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Plateau Collapse Model for the Transantarctic Mountains–West Antarctic Rift System: Insights from Numerical Experiments Robert W

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Plateau Collapse Model for the Transantarctic Mountains–West Antarctic Rift System: Insights from Numerical Experiments Robert W Plateau collapse model for the Transantarctic Mountains–West Antarctic Rift System: Insights from numerical experiments Robert W. Bialas W. Roger Buck Department of Earth and Environmental Science, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA Michael Studinger Lamont-Doherty Observatory of Columbia University, Palisades, New York 10964, USA Paul G. Fitzgerald Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA ABSTRACT The high elevation and considerable length of the Transantarctic Mountains have led to speculation about their origin. To date, no model has been able to adequately reconcile the jux- taposition of the high, curvilinear Transantarctic Mountains with the adjacent West Antarctic Rift System, a broad region of thin extended continental crust exhibiting wide rift character- istics. We present a fi rst-order investigation into the idea that the West Antarctic Rift System– Transantarctic Mountains region was a high-elevation plateau with thicker than normal crust before the onset of continental extension. With major Cretaceous extension, the rift underwent a topographic reversal, and a plateau edge with thickened crust, representing the ancestral Transantarctic Mountains, remained. In the Cenozoic, minor extension and major denuda- tion reduce the crustal root while simultaneously uplifting peak heights in the mountains. The Cretaceous stage of this concept is investigated using two-dimensional numerical models to determine under what conditions plateau collapse is plausible. Model results indicate that elevation of a remnant plateau edge decreases with increasing initial Moho temperature. Very cold initial Moho temperatures, <675 °C, under the plateau leave a thick plateau edge but do not exhibit wide rifting. A cold to moderate initial thermal structure, Moho temperatures of 675–850 °C, is needed to retain the plateau edge and still exhibit wide rifting in the middle of the plateau. We conclude that this plateau collapse concept is possible using these numerical experiments, and that application of this idea to the West Antarctic Rift System–Transantarctic Mountains system is also supported by geological and geophysical evidence. Keywords: Transantarctic Mountains, West Antarctic Rift System, rifting, numerical modeling, plateau collapse, Antarctica. INTRODUCTION While the spatial relationship between the The Transantarctic Mountains are one of the mountains and rift system is obvious, many dominant features of the Antarctic continent aspects of their formation remain unclear. Large- and defi ne the boundary between cratonic East magnitude extension in the West Antarctic Rift Antarctica and the West Antarctic Rift System System during the Cretaceous accompanied (Fig. 1). The Transantarctic Mountains are more relatively little denudation in the Transantarctic than 3000 km long and have local peak eleva- Mountains at that time. Conversely, minor Ceno- tions in excess of 4 km, making them the larg- zoic extension adjacent to the mountains accom- est and longest rift-related mountain belt in the panied or is younger than most of the denudation world. The West Antarctic Rift System is adja- (Fitzgerald, 2002; Karner et al., 2005; Huerta and cent to the mountains, and is a broad region of Harry, 2007). Geological and geophysical evi- thin, extended continental crust 750–1000 km dence arguing against the often-cited broken plate wide, comparable to the Basin and Range Prov- model for Transantarctic Mountains uplift (ten ince of North America (Fitzgerald et al., 1986). Brink et al., 1997) is mounting (e.g., Studinger Previous quantitative models for the Trans- et al., 2004, 2006; Lawrence et al., 2006), most antarctic Mountains have mostly been kine- notably, the absence of a laterally continuous matic and include mainly thermal uplift of the crustally penetrating fault at the mountain–rift mountains (e.g., Smith and Drewry, 1984), a system interface required by such a model. Figure 1. Topographic and bathymetry map of combination of thermal and mechanical uplift Distributed extension in the West Antarctic Transantarctic Mountains and Ross Embay- related to rifting (e.g., Fitzgerald et al., 1986; Rift System may require hot, thin lithosphere ment; reference location map is in upper ten Brink et al., 1997), and mechanical effects (e.g., Buck, 1991), while rift shoulder uplift of the right. Ross Sea is characterized by a series of basins and basement highs and elevated of rifting thick lithosphere (van der Beek et al., Transantarctic Mountains may require a region of heat fl ow. Ross Embayment—Ross Sea plus 1994; Busetti et al., 1999). Stern et al. (2005) uniformly cold, thick lithosphere (Fig. 2). Distrib- Ross Ice Shelf. WARS—Ross Embayment attributed as much as 50% of peak height to the uted wide rifting in the Cretaceous would leave a plus extended parts of West Antarctica. effects of glacial erosion. broad area of hot, thinned lithosphere, similar to © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGYGeology, August, August 2007; 2007 v. 35; no. 8; p. 687–690; doi: 10.1130/G23825A.1; 4 fi gures; Data Repository item 2007177. 687 West Antarctic Rift System and to determine the Zealand to the Pacifi c plate, and marking the conditions for which retention of a plateau edge onset of major extension in the West Antarctic with thick crust is plausible. Rift System (Lawver and Gahagan, 1995). Wide rifting (~400 km) ended with the 84–79 Ma sepa- TECTONIC SETTING ration of New Zealand and the Campbell Plateau The Transantarctic Mountains and West Ant- from Antarctica (Stock and Cande, 2002). Denu- arctica have been the site of repeated orogenies, dation (1–2 km magnitude) during the Creta- including Rodinian assembly, Rodinian breakup, ceous synchronous with this major extension and a transition from a passive to an active mar- is recorded along the Transantarctic Mountains Figure 2. Two classes of models of Trans- gin during Gondwana assembly (e.g., Goodge, (e.g., Fitzgerald, 2002, and references therein). antarctic Mountain uplift and Ross Embay- 2002). During the Cambrian–Ordovician Ross The major phase of denudation, 4−9 km along ment subsidence. Dashed lines approximate orogeny the region was part of the active mar- the Transantarctic front, began in the Eocene, brittle-ductile transition. Top panel shows pre-rift confi guration; bottom panels show gin of Gondwana (Goodge, 2002, and refer- spatially correlative with but temporally preced- post-rift confi guration. Class A demonstrates ences therein). Subduction moved outboard to ing minor extension localized in the Ross Sea extension of cold, thick lithosphere. Rift fl ank the Pacifi c margin of Gondwana from at least adjacent to the mountains (e.g., Fitzgerald, 2002, uplift raises the edges of the rift due to ther- 320 Ma to ca. 110 Ma (Mukasa and Dalziel, and references therein). The nature and extent of mal effects. Extension is characterized by narrow rifting, and isotherms are shallowed 2000). Reconstructions (e.g., Foster and Gray, Cenozoic rifting as well as its relationship to the locally. Class B, proposed in this paper, dem- 2000) indicate that the along-strike equiva- thermochronology data are still uncertain. Nar- onstrates extension of a high plateau under- lent to the region between Marie Byrd Land row rifting in the Terror Rift, transtension in Vic- lain by hot thick crust. Extension occurs over and the Transantarctic Mountains, what would toria Land (Wilson, 1995; Rossetti et al. 2006), a broad region in the former plateau, and rift have been the majority of the unextended Ross Adare Trough spreading projected into the Ross fl anks retain much of their elevation through retention of thick crustal roots. Isotherms are Embayment, from the Cambrian to the Middle Sea (Davey et al., 2006, and references therein), shallowed over a wide region. Devonian was the Lachlan Fold Belt (orogen) of and minimal extension with mostly climate- southeastern Australia, then adjacent to Antarc- controlled denudation (Karner et al., 2005) have tica. This correlation implies high paleotopog- all been proposed as key mechanisms active in the the Basin and Range. The time between the end raphy and thickened crust in the West Antarctic Cenozoic. Cenozoic alkaline magmatism associ- of Cretaceous and beginning of Cenozoic rifting Rift System into the Devonian. ated with Cenozoic rifting (e.g., LeMasurier and is insuffi cient to cool and thicken the lithosphere Paleocurrent orientations, lithofacies, and Thomson, 1990) has a much smaller volume and under the mountain–rift system interface to the stratigraphic relationships of the Devonian– limited extent compared to Jurassic magmatism, level required for rift shoulder uplift. Triassic Beacon Supergroup in the central and is not a viable mechanism for thickening the As a way to reconcile the structures, distri- Transantarctic Mountains suggest that deposi- Transantarctic crust. bution of rifting, and the great elevation of the tion began within two intermontane or succes- Studinger et al., (2004, 2006) demonstrated mountains, we explore the possibility that the sor basins after postorogenic uplift of the Ross that gravity anomalies across southern Victoria West Antarctic Rift System was a high-elevation terrain (e.g., Isbell, 1999). These data and the Land and the Scott and Reedy Glacier region plateau with thicker than normal crust before presence of sub-Beacon relief in the central are
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