Vol. 9, No. 10 October 1999 INSIDE • 1999 Honorary Fellows, p. 16 • Awards Nominations, p. 18, 20 • 2000 Section Meetings GSA TODAY North-Central, p. 27 A Publication of the Geological Society of America Rocky Mountain, p. 28 Cordilleran, p. 30 Refining Rodinia: Geologic Evidence for the Australia–Western U.S. connection in the Proterozoic Karl E. Karlstrom, [email protected], Stephen S. Harlan*, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131 Michael L. Williams, Department of Geosciences, University of Massachusetts, Amherst, MA, 01003-5820, [email protected] James McLelland, Department of Geology, Colgate University, Hamilton, NY 13346, [email protected] John W. Geissman, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, [email protected] Karl-Inge Åhäll, Earth Sciences Centre, Göteborg University, Box 460, SE-405 30 Göteborg, Sweden, [email protected] ABSTRACT BALTICA Prior to the Grenvillian continent- continent collision at about 1.0 Ga, the southern margin of Laurentia was a long-lived convergent margin that SWEAT TRANSSCANDINAVIAN extended from Greenland to southern W. GOTHIAM California. The truncation of these 1.8–1.0 Ga orogenic belts in southwest- ern and northeastern Laurentia suggests KETILIDEAN that they once extended farther. We propose that Australia contains the con- tinuation of these belts to the southwest LABRADORIAN and that Baltica was the continuation to the northeast. The combined orogenic LAURENTIA system was comparable in length to the AUSWUS Nd MODEL AGE modern American Cordilleran or 1.0 - 1.3 Ga Alpine-Himalayan systems. This plate ARUNTA MI 1.3 - 1.5 Ga MUSGRAVE 1.6 - 1.8 Ga YAVAPAI ENVILLE reconstruction of the Proterozoic super- J R Ga G 1.8 - 2.0 Ga AUSTRALIA L 1.5 ZA continent Rodinia called AUSWUS AT - 2.0 - 2.3 Ga BH AZ 1.3 (Australia–Southwest U.S.) differs from M > 2.5 Ga the well-known SWEAT (Southwest Massive Sulfides U.S.–East Antarctic) reconstruction in Rift Margins that Australia, rather than northern OAXACA 1000 Km Canada, is adjacent to the southwestern ANTARCTICA Thrust Faults United States. The AUSWUS reconstruc- tion is supported by a distinctive CRUSTAL AGE “fingerprint” of geologic similarities and tectonic histories between Australia Figure 1. AUSWUS reconstruction for 1.7 to 0.8 Ga, modified from Brookfield (1993). The Tasman line and the southwestern United States forms the eastern edge of Proterozoic Australia (Myers et al., 1996); the 87Sr/86Sr = 0.706 line marks the from 1.8 to 0.8 Ga, and by a better west edge of Proterozoic Laurentia. Continents were rotated to this configuration about an Euler pole agreement between 1.45 and 1.0 Ga located at 51.46°N 106.70°E, rotation angle 114.33°. Both continents appear in equal-area projection in North American coordinates. The position of Australia in the SWEAT reconstruction is shown for compar- paleomagnetic poles for Australia and ison (from Moores, 1991). Crustal age provinces inferred from Nd data. Massive sulfide deposits of Bro- Laurentia. ken Hill (BH) are to similar deposits in Jerome (J) in central Arizona and Mount Isa (MI) is across from the Carlin area of Nevada. INTRODUCTION chemistry, and long-lived mantle convec- continents, and later modifications to Many recent papers have concluded tion patterns (Dalziel, 1997; Hoffman et Precambrian plate margins during the that a supercontinent called Rodinia al., 1998; Evans, 1998). However, the dura- Phanerozoic. Thus, the implications of existed in the Neoproterozoic between 1.0 tion and configuration, and even the exis- possible pre-Pangean supercontinents for and 0.8 Ga. There is speculation that the tence (Piper and Zhang, 1999), of this late the evolution of Earth systems in general breakup of Rodinia may have been related Proterozoic supercontinent remain uncer- are difficult to evaluate. to dramatic changes in Earth systems such tain. Detailed reconstructions are hindered One approach to supercontinent as diversification of life, multiple low- by absence of a sea-floor record, lack of reconstructions is to try to match rifted latitude glaciations, fluctuating ocean sufficient geochronologic information to margins of a given age. Reconstructions of show synchroneity of supercontinent the Mesozoic supercontinent of Pangea *Present address: P.O. Box 25046, MS 980, assembly and breakup, lack of high- account for the shape and length of rift Federal Center, Denver, CO 80225, quality apparent polar wander paths [email protected] (APW) for Precambrian rocks of many Refining Rodinia continued on p. 2 IN THIS ISSUE GSA TODAY October Vol. 9, No. 10 1999 Refining Rodinia: Geologic Evidence Call for Nominations for 2000 GSA TODAY (ISSN 1052-5173) is published monthly for the Australia–Western U.S. GSA Distinguished Service Award ....... 22 by The Geological Society of America, Inc., with offices at 3300 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box connection in the Proterozoic ...... 1 GSA Division and Section Grants 1999 . 23 9140, Boulder, CO 80301-9140, U.S.A. Periodicals postage Memorial Preprints . .......... 2 paid at Boulder, Colorado, and at additional mailing offices. Call for Nominations—2000 John C. 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