RESEARCH ARTICLE Post Middle Miocene Tectonomagmatic and Stratigraphic 10.1029/2019GC008568 Evolution of the Victoria Land Basin, West Antarctica Special Section: Christopher P. Wenman 1, Dennis L. Harry 1 , and Sumant Jha 1 Polar region geosystems 1Department of Geosciences, Colorado State University, Fort Collins, CO, USA Key Points: • Neogene extension in the Victoria Land Basin included a middle Abstract Seismic re flection and borehole data are used to create structure maps of four regional and Miocene amagmatic phase and a three local unconformities that constrain the post middle Miocene evolution of the Victoria Land Basin post ‐Miocene magmatic phase (VLB), which is located in the western Ross Sea within the Late Cretaceous through Quaternary West • The sedimentary moat around Ross Island is a composite of flexural Antarctic Rift System. Isochore maps of the strata between unconformities show that rifting was mostly basins formed by the major volcanic amagmatic between 12 to 7.6 Ma, with subsidence controlled by faults bordering the northwest margin of the centers on the island basin and in a tectonic zone along the southern basin axis known as the Terror Rift. Depocenters • The elastic thickness of the lithosphere ranges from 0.6 –2.4 km surrounding volcanic features in strata younger than 4.3 Ma indicate an increasing in fluence of flexure due to volcanic loading on the subsidence pattern in the southern VLB after this time. The intervening period, Supporting Information: from 7.6 to 4.3 Ma, was a transitional period during which both extensional tectonism and magmatism •Supporting Information S1 exerted strong in fluences on basin morphology. Since 4.3 Ma, a series of flexural subbasins formed successively at different times and positions as the different volcanic centers that built Ross Island erupted. In composite, these subbasins form a flexural moat surrounding Ross Island and smaller volcanic centers Correspondence to: D. L. Harry, immediately to the north. The widths of these basins indicate that the flexural rigidity of the lithosphere 19 19 [email protected] ranges from 0.20 × 10 to 12.96 × 10 N‐m (elastic thickness 0.6 to 2.4 km). Plain Language Summary Seismic data from the Ross Sea of West Antarctic are used with data Citation: from past drilling expeditions to develop an understanding of the subsidence history of the Victoria Land Wenman, C. P., Harry, D. L., & Jha, S. (2020). Post middle Miocene Basin since middle Miocene time, about 12 million years ago. This is a time when continental rifting in the tectonomagmatic and stratigraphic Ross Sea had become focused on the edges of the long ‐lived West Antarctic Rift System, and a time that evolution of the Victoria Land Basin, followed a change in the direction of rifting from east ‐west to north ‐northeasterly. Maps of sediment West Antarctica. Geochemistry, Geophysics, Geosystems ,21 , thickness show that the Victoria Land Basin transitioned from one in which subsidence was controlled e2019GC008568. https://doi.org/ primarily by faulting between about 14 and 4 Ma. After this time, faulting continued to control subsidence in 10.1029/2019GC008568 the northern part of the basin, but bending of the lithosphere around Ross Island and other emerging volcanic centers played an increasing role in the southern part of the basin. The overall trend of the major Received 12 JUL 2019 Accepted 6 MAR 2020 depocenters in the basin reoriented from north to north ‐northeasterly to accommodate the post middle Accepted article online 9 MAR 2020 Miocene change in extension direction, but individual structures within the basin continued to exploit older structural trends. 1. Introduction This paper describes the post middle Miocene evolution of the Victoria Land Basin (VLB). The VLB is located in West Antarctica and is the westernmost of four major sedimentary basins that form a broad con- tinental extensional province beneath the Ross Sea and Ross Ice Shelf known as the West Antarctic Rift System (WARS) (Figure 1). From Late Cretaceous through Late Paleogene time, extension was in a westerly direction and spanned the breadth of the WARS, accommodated by faulting and rapid subsidence in each of the four Ross Sea basins (Cooper, Davey, & Hinz, 1991; Davey & Brancolini, 1995; Decesari, Sorlien, et al., 2007; Hinz & Block, 1983; Siddoway, 2007; D. S. Wilson & Luyendyk, 2009). Since Late Paleogene time, extension has been restricted mostly to the Bentley Subglacial Trough on the eastern flank of the rift and to the VLB on the western flank (Busetti et al., 1999; Davey et al., 2006, 2016; Decesari, Sorlien, et al., 2007; LeMasurier, 2008; Lloyd et al., 2015). Cenozoic extension on the western flank of the rift was accompanied by uplift of the Transantarctic Mountains along the western coast of the Ross Sea, alkaline magmatism in Victoria Land and within the southern VLB, and a change in extension direction from east ‐west to north ‐northwest (Paulsen et al., 2014; Wilson, 1995; and references above). ©2020. American Geophysical Union. Subsidence in the VLB accelerated in middle Miocene time following the change in extension direction, All Rights Reserved. accompanied by faulting along the western edge of the basin and along the basin axis within a tectonic WENMAN ET AL. 1of25 Geochemistry, Geophysics, Geosystems 10.1029/2019GC008568 Figure 1. Sedimentary basins and basement highs in the Ross Sea. Location of the Ross Sea, West Antarctic Rift System (WARS), and Transantarctic Mountains (TAMS) shown in inset. CoH, southern extension of Coulman High; red, Cenozoic alkaline rocks of the Ross Volcanic Group. Area shown in Figure 3 is indicated with dashed outline. Bedmap2 relief from Fretwell et al. (2013). Borders of basement highs from Fielding et al. (2008). zone known as the Terror Rift (Cooper, Davey, & Behrendt, 1987; Hall et al., 2007; Paulsen et al., 2014) (Figure 1). This post middle Miocene Terror Rift phase of extension (adopting the term of Fielding et al., 2006) was accompanied by alkaline volcanism in the southern part of the basin and within the Terror Rift (Granot et al., 2010; Hall et al., 2007; Hamilton et al., 2001; Henrys et al., 2007). Alkaline magmatism continues to the present on Ross Island, which is a large volcanic island straddling the Terror Rift in the southern VLB (Kyle, 1990; Rilling et al., 2007). Flexural subsidence due to volcanic loading of the lithosphere has dominated the post Late Miocene evolution of the southern VLB, particularly around Ross Island (Aitken et al., 2012; Henrys et al., 2007; Horgan et al., 2005; Stern et al., 1991). In this paper, we describe the spatial and temporal patterns of sedimentation and subsidence in the VLB dur- ing the post middle Miocene Terror Rift phase of extension that followed the middle Cenozoic change in extension direction. This spans a period of major tectonic and climatic changes during which (i) the basin reoriented from a north to a north ‐northeast strike, with preexisting structures controlling subsidence pat- terns within the basin; (ii) the basin transitioned from an amagmatic rift (excluding the rift flanks) in which subsidence was controlled primarily by extensional tectonics to a magmatic rift in which flexure around vol- canic centers strongly in fluenced the pattern of subsidence; and (iii) the sediment supply into the basin was increasingly in fluenced by waxing and waning ice sheets and variable volcaniclastic input. Our goals are (i) to distinguish patterns of flexural subsidence associated with volcanism from extensional subsidence; (ii) to identify periods of flexural subsidence and associate those with speci fic volcanic events occurring within the rift and in surrounding areas; (iii) to quantify variations in the sediment flux through time; and (iv) to assess the in fluence that changes in sediment supply had on filling of the basin. We use regional two ‐dimensional seismic re flection pro files and data from nearby boreholes to infer subsidence patterns in the VLB and within the flexural moat around Ross Island. Regional unconformities near the top and bottom of the strata filling the flexural moat are correlated throughout the VLB to constrain the moat geometry. Two additional regional unconformities are mapped below the strata filling the flexural basin in order to constrain subsi- dence in the VLB during the Terror Rift phase of extension prior to the onset of Ross Island volcanism. Three local unconformities that bound strata deposited during distinct subsidence episodes within the flex- ural basin are also mapped in order to determine the subsidence history of the moat. A series of depth ‐structure and isochore (vertical thickness) maps are presented that show the evolution of the southern VLB, Terror Rift, and Ross Island flexural moat since approximately 12 Ma. WENMAN ET AL. 2of25 Geochemistry, Geophysics, Geosystems 10.1029/2019GC008568 2. Geology of the VLB and WARS 2.1. Location and Tectonic Setting The WARS is an 800 ‐to 1,000 ‐km wide Late Cretaceous through Quaternary continental extensional province located in the Ross Embayment between Marie Byrd Land on the east (Paci fic) side of the rift and the Transantarctic Mountains on the west, bordering the East Antarctic craton (Figure 1). The rift extends beneath the Ross Ice Shelf and West Antarctic Ice Sheet toward the Weddell Sea, but its boundaries beneath the ice sheet are poorly known. The area encompassing the WARS previously lay within the arc and backarc on the overriding plate of a subduction system that bordered the Gondwana Paci fic margin from Early Paleozoic through Late Cretaceous time (Larter et al., 2002; Lawver et al., 1992; Lawver & Gahagan, 1994; Luyendyk, 1995; Storey et al., 1988; Storey & Alabaster, 1991; Torsvik et al., 2007). Convergence ended with subduction of the Phoenix ‐Paci fic spreading ridge during the Late Cretaceous Period, and rifting between Zealandia and Antarctica began soon thereafter (Eagles, 2003; Luyendyk, 1995; Storey et al., 1992; Figure 2.