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

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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.

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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. Paleogeographic map of the East Gondwana Paci fic margin Weaver et al., 1994) (Figure 2). Extension in the Ross Sea sector of the during the Late Cretaceous. The onset of extension in the WARS coincided WARS began about 105 Ma when the spreading ridge collided with with subduction of the Paci fic‐Phoenix spreading ridge beneath the East Marie Byrd Land, as indicated by plate motion models (Eagles Gondwana margin and with separation of Zealandia from West Antarctica. et al., 2004, 2009; Lawver & Gahagan, 1994), paleomagnetic data Red dashed line indicates trend of incipient WARS. AUS, Australia; IND, India; EA, East Antarctica; MAD, Madagascar; AFR, Africa; SA, South (Luyendyk et al., 1996), mylonites dredged from the eastern Ross Sea America; NZ, New Zealand; MB, Marie Byrd Land; T, Thurston Island (Luyendyk et al., 2001, 2003; Siddoway et al., 2004), and a transition from block; E, Ellsworth Island block; AP, Antarctic Peninsula block. Modi fied subduction ‐related calc ‐alkaline to extension ‐related alkaline magmatism from Fitzgerald (2002) and Torsvik et al. (2007). in Marie Byrd Land (Mukasa & Dalziel, 2000; Weaver et al., 1994). Seismic re flection and refraction pro files show that the structure of the WARS in the Ross Sea is dominated by four major north ‐striking basins and interbasin basement highs that were established during the Late Cretaceous through Paleogene period of westerly oriented extension (Figure 1) (Cooper, Davey, & Hinz, 1991; Cooper et al., 1995; Cooper, Davey, & Behrendt, 1987; Davey, 1981; Decesari, Sorlien, et al., 2007; Hayes & Davey, 1975; Hinz & Block, 1983). The VLB is the wes- ternmost of these basins. Further north, and separated from the VLB by a basement high, is the Northern Basin. Eastward are the Coulman High, the Central Trough, the Central High, and the Eastern Basin. Seismic refraction and re flection data indicate that syn ‐and post ‐rift strata reach thicknesses of 8 km in the Eastern Basin, 7 km in the Central Trough, and 14 km in the VLB (Busetti et al., 1999; Cooper, Davey, & Hinz, 1991; Cooper, Davey, & Behrendt, 1987, 1991). Gravity modeling, seismic refraction surveys, and body and surface wave studies show that the crust thickness varies from 10 –21 km in the basins up to 24 km beneath the basement highs (Bannister et al., 2003; Behrendt, 1999; Danesi & Morelli, 2000; Davey, 1981; Davey & Brancolini, 1995; Guterch et al., 1985; Llubes et al., 2003; Ritzwoller et al., 2001; Trey et al., 1999). The crust is thicker on the flanks of the rift, ranging from 25 –30 km in Marie Byrd Land on the east and from 35 –40 km beneath the Transantarctic Mountains on the west (An et al., 2015; Bannister et al., 2000; Busetti et al., 1999; Chaput et al., 2014; Finotello et al., 2011; Graw et al., 2016; Hansen et al., 2009, 2016; Lawrence et al., 2006; Llubes et al., 2003; O'Donnell & Nyblade, 2014; Pyle et al., 2010; Studinger et al., 2006; Winberry & Anandakrishnan, 2004). Comparison of the modern crust thickness in the Ross Sea basins with estimated prerift thickness, as well as plate reconstructions, suggests that the region has widened by 25% to 50% (250 –500 km) since the Late Cretaceous Period (Behrendt, 1999; Davey & Brancolini, 1995; Decesari, Wilson, et al., 2007; D. S. Wilson & Luyendyk, 2009).

2.2. Magmatism Cenozoic alkaline rocks are present on the eastern side of the rift in Marie Byrd Land and on the western side of the rift in the VLB, Transantarctic Mountains, and Northern Victoria Land. On the western side of the rift, these rocks comprise the early Eocene (48 Ma) to Holocene McMurdo Volcanic Group and Meander Intrusive Group (Kyle, 1990; Kyle & Cole, 1974; Tonarini et al., 1997). The McMurdo Volcanic Group is sub- divided into three geographic provinces (Kyle, 1990; Kyle & Cole, 1974). These include (i) the Hallett

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Province on the northwestern margin of the WARS, adjacent to the Northern Basin (13 to <10 Ma); (ii) the Melbourne Province located onshore north of the VLB (25 to <0.5 Ma); and (iii), the most recently active Erebus Province in the southern part of the VLB (18.7 Ma to pre- sent) (Armstrong, 1978; Kyle, 1990). The Erebus Volcanic Province includes Ross Island, which formed during three major eruptive phases since Late Miocene time, a group of early through Late Miocene volcanic centers located 40 –100 km southeast of Ross Island (White Island, Black Island, Brown Peninsula, Minna Bluff, Mt. Discovery, and Mt. Morning), and several volcanic seamounts and islands located near the axis of the VLB north of Ross Island (Figure 3) (Table 1). Ross Island was constructed in four main eruption phases associated with four distinct volcanic centers, including dormant or extinct Mt. Bird (4.6 –3.1 Ma), Mt. Terror (1.8 –0.8 Ma), Hut Point Peninsula (1.3 –0.4 Ma), and active Mt. Erebus (1.3 Ma to present).

2.3. Stratigraphy of the VLB Seismic re flection and refraction data show that the VLB contains at least 5 km of Cenozoic syn ‐rift strata (Cooper & Davey, 1985; Cooper, Davey, & Behrendt, 1987). The Cenozoic rocks overlay 9 –10 km of presumed Late Figure 3. Seismic pro files, boreholes, and major tectonic features in the Paleozoic and Mesozoic strata, which in turn lie atop Precambrian(?) Ross Sea. Gray lines, seismic re flection pro files used in this study; thick through Paleozoic metamorphic basement. The Cenozoic strata thin and blue lines (and corresponding blue numbers), pro files shown in Figure 4; onlap onto the Coulman High on the east side of the basin and onto the green circles, CRP, CIROS, AND ‐1, AND ‐2, and numbered DSDP drill sites; Western Shelf bordering the west flank of the basin (Brancolini et al., 1995; red, Erebus Volcanic Province of the McMurdo Volcanic Group, including Ross Island (RI), White and Black Islands (WB), Minna Bluff area (MB) Cooper, Davey, & Behrendt, 1991; Decesari, Sorlien, et al., 2007; Decesari, including Mt. Discovery and Mt. Melbourne, Beaufort Island (BI), and Wilson, et al., 2007; Fielding et al., 2008). Drilling on the western flank of Franklin Island (FI); pink, remainder of McMurdo Volcanic Group, exclu- the basin (Cenozoic Investigations in the Ross Sea [CIROS] ‐1; Cape sive of Erebus Volcanic Province; CoH, Coulman High; CeH, Central High; Roberts Project [CRP] ‐1, ‐2A, and ‐3; and McMurdo Sound Sediment VLB, Victoria Land Basin; CT, Central Trough; NVL, Northern Victoria and Tectonic Studies boreholes) and in the southern part of the basin on Land; SVL, Southern Victoria Land. Box indicates area shown in Figures 6 and 7 and 9 and 10. The Ross Sea Polar Stereographic 2000 projection (New the east and west sides of Ross Island (Antarctic Drilling Program Zealand Of fice of the Surveyor ‐General, 2013) is used here and in all sub- AND ‐1B and AND ‐2 boreholes) encountered Late Eocene through sequent maps. Holocene marine, glaciomarine, and volcaniclastic sedimentary strata composed of stacked unconformity bounded packages that seismic data show to thicken toward the basin axis (Cooper & Davey, 1985; Cooper, Davey, & Behrendt, 1987; Davey et al., 2000; Fielding, 2018; Fielding et al., 2006, 2008; Henrys et al., 2000).

Fielding et al. (2008) recognized 11 regional unconformities (named R athrough R k) in seismic re flection pro- files spanning the VLB (Table 2). These seismic unconformities bound basinward thickening packages of strata interpreted by Fielding et al. to have been deposited during five distinct Cenozoic tectonic episodes

in the basin. These include an Eocene Early Rift phase bounded by unconformities R aand R cand an early Oligocene Main Rift phase bounded by unconformities R cand R e. The Early and Main Rift phases coincide with the appearance of alkaline magmatic rocks of the McMurdo Volcanic Group in Victoria Land on the western flank of the VLB and with extension in the Northern Basin and Adare Trough further north (Davey et al., 2006; Granot et al., 2010; Hamilton et al., 2001; Selvans et al., 2014). Fielding and coworkers (Fielding, 2018; Fielding et al., 2008) inferred that the Main Rift phase was followed by a Late Oligocene

through middle Miocene period of tectonic quiescence (between seismic unconformities R eand R g), although others argue for continued extension during this time (Decesari, Sorlien, et al., 2007; Decesari, Wilson, et al., 2007; Hamilton et al., 2001; Johnston et al., 2008). The onset of the current phase of rifting in the middle Miocene coincided with a change in rift kinematics that was probably linked to a change in regional plate motions (Granot et al., 2010). Fielding et al. (2008) refer to this as the Terror Rift phase of

extension and correlate its onset with seismic unconformity R g. Unconformities R hand R iare associated with the onset of volcanism on White Island and Ross Island in the Late Miocene and Early Pliocene, respec-

tively, and unconformity R kapproximately corresponds with the end of the Pliocene warm period and expansion of the West Antarctic Ice Sheet.

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Table 1 Reported Ages of Eruptions in the Erebus Volcanic Province Volcanic center Age (Ma) Reference

Minna Bluff area

Mt. Morning 18.7 to 11.7, 6.13 to 0.02 a Martin et al. (2010) Minna Bluff 12 to 4 Wilch et al. (2011); Wright and Kyle (1990c) Mt. Discovery 5.44 to 1.87 Armstrong (1978); Wright and Kyle (1990d) White Island 7.65 to 0.17 Cooper et al. (2007) Black Island 10.19 to 1.69 Armstrong (1978); Timms (2006) Brown Peninsula 2.7 to 2.1 Armstrong (1978) Ross Island Mt. Bird 4.62 to 3.08 Wright and Kyle (1990a) Mt. Terror 1.75 to 0.82 Armstrong (1978); Wright and Kyle (1990b) Hut Point Peninsula 1.34 to 0.44 Armstrong (1978); Kyle (1981a, 1981b) Mt. Erebus 1.31 to present Esser et al. (2004) Terror Rift area b Beaufort Island 6.80 to 1.94 Rilling et al. (2009) Franklin Island 3.70 to 0.09 Rilling et al. (2007); Rilling et al. (2009) Seamounts 3.96 to 0.12 Rilling et al. (2007); Rilling et al. (2009) aTwo volcanic phases are recognized on Mt. Morning. bBeaufort and Franklin Island ages include nearby dredge sam- ples. Seamount ages span ranges of dredge samples from different areas within the terror rift.

In this paper, we focus on strata that were deposited during the Terror Rift phase of extension, above uncon-

formity R g(since about 12 Ma). The seismic and borehole data show that flexural subsidence around volca- nic centers belonging to the McMurdo Volcanic Group began to in fluence the subsidence pattern in the VLB beginning ca. 7.6 Ma, corresponding to the onset of volcanism on White Island and formation of unconfor-

mity R h. Volcanism on Ross Island began ca. 4.6 Ma, approximately coinciding with the formation of uncon- formity R iand ultimately leading to formation of a flexural sedimentary moat containing more than 1.5 km of post ‐Miocene strata (Aitken et al., 2012; Henrys et al., 2007; Horgan et al., 2005; Melhuish et al., 1995; Stern et al., 1991).

3. Methods 3.1. Seismic Data Interpretation The study area is a 320 × 320 ‐km region spanning from the East Antarctica coast to the eastern slope of the Coulman High and from Hut Point Peninsula to the north end of the VLB (Figure 3). Stacked multichannel seismic lines were obtained from the Antarctic Seismic Data Library System, supplemented with data from the Offshore New Harbor, Table 2 McMurdo Ice Sheet, and Hut Point Peninsula Projects (Betterly et al., 2007; Correlation of Unconformities With Previous Studies Horgan et al., 2005; Pekar et al., 2013) (Figure 3; Supporting Information Unconformity name and source Age (Ma) Table S1). The vertical resolution of the data (taken to be one fourth of the wavelength) ranges from 15 to 35 m. Brancolini Fielding This Nominal a et al. (1995) et al. (2008) paper Range age We mapped regional unconformities R g,R h,R i, and R k, which span the RMU3 ~1 Terror Rift phase of extension in the VLB (Fielding et al., 2008) (Table 2; RSU1 Rk Rk 1.7 to >2.1 1.8 Figure 4). Unconformity R jcould not be correlated across the seismic data RMU2 ~2.5 set with con fidence and so maps of this surface are not discussed here. As RMU1 ~3 noted previously, strata above unconformity R h(which coincides with the R R 2.8 –4.1 3.5 j j onset of volcanism on White Island) were deposited after flexural subsi- RSU2 Ri Ri 4.0 –4.6 4.3 b RSU4? Rh Rh 7.3 –7.8 7.6 dence around volcanic centers in the southern VLB began to in fluence b RSU4a? Rg Rg 10.8 –13.6 12.2 the basin morphology. Strata above R iwere deposited after Ross Island aAge ranges from Wilson et al. (2007, 2012) and Naish et al. (2007). began to form. We also mapped three local unconformities lying within b Fielding et al. (2008) correlated R hand R gwith RSU4a and RSU5 of the Ross Island flexural moat. From deepest to shallowest, these are Brancolini et al. (1995), respectively. RMU1 and RMU2, located between R iand R k, and RMU3, located

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Figure 4. Selected seismic pro files crossing the Victoria Land Basin. Top: composite pro file in the central VLB north of Ross Island, crossing from the Western Shelf to the Coulman High. Bottom left: pro file from McMurdo Sound, extending westward onto the western slope. Bottom right: pro file from northern VLB crossing from the Western Shelf across the Discovery Graben. Seismic lines are labeled at bottom, with numbers indicating pro file locations shown in Figure 3. Subvertical black lines, normal faults; vertical red line, location of CRP ‐3 drillhole. Outline of small igneous intrusion indicated in red on lower left panel.

between R kand the sea floor. RMU1 approximately coincides with but is stratigraphically above R j. The regional R ‐series unconformities and Ross Moat RMU ‐series unconformities bound wedges of strata that thicken toward Ross Island (Figure 5) and commonly downlap onto the unconformity that de fines the bottom of the respective sequence. Strata bounded by the Ross Moat unconformities merge and pinch out at the edges of the flexural moat, whereas strata between the regional unconformities onlap onto underlying strata and prograde toward the basin margins. The Ross Moat unconformities are interpreted to separate strata deposited during distinct subsidence episodes within the Ross Island flexural moat.

Identi fication of the R g,R h,R i,R j, and R kre flectors is based primarily on re flector positions shown in Fielding et al. (2008) and on published ties between the seismic pro files and synthetic seismograms con- structed from borehole logs at sites CIROS ‐1 (Pekar et al., 2013), AND ‐1B (Naish et al., 2007), and AND ‐2A (Akan, 2016; Pekar et al., 2013). Correlation of the unconformities between wells was aided by age and time ‐depth models for CIROS ‐1 (Henrys et al., 1998; Roberts et al., 2003; G. S. Wilson et al., 1998), the CRP boreholes (Davey et al., 2000; Fielding et al., 2008; Henrys et al., 2000; Roberts et al., 2013), AND ‐1B (G. S. Wilson et al., 2007, 2012) and AND ‐2A (Acton et al., 2008).

3.2. Map and Grid Construction Time ‐structure and isochron (interval transit time) maps were constructed for each unconformity and for each interval between unconformities, respectively. Two ‐way travel times were measured on each seismic line and contours drawn by hand at 50 ‐ms contour interval. In areas between seismic lines, the time ‐structure maps and isochron maps were iteratively adjusted to maintain conformability between maps of surfaces bounding the strata and maps of the strata thickness. The contour maps lack strong seismic con- trol on the east, south, and southwest sides of Ross Island. East and southwest of the island, the contour trends are constrained primarily by the assumptions that contours close to Ross Island and the Southern Victoria Land shorelines trend parallel to the coastline (as in areas where seismic control is available) and

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Figure 5. Correlation of unconformities between boreholes in the Central and Western Ross Sea. Unconformity names follow Fielding et al. (2008) west of Coulman High and Brancolini et al. (1995) east of Coulman High. Solid lines, correlations between wells based on seismic horizons mapped in this study. Dashed lines, tentative correlation of horizons not mapped in this study based on unconformity ages identi fied in boreholes. Lithologies and ages are from CRP ‐1/2/3 (Barrett, 2007; Lavelle, 1998), CIROS ‐1 (Pekar et al., 2013; Roberts et al., 2003; Wilson et al., 1998), AND ‐1B (Naish et al., 2007; Wilson et al., 2012) AND ‐2A (Acton et al., 2008; Pekar et al., 2013), and DSDP 273 (Bart, 2003; Hayes & Davey, 1975; Hayes & Party, 1975; Hinz & Block, 1983; Savage & Ciesielski, 1983). Ages are indicated above and below unconformities where available. Tops of boreholes are aligned at sea floor except CRP ‐2A and ‐3.

that they approximately maintain the contour spacing present in nearby areas further north that are well constrained by seismic data. Contours east of Ross Island are weakly constrained by subhorizontal dips imaged on seismic data further west on the Coulman High (Figure 3), which provide a limit to the width of the basin on the northeast side of Ross Island. The basin morphology south of Ross Island is uncon- strained except near the AND ‐1B borehole east of Hut Point Peninsula and is not shown on the contour maps. Faults are interpreted on the seismic pro files, but only the largest faults were correlated between pro- files. Consequently, faults are not included on the maps and instead appear as areas of tightly spaced contours. Grids were constructed from the digitized time ‐structure and isochron contours using the algorithm of Koshel (2012), which produces grid values evenly interpolated between contours. A uniform 1 × 1 ‐km grid spacing was used, which captures the most closely spaced contours. Pro files extracted from the grids agree with travel times on the seismic lines to within 20 ms and in most places to within 5 ms (less than the uncer- tainty in re flector ties between crossing seismic lines). The time ‐structure grids were converted to depth ‐structure grids using a linear velocity ‐depth function with the slope and intercept based on a vertical seismic pro file and core velocities from the AND ‐1B borehole (Hansaraj, 2008; Morin et al., 2007; Niessen et al., 2007):

vzðÞ¼ a1þa2z; (1)

−1 −1 where v(z) is the p ‐wave velocity, z is depth below sea floor, a 1= 1,791 m s , and a 2= 0.92 s (Chen, 2015). Given a linear velocity ‐depth relation, the depth of a re flection is

vsw TSF a1 a2T Z¼ þ e2 −1; (2) 2 a2

where T is the two ‐way travel time between the sea floor and the re flection, T SF is the arrival time of the sea- −1 floor re flection, and v sw is the p ‐wave velocity in seawater (1,480 m s ) (Al ‐Chalabi, 1997). Isochore thick- ness grids were constructed from the isochron grids. From equation 2, the isochore thickness is found to be

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dZ ΔTa 1 ΔZ¼ΔT ¼ ea2 =2; (3) dT 2

where ΔT is the two ‐way travel time between the horizons and is the travel time between the sea floor and the center line of the isochore interval. The fidelity of the depth grids was tested by comparison to unconformity depths in the boreholes and by verifying a match between the shapes of the depth ‐structure and time ‐structure contours and between the isochron and isochore contours.

The bathymetry grid used in this study (T SF in equation 2) was constructed from the sea floor re flection on the seismic pro files in order to assure colocation of the sea floor depths and the data points measured from the seismic lines that were used to construct the depth structure and isochore maps. The Bedmap2 bathymetry was used to guide interpolation of bathymetric contours between seismic pro files (Fretwell et al., 2013).

4. Results 4.1. Re flector Identi fication and Correlation Ages of the unconformities mapped in this study and their relation to previous studies are shown in Table 2. Our correlation of unconformities between boreholes is shown schematically in Figure 5. Composite seismic pro files showing the re flector correlations between boreholes are shown in Figures S1 –S5. Here, we sum- marize the borehole constraints and seismic relations used to establish the correlations and age model used in this study. AND ‐1B is the only borehole to penetrate all the unconformities that were mapped in this study, but unconformities identi fied in that borehole cannot be de finitively correlated into the rest of the study area due to a lack of connecting seismic pro files. Direct correlation from the study area to Deep Sea Drilling Project (DSDP) Site 273 is similarly not possible due to erosion on the Coulman High. Consequently, identi fication of the R ‐series unconformities in these boreholes is based primarily on match- ing the ages of the unconformities with those in the CIROS ‐1, CRP ‐1, and AND ‐2A boreholes, aided by jump correlation of seismic re flections a distance of 80 km across the Coulman High to DSDP Site 273 (Figures S1 –S5).

In the western VLB, the R kunconformity imaged on seismic pro files correlates with the top of a ca. 15 million year hiatus overlain by 1.8 Ma strata at 44 meters below the sea floor (mbsf) in the CRP ‐1 borehole and 27 mbsf in the CRP ‐2A borehole (Barrett, 2007; Florindo et al., 2005) (Figures S2 and S3). Following Acton et al. (2008), we correlate that with a lower Pleistocene unconformity at 38 mbsf in the AND ‐2A borehole separating 0.78 and 2.45 Ma strata. Based on age and following Naish et al. (2007) and Wilson et al. (2012),

we identify R kin the AND ‐1B borehole as an unconformity encountered at 150 mbsf separating 1.7 Ma strata from strata older than 2.1 Ma. Seismic data show that R kis truncated by the sea floor on both flanks of the VLB (the Western Shelf and Coulman High) (Figures 4 and 5). It is possible to jump correlate seismic hor-

izon R kacross the Coulman High to unconformity RSU1 at the base of the Pleistocene section at DSDP Site 273 (Brancolini et al., 1995) (Figure S1), although an alternative interpretation is that R kis missing at DSDP Site 273 due to sea floor erosion. Considering the age range in all the boreholes, we assign a nominal

age of 1.8 Ma to unconformity R k, with an estimated range of 1.7 –2.1 Ma.

Seismic horizon R jis truncated by R kon the Western Shelf and Coulman High and is thus not easily identi- fied in all parts of the basin. Consequently, its stratigraphic position in many areas is deduced primarily from

the observation that, where clearly evident, horizon R jis conformable with the more easily mapped bound- ing horizons R kand R i. Following Acton et al. (2008), we correlate seismic horizon R jwith the top of an interval in the AND ‐2A borehole containing a series of unconformities between 48 and 128 mbsf that sepa-

rate 2.8 Ma strata above R jfrom 11.4 Ma strata below (Figures S3 and S4). Unconformity R jis identi fied at 285 mbsf in AND ‐1B, where it corresponds to a hiatus between ca. 3 and 4 Ma (Naish et al., 2007; G. S.

Wilson et al., 2012) (Figure S5). We assign a nominal age of 3.5 Ma to unconformity R j, with an estimated range of 2.8 –4.1 Ma.

Seismic data show unconformity R iis truncated by R kon the Coulman High and by either R kor R jon the western shelf. Fielding et al. (2008) correlated the R iseismic horizon to an interval dated between 4.0 to 4.6 Ma at 20 mbsf in the McMurdo Sound Sediment and Tectonic Studies ‐1 borehole on the Western

Shelf. In the AND ‐2A borehole, R iis interpreted to lie within a series of unconformities between 48 and 128 mbsf that separate 3 and 11 Ma strata (Figures S3 and S4) (Acton et al., 2008). Based on age,

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unconformity R iis interpreted to correspond to an unconformity separat- ing 3.6 and 4.4 Ma strata in the AND ‐1B borehole at 350 mbsf (Naish et al., 2007; G. S. Wilson et al., 2012). In agreement with Fielding

et al. (2008), we jump correlate R iacross the Coulman High with the RSU2 unconformity of Brancolini et al. (1995) at DSDP Site 273

(Figure S1). We assign a nominal age of 4.3 Ma to unconformity R i, with an estimated range of 4.0 –4.6 Ma.

Fielding et al. (2008) estimated the age of the R hunconformity to be 7.6 Ma based on the stratigraphic position of the seismic horizon, which is immediately beneath the oldest White Island volcanic strata. They noted, however, that the only firm maximum age constraint is that it must

be younger than the deeper horizon R f, which was found to be 17 Ma in the CRP boreholes. We correlate R hto the base of the series of unconfor- mities between 48 to 128 mbsf in the AND ‐2A, where it overlays 11.4 Ma strata (Figures S3 and S4) (Acton et al., 2008). Based on the age and follow- ing Naish et al. (2007) and Wilson et al. (2012), we identify unconformity

Rhin the AND ‐1B borehole at 757 mbsf, where it separates 7.26 and 7.81 Ma strata (Figure S5). R his erosionally truncated by unconformity Rkon the western shelf and by the sea floor on the western flank of the Central High. We are not able to identify a correlative horizon at DSDP

Site 273. We assign a nominal age of 7.6 Ma to unconformity R h, with an estimated range of 7.3 –7.8 Ma.

Fielding et al. (2008) note that the age of the R gunconformity is uncertain, Figure 6. Bathymetry map of the VLB and surrounding area. Map is based although it must be younger than 17 Ma (the age of the underlaying R f on arrival times to sea floor seismic re flector and a water velocity of unconformity encountered in the CRP boreholes). They assign a nominal 1,480 m s −1. Red shading, igneous rocks of Erebus Volcanic Province, including Ross Island, Franklin Island (FI), Beaufort Island (BI), and the age of 13 Ma based on the age model of Naish et al. (2007) from the northern ends of Black and White islands (south of Ross Island). Black dots AND ‐1B borehole. In the AND ‐1B borehole, Naish et al. (2007) and indicate edge of Ross Ice Shelf, with area lacking seismic coverage indicated Wilson et al. (2012) identify the R gunconformity at 1,223 mbsf, where it by diagonal lines. B, Mt. Bird; T, Mt. Terror; E, Mt. Erebus; C.I., contour separates 10.82 Ma and 13.57 Ma strata (Figure S5). Based on age, we cor- interval. relate this with an unconformity in the AND ‐2A borehole at 225 mbsf that separates 11.4 Ma strata from underlying 14.2 to 15.7 Ma strata (Acton

et al., 2008) (Figures S3 and S4). Horizon R gis truncated by unconformity R ion the western shelf and by the sea floor on the western flank of the Coulman High. We tentatively jump correlate R gacross the Coulman High with the RSU4 unconformity of Brancolini et al. (1995) at DSDP Site 273 (Figure S5). We

assign a nominal age of 12.2 Ma to unconformity R g, with an estimated range of 10.8 –14.2 Ma.

The depths to the R gand R hhorizons adjacent to the Western Shelf in the northwest part of the study area are poorly constrained. Here, the seismic pro files cross a west dipping normal fault with unknown throw.

The R gand R hre flectors on the east side of this fault are jump correlated across the fault (a distance of <3 km) to two unconformities recognized in the seismic data on the west side, but an alternative interpreta-

tion is that Rg and R hare much shallower (or missing due to erosion) in the Discovery Graben. We discuss this further in section 5. AND ‐1B is the only borehole that penetrates the RMU series of unconformities that are identi fied in this paper. At that borehole, the seismic horizons marking unconformities RMU1, RMU2, and RMU3 correlate with thin clastic intervals separating diamictite beds (Figures 5 and S5). We discuss the ages of the RMU ser- ies of unconformities in section 5.2.

4.2. Structure Maps The bathymetry map constructed from the seismic pro files is similar to the Bedmap2 bathymetry (Fretwell et al., 2013) but of lower resolution. The VLB underlays an elongate bathymetric trough, with sea floor depths ranging up to 980 m (Figure 6). The bathymetric basin is bordered on the west by sharp relief adjacent to the Western Shelf and on the east and northeast by more gradational slopes onto the Coulman High. Rugged relief is present around Beaufort Island and smaller volcanic features to its east and north, attesting to

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Figure 7. Depth ‐structure maps of regional R ‐series unconformities in order of increasing depth. (a) Horizon R k. (b) Horizon R i. (c) Horizon R h. (d) Horizon R g. Stippled areas indicate strata missing due to erosion on uplifts in the Victoria Land Basin and on the Coulman High and erosion or nondeposition on the Western Shelf. Black dots indicate edge of Ross Ice Shelf, with area lacking seismic coverage indicated by diagonal lines. CoH, Coulman High; B, Mt. Bird; T, Mt. Terror; E, Mt. Erebus; FI, Franklin Island; BI, Beaufort Island; C.I., contour interval.

their young ages. Small subbasins located east of Hut Point Peninsula and on the west and north sides of Ross Island form a semicontinuous bathymetric moat around Ross Island, 25 km wide on the west side and approximately 50 km wide on the east side (Figure 6).

The depth structure maps of regional unconformities R k,R i,R h, and R greveal a morphologically partitioned basin, with relief becoming more pronounced with depth (Figure 7; time ‐structure maps are shown in

Figure S6). South of Beaufort Island, relief on horizon R kis dominated by an irregular low surrounding Ross Island and extending northward from Mt. Bird Peninsula (Figure 7a). North of Beaufort Island, the

VLB at the level of the R khorizon is divided into two north ‐trending troughs separated by a basement high known as the Lee Arch (Cooper, Davey, & Cochrane, 1987). The westernmost of the two troughs forms a nar- row fault ‐bounded basin trending parallel to the Western Shelf known as the Discovery Graben (Figure 4,

lower right). As noted in section 4.1, the depths to the R gand R hhorizons in the Discovery Graben are uncer- tain as the seismic pro files cross a normal fault with unknown throw. We consider our placement of the R h and R ghorizons in the Discovery Graben to be the deepest that can reasonably be allowed by the data, and note that the depths to these horizons and the thickness of the R gto R hisochore interval may be overesti- mated in this area.

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The southern VLB, from Ross Island northward past Beaufort Island, becomes broader and more symmetric on successively deeper horizons,

forming an approximately rectangular basin at the level of horizon R g (Figures 7b –7d). The Discovery Graben becomes broader on successively deeper horizons, and the intervening Lee Arch divides into two distinct narrow north ‐trending en echelon highs (Figures 6c and 6d). Overall, the VLB morphology becomes less partitioned downward. At the level

of horizon R k, the VLB is comprised of a broad southern basin that is linked to two narrow north ‐trending troughs to the north on either side of the low relief Lee Arch. The greatest relief is around Ross Island and

along the edge of Western Shelf. At the level of horizon R g, the overall VLB morphology resembles that of an elongate asymmetric trough, with steep dips along its western margin, gentle dips along its eastern margin, and two narrow highs (the Lee Arch) present in the central part of the basin. Relief in the northern and central VLB is most pronounced along

the western side of the basin above horizon R iand is more evenly distrib- uted between the troughs on either side of the Lee Arch between horizons

Riand R g. Unconformities RMU1, RMU2, and RMU3 merge with or onlap onto the underlying re flectors, and the intervening strata pinch out or thin below seismic resolution on the edges of the Coulman High and Western Shelf and in the northern part of the VLB (Figure 4). These unconformities were not mapped in areas where the thickness of the intervening strata is below seismic resolution (about 15 m), restricting their mapped subcrops to a roughly elliptical area that encompasses the southern VLB around Ross Island, the region to the north surrounding the intrusive centers in the central VLB (the southern Terror Rift), and the southern part of the Discovery Graben (Figures 8 and S7). The basin de fined by the RMU ‐series subcrops is partitioned into a relatively deep southern end and a shallower northern end, with closed subbasins located (i) east of Hut Point Peninsula, (ii) in McMurdo Sound west of Ross Island, (iii) north of Ross Island wrapping around Mt. Bird and Beaufort Island, and (iv) extending northward into the southern part of the Discovery Graben. An arm of the basin north of Beaufort Island also extends as a broad low northeastward onto the flank of the Coulman High. In general, the features described above become more subdued upward in the stratigraphy.

4.3. Isochore Maps The isochore maps show that the northern VLB accumulated approxi- mately 1,800 and 1,000 m of strata on the western (Discovery Graben)

and eastern sides of Lee Arch since middle Miocene time (horizon R g, 12 Ma) (Figure 9a; Table 3). Approximately 1,400 m of strata were depos- ited in the southern Terror Rift southeast of Beaufort Island during this time and 1,400 to 2,000 m of strata in the southern VLB around Ross Island (the Mt. Terror subbasin is excluded here, as seismic data are lim- ited to the northwest flank of the structure). In the Discovery Graben, Figure 8. Depth ‐structure maps of Ross Moat RMU ‐series unconformities most of these strata (1,600 m) accumulated prior to 4.3 Ma (horizon R i) in order of increasing depth. (a) Horizon RMU3. (b) Horizon RMU2. (c) Horizon RMU1. Stippled areas indicate strata missing due to erosion on (Figure 9b). The accumulation of post middle Miocene strata is evenly dis- the Coulman High and erosion or nondeposition on the Western Shelf. tributed before and after 4.3 Ma in the northeastern VLB (east of Lee Black dots indicate edge of Ross Ice Shelf, with area lacking seismic coverage Arch) and southern Terror Rift. In the southern VLB, around Ross indicated by diagonal lines. B, Mt. Bird; T, Mt. Terror; E, Mt. Erebus; FI, Island, most of the strata (60 –75%, excluding the Mt. Terror subbasin) Franklin Island; BI, Beaufort Island; C.I., contour interval. accumulated after 4.3 Ma (Figure 9c). As indicated by the structure

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maps (Figure 7), relatively thick sediment accumulation occurred across

the breadth of the VLB between formation of horizons R g and R i (although it is most intense along the western margin of the basin in the

northern VLB). After formation of unconformity R i, the sediment accu- mulation was more concentrated in narrow troughs in the central and northern VLB and around Ross Island and smaller volcanic centers in the southern VLB.

5. Discussion 5.1. Subsidence Patterns Through Time The evolution of the basin through time is inferred from variations in sedi- ment thickness within the seven stratigraphic intervals bounded by the seismic horizons mapped in this study (Figures 10 and 11; see also regio- nal seismic pro files in Figures S1 –S5). Areas of thickened strata are taken to indicate areas of subsidence. This is somewhat problematic, given the effects of sediment loading and compaction, the lack of paleo water depth information, and the presence of the modern bathymetric moat around Ross Island that indicates the basin has been under filled since perhaps

early Pliocene time (the age of unconformity R k). It is possible that the VLB was also under filled at other times. However, we note that strata between each of the RMU series of unconformities thin or pinch out at approximately the same locations on the western shelf (Figure 4). This suggests the basin was consistently filled to the same level, which is most simply explained with a completely filled basin. If the basin was under- filled, the amount of subsidence estimated from sediment thicknesses will be underestimated but the position and relative patterns of subsidence should still be indicative of where the basin was most tectonically active. Similarly, although sediment loading and compaction may bias estimates of tectonic subsidence obtained from sediment thickness maps, this also primarily impacts the amount of subsidence and not the spatial and tem- poral pattern. Differential sediment transport into the basin presents a more dif ficult issue. This primarily affects the isopach maps near Ross Island and Coulman Island, where thick gravity deposits are evident on seismic pro files within 5 km of the coast. Consequently, the isochore maps are not used to infer subsidence close to or beneath these volcanic features.

From about 12.2(?) to 7.6 Ma (interval R gto R h), the VLB was partitioned into two subbasins and accumulated up to 1,630 m of sediment

(Figure 10a). The age for R gis uncertain and may be as old as 14 Ma, as discussed in section 4.1. In the northern VLB, subsidence was concen- trated along the western margin of the basin, where the Discovery Graben formed an elongate narrow trough trending parallel to the Western Shelf. The northeastern VLB accumulated little sediment during this time. The southern VLB formed a roughly rectangular basin, offset 100 km eastward relative to the Discovery Graben and with gentler dips Figure 9. Isochore maps of post middle Miocene strata in the VLB. (a) Total on the flanks. This depocenter encompasses the area around Ross Island post middle Miocene strata between horizon R gand the sea floor (12.2 Ma and the axial part of the southern Terror Rift around Beaufort Island. to present). (b) Late Miocene through early Pliocene strata between Extension in the Terror Rift during this time is evidenced by faults cutting horizons R gand R i(12.2 to 4.3 Ma). (c) Early Pliocene and younger strata horizons R and R (Figure 4). Many of the faults cutting horizon R also between horizon R iand the sea floor. Black dots indicate edge of Ross Ice g h h Shelf, with area lacking seismic coverage indicated by diagonal lines. B, Mt. offset horizon R i, but many do not, indicating at least two phases of post Bird; T, Mt. Terror; E, Mt. Erebus; FI, Franklin Island; BI, Beaufort Island; middle Miocene tectonism. Small depocenters around Beaufort Island C.I., contour interval. and the seamount to the north are attributed to flexural subsidence due

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Table 3 to volcanic loading of the lithosphere, but evidence of a similar flexural Subbasin Isochore Thicknesses moat around Ross Island is lacking during this time. Relatively thick Isopach interval Thickness (m) a sediment accumulations southeast of Hut Point Peninsula are attributed Northern VLB and Ross moat to flexural subsidence associated with volcanism on White Island (active Terror Rift b subbasins c beginning 7.65 Ma; Table 1), as seismic data show strata near Hut Point Peninsula in this interval thicken southward away from Ross Island. NDG ELA STR B T d HPP E We interpret the period 12.2 to 7.6 Ma to have been dominated by Rg:SF 1800 1050 1400 1800 1000 2000 1400 tectonic subsidence associated with extension along the western Rg:Ri 1600 550 700 700 350 500 400 margin of the northern VLB and in a symmetric graben spanning R:SF 200 500 700 1100 650 1500 1000 i the breadth of the southern VLB. Minor flexural subbasins formed aIsochore thicknesses are measured on seismic lines passing closest to along the axis of the Terror Rift and south of Ross Island in response deepest part of subbasin. bNDG = Northern Discovery Graben; ELA = east of Lee Arch (north of Franklin Island); STR = southern to volcanism in these areas. Terror Rift (west of Beaufort Island). cB = Mt. Bird Subbasin; T = Mt. Terror Subbasin; HPP = Hut Point Peninsula Subbasin; E = Mt. The VLB accumulated up to 890 m of sedimentary strata between 7.6 to d Erebus Subbasin. Mt. Terror subbasin is minimal thickness, as seismic 4.3 Ma (interval R hto R i) (Figure 10b). The basin retained the overall data do not extend into likely deepest part of depocenter. north ‐trending elongate morphology present in the underlying R gto R h interval, but the enclosed depocenters within the basin are smaller, more irregularly shaped, and less connected along the basin axis. Subsidence in the northern VLB continued to concentrate along its western flank, with the thickest strata accumulating in the northern part of the Discovery Graben. In the southern VLB, the thickest strata are concentrated in a narrower region than in the underlaying isochore interval, primarily along the axis of the Terror Rift north of Mt. Bird and around Beaufort Island and the small seamount to its north. We interpret the period 7.6 to 4.3 Ma to have been one of continuing but perhaps slower extension in the VLB, with extension in the southern VLB concen- trated primarily along the axis of the Terror Rift. Small depocenters are interpreted to indicate flexural sub- sidence associated with continued relatively minor magmatism in the Terror Rift north of Ross Island.

Horizons above R irecord four separate intervals of deposition during periods in which volcanism on Ross Island was underway. The R ito RMU1 interval is up to 420 m thick and shows subsidence within a roughly elliptical basin in the southern and central VLB encompassing the southern part of the Discovery Graben, Ross Island, Beaufort Island, and smaller volcanic features to the northwest and south of Beaufort Island

(Figure 11a). The ages of R i(4.0 –4.6 Ma; Table 2) and the igneous rocks on Mt. Bird (4.6 –3.1 Ma; Table 1) indicate that subsidence north of Ross Island during the R ito RMU1 time interval is likely due to incipient Mt. Bird magmatism. The small depocenters north of Ross Island are interpreted to be minor flexural basins

associated with magmatism in the Terror Rift. As in the R gto R hisochore, we interpret the depocenter east of Hut Point Peninsula to be due to volcanism on White and/or Black islands, consistent with southward thick-

ening strata in this area (Figure S5). The Discovery Graben is a less pronounced feature in the R ito RMU1 interval than in underlaying isochore intervals, and many faults that offset the R gand R hhorizons in the Terror Rift and on the flanks of the VLB do not offset or barely offset the R iunconformity (Figure 4). We thus interpret the time between formation of the R iand RMU1 unconformities to be one of relative tectonic quies- cence in the VLB, with subsidence dominated by flexure due to volcanic loading around intrusive centers in the Terror Rift, at Mt. Bird on Ross Island, and south of Ross Island on White and Black islands and near Minna Bluff.

The RMU1 to RMU2 isochore interval is up to 230 m thick (Figure 11b). In comparison to the underlying isochore interval, sediment thickness variations are much more subdued and depocenters are not as well developed. The anomalously thick sediment accumulations around the volcanic centers north of Ross Island are substantially smaller, but depocenters northwest and east of Mt. Bird are well developed and are attributed to flexure associated with continuing volcanism on Mt. Bird. The depocenter southeast of Hut Point Peninsula present in the underlaying isochore interval has shifted position in the RMU1 to RMU2 interval, with a new smaller depocenter appearing closer to the coast in this area. Seismic pro files show a change from southward thickening strata (away from Ross Island) below RMU1 to thickening toward Ross Island in the RMU1 to RMU2 interval (Figure S5). Subsidence near Hut Point Peninsula during the RMU1 to RMU2 interval is thus attributed to volcanism on the southeastern part of Ross Island. Minor thickening of strata in the southern part of the Discovery Graben during this time interval is attributed to extensional faulting. We interpret the time between deposition of horizons RMU1 and RMU2 to have

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been a period of minor extensional tectonism in the VLB. Volcanism on Mt. Bird and on the southwest side of Ross Island produced flexural sub- basins north of the island and east of Hut Point Peninsula.

The RMU2 to R k(1.8 Ma) interval is up to 220 m thick (Figure 11c). The depocenters north of Ross Island are smaller and fewer than in the under- lying isochore intervals, and the depocenters on either side of Mt. Bird are absent. A new depocenter, not present in deeper isochore intervals, is pre- sent north of the Mt. Terror coast of Ross Island. This depocenter is docu- mented on its western side by seismic pro files east of Mt. Bird Peninsula, but its maximum depth and northern and eastern extents are uncertain. The primary constraints on the width of this depocenter are from seismic pro files north of the basin, which show the dips to steepen toward Ross Island, and pro files to the east over Coulman High, which show relatively flat strata. We interpret the disappearance of the depocenters around Mt. Bird peninsula to indicate a cessation of volcanism on Mt. Bird prior to this time interval, and the appearance of the depocenter north of the Mt. Terror coast is attributed to flexural subsidence associated with the onset of magmatism at Mt. Terror. Minor subsidence continued in the Discovery Graben and in the southern Terror rift north of Ross Island, where a north ‐trending trough extends from Bird Peninsula into the

northeastern VLB. Faults in the Terror Rift that extend above the R ihor- izon also offset the RMU2 re flector and in many instances show growth at

least through horizon R k, indicating ongoing extension in the Terror Rift during this time (Figure 4, top). Similar faults offset horizon R kon the western flank of the VLB (Figure 4, lower right). We interpret the time

between formation of horizons RMU2 and R k(about 1.8 Ma) to have been a period of ongoing extension accommodated by minor faulting in the Terror Rift and on either side of the Lee Arch in the northern VLB. Subsidence in the southern VLB was dominated by flexure around Ross Island, with the main depocenter shifting eastward as volcanism shifted from Mt. Bird to Mt. Terror.

The R kto RMU3 interval is up to 235 m thick and shows a more organized pattern of subsidence than the underlying three intervals (Figure 11d). Subsidence in the Terror Rift de fines a relatively narrow trough trending northward under Ross Island from Hut Point Peninsula, through Bird Peninsula, and past Beaufort Island into the northeastern VLB. A parallel striking depocenter is present in the southern extension of the Discovery Graben. These two narrow rift basins are linked by a south ‐southwest Figure 10. Isochore maps between unconformities in middle Miocene trending depocenter in the central VLB, north of Beaufort Island. The through early Pliocene strata in the VLB in order of decreasing depth. depocenter north of the Mt. Terror coast that is present in the RMU2 to (a) Strata between horizons R gand R h(12.2 –7.6 Ma). (b) Strata between Rkisochore interval is absent in the R kto RMU3 interval, suggesting an horizons R hand R i(7.6 –4.3 Ma). Black dots indicate edge of Ross Ice end of Mt. Terror volcanism by this time. A new enclosed depocenter on Shelf, with area lacking seismic coverage indicated by diagonal lines. B, the southeastern side of Ross Island is interpreted to result from flexural Mt. Bird; T, Mt. Terror; E, Mt. Erebus; FI, Franklin Island; BI, Beaufort Island; C.I., contour interval. subsidence associated with the onset of volcanism at Hut Point Peninsula. Another small depocenter west of Ross Island is interpreted to be a result of flexure associated with incipient volcanism at Mt.

Erebus. We interpret the time between formation of horizons R kand RMU3 to be a period during which extensional tectonism in the VLB focused within narrow depocenters in the Discovery Graben and Terror Rift, extending beneath Ross Island. Volcanism on Ross Island shifted from Mt. Terror to Hut Point Peninsula, creating a new flexural subbasin southeast of Ross Island.

The RMU3 to sea floor isochore thickness ranges from 10 to 275 m (Figure 11e). Thickness changes in this

interval are more subdued than in the R kto RMU3 interval, which is interpreted to indicate decreased

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Figure 11. Isochore maps between unconformities in post early Pliocene strata in the VLB in order of decreasing depth. Maps are limited to the subcrop of local Ross Moat unconformities RMU1, RMU2, and RMU3. (a) Strata between horizons Ri(4.3 Ma) and RMU1. Note the small depocenters around volcanic features northwest and south of Beaufort Island, which are interpreted to be flexural basins. (b) Strata between horizons RMU1 and RMU2. (c) Strata between horizons RMU2 and R k(1.8 Ma). (d) Strata between horizons R kand RMU3. (e) Strata between horizons RMU3 and the sea floor. Black dots indicate edge of Ross Ice Shelf, with area lacking seismic coverage indicated by diagonal lines. B, Mt. Bird; T, Mt. Terror; E, Mt. Erebus; FI, Franklin Island; BI, Beaufort Island; C.I., contour interval.

tectonism during this period (Figure 11e). The subbasin east of Hut Point Peninsula that is present in the R k to RMU3 interval is reduced in size and amplitude in the RMU3 to sea floor interval, which is interpreted to indicate waning volcanism in this area after deposition of horizon RMU3. The small sub ‐basin that is present west of Ross Island in the underlying isochore interval is larger and deeper in the RMU3 to sea floor interval,

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which is interpreted to indicate continued flexural subsidence due to ongoing magmatism at Mt. Erebus. Depocenters in the southern Discovery Graben and Terror Rift are absent or less pronounced than in the underlaying isochore interval, and thickening of strata around intrusions in the Terror Rift is also absent. We interpret the time between formation of horizon RMU3 and the present to have been a period of dimin- ished tectonism in the VLB, with subsidence dominated by flexure on the west side of Ross Island as a result of magmatism on Mt. Erebus. 5.2. Ages of the Ross Moat Unconformities The ages of unconformities RMU1, RMU2, and RMU3 are inferred from their position in the AND ‐1B bore-

hole (the only borehole penetrating these horizons), the ages of the bracketing R i, Rj, and R kunconformities (Table 2), and by comparison of the subsidence patterns in each isochore interval (Figure 11) with ages of the

major volcanic centers on Ross Island (Table 1). Unconformities R i(4.0 –4.6 Ma) and RMU2 bound most of the subsidence that occurred in the subbasins around Mt. Bird, which was active from 4.6 –3 Ma. The interval

between unconformities RMU2 and R k (1.7 –2.4 Ma) shows subsidence north of Mt. Terror (active 1.8 –0.8 Ma), waning subsidence around Mt. Bird, and little subsidence around the rest of Ross Island. The

interval R kto RMU3 shows little subsidence north of Mt. Terror, pronounced subsidence east of Hut Point Peninsula (1.3 –0.4 Ma), and minor subsidence west of the Mt. Erebus coast in McMurdo Sound. The interval RMU3 to the sea floor shows diminished subsidence near Hut Point Peninsula and pronounced subsidence west of Mt. Erebus (1.3 Ma to present). We infer that unconformity RMU1 formed near the end of

Mt. Bird magmatism, approximately 3 Ma. This is approximately the age of the R junconformity, although RMU1 must be the younger of the two as it lies stratigraphically above R j. Unconformity RMU2 formed sometime between the cessation of volcanism at Mt. Bird and its onset at Mt. Terror (approximately 2.5 Ma). Unconformity RMU3 formed near the end of magmatism at Hut Point Peninsula and the onset of magmatism at Mt. Erebus (approximately 0.8 to 1 Ma). The seismic re flectors identi fied as unconformities RMU1, RMU2, and RMU3 correlate with clastic units separating diamictite intervals in the AND ‐1B borehole (Figure 5).

6. Implications for the Post Middle Miocene Evolution of the VLB and the Strength of the Lithosphere 6.1. Tectonomagmatic Evolution of the VLB The post middle Miocene tectonic and magmatic evolution of the VLB is inferred from (i) the subsidence his- tories inferred from the isochore maps (Figures 9 –11), (ii) the relative ages of fault movements observed in the seismic data (Figure 4), (iii) the ages of the regional R ‐series unconformities as determined from ties to boreholes (Figure 5 and Table 2), and (iv) the ages of the local RMU series of unconformities as deduced by matching the space ‐time pattern of subsidence around Ross Island with eruption ages (Table 1). We also con- sider changes in the sedimentation rate between each adjacent pair of R ‐series unconformities, which were estimated by normalizing the sediment thickness by the duration of the respective isochore interval (Figure 12). The sedimentation rates are believed to be overestimated close to Ross Island, particularly north

of the Mt. Terror coast, where seismic data are sparse. Sedimentation rates in the R gto R hand R hto R iinter- vals may be overestimated everywhere given the uncertainty in the ages of the R gand R hunconformities dis- cussed in section 4.1. The VLB was a quasi ‐rectangular north to north ‐northwest striking basin during the period between forma-

tion of unconformities R gand R i(Figure 10). We assign this to the age range 12.2 to 4.3 Ma, but note that unconformity R gmay be as old as 14.2 Ma based on the AND ‐2A core (section 4.1). Sedimentation rates dur- ing the R gto R iinterval were relatively uniform throughout most of the basin (Figures 12a and 12b). Higher sedimentation rates were present in the Discovery Graben on the western edge of the northern VLB and in the Terror Rift that trends along the axis of the southern VLB (although the thickness and sedimentation rates in this area may be overestimated, as noted in section 4.2). Numerous faults in the Discovery Graben

and Terror Rift offset horizons R gand R hand terminate near horizon R i, indicating that the basin was tec- tonically active during this time (Figure 4). The relatively high sedimentation rates in the Discovery Graben indicate more rapid subsidence along the western margin of the northern VLB in comparison to the south- ern VLB, which is attributed to the more focused nature of extension in the northern VLB (particularly dur-

ing the R gto R hinterval; Figures 10 and 12a). Sedimentation rates decreased in the southern Discovery

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Figure 12. Sedimentation rate during intervals between R ‐series unconformities. (a) Isochore interval R gto R h(12 to 7.6 Ma). If an older age of 14 Ma is chosen for unconformity R gas discussed in the text, sedimentation rates should be scaled by 72%. (b) R hto R i(7.6 to 4.3 Ma). If an older age of 11.3 Ma is chosen for unconformity R has discussed in the text, sedimentation rates should be scaled by 47%. (c) R ito R k(4.3 to 1.8 Ma). (d) R kto sea floor (1.8 Ma to present). Black dots indicate edge of Ross Ice Shelf, with area lacking seismic coverage indicated by diagonal lines. B, Mt. Bird; T, Mt. Terror; E, Mt. Erebus; FI, Franklin Island; BI, Beaufort Island; C.I., contour interval.

Graben between formation of unconformities R hand R i(7.6 to 4.3 Ma) and increased in the northeastern VLB, indicating broadening of the region of extension in the northern VLB in Late Miocene and Early Paleogene time (Figures 10 and 12b). This was accompanied by narrowing of the zone of extension in the southern VLB, as inferred from the decreased sedimentation rate on the east and west flanks of the basin.

The subsidence pattern changed after formation of unconformity R i(4.3 Ma). From 4.3 to 1.8 Ma (unconfor- mities R iand R k), the central and southern parts of the basin (southward from a position midway between Beaufort and Franklin Islands) maintained the overall north ‐northwest striking rectangular shape present in the underlaying isochore intervals (Figures 9c and 12c). Sediment accumulations in the southern part of the basin are concentrated most strongly in quasi ‐elliptical depocenters around Ross Island, Beaufort Island, and smaller volcanic centers in the southern Terror Rift that are interpreted to be the result of flex- ural subsidence due to volcanic loading of the lithosphere. Flexural subbasins formed sequentially around

Mt. Bird (between unconformities R iand RMU2), Mt. Terror (between RMU2 and R k), Hut Point Peninsula (R kand RMU3), and Mt. Erebus (RMU3 and the sea floor) at times corresponding to the eruption ages of the respective volcanic center (Figure 11). In the northern VLB, the broad ‐scale strike of the basin

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Table 4 Sedimentation Rates in the Victoria Land Basin Isopach interval Duration (million years) Volume (m 3)a Rate (m 3/year)

Total b 13 6 6 Rgto sea floor 12.2 (14) 3.3 × 10 2.7 × 10 (2.4 × 10 ) Major subsidence phases b 13 6 6 Rgto R i 7.9 (9.7) 2.2 × 10 2.8 × 10 (2.3 × 10 ) 13 6 Rito Sea floor 4.3 1.1 × 10 2.6 × 10 R‐series unconformities b 13 6 6 Rgto R h 4.6 (6.4) 1.3 × 10 2.8 × 10 (2.0 × 10 ) b 13 6 6 Rhto R i 3.3 (6.7) 0.8 × 10 2.5 × 10 (1.2 × 10 ) 13 6 Rito R k 2.5 0.5 × 10 2.0 × 10 13 6 Rkto sea floor 1.8 0.6 × 10 3.4 × 10 RMU ‐series unconformities 12 6 Rito RMU1 1.3 3.1 × 10 2.4 × 10 RMU1 to RMU2 0.5 9.5 × 10 11 1.9 × 10 6 11 6 RMU2 to R k 0.7 9.8 × 10 1.4 × 10 11 6 Rkto RMU3 0.8 6.6 × 10 0.8 × 10 RMU3 to sea floor 1.0 8.0 × 10 11 0.8 × 10 6 aSediment volume calculated in area between Coulman High and Western Shelf shown in Figure 12. bAlternate inter- val durations discussed in text and corresponding rates shown in parentheses.

shifted from north ‐northwest to north ‐northeast during the R ito R kisochore interval (Figure 12c). Narrow elongate north ‐northwest striking depocenters formed within the basin at high angles to the overall strike of the basin and following the trend of the older Discovery Graben. These depocenters are separated by parallel north ‐northwest striking highs that formed on the formerly south striking Lee Arch. Since 1.8 Ma (horizon

Rk), subsidence in the northern VLB has concentrated primarily in relatively narrow troughs on either side of the Lee Arch (Figure 12d). Subsidence in the southern VLB has concentrated around Ross Island, Beaufort Island, and in a broad north ‐northeasterly striking trough trending northeastward from Ross Island toward the Western Shelf. Given the north ‐northeast regional extension direction, we interpret the northeast trending trough to be an extensional transfer linking zones of transtensional faulting in the Discovery Graben and Terror Rift. The orientation of these structures, as well as the smaller scale depocenters within the northern VLB, appear to be controlled by the older north striking fabrics formed during the

earlier period of westerly directed extension. Faults cutting horizon R k(and in some places, the sea floor) in the Discovery Graben and Terror Rift indicate extension continued in these areas throughout the Pliocene and Quaternary periods (Figure 4). Together, the isochore and sedimentation rate maps show the time interval between about 12.2(?) to 4.3 Ma

(horizons R gand R i) to have been a period in which extensional tectonics dominated the subsidence pattern in the VLB. The southern VLB formed a roughly symmetric fault bounded basin during this time, with the most rapid sedimentation occurring along the axis of the basin in the Terror Rift. Extension in the northern part of the VLB was initially concentrated along the western margin of the basin in the Discovery Graben,

spreading into the eastern half of the northern VLB after 7.3 Ma (horizon R h). After 4.3 Ma, the northern VLB reoriented to a north ‐northeasterly strike, and deformation within the basin became partitioned first into a series of distinct north ‐northwest striking elongate depocenters separated by structural highs

(between horizons R iand R k, 1.8 Ma) and finally into two narrow north ‐striking troughs on either side of the Lee Arch (between horizon R k and the sea floor). The southern VLB shifted from a north to a north ‐northwesterly strike during this time, and subsidence in this part of the basin became increasingly dominated by flexure around Ross Island and other volcanic centers.

6.2. Sediment Flux Sediment flux into the VLB was estimated by spatially integrating the sedimentation rate (Figure 12) within

the area bounded by the Western Shelf and Coulman High at the R khorizon level (Figures 12c and 12d), excluding the area beneath Ross Island (Table 4). Sediment flux is nearly uniform when integrated over time 6 3 −1 scales of ca. 5 million years or more, averaging 2.7 × 10 m year since the formation of unconformity R g

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Table 5 Flexural Rigidity for Subbasins Around Ross Island a b Volcano Subbasin width, X 0(km) Flexural rigidity, D (N ‐m) Elastic thickness, T e(km) Mt. Bird 40 4.90 × 10 19 1.8 Mt. Terror 50 12.96 × 10 19 2.4 Hut Point Peninsula 18 0.20 × 10 19 0.6 Mt. Erebus 30 1.55 × 10 19 1.2 aFlexural rigidity calculated for a point load on an unbroken elastic plate according to D = α4Δρ g, where gis 9.8 m s −2, 3 −3 Δρ = 0.5 × 10 kg m is the assumed density contrast between the mantle and basin fill, and α≈X0/4 is the flexural b 2 1/3 parameter (Lambeck & Nakiboglu, 1980). Effective elastic thickness is given by T e= [12D(1 ‐ν)/E] , where ν= 0.25 is Poisson's ratio and E = 10 11 Pa is Young's modulus.

(about 12.2 Ma). A moderately lower rate of 2.4 × 10 6m3year −1is obtained if an older age of 14 Ma is

assumed for unconformity R g(section 4.1). The rate is nearly the same in the middle Miocene through early Pliocene relatively amagmatic period of extension (horizons R gto R i) and the early Pliocene to modern period (horizon R ito the sea floor) in which magmatism played a major role in the formation of depocenters in the southern VLB. The nearly uniform sedimentation rate may indicate a relatively constant time ‐averaged sediment supply into the basin since the middle Miocene Period. Sediment fluxes show a moderately higher variation over the finer time scales given by the isochore intervals de fined 6 6 3 −1 between each set of R ‐series of unconformities, ranging from 2.0 × 10 to 3.4 × 10 m year (the R hto 6 3 −1 Riinterval has an anomalously low rate of 1.2 × 10 m year if an age of 11.3 Ma is assumed for unconformity R h). Relatively high sediment flux occurred during the Late Miocene R gto R hisochore interval (12.2 to 7.6 Ma), when subsidence is interpreted to have been primarily controlled by tectonic extension. Sediment fluxes into the basin decreased with time between the middle Miocene and Late

Pliocene Periods (unconformity R k) before increasing again in the Quaternary. The increased magmatism and associated flexural subsidence after formation of horizon R i(4.3 Ma) did not measurably change the sediment flux into the basin when averaged over million year time scales or more (although we note that we did not include the volcanic edi fices in the flux calculation). We infer that volcaniclastic strata either constitute a relatively minor part of the sediments in the basin, or the in flux of volcaniclastic strata was balanced by a decrease in pelagic sedimentation and/or a decrease in sediment transported into the basin. The sediment flux shows a modest decline over time within the area de fined by the subcrop of the RMU 6 3 −1 series of unconformities (Figure 11), from 2.4 × 10 m year during the early Pliocene (R ito RMU1, 4.3 6 3 −1 to 3 Ma) to 0.8 × 10 m year since the early Pleistocene (intervals R kto RMU3 and RMU3 to the sea floor, 1.8 to 1 Ma and 1 Ma to present, respectively). This may be due to a decrease in volcanism on Ross Island, with smaller volume younger eruptions creating smaller flexural basins than the earlier

eruptions (Mt. Bird and Mt. Terror). The net sediment flux into the VLB during the R ito R ktime interval was comparable to the flux during the earlier R gto R hand R hto R iintervals (Table 4), suggesting the flexural subbasins that formed after the R iunconformity were filled to the spill level, with excess sediment transported into other parts of the VLB.

6.3. Flexure and Strength of the Lithosphere The Ross Island flexural moat is shown to be a composite of smaller flexural subbasins that formed at differ- ent times and positions in response to loading of the lithosphere as the different volcanoes comprising Ross Island erupted. The widths of these flexural subbasins range from 18 to 50 km, including the distance from the coastline to the respective volcano. This corresponds to flexural rigidities ranging from 0.20 × 10 19 to 12.96 × 10 19 N‐m, assuming an unbroken elastic plate and a point load located at the position of the respec- tive volcano (Table 5). This is about one to three orders of magnitude lower than flexural rigidities estimated from the shape of the composite moat or from regional studies. A higher rigidity, 10 23 N‐m, is obtained by modeling the bathymetric moat and treating the lithosphere as a semi ‐in finite (broken) plate with the edge laying beneath Ross Island (Stern et al., 1991). Intermediate values, ranging from 0.4 × 10 21 to 1 × 10 21 N‐m, are obtained from unbroken plate models of the stratigraphic surfaces within the basin and gravity data (Aitken et al., 2012; Chen, 2015; Ji et al., 2017; Ten Brink et al., 1997). The intermediate values are similar to the rigidity estimates of 0.8 × 10 21 to 2.2 × 10 21 N‐m that are obtained in more regional models close to

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the Transantarctic Mountains front (Ji et al., 2017). The results here thus favor a relatively weak lithosphere in comparison to previous studies and suggest that models based on the total subsidence in the composite moat are biased toward high values. The rigidity estimates obtained here (below 10 20 N‐m, with elastic thick-

ness T e< 3 km) are low in comparison to typical values for oceanic and continental lithosphere (which are 21 generally greater than 10 N‐m, or T e> 5 km) but are comparable to values estimated for active midocean ridge systems, oceanic lithosphere under seamounts that were emplaced near midocean ridges (but have since moved), and seamounts emplaced on thermally rejuvenated oceanic lithosphere (e.g., the French Polynesia seamounts) (McKenzie & Fairhead, 1997; Watts & Burov, 2003; Watts & Zhong, 2000). Values that 19 are only slightly higher (D = 86.5 × 10 N‐m, T e= 4.6 km) are obtained in the broad basin and range con- tinental extensional province of western North America (Lowry & Smith, 1994). The results obtained here are thus consistent with emplacement of the Ross Island volcanoes on relatively warm (thermally juvenile) lithosphere within an active continental extensional province on the western side of the WARS.

7. Summary Two ‐dimensional multichannel seismic re flection pro files are used to map four Late Miocene and younger

regional unconformities in the VLB (from oldest to youngest: R g,R h,R i, and R k) and three intervening local unconformities (RMU1, RMU2, and RMU3) that are interpreted to have formed between periods of volcan- ism on Ross Island. The ages of the regional unconformities are established from previously published ties to the DSDP 273; CIROS ‐1; CRP ‐1, ‐2, and ‐3; and AND ‐1B and 2A drillholes. The ages of the regional uncon- formities are estimated from their stratigraphic position and by comparison of space ‐time patterns of subsi- dence with the timing of volcanic eruptions on Ross Island. Depth ‐structure and isochore maps constructed for each unconformity and the intervening strata reveal a two ‐phase subsidence history of the VLB since middle Miocene time. The maps show that subsidence in the VLB between about 12 to 7.6 Ma was controlled primarily by faulting on the flanks of the Terror Rift in the southern VLB and in the Discovery Graben on the northwest flank of the basin in the northern Acknowledgments VLB and that rifting was largely amagmatic during that time. The extension and subsidence rates slowed We thank Terry Wilson for suggesting we look at flexural subsidence around after 4.3 Ma, and subsidence in the southern VLB has since been dominated by flexure around Ross Ross Island and Derek Witt and Jace Island and smaller intrusions in the Terror Rift to the north. A series of small flexural subbasins developed Koger for their insights during many successively around Ross Island since 4.3 Ma, at positions near Mt. Bird, Mt. Terror, Hut Point Peninsula, discussions. We thank three anonymous reviewers, whose detailed and finally Mt. Erebus. In composite, these subbasins form a semicontinuous sedimentary moat surrounding comments helped us to improve the the eastern, northern, and western sides of Ross Island. The widths of the subbasins suggest the flexural presentation of arguments and to more strength of the lithosphere around Ross Island during the Neogene has ranged from 0.20 × 10 19 to thoroughly assess the implications. We 19 are grateful to Chris Sorlien for his 12.96 × 10 N‐m. The period 7.6 to 4.3 Ma was a transitional period, during which extensional tectonism helpful review on an early version of dominated basin morphology, but volcanism on Ross Island and White Island began to in fluence the pattern this paper and for sharing his of subsidence in the southern VLB. Relative uplift of the Lee Arch began during this time and continued into preliminary interpretations, and to the Ross Map team, Huw Horgan, Marvin the Pliocene Period, partitioning the northern VLB into two elongate north ‐trending troughs along the east- Speece, and Stephen Pekar for sharing ern and western margins of the basin. The easternmost of these depocenters became inactive by early their seismic data sets. This material is Pleistocene time, and a southeast trending transfer zone developed linking extension in the Terror Rift in based upon work supported by the National Science Foundation under the southern VLB to extension in the Discovery Graben along the western margin of the northern VLB. Grant numbers 1043700 and 1169553 Except for continued subsidence in the Mt. Erebus flexural basin east of Ross Island, the VLB has been lar- and under Cooperative Agreement gely quiescent since approximately 1 Ma. number 0342484 through subawards administered and issued by the ANDRILL Science Management Of fice at the University of Nebraska ‐Lincoln, References as part of the ANDRILL U.S. Science Acton, G., Crampton, J., Di Vincenzo, G., Fielding, C. R., Florindo, F., Hannah, M., et al. (2008). Preliminary integrated chronostratigraphy Support Program. Seismic data used in of the AND ‐2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica. Terra Antarctica ,15 (1), 211 –220. this project can be obtained from the Aitken, R. A., Wilson, G. S., Jordon, T., Tinto, K., & Blakemore, H. (2012). Flexural controls on late Neogene basin evolution in southern Antarctic Seismic Data Library System McMurdo Sound, Antarctica. Global and Planetary Change ,80 ‐81 ,99 –112. https://doi.org/10.1016/j.gloplacha.2011.08.003 (SDLS). Digital records of seismic Akan, M. 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