990 Ma, orogenic period, in contrast with the mixture pey et al., 1997), the cooling rates yield an possibly reflecting the thermal aureole of the of pre-Grenvillian ages, syncollisional defor- exhumation rate for each tectonic domain, sig- Saõ Domingos granite. In contrast, calculated mation ages, and postorogenic cooling ages nifying that cooling ages from different min- paleobaths of the central Grenville Province of observed in the southwest Amazon. A corol- erals (with different Tc) can be normalized for Laurentia at 1.0 Ga demonstrate that much lary for deciphering Grenvillian events is that a given time to a common dimension, depth. deeper crustal levels are exposed. From north- the southwest Amazon exhibits excellent pres- Clearly, calculated exhumation rates have not west to southeast, the average paleodepth at ervation of syncollisional structures, in con- been constant since the end of the Grenville 1.0 Ga of the Central Gneiss Belt and Central trast with the widespread reactivation of duc- episode; the presence of Cambrian– Metasedimentary Belt is 15–25 km, versus tile structures in the postorogenic extensional Ordovician sediments on top of Grenvillian 10–15 km for rocks of the Frontenac- phase in the Laurentian belt. This reactivation metamorphic rocks in Laurentia and the Neo- Adirondack Lowlands, and 15–30 km for the has not completely obliterated the record of proterozoic–Paleozoic sedimentation in the Adirondack Highlands, consistent with P-T along-strike motion in the central Grenville southwest Amazon craton establish a cut-off estimates (e.g., Streepey et al., 1997). Where- Province documented by the northeast- time for postorogenic exhumation. Calculating as relative motion between these fault- directed thrusting parallel to strike (Hanmer,

GEOLOGY, August 2006 671 1988) or sinistral motion across prominent brian Research, v. 139, p. 121–146, doi: 10.1016/ ville orogeny: Canadian Journal of Earth Scienc- mylonitic shear zones (e.g., Martignole and j.precamres.2005.05.010. es, v. 31, p. 1714–1726. Busch, J.P., Mezger, K., and van der Pluijm, B.A., 1997, Mezger, K., Essene, E.J., van der Pluijm, B.A., and Hal- Pouget, 1994; Busch et al., 1997; Zhao and Suturing and extensional reactivation in the liday, A.N., 1993, U-Pb geochronology of the Martignole, 1997; Martignole and Friedman, Grenville orogen, Canada: Geology, v. 25, Grenville orogen of Ontario and New York: Con- 1998; Streepey et al., 2001; O’Dowd et al., p. 507–510, doi: 10.1130/0091-7613(1997) straints on ancient crustal tectonics: Contributions 2004). 025Ͻ0507:SAERITϾ2.3.CO;2. to Mineralogy and Petrology, v. 114, p. 13–26, Cosca, M.A., Sutter, J.F., and Essene, E.J., 1991, Cool- doi: 10.1007/BF00307862. The broad differences in strain pattern ing and inferred uplift/erosion history of the Mosher, S., 1998, Tectonic evolution of the Southern across the restored Grenville belt may reflect Grenville orogen, Ontario: Constraints from Laurentian Grenville Orogenic Belt: Geologi- the deep crustal profile of other collisional 40Ar/39Ar thermochronology: Tectonics, v. 10, cal Society of America Bulletin, v. 110, belts. A similar orogenic asymmetry charac- p. 959–977. p. 1357–1375, doi: 10.1130/0016-7606(1998) 110Ͻ1357:TEOTSLϾ2.3.CO;2. terizes the Alpine-Pyrenees belt, where con- Culshaw, N.G., Reynolds, P.H., and Check, G., 1991, An 40Ar/39Ar study of post-tectonic cooling and O’Dowd, C.R., Eaton, D., Forsyth, D., and Asmis, vergence precluded extensive strike-slip mo- uplift in the Britt domain of the Grenville Prov- H.W., 2004, Structural fabric of the Central Me- tion (Beaumont et al., 2000). Numerical ince, Ontario: Earth and Planetary Science Let- tasedimentary Belt of southern Ontario, Canada, models of convergent orogens predict a broad ters, v. 105, p. 405–415, doi: 10.1016/0012- from deep seismic profiling: Tectonophysics, 821X(91)90181-G. v. 388, p. 145–159, doi: 10.1016/j.tecto.2004. prowedge marked by late exhumation of high- 07.041. Dalziel, I.W.D., 1991, Pacific margins of Laurentia and Payolla, B.L., Bettencourt, J.S., Kozuch, M., Leite, grade rocks, and a confined retrowedge that East Antarctica–Australia as a conjugate rift pair; W.B., Fetter, A.H., and Van Schmus, W.R., 2002, undergoes early exhumation from a shallower evidence and implications for an Eocambrian su- Geological evolution of the basement rocks in the crustal level (Jamieson et al., 2002), in keep- percontinent: Geology, v. 19, p. 598–601, doi: east-central part of the Rondoˆnia Tin Province, ing with our empirical model of Laurentia and 10.1130/0091-7613(1991)019Ͻ0598:PMO- Ͼ SW Amazonian craton, Brazil: U-Pb and Sm-Nd southwest Amazon, respectively. This agree- LAE 2.3.CO;2. isotopic constraints: Precambrian Research, Davidson, A., 1984, Identification of ductile shear v. 119, p. 141–169, doi: 10.1016/S0301-9268 ment leads us to speculate that the orogenic zones in the southwestern Grenville Province of asymmetry of the Grenville belt reflects slab (02)00121-3. the Canadian Shield, in Kro¨ner, A., and Greiling, Rivers, T., and Corrigan, D., 2000, Convergent margin polarity with hinterland-directed subduction R., eds., Precambrian tectonics illustrated: Stutt- on southeastern Laurentia during the Mesoproter- (i.e., Laurentia under the conjoined Amazon- gart, Verlag, p. 207–235. ozoic: tectonic implications: Canadian Journal of Paragua craton). Another explanation is that Dewey, J.F., and Burke, K., 1973, Tibetan, Variscan and Earth Sciences, v. 37, p. 359–383, doi: 10.1139/ Precambrian basement reactivation: Products of cjes-37-2-3-359. the more uniform strain of Laurentia reflects continental collision: Journal of Geology, v. 81, Rivers, T., Martignole, J., Gower, C.F., and Davidson, its preexisting thermal structure, softened by p. 683–692. C.F., 1989, New tectonic divisions of the Gren- long-lived convergence and magmatism Forsyth, D.A., Milkereit, B., Davidson, A., Hanmer, S., ville Province, southeast Canadian Shield: Tec- throughout the Mesoproterozoic (e.g., Rivers Hutchinson, D.R., Hinze, W.J., and Mereu, R.F., tonics, v. 8, p. 63–84. and Corrigan, 2000). The older model ages of 1994, Seismic images of a tectonic subdivision of Streepey, M.M., Essene, E.J., and van der Pluijm, B.A., the Grenville Orogen beneath Lakes Ontario and 1997, A compilation of thermobarometric data the southwest Amazon craton suggest that the Erie: Canadian Journal of Earth Sciences, v. 31, from the Metasedimentary Belt of the Grenville colder, stronger Amazon lithosphere prevailed p. 229–242. Province: Ontario and New York State: Canadian during the Grenvillian collision, regardless of Geraldes, M.C., Figueiredo, B.R., Tassinari, C.C.G., Mineralogist, v. 35, p. 1237–1247. subduction geometry. and Ebert, H.D., 1997, Middle Proterozoic vein- Streepey, M.M., Johnson, E.L., Mezger, K., and van der hosted gold deposits in the Pontes e Lacerda re- Pluijm, B.A., 2001, The early history of the gion, south-western Amazonian craton, Brazil: In- Carthage-Colton Shear Zone, Grenville Province, ACKNOWLEDGMENTS ternational Geology Review, v. 39, p. 438–448. New York: Journal of Geology, v. 109, This research was supported by National Science Geraldes, M.C., Van Schmus, W.R., Condie, K.C., Bell, p. 479–492, doi: 10.1086/320792. Foundation grants to Tohver (INT-0301807) and van S., Teixeira, W., and Babinski, M., 2001, Prote- Tohver, E., van der Pluijm, B.A., Van der Voo, R., Riz- der Pluijm (EAR-0230059), a Geological Society of rozoic geologic evolution of the SW part of the zotto, G., and Scandolaˆra, J.E., 2002, Paleoge- ography of the Amazon craton at 1.2 Ga: Early America student grant to Tohver, and the Stichting Dr. Amazonian Craton in Mato Grosso state, Brazil: Grenvillian collision with the Llano segment of Shurmannfonds. Teixeira received support from Con- Precambrian Research, v. 111, p. 91–128, doi: Laurentia: Earth and Planetary Science Letters, selho Nacional de Desenvolvimento Cientı´fico e Tec- 10.1016/S0301-9268(01)00158-9. v. 199, p. 185–200, doi: 10.1016/S0012-821 nolo´gico (470373/2004) 0. This work benefited from Hanmer, S., 1988, Ductile thrusting at mid-crustal level, X(02)00561-7. discussions with M. Nyman, K. Mezger, and J. Rahl. southwestern Grenville Province: Canadian Jour- Reviews by J. Martignole, S. Boger, and P. Cawood are Tohver, E., van der Pluijm, B.A., Mezger, K., Scando- nal of Earth Sciences, v. 25, p. 1049–1059. gratefully acknowledged. lara, J.E., and Essene, E.J., 2004, Significance of Hoffman, P.F., 1991, Did the breakout of Laurentia turn the Nova Brasilaˆndia Metasedimentary Belt in Gondwanaland inside out?: Science, v. 252, western Brazil: Redefining the Mesoproterozoic REFERENCES CITED p. 1409–1411. boundary of the Amazon craton: Tectonics, v. 23, Beaumont, C., Munoz, J.A., Hamilton, J., and Fullsack, Jamieson, R.A., Beaumont, C., Nguyen, M.H., and Lee, p. TC6004, doi: 10.1029/2003TC001563. P., 2000, Factors controlling the Alpine evolution B., 2002, Interaction of metamorphism, defor- Tohver, E., van der Pluijm, B.A., Scandolaˆra, J.E., and of the central Pyrenees inferred from a compari- mation and exhumation in large convergent oro- Essene, E.J., 2005, Late Mesoproterozoic defor- son of observations and geodynamical models: gens: Journal of Metamorphic Geology, v. 20, mation of SW Amazonia (Rondoˆnia, Brazil): Journal of Geophysical Research, v. 105, no. B4, p. 9–24. Geochronological and structural evidence for col- p. 8121–8145. Litherland, M., Annells, R.N., Darbyshire, D.P.F., lision with Southern Laurentia: Journal of Geol- Beaumont, C., Jamieson, R.A., Nguyen, M.H., and Lee, Fletcher, C.J.N., Hawkins, M.P., Klinck, B.A., ogy, v. 113, p. 309–323, doi: 10.1086/428807. B., 2001, Himalayan tectonics explained by ex- Mitchell, W.I., O’Conner, E.A., Pitfield, P.E.J., Windley, B.F., 1986, Comparative tectonics of the West- trusion of a low-viscosity crustal channel coupled Power, G., and Webb, B.C., 1989, The Protero- ern Grenville and the Western Himalaya, in to focused surface denudation: Nature, v. 414, zoic of eastern Bolivia and its relationship to the Moore, J.M., et al., eds., The Grenville Province: p. 738–742, doi: 10.1038/414738a. Andean mobile belt: Precambrian Research, Geological Association of Canada Special Paper Bettencourt, J.S., Tosdal, R.M., Leite, W.B., and Pay- v. 43, p. 157–174, doi: 10.1016/0301-9268 31, p. 341–348. olla, B.L., 1999, Mesoproterozoic rapakivi gran- (89)90054-5. Zhao, X., Ji, S., and Martignole, J., 1997, Quartz c-axis ites of the Rondoˆnia Tin Province, southwestern Martignole, J., and Friedman, R., 1998, Geochronolog- fabrics and their tectonic implications for high border of the Amazonian craton, Brazil. I. Recon- ical constraints on the last stages of terrane as- temperature deformation of the Morin granulite naissance U-Pb geochronology and regional sembly in the central part of the Grenville Prov- complex, Grenville Province: Canadian Journal of implications: Precambrian Research, v. 95, ince: Precambrian Research, v. 92, p. 145–164, Earth Sciences, v. 34, p. 819–832. p. 41–67. doi: 10.1016/S0301-9268(98)00072-2. Manuscript received 30 December 2005 Boger, S.D., Raetz, A., Giles, D., Etchart, E., and Fan- Martignole, J., and Pouget, P., 1994, A two-stage em- Revised manuscript received 21 March 2006 ning, C.M., 2005, U-Pb age data from the Sunsas placement for the Cabonga allochthon (central Manuscript accepted 28 March 2006 region of Eastern Bolivia, evidence for the allo- part of the Grenville Province): Evidence for or- chthonous origin of the Paragua block: Precam- thogonal and oblique collision during the Gren- Printed in USA

672 GEOLOGY, August 2006