Unzipping the Patagonian Andes Long-lived influence of rifting history on foreland basin evolution

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1 SHORT RESEARCH Unzipping the Patagonian Andes Long-lived influence of rifting history on foreland basin evolution Matthew A. Malkowski, Marty Grove, and Stephan A. Graham DEPARTMENT OF GEOLOGICAL SCIENCES, STANFORD UNIVERSITY, STANFORD, CALIFORNIA 930, USA ABSTRACT The Andean Cordillera is widely considered to be one of the type examples of a convergent margin setting. In the southernmost Andes, however, rifting and volcanism predated mid-cretaceous breakup of Gondwana and formation of the South Atlantic Ocean by up to 0 m.y. and culminated in the opening of the Rocas Verdes backarc basin east of the Mesozoic Patagonian Batholith. We present new U-Pb geochronology from the Austral sector (9 S 0 S) that indicates rift volcanism occurred between 1 and 17 Ma near the northern terminus of the basin. Available data and observations from the southern Rocas Verdes Basin indicate larger-magnitude and longer-duration extension compared to the northern basin region. The Rocas Verdes Basin underwent progressive northward propagation and opening and was later backfilled concomitantly with the opening of the southern Atlantic Ocean by north-to-south deposition within a retroarc foreland setting. The influence of the inherited tectonic fabric of the Rocas Verdes backarc basin on the subsequent foreland basin explains many unique characteristics of the Patagonian Andes, such as a protracted deep basin that formed atop the previously rifted and weakened crust. Moreover, the early rift history helps account for intraplate deformation of southernmost South America during the opening of the South Atlantic Ocean. LITHOSPHERE; v. ; no. 1; p. 3 ; GSA Data Repository Item Published online October 01 doi:.1130/l9.1 INTRODUCTION The southernmost Andes preserve a nearly complete stratigraphic record of a tectonic transition from a backarc rift basin (Rocas Verdes Basin) to a foreland basin (Magallanes Austral Basin; Dalziel et al., 197; Wilson, 1991; Fildani and Hessler, 00). In southern Patagonia, the Jurassic earliest Cretaceous backarc extension was sufficient to form a fully developed ocean basin >0 km wide (Stern and de Wit, 003). The ensuing transition from rifted ocean basin to a contractional foreland fold-and-thrust belt has been well documented in the Ultima Esperanza sector of the basin (Fig. 1; e.g., Wilson, 1991; Fildani and Hessler, 00). However, along-strike variations in the relative timing of these tectonic phases remain poorly constrained and a subject of debate. Previous workers have suggested a northward diachronous opening of the Rocas Verdes Basin on the basis of variations in the geochemistry and geochronology of obducted ophiolitic complexes (Stern et al., 199; Mukasa and Dalziel, 199; Stern and de Wit, 003). In contrast, some workers have interpreted studies of rift volcanism south of ~1 S to indicate that the basin opened simultaneously along its length (e.g., Klepeis et al., 0). Resolving this discrepancy is important for understanding how spatial variations in tectonic evolution relate to sedimentary basin evolution, including sediment dispersal, depositional facies, and sediment accommodation (e.g., Romans et al., 0; Fosdick et al., 01; Malkowski et al., 01). On a plate scale, constraining the tectonic evolution of the Rocas Verdes Basin prior to opening of the South Atlantic Ocean may help account for kinematic gaps and/or overlaps between southernmost South America and Africa and recent attempts to reconstruct the paleogeography of Gondwana (e.g., Torsvik et al., 009; Moulin et al., 0). We present new field observations and geochronologic data that document the onset of rift volcanism in a segment of the Rocas Verdes Basin that was previously unstudied for this purpose. Our results are most consistent with a northwardpropagating onset of rift volcanism for the entire basin. This trend was reversed during subsequent basin inversion and formation of the successor foreland basin. In this model, the southern end of the Rocas Verdes Basin was characterized by a deeper basin floored by oceanic crust. The Rocas Verdes Basin narrowed northward and was more transitional in oceanic character. The northern basin filled progressively from north to south as defined by the onset of coarse clastic deposition associated with the successor foreland basin phase. Overall, this work highlights the longlived spatial and temporal influence that features inherited from past tectonic episodes can have on successive phases of tectonic and basin evolution. JURASSIC EARLY CRETACEOUS RIFTING Regional crustal extension initiated as large volumes of plume-related silicic magmatism referred to as the Chon Aike silicic large igneous province (e.g., Pankhurst et al., 199, 000). This widespread event is recorded by metaluminous assemblages of felsic volcanic and volcaniclastic rocks, which are present throughout the Patagonia region and along the Antarctic Peninsula (Fig. 1; Pankhurst et al., 199). In southern Patagonia, these rocks are referred to as the El Quemado complex and Tobífera Formation (Fig. ). Ages of volcanic rocks associated with the Chon Aike large igneous province range from ca. 190 to 10 Ma (Pankhurst et al., 000). South of 0 S, crustal extension continued at least until 139 Ma (Stern et al., 199), possibly as late as 11 Ma (Calderón et al., 013), and resulted in generation of oceanic crust and the Rocas Verdes marginal basin (Dalziel et al., 197). Obducted Jurassic ophiolitic complexes are exposed south of 1 S and include the Sarmiento, Capitán Aracena, Carlos III, and Tortuga ophiolites (Fig. 1). Geochemical and compositional differences between these complexes suggest that there was a greater degree of crustal extension in the southern end of the basin (de Wit and Stern, 191; Calderón et al., 013). The Sarmiento ophiolite is exposed at 1 S S and consists of gabbro, sheeted dikes, and lava with lesser trondhjemite and plagiogranite. About 300 km to the south (~ S), the Tortuga ophiolite largely consists of gabbro, sheeted basaltic dikes, and lavas, but it exhibits a more continuous transition to diabase (de Wit and Stern, 191). Geochemical results indicate that LITHOSPHERE 01 Geological Volume Society of America Number 1 For permission to copy, contact editing@geosociety.org 3

2 MALKOWSKI ET AL. U. JUR. L. CRET. U. CRET. S Fuegian Sector Ult. Esp. Sector Austral Sector S MAB RVB Patagonian batholith Sarmiento Complex Patagonia ice field Alta Vista Fm. (Tres Pasos equiv.) Lago Viedma/Cerro Toro Fm. (Punta Barrosa equiv.) (~1-9 Ma) Rio Mayer Fm. U-Pb samples EC10 (DZ) EC13 EQC73 EC130 LT3 EQC0B CS7C CS7A (DZ) Pre-Jurassic metasedimentary basement El Quemado Complex Nazca RVB structure high (-00 m) Plate 30 S Study Area 1 Study Area L. Viedma L. Argentino Argentina Chile Lithologies Tertiary (sedimentary) Upper Cretaceous (sedimentary) Lower Cretaceous Capitán Aracena (sedimentary) and Carlos III Complexes Ophiolite complex Upper Jurassic Rift ages (samples) (volcaniclastic) This study Paleozoic basement (metasedimentary) Previous studies 7 W 70 W Magallanes Austral foreland basin COMPRESSIONAL PHASE Basin transition 0 S Antarctic Plate Rocas Verdes backarc rift basin EXTENSIONAL PHASE GONDWANA INTACT South America Antarctic Peninsula FI (IM) Scotia Plate 0 0 km Tortuga Complex Atlantic Ocean Weddell Sea Rio Chico Dungeness Arch N SGI 0 W 30 W (SGI) Figure 1. Study area map highlighting the geology of the Jurassic Cretaceous Rocas Verdes Basin (RVB) and the Cretaceous fill of the Magallanes Austral Basin (MAB). SGI South Georgia Island. We divide the Rocas Verdes Basin into three sectors: the Austral sector (Argentina), the Ultima Esperanza sector (Chile), and the Fuegian sector (Argentina and Chile). Map is modified from Fildani and Hessler (00). Structure contour is after Biddle et al. (19). Inset map shows the relative study area location (boxed region) in the context of the regional plate configuration. Shaded regions outline the continuation of transitional and continental crust. FI (IM) Falkland Islands (Islas Malvinas). Figure. Schematic stratigraphic section from the Austral sector (~9 S 0 S) of the Rocas Verdes Basin and Magallanes Austral Basin (modified from Malkowski et al., 01). Circles indicate approximate stratigraphic position of U-Pb detrital zircon samples (gray circles) and igneous samples (white circles). Forma tion thicknesses are not to scale. The 1 9 Ma ages are from Malkowski et al. (01). the Tortuga ophiolite evolved by igneous fractionation with a steady input of undifferentiated magma from a direct mantle source, whereas the Sarmiento ophiolite is best modeled by closedsystem igneous fractionation, which was isolated from continuous input of new batches of mantle melt (de Wit and Stern, 191). MAGALLANES AUSTRAL FORELAND BASIN The transition from the Rocas Verses backarc basin to foreland fold-and-thrust belt deformation is indicated by marine deepening triggered by foreland flexure in the compressive regime. Foreland basin initiation is marked by the deposition of coarse clastic detritus associated with the Punta Barrosa Formation and equivalent units (Wilson, 1991; Fildani and Hessler, 00; Malkowski et al., 01). In the Ultima Esperanza District of Chile, paleobathymetric indicators suggest a marine deepening from 300 m water depth during Albian Cenomanian time to up to 000 m in Cenomanian Coniacian time (Natland et al., 197; Biddle et al., 19). Sustained delivery of arc-derived coarse-clastic sediment was established by ca. 9 Ma in this basin sector (Fildani et al., 003). While equivalent paleobathymetric data are not available from other basin sectors, there is a clear diachronous north-south trend in the initiation of coarse clastic deposition within the entire basin (Malkowski et al., 01). Deposition of coarse-grained submarine fan systems began as early as latest Aptian time (ca Ma) in the Austral Basin sector (~9 S; Malkowski et al., 01). Conversely, in the Fuegian sector (~ S), deep-water deposition of coarse clastics initiated much later (9 Ma; Fig. ; McAtamney et al., 011; Malkowski et al., 01). Deposition of the Yahgan Formation (Zapata Rio Mayer Formation equivalent) may have persisted as late as Campanian time in the Tierra del Fuego region (~ S; Barbeau et al., 009a). The north-south diachronous nature of basin filling continued at least through the remainder of Cretaceous time with the deposition of the Cerro Toro, Tres Pasos/Alta Vista, and Dorotea Formations (e.g., Romans et al., 0; Bernhardt et al., 0). U-Pb ZIRCON GEOCHRONOLOGY We report new field observations and zircon U-Pb ages from volcanic and volcaniclastic units of the El Quemado complex and detrital zircon data from the east Andean metamorphic complex and the lower Rio Mayer Formation (Fig. ). Specific sample locations, analytical results, and methods are available in the GSA Volume Number 1 LITHOSPHERE

3 Geochronology of the Jurassic Cretaceous Rocas Verdes backarc rift basin SHORT RESEARCH Data Repository. 1 U-Pb zircon analyses were conducted by laser ablation inductively coupled plasma mass spectrometry at the University of Arizona LaserChron Center. All ages and uncertainties reported in the text are weighted means of overlapping single-grain dates and s standard errors, respectively. New data come from key sample locations, including the unconformable contact between the Jurassic El Quemado volcanics and the Paleozoic East Andean metamorphic complex exposed along Cerro Polo in the Parque Nacional Los Glaciares near El Chaltén (~9 S). The upper gradational contact between the El Quemado volcanics and the overlying Rio Mayer Formation was sampled north of Lago Argentino, adjacent to the Upsala Glacier (~0 S; Fig. 1). Data from these locations bracket the timing of rift-related felsic volcanism in the study area (Fig. ). Sample CS7A, from just beneath the unconformity, yields detrital zircon age spectra that are consistent with derivation from the East Andean metamorphic complex, with age maxima at ca. 0 and 0 Ma (Fig. 3; Hervé et al., 003). Sample CS7C, from a rhyolite unit deposited upon the east Andean metamorphic complex, yields an eruptive age of 1.0 ±.0 Ma (Fig. 3). We interpret this age as the onset of rift volcanism in this region. Samples LT3, EQC0b, and EQC73 were also collected near El Chaltén and yield model ages of 10. ±. Ma, 19.7 ± 1.3 Ma, and 19.1 ± 1. Ma, respectively (Fig. 3). Samples EC130, EC13, and EC10 were collected from good exposures of the transition between the El Quemado volcanics to the Rio Number Number A lower Rio Mayer Fm. EC10 (N=3) youngest ages El Quemado Complex (lower contact) 10 CS7C (N=3) interpreted eruption age (see Figure 3B) Relative probability Relative probability B EC13 1. ± 1. Ma (σ) MSWD = 0. N = 1 EC ± 1. Ma (σ) MSWD = 1.7 N = 19 EQC ± 1. Ma (σ) MSWD = 0. N = EQC0b 19.7 ± 1.3 Ma (σ) MSWD = 0. N = 73 Number East Andean Metamorphic Complex (EAMC) CS7A (N=93) youngest ages Age (Ma) Relative probability error bars at 1σ LT3 10. ±. Ma (σ) MSWD = 1.0 N = 37 CS7c 1.0 ±.0 Ma (σ) MSWD = 0. N = Age (Ma) Figure 3. U-Pb zircon geochronology results. (A) Histograms and age probability plots of detrital zircon ages from the East Andean metamorphic complex (EAMC), a rhyolitic volcanic unit of the El Quemado complex deposited upon the unconformity with the East Andean metamorphic complex, and the lower Rio Mayer Formation. (B) Results and interpreted ages of igneous zircon samples from felsic volcanics of the El Quemado complex. MSWD mean square of weighted deviates. 1 GSA Data Repository Item 01337, DR1: Table of U-Pb sample locations; DR: Description of analytical methods in U-Pb zircon geochronology; DR3: Table of analytical results and isotope ratios; DR: Imagery examples of zircon cathodoluminescence and analytical spot locations, is available at or on request from editing@geosociety.org, Documents Secretary, GSA, P.O. Box 910, Boulder, CO , USA. LITHOSPHERE Volume Number 1

4 MALKOWSKI ET AL. Mayer Formation at the northwest end of Lago Argentino (~0 S). Samples EC130 and EC13, from submarine volcanic deposits, yield model ages of 1. ± 1. Ma and 1. ± 1. Ma, respectively (Fig. 3). Sample EC13, the stratigraphically youngest identifiable volcanic unit, represents the upper bound of rift volcanism. Nearly 0% of the detrital zircon ages from the lower Rio Mayer Formation sample EC10 are between 1 and 1 Ma (Fig. 3). This suggests that El Quemado volcanics were the primary source of detritus for EC10. In summary, these new U-Pb geochronology results indicate that rift volcanism was active from ca. 1 to 17 Ma (accounting for s uncertainties) in the Austral sector of the basin. DISCUSSION Opening of the Rocas Verdes Basin may represent an early attempt to break apart Gondwana as a temporary proto Atlantic Ocean. The driving mechanism of this process whether it was driven principally by mantle plume processes (e.g., Storey et al., 001, and references therein), or it was related to backarc extension behind the same subduction margin responsible for the Patagonian Batholith remains a contentious topic (Stern and de Wit, 003). In the Fuegian and Ultima Esperanza sectors of southern Patagonia, the Rocas Verdes Basin is characterized by bimodal volcanism. This includes felsic volcanic units of the Tobífera Formation and mostly mafic rocks associated with the ophiolite complexes. Geochemical and compositional variations between ophiolitic complexes indicate that crustal extension was greater for the Tortuga ophiolite than the more northerly Sarmiento ophiolite (de Wit and Stern, 191). The absence of ophiolite within the Austral sector indicates that extension was insufficient to generate oceanic crust at this latitude. At least by Late Jurassic time, a backarc setting for the Rocas Verdes Basin is supported by coeval arc magmatism and early emplacement of the southern Patagonia Batholith both before and after mafic magmatism of the ophiolite complexes (Hervé et al., 007). The south-to-north decrease in the magnitude of rifting is also associated with a southto-north decrease in the timing of initial rifting. The upper and lower contacts of the El Quemado volcanics are well exposed between latitudes 9 S and 0 S, near the inferred northern terminus of the Rocas Verdes Basin. Our U-Pb ages from volcanic deposits indicate that rift volcanism initiated at 1.0 ±.0 Ma at this latitude. The oldest ages obtained from the equivalent Tobífera Formation in the Ultima Esperanza and Fuegian sectors are ca. 17 Ma and 17 Ma, respectively (Fig. ; Pankhurst et al., 000). Thus, available geochronology data among all three basin sectors support a northward-progressing initiation (ca Ma) of rift volcanism from ~ S to ~9 S (Fig. ). The duration of rift volcanism also decreases northward, with ages in the Ultima Esperanza and Fuegian sectors spanning at least 30 m.y., and volcanism in the Austral sector only lasting ~7 m.y. The exact timing and mechanism for the cessation of rifting in the Rocas Verdes Basin remain unclear. The southern Atlantic Ocean may have begun to form as early as 13 Ma or earlier (Rabinowitz and LaBrecque, 1979; Nürnberg and Müller, 1991). A northward-propagation model has been proposed for the opening of the southern Atlantic Ocean. The initial phase in this model begins along the southern tip of South America between 10 and 130 Ma (Nürnberg and Müller, 1991). Although precise age resolution is lacking, this timing corresponds closely with late-stage rifting and the formation Latitude 0 S S S 9 Arc volcanism / magmatism 11 Diachronous initiation of coarse-clastic deposition (foreland basin) 11 Deposition of Rio Mayer Fm. (and equivs.) mudstone of oceanic crust beneath the Rocas Verdes Basin between 10 and 139 Ma (Stern et al., 199; Mukasa and Dalziel, 199). Thus, if seafloor spreading initiated by ca. 10 Ma and was continuous until ca. 13 Ma, even very slow spreading rates (e.g., 1 cm/yr) could have resulted in an ocean basin that was nearly 00 km wide. Cretaceous closure of the Rocas Verdes Basin required significant internal shortening in southern Patagonia and ultimately resulted in obduction of the ophiolitic complexes. Kinematic thrust belt reconstructions show a consistent north to south increase in shortening, including minimum estimates that range from >0 km (Klepeis et al., 0) to km (Kraemer, 003) at the southern end of the foldthrust belt (Betka et al., 01). Both the initial rifting and subsequent closure of the basin were of sufficient magnitude that they need to be accounted for in kinematic models for the opening of the southern Atlantic Ocean. In particular, accounting for this intraplate deformation may Opening SAO Unconformity Continental Crust This study Intermediate Crust 1 1 Intermediate Oceanic Crust Age (Ma) Geochronology samples interbedded ash felsic volcanic (Tobifera / El Quemado) sandstone (max. depositional age) gabbro or plagiogranite (ophiolite complex) Oceanic Crust Rift volcanism 9 3 Diachronous initiation of rift volcanism bimodal volcanism Figure. Summary plot of geochronology data for rift volcanism associated with the Rocas Verdes Basin and initiation of coarse clastic deposition in the Magallanes Austral Basin. This figure highlights the northward diachronous trend observed in the initiation of rift volcanism and the timing of successive events such as bimodal volcanism and the initiation of coarse clastic deposition during foreland basin sedimentation. Depicted spreading centers are schematic for the magnitude of extension. SAO South Atlantic Ocean. Geochronology sample references: 1 Stern et al. (199), Mukasa and Dalziel (199), 3 Pankhurst et al. (000), Fildani et al. (003), Calderón et al. (007), Barbeau et al. (009a), 7 Barbeau et al. (009b), Klepeis et al. (0), 9 McAtamney et al. (011), Calderón et al. (013), 11 Malkowski et al. (01), this study. Ult. Esp. Ultima Esperanza Fuegian Sector Ult. Esp. Sector Austral Sector Volume Number 1 LITHOSPHERE

5 Geochronology of the Jurassic Cretaceous Rocas Verdes backarc rift basin SHORT RESEARCH help to ameliorate highly overlapping plate reconstructions for southernmost South America and Africa (e.g., Torsvik et al., 009). The postrift phase of the Rocas Verdes Basin is recorded primarily by the widespread deposition of the fine-grained Rio Mayer Formation (Zapata Formation equivalent; Fig. ). The thickness of this unit varies from 00 to 00 m in the northern sector to greater than 00 m in the southern basin (Biddle et al., 19; this study). The 30 0 m.y. period of fine-grained Rio Mayer Formation deposition was followed by the reappearance of coarse clastic sedimentation related to the successor foreland basin (Malkowski et al., 01). Consistent coarse-clastic deposition began first in the north by 1 Ma (Malkowski et al., 01), but did not appear in the Fuegian sector of the basin until 9 Ma (McAtamney et al., 011) or possibly at late as 1 73 Ma (Fig. ; Barbeau et al., 009a). This may indicate that the initiation and development of the foreland basin system evolved diachronously in a way that mirrored the precursor basin history (Fig. ). Regional basin-filling patterns show that progressive southward infilling of the successor basin continued through the remainder of the Cretaceous (Romans et al., 0; Bernhardt et al., 0). We favor a model of diachronous rift tectonism for the Rocas Verdes Basin that preceded (but may have overlapped and been related to) opening of the South Atlantic Ocean. Northward propagation of the Rocas Verdes Basin occurred during Middle to Late Jurassic time and generated an ocean basin that widened and deepened southward. The Rocas Verdes ocean basin likely opened by backarc spreading in a setting similar to the modern Sea of Japan (Dalziel et al., 197). Cretaceous convergence closed this basin and resulted in obduction of ophiolites before 0 Ma (Calderón et al., 013). Lithospheric shortening during this convergence resulted in a fold-and-thrust belt and flexural loading of a successor basin underlain by oceanic and attenuated continental crust of the Rocas Verdes Basin. This weakened crust enhanced the effects of flexural loading to permit anomalously thick deep-water foreland basin stratigraphy (Fosdick et al., 01) and may have facilitated increased thrust belt related shortening from north to south (e.g., Betka et al., 01). SUMMARY A north-south gradient in the timing and extent of rift volcanism is associated with the opening of the Rocas Verdes Basin. New zircon U-Pb geochronology data from key exposures show that rift volcanism occurred between 1 and 17 Ma in the Austral Basin sector. Combined with previously published ages, our results suggest diachronous initiation of rifting that propagated northward through time and culminated in an ocean basin that widened southward. These along-strike variations in the geometry and paleogeography of the Rocas Verdes Basin should be accounted for in plate models associated with the opening of the southern Atlantic Ocean. Diachronous opening also characterized the southernmost Atlantic Ocean, beginning in Early Cretaceous time. Cretaceous shortening ultimately closed the Rocas Verdes Basin and established a successor foreland basin in its place. Many characteristics of the foreland basin, including north to south variations in crustal shortening, the timing of coarse sediment dispersal, and the distribution of depositional facies, are in large part due to the earlier rifting history. ACKNOWLEDGMENTS We thank the Parque Nacional Los Glaciares of Argentina and the Estancias Cristina and La Quinta for their long-standing cooperation in granting outcrop access. Funding was provided by the Stanford Project on Deep-Water Depositional Systems and grants from the Stanford School of Earth Sciences. Mark Pecha and the laboratory personnel at the University of Arizona provided excellent expertise and assistance with U-Pb data collection. The Arizona LaserChron Center is funded by National Science Foundation grant EAR We thank Marcos Calo, Theresa Schwartz, Glenn Sharman, and Zach Sickmann, Corey Steimel, and Mariano Valdez for field assistance and Trevor Dumitru for assistance with heavy mineral separations. We are grateful to Julie Fosdick, Sam Johnstone, and Theresa Schwartz for early revisions and discussions. Finally, we thank Massimo Tiepolo, an anonymous reviewer, and Science Editor Kurt Stüwe for their helpful reviews, as well as the Lithosphere editorial staff for their assistance throughout the review process. 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6 MALKOWSKI ET AL. ince of Patagonia and related rocks in West Antarctica: A silicic large igneous province: Journal of Volcanology and Geothermal Research, v. 1, no. 1, p , doi:.1 /S (97) X. Pankhurst, R.J., Riley, T.R., Fanning, C.M., and Kelley, S.P., 000, Episodic silicic volcanism in Patagonia and the Antarctic Peninsula: Chronology of magmatism associated with the break-up of Gondwana: Journal of Petrology, v. 1, p. 0, doi:.93/petrology /1..0. Rabinowitz, P.D., and LaBrecque, J., 1979, The Mesozoic South Atlantic Ocean and evolution of its continental margins: Journal of Geophysical Research, v., no. B11, p , doi:.9/jb0ib11p0973. Romans, B.W., Fildani, A., Graham, S.A., Hubbard, S.M., and Covault, J.A., 0, Importance of predecessor basin history on sedimentary fill of a retroarc foreland basin: Provenance analysis of the Cretaceous Magallanes Basin, Chile: Basin Research, v., no., p. 0, doi:.1111 /j x. Stern, C.R., and de Wit, M.J., 003, Rocas Verdes ophiolites, southernmost South America: Remnants of progressive stages of development of oceanic-type crust in a continental margin back-arc basin, in Dilek, Y., and Robinson, R.T., eds., Ophiolites in Earth History: Geological Society of London Special Publication 1, p. 3, doi:.11 /GSL.SP Stern, C.R., Mukasa, S.B., and Fuenzalida, P., 199, Age and petrogenesis of the Sarmiento ophiolite complex of southern Chile: Journal of South American Earth Sciences, v., no. 1, p. 97, doi:.1 / (9) 9000-Y. Storey, B.C., Leat, P.T., and Ferris, J.K., 001, The location of mantle-plume centers during the initial stages of Gondwana breakup, in Ernst, R.E., and Buchan, K.L., eds., Mantle Plumes: Their Identification Through Time: Geological Society of America Special Paper 3, p. 71 0, doi:.1130/ Torsvik, T.H., Rousse, S., Labails, C., and Smethurst, M.A., 009, A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin: Geophysical Journal International, v. 13, p. 9 3, doi:.1111 /j.13 -X.0.07.x. Wilson, T.J., 1991, Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile: Geological Society of America Bulletin, v. 3, p , doi:.1130 / (1991)3<009:TFBATF>.3.CO;. MANUSCRIPT RECEIVED AUGUST 01 REVISED MANUSCRIPT RECEIVED 17 SEPTEMBER 01 MANUSCRIPT ACCEPTED SEPTEMBER 01 Printed in the USA Volume Number 1 LITHOSPHERE