Kinematics of the Neogene Terror Rift: Constraints from Calcite Twinning Strains in the ANDRILL Mcmurdo Ice Shelf (AND-1B) Core, Victoria Land Basin, Antarctica

The ANDRILL McMurdo Ice Shelf (MIS) and Southern McMurdo Sound (SMS) Drilling Projects themed issue

Kinematics of the Neogene Terror Rift: Constraints from calcite twinning strains in the ANDRILL McMurdo Ice Shelf (AND-1B) core, Victoria Land Basin, Antarctica

Timothy S. Paulsen1,†, Terry J. Wilson2, Christie Demosthenous1, Cristina Millan2, Rich Jarrard3, and Andreas Läufer4 1Department of Geology, University of Wisconsin Oshkosh, 800 Algoma Boulevard, Oshkosh, Wisconsin 54901, USA 2Byrd Polar Research Center and School of Earth Sciences, The Ohio State University, 108 Scott Hall, 1090 Carmack Road, Columbus, Ohio 43210, USA 3Department of Geology and Geophysics, 717 WBB, University of Utah, 135 S. 1460 East, Salt Lake City, Utah 84112-0111, USA 4Bundesanstalt fü r Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover, Germany

ABSTRACT INTRODUCTION Neogene sedimentary and volcanic rocks recovered by the ANDRILL (ANtarctic geological We report new strain analyses of mechani- The Antarctic continental interior has under- DRILLing) McMurdo Ice Shelf (MIS) AND-1B cally twinned calcite in veins hosted by Neo- gone repeated rifting events within the West drill core from the Terror Rift of the West Ant- gene (13.6–4.3 Ma) sedimentary and volcanic Antarctic Rift system (Fig. 1) since the creation arctic Rift (Fig. 1) to better defi ne strain patterns rocks recovered from the Terror Rift system of the Antarctic plate by ca. 80 Ma during Gond- and their relation to existing kinematic models in the southern Ross Sea, Antarctica, by the wana breakup (LeMasurier, 1990; Tessensohn for the evolution of the rift system. ANDRILL (ANtarctic geological DRILLing) and Wörner, 1991; Boger, 2011). The rift system McMurdo Ice Shelf (MIS) Project. Strain is fl anked by the Transantarctic Mountain uplift GEOLOGIC BACKGROUND analyses of the ANDRILL MIS AND-1B along the edge of the East Antarctic craton (ten drill core samples yield prolate and oblate Brink et al., 1997), shows multiple episodes of Rifting within the West Antarctic Rift system ellipsoids with principal shortening and faulting (Davey and Brancolini, 1995; Salvini commenced during Mesozoic breakup of Gond- extension strains ranging from –7% to 9%, et al., 1997), and is marked by Miocene to active wana and has continued episodically into the respectively. The majority of samples show volcanoes (LeMasurier, 1990). Deformation Neogene (Cooper et al., 1987; Tessensohn and ≤25% negative expected values, indicating within the rift system is of widespread inter- Wörner, 1991; Wilson, 1993; Encarnación et al., homogeneous coaxial strain characterized est because of its signifi cance for understand- 1996; Cande et al., 2000; Rocchi et al., 2002, predominantly by subvertical shortening. We ing the breakup of Gondwana and constrain- 2003; Henrys et al., 2007; Tonarini et al., 1997; attribute the subvertical shortening strains ing Mesozoic to Cenozoic global plate circuits Fielding et al., 2008; Sutherland, 2008; Wilson to mechanical twinning at relatively shallow (Steinberger et al., 2004; Granot et al., 2010, and Luyendyk, 2009; Granot et al., 2010, 2013). depths in an Andersonian normal faulting 2013). Rocks within the rift system have been Tectonic models for the rift system typically stress regime induced by sedimentary and ice the subject of geophysical studies (Bosum et al., invoke phases of extension in the Cretaceous sheet loading of the stratigraphic sequence 1989; Behrendt et al., 1991, 1996; Damaske and Cenozoic (Cooper et al., 1987; Tessen- and characterized by low stress magnitudes. et al., 1994; Davey and Brancolini, 1995; Salvini sohn and Wörner, 1991; Davey and Brancolini, Oriented samples yield a northwest-southeast et al., 1997; Luyendyk et al., 2001, 2003; Finn 1995; Salvini et al., 1997; Fitzgerald, 2002; average extension direction that is subparallel et al., 2005; Karner et al., 2005; Hall et al., 2007; Karner et al., 2005; Huerta and Harry, 2007; to other indicators of Neogene extension. This Henrys et al., 2007; Fielding et al., 2008), out- Bialas et al., 2007; Siddoway, 2008; Wilson northwest-southeast extension is consistent crop-based structural analyses (Wilson, 1995; and Luyendyk, 2009). Seismic studies indicate with strain predicted by Neogene orthogonal Storti et al., 2001, 2008; Rossetti et al., 2000, at least two distinct periods of Cenozoic rifting rifting in a north-northeast–trending rift seg- 2002, 2003, 2006; Läufer et al., 2003) and (Paleogene and Neogene) within the western ment, as well as models of right-lateral trans- thermochronological work (Fitzgerald, 1992, Ross Sea. The youngest phase produced a Neo- tensional rifting. The overall paucity of a non- 2002; Lisker, 2002; Lisker and Läufer, 2013). gene fault system superimposed on the Creta- coaxial layer-parallel shortening signal in the However, there are few structural analyses of ceous(?) to Paleogene Victoria Land rift basin in AND-1B twin populations favors orthogonal rift basin strata sampled by drilling (Wilson and the western Ross Sea known as the Terror Rift extension in the Neogene Terror Rift system, Paulsen, 2000, 2001; Millan et al., 2007; Wilson (Fig. 1; Cooper et al., 1987; Davey and Branco- but could also be due to spatial partitioning of et al., 2007), despite the fact that such data have lini, 1995; Salvini et al., 1997; Hall et al., 2007; strain in a transtensional rift regime. the potential to provide important insights into Henrys et al., 2007; Fielding et al., 2008). Neo- the geodynamic evolution of rifting. The intent gene faulting within the Terror Rift has occurred †Email: [email protected] of this paper is to present new strain data from concomitant with the eruption of an extensive

Geosphere; October 2014; v. 10; no. 5; p. 828–841; doi:10.1130/GES01002.1; 9 ﬁ gures; 2 tables. Received 22 November 2013 ♦ Revision received 18 April 2014 ♦ Accepted 4 June 2014 ♦ Published online 18 August 2014

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alkali volcanic province along the Transant- Transantarctic Mountain rift fl ank to right-lateral their model, encompasses reverse, strike-slip, arctic Mountain rift fl ank and offshore locali- oblique rifting during the Cenozoic (Wilson, and normal faulting in the Transantarctic Moun- ties collectively known as the McMurdo Vol- 1995; Salvini et al., 1997; Storti et al., 2001, tain rift fl ank uplift and offshore regions of the canic Group (Bosum et al., 1989; LeMasurier, 2008; Rossetti et al., 2000, 2002, 2003, 2006). western Ross Sea (Fig. 1). In this model, Paleo- 1990; Kyle, 1990; Behrendt et al., 1991, 1996; Other geodynamic models for Mesozoic–Neo- gene to recent faulting accommodates right- Damaske et al., 1994). Active volcanism charac- gene rifting call for simple orthogonal extension lateral oblique extension induced by a transfer terizes the Terror Rift today, and interpretations (Busetti et al., 1999; Trey et al., 1999; Cande of shear from right-lateral transform faulting of marine seismic data indicate that faulting is et al., 2000; Karner et al., 2005; Davey and De along the mid-ocean ridge in the Southern Ocean locally as young as the Pliocene to Pleistocene Santis, 2006; Davey et al., 2006; Wilson and (Salvini et al., 1997; Storti et al., 2007). (Hall et al., 2007; Henrys et al., 2007). Luyendyk, 2009; Granot et al., 2010, 2013). The debate surrounding the nature of rift Previously workers have attributed the pres- Storti et al. (2008) challenged Cenozoic orthog- kinematics in the western Ross Sea exists ence of transtensional fault arrays in the south- onal rifting models, and argued for a Paleogene because spatial and temporal patterns of strains ern and northern Victoria Land sectors of the inception of the Terror Rift, which, according to recorded within the rift system are poorly documented. Kinematic studies of fault arrays within the Transantarctic Mountains rift fl ank Neogene volcanics are hampered because faults in these areas typi- 165° E Devonian-Jurassic cally occur in rocks that are no younger than Jurassic (Warren, 1969). Seismic studies of fault Beacon Supergp. systems in the submarine rift basins are ham- Neoroterozoic- pered because of limited information on fault NVL early Paleozoic kinematics. There have, however, been drilling Ross orogenic belt projects that have recovered deformed Cenozoic 180° strata from the western sector of the rift system (Fig. 1). Transantarctic Mountains West Antarctic One of the most important drill cores yet Rift System obtained of Neogene strata fi lling the Terror Rift is the ~1285 m ANDRILL MIS AND-1B Mount drill core (Naish et al., 2009). The AND-1B Melbourne 75° S drill core was recovered in the Windless Bight region of the Ross Ice Shelf in 2006–2007 Victoria (Figs. 1 and 2; Naish et al., 2007). The core Land includes interbedded late Miocene to Pleisto- Basin Central cene sedimentary (diatomite, diamictite, sand- Basin stone, and shale) and volcanic rocks (Fig. 3; Terror Rift Ross 180 AND-1B WARS CRP x Ross Sea NWMIS-1 SE Island WANT EANT AND-1B 1.0 s AND2A 100 km 90 E 90 W MIS1 seafloor TAM SVL Ross Ice Shelf Ri = 4.3–3.6 Ma 0 Mount Morning two-way traveltime two-way Figure 1. Simplifi ed regional map of Terror Rift faults and Neo- gene volcanoes in the offshore Victoria Land Basin and the adjacent 2.0 s Transantarctic Mountains rift fl ank uplift within the West Antarc- tic Rift system of the western Ross Sea. Arrows indicate shear sense samples along faults that have been previously proposed to involve compo- analyzed nents of right-lateral slip. Open circles mark the locations of the ANDRILL (ANtarctic geological DRILLing) and Cape Roberts Figure 2. Interpreted seismic section MIS-1 drill sites. MIS-1 shows the location of the interpreted seismic sec- showing the location of the AND-1B bore- tion in Figure 2. AND-1B—ANDRILL McMurdo Ice Shelf Drilling hole with respect to the seismic refl ectors Project, AND-2A—ANDRILL Southern McMurdo Sound Drilling and faults that offset refl ectors below the Project, CRP—Cape Roberts Drilling Projects, EANT—East Ant- Ri seismic refl ector, which is interpreted as arctica, NVL—northern Victoria Land, SVL—southern Victoria a ca. 4.3–3.6 Ma regional unconformity in Land, TAM—Transantarctic Mountains, WANT—West Antarctica, the Victoria Land Basin (Wilson et al., 2012; WARS—West Antarctic Rift system (compiled from Salvini et al., modifi ed from Naish et al., 2005). See Fig- 1997; Wilson , 1999; Storti et al., 2008). ure 1 for location of seismic line.

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Figure 3. Stratigraphic column of the AND- (≤20 MPa) (Lacombe and Laurent, 1996; Ferrill, 1B drill core (Krissek et al., 2007) showing 1998; González-Casado et al., 2006). This prop- the locations of the samples (stars) ana- erty makes twinning strain analyses a powerful lyzed in this paper with respect to the main tool that has been used to study paleostrain and lithologic units, stratigraphic ages, and the stress patterns of rock cements and secondary Ri seismic refl ector that is interpreted as veins found in outcrops and drill cores from a regional 4.3–3.6 Ma unconformity in the relatively undeformed sectors of plate interiors Victoria Land Basin. Dark arrow shows the (Lacombe et al., 1990; Craddock et al., 1993; main phase of Terror Rift faulting based on Craddock and Pearson, 1994; van der Pluijm interpretation of seismic data in the Victoria et al., 1997), as well as orogenic belts (Engelder, Land Basin (Fielding et al., 2008). Letters 1979; Teufel et al., 1984; Craddock et al., 1988, A–F mark clusters of samples from differ- 2000, 2007; Kilsdonk and Wiltschko, 1988; ent depth intervals (mbsf—meters below Evans and Dunne, 1991; Lacombe et al., 1994; seafl oor) analyzed in this study that corre- González-Casado and García-Cuevas, 1999; late with stereonets shown in Figure 7. Light Craddock and Relle, 2003; González-Casado arrow marks time span over which less-pro- et al., 2003) and rift systems (Friedman and nounced faulting occurred within the basin. Heard, 1974; Lomando and Engelder, 1984; LSU 1-8—Lithostratigraphic units after Craddock et al., 1997). Krissek et al. (2007). Ri Reflector Other than one study of calcite twinning strains in cements and veins within early Paleo- A zoic rocks of the Ellsworth Mountains (Crad- * dock et al., 1998), there have been no other Krissek et al., 2007; McKay et al., 2009; Di calcite strain analyses reported from Antarctica. Roberto et al., 2010; Cody et al., 2012; Ross In the case of the Terror Rift, calcite twinning et al., 2012; Williams et al., 2012; Wilson et al., analyses allow us to evaluate the problem of 2012). Systematic fracture logging of the core B Neogene rifting strains in a new way, namely identifi ed ~1400 natural fractures (i.e., preexist- by determining whether mechanical twins in ing fractures in the rock intersected by coring) * calcite recovered from the faulted stratigraphic that are dominated by normal faults and calcite * section record strain patterns expected if defor- veins (Figs. 4 and 5; Wilson et al., 2007). Steep mation occurred within Andersonian strike-slip extension veins folded by compaction (Fig. or normal faulting stress regimes. If deforma- 5A), fault breccias (Fig. 4B), and unfolded tion of the calcite occurred due to horizontal extension veins (Fig. 5B) indicate deforma- compression associated with strike-slip tec- tion of a continuum of sublithifi ed to lithifi ed * C tonism, then we expect mechanical twinning to rock during burial and lithifi cation of the strati- have recorded subhorizontal maximum short- graphic sequence (Wilson et al., 2007; Millan, D ening strains. In contrast, deformation due to 2013). Natural fractures are present in the high- * the lithostatic loading associated with a normal est core that the Core Structure Measurement faulting stress regime predicts subvertical maxi- Group was able to log (i.e., not retrieved in * mum shortening strains. Calcite strain analyses

plastic liners) at the ANDRILL McMurdo Ice Rift Faulting Terror can also yield important information about the Shelf Drill Site Laboratory, with steep conju- direction of extension within the Terror Rift at gate faults occurring ~42 m below seafl oor E the time of twinning. Here we provide new con- (mbsf) (Wilson et al., 2007). There is a zone * straints on strains associated with deformation of high fracture density in an interval between * within the Terror Rift, through calcite twinning ~125 and 300 mbsf that is estimated to range in strain analyses performed on 19 samples col- age from ca. 1.6 to 3 Ma (Wilson et al., 2012). lected from the AND-1B core. Natural fractures are ubiquitous in the core * below ~450 mbsf (Wilson et al., 2007), consis- F METHODS tent with the presence of normal faults detected * on seismic sections below the Ri seismic refl ec- * We collected samples from the AND-1B core tor (Figs. 2 and 3; Horgan et al., 2005) that has with the purpose of studying fracture fi lls to been interpreted as a regional ca. 4.3–3.6 Ma supplement fracture logging conducted at the unconformity in the Victoria Land Basin (Field- AND-1B drill site during coring operations. We ing et al., 2008; Wilson et al., 2012). refer to the samples by their depths (in mbsf) The natural fractures recovered in the AND- in the core. From this sample suite we selected 1B core assume regional signifi cance in the recorded by mechanically twinned calcite found 19 samples of vein calcite from a depth range problem of West Antarctic Rift system tectonics along veins, faults, clast margins, and in cavities of 473 to 1279 mbsf that appeared to have suf- because they are rare examples of rift structures at depths ≥473 mbsf within the AND-1B core. fi cient numbers of twinned calcite grains to that crosscut Neogene strata and can be directly Calcite acts as an extremely sensitive strain determine three-dimensional (3-D) strain ellip- studied. This paper focuses on Neogene strains gauge by twinning at low differential stresses soids using the calcite strain-gauge technique

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A B ple 674.82), a calcite-lined open cavity in a volcanic fl ow (Fig. 6A; sample 648.13), and a calcite-rimmed sedimentary clast margin (sample Up Up 473.84). All of the samples were oriented with Core Core respect to the up direction and an arbitrary north direction marked by a red scribe line drawn along the length of the core (e.g., Figs. 4B, 4C, 5A, and 5B). striated calcite We followed standard procedures for con- along fault ducting the optical universal stage microscope calcite measurements of c-axis and twin orientations, cemented twin and grain thicknesses, and the number of twins within mechanically twinned calcite fault breccia grains (Turner and Weiss, 1963; Evans and Groshong, 1994). We measured and analyzed thin and thick twin sets and assigned an average thickness of 0.5 μm for thin twins. Cal- cite has three possible twin glide horizons that can be activated if the principal stresses have ~10 mm ~20 mm 911.97 1279.2 suitable orientation and suffi cient magnitude to achieve the critical resolved shear stress C D (~10 ± 5 MPa) required to induce mechanical calcite filled normal fault twinning in optimally oriented grains (Turner normal fault et al., 1954; Jamison and Spang, 1976; Ferrill, 1998; Lacombe and Laurent, 1996; Lacombe, Up Up 2001; González-Casado et al., 2006). The cal- Core Core cite strain-gauge technique can yield accurate strain tensor results for as few as fi ve twin sets in the absence of a complex strain history and measurement errors. However, in practice, more robust ellipsoid results are assured with the measurement and analysis of higher numbers vein web subsidiary of twin sets, which helps maximize 3-D cover- extension age of twin optic orientations (Groshong, 1974; Groshong et al., 1984). In cases where we had veins ample sample material from the drill core (n = 5; samples 473.84, 674.82, 905.24, 1115.66, and 1279.57), we analyzed twins in as many as 25 grains in 2 orthogonal thin sections (Groshong et al., 1984). For the remaining samples (n = ~20 mm ~10 mm 14), we measured twins in as many as 50 grains 905.24 1265.66 in a single or multiple parallel thin sections. We used the CSG99 strain-gauge software Figure 4. Photos showing examples of fractures that contain mechanically twinned calcite (Evans and Groshong, 1994) to conduct calcite sampled and analyzed as part of this study. Sample numbers in lower left. (A) Striated cal- strain-gauge analyses on the optically cleaned cite along a normal fault. (B) Calcite-cemented fault breccia. (C) A web of extension veins data sets. The fi rst step in a calcite strain-gauge associated with a normal fault. (D) Subsidiary extension veins associated with a normal analysis involves determination of a bulk strain fault. Note that in B and C, the arbitrary north direction used for initial fracture logging is tensor using all of the data collected (referred marked by a red scribe line drawn along the length of the core. to as ALL) from a calcite aggregate (Gro- shong, 1972, 1974; Groshong et al., 1984). The technique determines the expected value (Groshong, 1972, 1974). Figure 3 shows the veins subsidiary to a normal fault (Fig. 4D; sam- of strain for each twin set given the calculated locations of the samples within the AND-1B ple 1265.66), and an ~55° dipping fracture clas- bulk strain tensor, as well as the deviation of core. Our sample suite includes twinned calcite sifi ed as a possible fault based on its dip angle the expected and measured values of strain for along striated faults (Fig. 4A; samples 911.97 during structural logging (sample 842.01). The each set (Groshong, 1974). These initial results and 1085.71), cemented fault breccias (Figs. 4B sample suite also includes 6, ~1–2-mm-thick are, in turn, used to guide the removal of 20% and 6C; samples 1279.2 and 1279.57), anasto- steeply dipping (>75°) folded extension veins of the data with the largest magnitude devia- mosing webs of extension veins that likely mark (Figs. 5A and 6D; samples 961.29, 1115.62, tions between expected and measured strains shear zones within the core (Fig. 4C; samples 1115.66, 1119.36, 1193.53, and 1274.34), an from the entire data set. This cleaning procedure 905.25, 918.74, 920.83, 930.04), extension unfolded extension vein (Figs. 5B and 6B; sam- improves strain ellipsoid accuracy by reducing

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A B 2007), indicating that the maximum principal strain axes of the strain ellipsoids and the c-axes Up Up of the data sets can be viewed in an Earth sur- Core Core face frame of reference. Stereonets in Figure 7 show the three princi- extension vein pal strain axes and c-axis distributions for the individual sample analyses of the ALL, LDR, PEV, and NEV data sets, as well as the optically culled OCLDR, OCPEV, and OCNEV data sets. Figure 8 shows cumulative stereonet plots of the principal shortening (e1) and extension (e3) strain axes determined from the individual sample analyses. Table 1 provides the details of the calcite strain data analyses for the raw data set (ALL). Table 2 provides the details folded of the calcite strain data analyses for the LDR, extension PEV, and NEV data sets, as well as the optically veins culled OCLDR, OCPEV, and OCNEV data sets. Tables 1 and 2 also show differential stresses ~20 mm ~20 mm inferred from average twin densities using the 1115.62 &1115.66 674.82 method of Rowe and Rutter (1990).

Figure 5. Photo and unrolled whole-core scan image of the outer surface of the AND-1B core RESULTS showing examples of extension veins that contain mechanically twinned calcite sampled and analyzed as part of this study. Sample numbers in lower left. (A) Steeply dipping, folded Twinned and untwinned calcite grains occur extension veins. Scale on left shows cm (major divisions) and mm (minor divisions). (B) An within fracture ﬁ lls in the samples and range unfolded extension vein. Note the arbitrary north direction used for initial fracture logging, in morphology from ﬁ brous to elongate blocky marked by a red scribe line drawn along the length of the core. and blocky calcite spar (Figs. 6A–6D). Grain sizes measured perpendicular to twins in the raw data set (ALL) range from ~85 µm to 780 µm. data scatter that is likely related to inhomoge- concentrations on the strain analyses by reduc- Twin densities of the 6 folded vein samples are neous strains and possible measurement errors ing c-axis concentrations by culling 0%–63% of generally higher than the majority of the twin (Groshong, 1974; Teufel, 1980; Groshong twin sets whose grains contributed to optic axis densities in the remaining 13 samples (e.g., et al., 1984). The remaining data set (largest concentrations until optic axes had ≤5 sigma 61 twins/mm versus 34 twins/mm average in deviations removed; referred to as LDR) is then distributions on Kamb (1959) contour plots. ALL; Table 1). Thick twins and bent twins tend used for a new strain analysis. Twin sets within These optically culled data sets (referred to as to be more common in the folded vein sample grains that are expected to twin due to the cal- OCLDR, OCPEV, and OCNEV) were then ana- suite. Overall, twin thicknesses (thin and thick), culated bulk strain tensor are known as posi- lyzed following the analytical procedure out- twin morphologies, and twin densities (e.g., 5 tive expected values (PEV), whereas twin sets lined here. twins/mm to 71 twins/mm in ALL) suggest that within grains that are not expected to twin due Strain ellipsoids for four samples (905.24, twinning generally occurred at temperatures to the calculated bulk strain (i.e., they have the 911.97, 920.83, and 930.04) were reoriented <200 °C (Burkhard, 1993; Ferrill, 1991; Fer- wrong sense of shear) are referred to as negative with respect to north by matching features in rill et al., 2004), consistent with AND-1B bore- expected values (NEV) (Groshong, 1974). LDR core imagery with correlative features observed hole logging studies that indicate a 76.7 °C/km strain-gauge analyses that yield >40% NEV in oriented borehole televiewer imagery (see contemporary temperature gradient at the drill could signal noncoaxial or inhomogeneous Jarrard et al., 2001a; Paulsen et al., 2002). Bore- site (Morin et al., 2010). Differential stresses strains (Groshong, 1974; Teufel, 1980). In such hole televiewer logging could not be conducted inferred from average twin densities range from a case, the original data (i.e., the entire data set deeper than 1018 mbsf because these borehole 68 to 265 MPa for the raw data set (ALL) (Rowe prior to LDR cleaning) can be divided into PEV depths exceeded the length of the logging cable and Rutter, 1990), but these values are likely and NEV data sets and analyzed separately to (Morin et al., 2007). Four samples (1265.66, unreliable overestimates because the twin den- evaluate the possibility of noncoaxial deforma- 1274.34, 1279.20, and 1279.57) from these sity technique is not applicable to low-tempera- tion (Groshong, 1974; Teufel, 1980). depths were therefore reoriented with respect ture (<200 °C) deformation (Burkhard, 1993; Calcite grain c-axes are not uniformly dis- to north by rotating the average bedding dip Ferrill, 1998). tributed in our data sets (they range from hav- direction of their respective intact core intervals Mechanical twinning of calcite due to the ing 5–10 sigma distributions on Kamb contour to match the average in situ bedding dip direc- application of noncoaxial vertical and hori- plots); this is probably related to our limited tion (320°) determined from oriented intact zontal loads predicts high NEV percentages numbers of orthogonal thin sections, the lim- core intervals. The remaining samples (n = 11) (>40%) following LDR analysis (Teufel, 1980), ited range of view offered by the universal stage and their respective strain ellipsoid results are but LDR data for 18 of 19 samples show ≤25% (Stauffer, 1966), and possibly to mineral growth oriented with respect to the up direction in the negative expected values (Table 1). Collectively, within fractures (Bons et al., 2012). We evalu- core, but not with respect to north. The AND-1B these results indicate homogeneous coaxial ated the impact of the presence of the c-axis borehole is within ~2° of vertical (Morin et al., strain and are consistent with a single or multi-

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A B ple approximately coaxial deformation episodes (Teufel, 1980). The sole exception is sample 473.84 (~30% NEV for LDR data), for which Up Up we divided and analyzed the PEV and NEV data Core Core separately. LDR strain analyses of our samples yield strain ellipsoid results that are similar to the strain results produced by analysis of the raw data set (ALL). LDR strain analyses yield prolate (58%), oblate (32%), and plane (10%) strain ellipsoids with principal shortening (e1) cavity and extension (e3) strains ranging from –6.7% to 8.6%, respectively (Table 2). LDR data from a majority of the samples (n = 17 of 19) and the PEV data split for sample 473.84 yield similar strain ellipsoid orientations (subvertical maximum shortening and subhorizontal maximum extension axes) (Table 2; Figs 7 and 8). The only two analyses that did not yield subvertical ~2 mm ~2 mm shortening directions include the LDR results 648.14 674.84 for sample 648.13 and the LDR/NEV data split for sample 473.84, both of which show shallow C D maximum principal shortening and stretching axes. The principal shortening directions (e1) Up Up yielded by the LDR data are similar to the e1 Core Core directions yielded by the raw data set (ALL) (i.e., they are also dominated by subvertical e1 axes). The LDR data e1 axes show less scatter with respect to the e1 axis population yielded by the raw data (ALL), as would be expected because of the increased accuracy produced by the reduction of inhomogeneous strains and possible measurement errors (Groshong, 1974; Teufel, 1980; Groshong et al., 1984). fault Optic axis populations of the optically culled data (OCLDR) (Fig. 7) can be divided into three breccia types based on their distribution. Type 1 samples (1115.66, 1265.66, 1274.34, and 1279.57) have optic axis populations that show nearly random distributions. Type 2 samples (473.84, ~2 mm ~0.5 mm 648.13, 674.81, 905.24, 911.97, 920.83, 961.29, 1279.57 1115.62 1085.7, 1193.53, and 1279.2) show good 3-D optic axis distributions that have weak girdle or Figure 6. Photomicrographs illustrating morphologies of mechanically twinned calcite broad bimodal patterns. Type 3 samples (842.01, grains within the AND-1B core. Sample numbers in lower left. (A) Euhedral blocky mor- 918.74, 930.04, 1115.62, 1119.36) show limited phology in an open cavity in a volcanic ﬂ ow. (B) Blocky morphology in an unfolded, steeply to fair 3-D optic distributions with girdle patterns dipping extension vein. (C) Blocky morphology in a calcite-cemented fault breccia. (D) Elon- for relatively small populations of optic axes gate blocky morphology in a steeply dipping folded vein. (≤17). These optic axis distributions are gener-

Figure 7 (on following page). Lower hemisphere equal area plots show calcite strain-gauge results for samples of twinned calcite along veins, faults, clasts, and cavities in the AND-1B drill core. Numbers refer to strain ellipsoidal axes determined from the calcite strain analyses: 1 = e1 (shortening), 2 = e2 (intermediate), and 3 = e3 (extension). Negative numbers are shortening strains. Filled circles are optic axis orientations, great circles show orientation of fractures and/or veins containing calcite with measured twins. PEV—positive expected values; NEV—negative expected values; ALL—entire raw data set; LDR—20% largest deviations removed from the entire data set; OCLDR— 20% largest magnitude deviations removed from data set that was ﬁ rst culled to minimize optical concentrations. Equal signs indicate where LDR and OCLDR analyses used identical data sets because optic axes in LDR analyses did not exceed 5 sigma concentrations (i.e., no culling of optical data was required). Letters A–F correlate with intervals of increasing depth within the AND-1B stratigraphic section shown in Figure 3.

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Calcite Strain Gauge Results -ALL- -LDR- -OCLDR- -1=principal shortening, 2=intermeditate, 3=principal extension 961 (folded vein)

-ALL- -PEV- -OCPEV- -1 -1 -1 -A- 473 (clast) -NEV- -OCNEV- -2 -2 3 3 -2 3 3 3 2 2-2 -1 -22 -1 -E- 1085 (normal fault) PEV -1 PEV -1 3 3 3 -1 3 3 NEV 3 NEV -2 -2 2 -1 -ALL- -LDR- -OCLDR- -1 -1 -B- 648 (flow cavity) 1115.62 (folded vein) -1 3 3 3 -2 -2 3 3 3 -2 -2 -2 -2 -1 -1 -1 -1 -1 = 674 (extension vein) 1115.66 (folded vein) -2 -2 -2 3 3 3 3 3 3 -1 -1 -1 -1 -1 -1 2 2 -2 -C- 842 (normal fault?) 1119 (folded vein) 3 3 -2 3 3 2 3 -1 -1 -2 -1 -2 -1 -1 3 -1 = -2 -2 1193 (folded vein) -D- 905 (vein web) N N N -2 -2 -2 3 3 3 3 -1 -1 -1 3 -1 -1 3 -1 2 2 -2 911 (normal fault) -F- 1265 (extension veins) N N N N N N -2 2 -1 3 3 3 2 3 -1 -1 3 -1 -2 -1 3 -1 2 2 918 (vein web) 1274 (folded vein) N N N 2 -2 2 -1 -1 -1 -1 -1 3 -1 3 3 3 3 3 2 -2 -2 920 (vein web) 1279.2 (fault breccia) N N N N N N -2 -1 -1 -1 -1 -1 -2 -2 2 3 3 -1 -2 3 3 3 3 -2 930 (vein web) 1279.57 (fault breccia) N N N N N N 2 2 3 3 -1 3 3 -1 -1 -1 -1 3 -1 3 2 2 -2 -2

Figure 7.

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AN/TN AN/TN TN (shortening) axes of two of these nine samples ALL (1115.62 and 1279.2) are independently con- ﬁ rmed by strain results from samples 1115.66 and 1279.57, which come from the same two intact core intervals and show subvertical e1 (shortening) axes that are the best-constrained strain axes. The subvertical e1 (shortening) 106° axes for sample 1115.62 and four other of the nine samples (961.29, 1119.36, 1193.53, and 1274.34) are independently supported by subvertical shortening implied by folding of their LDR respective veins around subhorizontal axes. PEV Reorientation of LDR and OCLDR strain NEV ellipsoids for samples 905.24, 911.97, and 930.04, with respect to north yields east-west to northwest-southeast maximum principal 104° extension axes, whereas sample 920.83 yields a north-south maximum principal extension (e3) axis (Fig. 7). Reorientation of LDR and OCLDR strain ellipsoids for samples 1265.66, 1274.34, 1279.20, and 1279.57 with respect to OCLDR north yields east-northeast–west-southwest to OCPEV northwest-southeast maximum principal exten- OCNEV sion (e3) axes.

DISCUSSION AND CONCLUSIONS

109° Deformation Timing

All of the calcite samples studied herein were collected below a basin-wide unconformity (i.e., e1 Shortening Axes e3 Extension Axes Oriented the Ri seismic refl ector) that is ca. 4.3–3.6 Ma at e3 Extension Axes 440 mbsf in the AND-1B core (Figs. 2 and 3). Figure 8. Lower hemisphere equal area plots e1 (shortening) and e3 (extension) axes yielded This correlates with the ca. 13 Ma to ca. 4 Ma by ALL, LDR, and OCLDR analyses (see text). ALL—entire raw data set; LDR—20% larg- stratigraphic interval that is dominated by normal est deviations removed from the entire data set; OCLDR—20% largest magnitude devia- faulting in marine seismic data of the Victoria tions removed from data set that was fi rst culled to minimize optical concentrations (OC); Land Basin, as well as normal faults and opening PEV—positive expected values; NEV—negative expected values. All of the e1 and e3 axes mode veins in the AND-1B core (Wilson et al., are oriented with respect to the vertical core axis and the up direction (i.e., Earth surface 2007, 2012; Hall et al., 2007; Henrys et al., 2007; frame of reference). Oriented e1 and e3 axes are shown with respect to true north (TN), Fielding et al., 2008; Millan, 2013). Twinned whereas unoriented e1 and e3 axes are shown with respect to an arbitrary north (AN) used calcite along or proximal to striated faults (sam- in initial fracture logging. Fisher mean directions (black squares) and alpha-95 cones of ples 911.97, 1085.7, and 1265.66) suggests that confi dence are shown for e3 axes that have been oriented with respect to TN. twinning might have occurred, at least in part, concomitant with faulting of the AND-1B stratigraphic sequence (e.g., Laurent, 1987). Age con- ally similar to other twin analyses of calcite veins show subvertical principal shortening and sub- straints permit faulting, tensile fracturing, and that have yielded interpretable paleostrain and horizontal principal extension directions. mechanical twinning of fracture fi lls to be as old stress results (Kilsdonk and Wiltschko, 1988; Experimental studies indicate that the best- as the ca. 13 Ma to ca. 4 Ma stratigraphic inter- Craddock et al., 2007; Lacombe et al., 2009). constrained principal strain axis is the axis that vals in which they are hosted. However, faulting The optically culled OCLDR, OCPEV, and has a magnitude with opposite sign with respect as young as Pliocene to Pleisto cene age has been OCNEV data sets yield strain ellipsoid axis to the other two principal axes (Groshong documented elsewhere for the Terror Rift (Hall magnitudes (–6% to 8%) and orientations that et al., 1984). Of 19 samples, 10 yield posi- et al., 2007; Henrys et al., 2007; Fielding et al., are virtually the same as the results yielded by tive (extensional) e2 axes as a result of LDR, 2008), and is possible for fractures (and mechani- the LDR, PEV, and NEV analyses, regardless of OCLDR, or PEV analyses, meaning that their cal twins) within the AND-1B core. The greater differences in optic axis distribution types (i.e., subvertical negative (shortening) e1 axes are the abundance of veins below the Ri unconformity types 1–3) (Table 2; Figs. 7 and 8). The optic best-resolved strain axes. The nine remaining and the interpretation that folded veins formed axis concentrations present in some of the sam- samples yield negative (shortening) e2 axes as during compactional volume loss infl uenced by ples therefore appear to have a negligible impact a result of LDR or OCLDR analyses, meaning diagenetic reactions and collapse of early cement on the fi rst-order results of the LDR, PEV, and that their shallow e3 (extension) axes are the framework (Millan, 2013) argue for a late Mio- NEV strain analyses, which predominantly best-resolved strain axes. The subvertical e1 cene to Pliocene age for vertical shortening.

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Stress Magnitudes

The ~10 ± 5 MPa critical resolved shear stress that is generally thought to be required for mechanical twinning of calcite (Turner et al., 1954; Jamison and Spang, 1976; Ferrill, 1998; Lacombe and Laurent, 1996; Lacombe, PLES 2001; González-Casado et al., 2006) would require a minimum differential stress of ~20 ± 10 MPa to cause twinning on optimally oriented twin glide horizons. The lack of precise —number of twin sets measured; NEV— 0 age constraints for the time of twinning within our samples precludes deﬁ nitive reconstruction of the lithostatic loads at the time of twinning. However, a conservative ﬁ gure for the present effective maximum vertical stress (lithostatic method Host rock

Orientation stress with an assumed hydrostatic pore pressure removed) at the depths from which we collected our samples (473–1279 mbsf with an additional ~900 m water depth) has been Fabric determined, using core grain density measure- interpretation ments and calculated fractional porosities, to range from 3.7 to 12.5 MPa (Niessen et al., 2013). Cyclic vertical loading of the crust by stress (MPa) —number of twin sets used in strain analysis; N Differential Differential

F the repeated grounding of continental ice sheets ing negative; B, P—bearing, plunge; LNS—layer-normal shortening; LPS—layer-parallel has also occurred since ca. 12 Ma in the Ross Sea (McKay et al., 2009; Naish et al., 2009) and he core axis and arbitrary north. probably increased the vertical stress during the Density

(twins/mm) Last Glacial Maximum by ~6.5 MPa at the drill site (Niessen et al., 2013), yielding effective e3 (%) vertical stresses ranging from ~10 to 19 MPa. Cumulative loads like these would place our

e2 samples within the depth window for twin- (%) ning in an ambient stress ﬁ eld characterized by vertical uniaxial compression or low minimum e1 (%) horizontal stresses. It is also possible that twinning reﬂ ects greater loads or smaller assumed oor; ALL—all data used in strain analysis; N oor; pore pressures than held by the assumptions, or (°) e3 stress concentrations associated with faulting and/or bulk vertical compaction of the sequence. Stress concentrations would permit twinning at (°)

e2 shallower depths earlier in the burial history, but even in this case, the subvertical shortening strains are still ultimately related to the force

Strain orientation (B, P) Principal strains imposed by the lithostatic load. (°) e1

Stress Regime and Rift Kinematics (%) NEV The principal stress axes at shallow depths

0 (<2 km) within the crust are generally expected /N F to have subvertical (SV) and subhorizontal orientations (Anderson, 1951; Engelder, 1993). The vertical stress in the crust is due to the gravita-

Data tional force exerted by the column of overlying analyzed N

TABLE 1. DETAILED RESULTS OF CALCITE STRAIN-GAUGE ANALYSES USING ALL DATA COLLECTED FROM THE ANDRILL MCMURDO ICE SHELF CORE SAM ANDRILL THE COLLECTED FROM DATA ALL USING ANALYSES OF CALCITE STRAIN-GAUGE RESULTS 1. DETAILED TABLE rock (i.e., the lithostatic pressure). The vertical stress is expected to induce a subvertical maxi- 4.8 ALL 59/59 30 143, 39 336, 50 238, 7 –2.34 0.32 2.02 54 244 IS? NA clast margin vein 6.48 ALL 24/24 8 6 114, 217, 65 021, 24 –0.14 –0.08 0.22 5 68 LPS? IS? NAow mineralized cavity in volcanic ﬂ 6.73 ALL 67/67 9 183, 63 345, 26 079, 8 –2.51 –0.10 2.62 29 198 LNS NA extension vein 9.10 ALL 52/52 12 101, 70 205, 05 296, 19 –2.49 0.32 2.18 45 231 LNS BHTV extension vein web associated with normal faults 9.10 ALL 35/35 20 304, 18 187, 55 045, 29 –1.68 –0.23 1.91 31 203 LPS BHTV calcite along striated normal fault 9.10 ALL 32/32 13 095, 86 317, 3 3 227, –3.84 0.05 3.79 50 239 LNS NA extension vein web 9.10 ALL 55/55 24 047, 46 257, 39 154, 16 –2.27 –0.10 2.37 32 206 LNS BHTV vein web 9.10 ALL 26/26 4 129,89 220, 0 2 310, –2.89 0.62 2.27 63 256 LNS BHTV extension vein web 9.10 ALL 55/55 9 58 323, 124, 31 219, 9 –2.61 –0.35 2.96 51 240 LNS NA folded vein 9.99 ALL 53/53 17 203, 42 057, 42 310, 18 –1.42 –0.00 1.42 25 187 LNS NA calcite along striated fault age 11.04 11.04 11.04 ALL 11.04 ALL 26/26 ALL 54/54 15 26/26 22 003, 8 15 043, 74 153, 81 157, 74 16 211, 272, 5 337, 16 302, 3 067, 0 –8.52 –6.58 –2.50 –3.96 1.77 11.02 –0.80 4.81 4.76 71 60 60 265 252 252 LPS LNS LNS NA NA NA folded vein folded vein folded vein 11.04 11.04 11.04 ALL ALL 22/22 48/48 23 12 121, 71 239, 86 002, 9 037, 8 17 269, 7 306, –4.53 –1.40 –0.21 –0.06 5.94 0.26 60 10 252 119 LNS LNS NA bedding extension veins branching from normal fault folded vein ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ 13.57 ALL 59/59 15 010, 52 178, 38 272, 6 –3.14 0.03 3.11 61 253 LNS bedding folded vein and blocky spar along normal fault 13.57 ALL 55/55 18 006, 48 192, 41 099, 3 –2.58 –0.65 3.23 34 210 LNS bedding calcite-cementing fault breccia 13.57 ALL 53/53 30 204, 81 347, 7 077, 5 –1.73 0.12 1.61 41 224 LNS bedding calcite-cementing fault breccia (Ma) ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ mum principal shortening strain as it drives the Fracture rearrangement of grains during porosity loss

ANDRILL—ANtarctic geological DRILLing Program; mbsf—meters below seaﬂ associated with burial (Ramsay and Wood, 1973). Mechanical twinning of calcite occurs *Strain axes oriented with respect to core axis (vertical) and true north; all other strain are t Note: Depth (mbsf) negative expected values; e1, e2, and e3 are the maximum, intermediate, minimum principal strains with shortening strain be shortening; IS—inhomogeneous strain; BHTV—borehole televiewer; NA—not applicable. 473.84 648.13 674.82 842.01905.24* <8.8 ALL 8/8 0 26 175, 314, 58 076, 19 –1.67 0.03 1.63 21 174 LPS NA mineralized fault? 911.97* 918.74 920.83* 930.04* 961.29 1085.7 1115.62 1115.66 1119.36 1193.53 1265.66* 1274.34* 1279.2* 1279.57*

836 Geosphere, October 2014

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/5/828/3333271/828.pdf by guest on 27 September 2021 Kinematics of the Neogene Terror Rift — F d minimum principal strains with rst optically culled to minimize optical concentrations; N method Host rock Orientation S N L Fabric interpretation data set that was fi 2 1 neous strain; NA—not applicable; BHTV—borehole televiewer. 2 stress (MPa) Differential Differential 5 he core axis and arbitrary north. 3 Density (twins/mm) 5 1 . e3 (%) 2 4 1 . e2 (%) 0 – 2 0 . e1 2 (%) – oor; LDR—largest magnitude deviations removed from the entire data set; OCLDR—largest 9 , 6 (°) e3 0 1 0 2 , (°) e2 0 0 2 8 6 , Strain orientation (B, P) Principal strains (°) e1 5 5 3 5 (%) —number of twin sets measured; NEV—negative expected values; PEV—positive e1, e2, e3—maximum, intermediate, an NEV 0 0 3 5 /N / F 0 2 R D L C NEV 18/59 0 066, 6 327, 60 160, 29 –2.02 –0.29 2.31 44 229 LPS? IS? NA Data TABLE 2. DETAILED RESULTS OF CALCITE STRAIN GAUGE ANALYSES USING DATA COLLECTED FROM THE ANDRILL MCMURDO ICE SHELF CORE SAMPLES ANDRILL THE COLLECTED FROM USING DATA ANALYSES OF CALCITE STRAIN GAUGE RESULTS 2. DETAILED TABLE OCLDR 23/67 9 197, 73 340, 14 072, 10 –2.09 –0.31 2.40 28 196 LNS OCLDR 26/52 15 70 114, 203, 00 293, 20 –1.50 –0.02 1.52 40 222 LNS OCLDR 24/35OCLDR 10/32 8 10 46 289, 181, 17 105, 58 077, 39 007, 5 –1.23 274, 31 0.48 –5.61 0.75 0.49 5.13 30 62 201 255 LNS LNS OCPEV 23/59 13 147, 53 321, 37 053, 3 –4.85 0.42 5.27 56 247 LNS OCLDR 8/26 0 019, 71 201, 19 1 111, –2.72 –0.71 3.43 65 258 LNS OCLDR 24/55 17 297,83 092, 6 183, 3 –1.73 0.49 1.24 29 198 LNS OCLDR 27/55 22 323, 60 129, 30 223, 6 –2.03 –0.31 2.35 47 234 LNS OCLDR 31/53 6 224, 51 060, 38 324, 8 –0.70 0.15 0.54 19 167 LNS OCLDR 17/26OCLDR 26/54 6 23 190, 57 033, 31 044, 72 297, 10 212, 18 303, 4 –6.29 –2.03 –4.32 8.32 –0.02 4.34 63 56 256 247 LNS LNS OCLDR 16/26 25 349, 70 151, 20 243, 6 –3.57 –1.30 4.88 54 244 LNS OCLDR 16/22 0 137, 68 003, 15 269, 15 –3.98 –1.96 5.94 61 253 LNS OCLDR 34/48OCLDR 12 20/59 197, 62 10 052, 23 017, 49 316, 14 172, 38 –0.22 272, 13 0.05 –2.29 0.16 –1.18 3.47 8 55 103 246 LNS LNS OCLDR 20/55 5 153, 80 335, 10 244, 0 –0.84 –0.24 1.08 17 159 LNS O OCNEV 9/59 0 061, 13 313, 54 160, 33 –1.46 –0.76 2.21 45 231 LPS? IS? analyzed N 4.8 PEV 41/59 12 146, 51 322, 39 054, 2 –3.97 0.03 3.94 59 251 LNS NA clast margin vein 6.48 LDR 20/24 10 6 119, 69 224, 027, 21 –0.12 –0.02 0.15 4 51 LPS? NAow mineralized cavity in volcanic fl 6.73 LDR 54/67 7 188, 73 348, 16 079, 6 –1.87 –0.26 2.13 26 190 LNS NA extension vein 9.10 LDR 42/52 10 096, 63 203, 09 297, 25 –1.54 0.06 1.48 38 218 LNS BHTV extension vein web associated with normal faults 9.10 LDR 28/35 7 45 276, 169, 17 064, 40 –1.00 0.10 0.91 27 193 LNS BHTV calcite along striated normal fault 9.10 LDR 26/32 8 82 146, 344, 8 254, 3 –2.52 –0.34 2.87 44 229 LNS NA extension vein web 9.10 LDR 44/55 14 064,68 264, 21 172, 7 –1.65 –0.25 1.90 26 190 LNS BHTV vein web 9.10 LDR9.10 21/26 LDR 44/55 0 16 063, 84 210, 5 334, 64 137, 25 300, 3 230, 7 –2.58 –1.86 0.22 –0.31 2.36 2.16 61 50 253 239 LNS LNS BHTV NA extension vein web folded vein 9.99 LDR 43/53 9 215, 40 074, 43 324, 21 –0.77 –0.07 0.84 22 178 LNS NA calcite along striated fault age 11.04 LDR11.04 21/26 LDR11.04 44/54 0 LDR 14 184, 56 21/26 031, 31 046, 74 14 294, 13 15 211, 301, 88 302, 4 –6.73 150, 2 –1.92 –4.79 060, 1 8.65 0.18 –3.79 4.61 68 –0.39 4.18 57 262 53 248 LNS 243 LNS NA LNS folded vein NA NA folded vein folded vein 11.04 LDR11.04 18/22 LDR 11 39/48 134, 67 23 008, 14 57 210, 274, 18 013, 32 –4.53 108, 8 –0.80 –0.19 5.33 0.01 56 0.18 7 247 LNS 93 LNS NA folded vein bedding veins branching from normal fault extension ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ 13.57 LDR 48/59 8 017, 62 177, 27 272, 8 –2.34 –1.03 3.38 56 247 LNS bedding folded vein and blocky spar along normal fault 13.57 LDR13.57 44/55 LDR 43/53 2 21 047, 79 178, 7 177, 68 339, 21 269, 8 071, 6 –1.29 –0.45 –1.24 1.74 0.13 1.12 27 36 193 214 LNS LNS bedding calcite-cementing fault breccia bedding calcite-cementing fault breccia (Ma) ≤ ≤ ≤ ≤ ≤ ≤ ≤ ≤ Fracture rst culled to minimize optical concentrations; OCPEV/NEV—positive and negative expected value data sets yielded by analysis of ANDRILL—ANtarctic geological DRILLing Program; mbsf—meters below seafl *Strain axes oriented with respect to core axis (vertical) and true north; all other strain are t Note: shortening strain being negative; B, P—bearing, plunge; LNS—layer-normal shortening; LPS—layer-parallel IS—inhomoge number of twin sets used in strain analysis; N Depth (mbsf) 648.13 473.84 674.82 842.01905.24* <8.8 LDR 7/8 14 77 217, 092, 7 001, 10 –3.72 –1.79 5.51 22 178 LNS NA mineralized fault? 911.97* 918.74 920.83* 961.29 930.04* 1085.7 1115.62 1115.66 1119.36 1193.53 1265.66* 1274.34* 1279.2* 1279.57* set that was fi

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under relatively low differential stresses (≤20 because of possible twin plane rotations dur- 165° E MPa; Lacombe and Laurent, 1996; Ferrill, 1998; ing folding has negligible impact on the Fisher González-Casado et al., 2006) and when litho- mean directions for the ALL (109°, 31° alpha- Terror Rift static loads are suffi cient and SV > SH (the maxi- 95), LDR (106°, 33° alpha-95), and OCLDR mum horizontal stress), twinning is expected (113°, 35° alpha-95) data sets. The anomalous to record a subvertical shortening strain (Fried- long axis (e3) of the strain ellipsoid for sample Ross man and Heard, 1974; Lomando and Engelder, 920.83 could be a signal of radial extension dur- Sea Transantarctic Mountains 1984, Craddock et al., 2000; González-Casado ing burial compaction (Millan, 2013). Alterna- et al., 2006). The fi rst-order results yielded by tively, it could mark an artifact related to the lim- CRP-Oligocene twinned calcite in the AND-1B samples record a ited number of oriented samples, errors, or noise Faults & Bedding Ross relatively uniform strain pattern characterized by that could cause a large strain ellipsoid rotation Island subvertical shortening and shallow (layer paral- because of small differences in strain magnitude lel) extension, indicative of twinning in a normal between the intermediate (approximately east- faulting stress regime (S > S ). Relatively small west; e2) and maximum (approximately north- HPV V H AND-1B bedding dips (~10° for stratigraphic intervals south; e3) axes of the strain ellipsoid. AND-2A deeper than ~900 mbsf; depositional ages ca. The dominant east-southeast–west-northwest 77.5° S 9 Ma or older) and differences in optical axis average extension direction yielded by individ- WIV distributions appear to have had little impact on ual analyses is consistent with the west-north- Ice Shelf these fi rst-order results. The subvertical shorten- west and east-southeast dip directions of north- Mount 50 km Morning Vent Alignments ing strains indicated by the majority of the cal- northeast–striking faults interpreted on marine Pleistocene cite data are therefore consistent with the normal seismic sections at depth at the drill site (by T.J. faulting stress regime indicated by the domi- Wilson and S. Henrys) as well as the dominant Figure 9. Simplifi ed regional map show- nance of normal faults within the AND-1B core northeast strike of folded veins from the AND- ing the AND-1B and Cape Roberts Project and demonstrated by marine seismic sections 1B core (Millan, 2013). Northwest-southeast (CRP) drill holes with respect to the major across the Terror Rift to the north of Ross Island extension is subparallel to the west-northwest structural and volcanic elements of the (Hall et al., 2007; Henrys et al., 2007). (~288° azimuth) deviation of the AND-1B bore- Erebus Volcanic Province in the western Vic- Twinning due to subvertical shortening (com- hole from vertical (from 700 to 1024 mbsf), toria Land rift basin (modifi ed from Storti paction) appears to be substantiated by varia- which might refl ect updip prodding of the drill et al., 2008). White arrows show extension tions in twin densities. Rock mechanics stud- bit by a population of fractures with a dominant directions indicated by different data sets. ies show that twin densities will increase with southeast dip direction within this section of Northwest-southeast extension indicated by increasing strain at low temperatures (<350 °C) the borehole (Morin and Wilkens, 2005; Morin calcite data in the AND-1B core is sub parallel (Rybacki et al., 2013). The correlation of high et al., 2010). Northwest-southeast extension to extension indicated by Pleistocene vent twin densities with folded veins is therefore is also subparallel to the northwest-southeast alignments on the Mount Morning volcano consistent with the accruement of greater twin- extension direction implied by the elonga- and faults hosted in Oligocene strata recov- ning strains in veins that formed earlier in the tion of the ca. 1.3 Ma Hut Point and younger ered by the CRP. Oligocene east-northeast compaction of their hosting stratigraphic inter- than 7.7 Ma White Island volcano complexes, extension is indicated by the dip direction of vals (Wilson et al., 2007; Millan, 2013). Lower Pleistocene to contemporary volcano align- bedding in the CRP borehole and drill cores twin densities in some or in all of the remain- ments at Mount Morning (Paulsen and Wilson, (Jarrard et al., 2001b). HPV—Hut Point vol- ing samples might therefore refl ect twinning 2009), and faults hosted in Oligocene strata in cano, WIV—White Island volcano. later in the burial history when smaller porosity the Cape Roberts drill core (Fig. 9; Wilson and reductions were insuffi cient to cause buckling of Paulsen, 2001). preexisting veins. All of the folded veins yield Northwest-southeast extension indicated by to horizontal compression within an Ander- subvertical shortening strains, which are con- the calcite twin data is oblique to the overall sonian strike-slip faulting stress regime, but sistent with the subvertical shortening implied north-northwest strike of longitudinal faults local in homogeneous strain-stress fi elds are also by the folds. This, coupled with the ≤25% in the Terror Rift system north of Ross Island. possible. The nearly orthogonal fl ip between NEV yielded by the folded veins, suggests that However, the northwest-southeast extension the e1 and e3 axes for PEV and NEV data from possible inhomogeneous strains associated with direction is at a high angle to a northeast-strik- sample 473.84 makes polyphase deformation twin plane rotation during folding were suc- ing rift segment that has been inferred in the dubious (Burkhard, 1993). The overall paucity cessfully reduced and/or that twinning occurred area of the drill site and to the south (Johnston of a noncoaxial layer-parallel shortening signal late with respect to folding (Groshong, 1974). et al., 2008) in the same general region where in the AND-1B twin populations indicates that However, we cannot preclude the possibility of the Transantarctic Mountain Front rift border the horizontal compressive stresses hypothe- some twin plane rotation in the folded vein sam- fault is inferred to change trend and step west- sized by transtensional kinematic models for the ples, and the folded vein strain ellipsoid results ward across a regional accommodation zone rift system have been absent or of insuffi cient should therefore be viewed with this in mind. (Wilson, 1999). Northwest-southeast extension magnitude to cause a widespread noncoaxial The azimuths of the long axes of strain ellip- is consistent with strain predicted by orthogo- strain overprint. The general lack of accompa- soids yielded by the ALL, LDR, and OCLDR nal rifting in a northeast-trending rift segment, nying horizontal shortening in the twin data sug- data sets yield virtually indistinguishable 106° as well as tectonic models that invoke Neogene gests that the Neogene Terror Rift system lacks (27° alpha-95), 104° (28° alpha-95), and 109° right-lateral transtensional rifting. a strong longitudinal strike-slip component (i.e., (30° alpha-95) Fisher mean directions, respec- Layer-parallel shortening in samples 473.84 it favors orthogonal extension), but it is possible tively (Fig. 8). Excluding sample 1274.34 (NEV) and 648.14 could refl ect twinning due that the virtual absence of horizontal shorten-

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ing could be due to spatial partitioning of strain Behrendt, J.C., Saltus, R., Damaske, D., McCafferty, A., fold and thrust belt [abs.]: Journal of African Earth Sci- Finn, C.A., Blankenship, D., and Bell, R.E., 1996, ences, v. 27, p. 49–50. within a transtensional rift system. Patterns of late Cenozoic volcanic and tectonic activ- Craddock, J.P., Nielson, K.J., and Malone, D.H., 2000, Given the young age for the calcite twinning ity in the West Antarctic rift system revealed by aero- Calcite twinning strain constraints on the emplace- strains, it is noteworthy that there is a rough cor- magnetic surveys: Tectonics, v. 15, p. 660–676, doi: 10 ment rate and kinematic pattern of the upper plate of .1029 /95TC03500 . the Heart Mountain Detachment: Journal of Structural relation between the average extension direc- Bialas, R.W., Buck, W.R., Studinger, M., and Fitzgerald, P., Geology, v. 22, p. 983–991, doi: 10 .1016 /S0191 -8141 tion recorded by the calcite and the present-day 2007, Plateau collapse model for the Transantarctic (00)00017 -1 . extension direction (070°–110°) predicted to be Mountains–West Antarctic Rift System: Insights from Craddock, J.P., McKiernan, A.W., and de Wit, M.J., 2007, numerical experiments: Geology, v. 35, p. 687–690, Calcite twin analysis in syntectonic calcite, Cape Fold induced at 100 km depth by geodynamic model- doi: 10 .1130 /G23825A .1 . 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Cooper, A.J., et al., eds., Geology and seismic stratig- Burkhard, M., 1993, Calcite twins, their geometry, appear- raphy of the Antarctic margin: American Geophysical to potential drivers such as mantle fl ow, there ance and signifi cance as stress-strain markers and Union Antarctic Research Series Volume 68, p. 167– is a clear need for continued acquisition of ori- indicators of tectonic regime: Journal of Structural 182, doi: 10 .1029 /AR068p0167 . ented strain data from the Terror Rift and adja- Geology, v. 15, p. 351–368, doi: 10 .1016 /0191 -8141 Davey, F.J., and De Santis, L., 2006, A multi-phase rifting (93)90132 -T . model for the Victoria Land Basin, western Ross Sea, cent sectors of the West Antarctic Rift system. Busetti, M., Spadini, F.M., Van der Wateran, F.M., Cloetingh , in Fü tterer, D.K., et al., eds., Antarctica: Contributions S., and Zanolla, C., 1999, Kinematic modeling of the to global earth sciences: Berlin, Springer, p. 303–308. ACKNOWLEDGMENTS West Antarctic Rift System, Ross Sea, Antarctica: Davey, F., Cande, S.C., and Stock, J., 2006, Extension in the We thank members of the ANDRILL (ANtarctic Global and Planetary Change, v. 23, p. 79–103, doi: 10 western Ross Sea region—Links between Adare Basin geological DRILLing) MIS (McMurdo Ice Shelf) .1016 /S0921-8181 (99)00052 -1 . and Victoria Land Basin: Geophysical Research Let- Project downhole logging team for their work that Cande, S.C., Stock, J.M., Muller, R.D., and Ishihara, T., 2000, ters, v. 33, L20315, doi: 10 .1029 /2006GL027383 . Cenozoic motion between East and West Antarctica: Di Roberto, A., Pompilio, M., and Wilch, T.I., 2010, Late enabled core reorientation studies after drilling and Nature, v. 404, p. 145–150, doi: 10 .1038 /35004501 . Miocene submarine volcanism in ANDRILL AND-1B the ANDRILL MIS Science Team for their contribu- Cody, R., Levy, R., Crampton, J., Naish, T., Wilson, G., drill core, Ross Embayment, Antarctica: Geosphere, tions to the project. We also thank Richard Groshong and Harwood, D., 2012, Selection and stability of v. 6, p. 524–536, doi: 10 .1130 /GES00537 .1 . and John Craddock for helpful reviews that improved quantitative stratigraphic age models: Plio-Pleistocene Encarnación, J., Fleming, T.H., Elliot, D.H., and Eales, H.V., this manuscript, Mark Evans for providing a copy of glacio marine sediments in the ANDRILL 1B drillcore, 1996, Synchronous emplacement of Ferrar and Karoo the calcite strain-gauge software, Rick Allmendinger McMurdo Ice Shelf: Global and Planetary Change, dolerites and the early breakup of Gondwana: Geology, for providing his stereonet program, and the many v. 96–97, p. 143–156, doi: 10 .1016 /j .gloplacha .2012 v. 24, p. 535–538, doi: 10 .1130 /0091 -7613 (1996)024 ANDRILL support personnel that made this project .05 .017 . <0535: SEOFAK>2 .3 .CO;2 . Cooper, A.K., Davey, F.J., and Behrendt, J.C., 1987, Seis- Engelder, T., 1979, Mechanisms for strain within the upper possible. Paulsen acknowledges the University of mic stratigraphy and structure of the Victoria Land Devonian clastic sequence of the Appalachian Pla- Wisconsin Faculty Sabbatical Program for release basin, Western Ross Sea, Antarctica, in Cooper, A.J., teau, western New York: American Journal of Science, time to work on this project and thanks the Antarc- and Davey, F.J., eds., The Antarctic continental mar- v. 279, p. 527–542, doi: 10 .2475 /ajs .279 .5 .527 . tic Research Centre at Victoria University of Wel- gin: Geology and geophysics of the western Ross Engelder, T., 1993, Stress regimes in the lithosphere: Prince- lington (New Zealand) for hosting this work. The Sea: Circum-Pacifi c Council for Energy and Mineral ton, New Jersey, Princeton University Press, 457 p. ANDRILL Program is a multinational collaboration Resources Earth Science Series, Volume 5B, p. 27–76. Evans, M.A., and Dunne, W.M., 1991, Strain factorization between the Antarctic Programs of Germany, Italy, Craddock, J.P., and Pearson, A.M., 1994, Non-coaxial hori- and partitioning in the North Mountain thrust sheet, New Zealand, and the United States. Antarctica New zontal shortening strains preserved in twinned amyg- central Appalachians, U.S.A: Journal of Structural dule calcite, DSDP Hole 433, Suiko seamount, north- Geology, v. 13, p. 21–35, doi:10 .1016 /0191 -8141 Zealand is the project operator, and has developed west Pacifi c plate: Journal of Structural Geology, v. 16, (91)90098 -4 . the drilling system in collaboration with Alex Pyne at p. 719–724, doi: 10 .1016 /0191 -8141 (94)90121 -X . Evans, M.A., and Groshong, R.H., Jr., 1994, A computer Victoria University of Wellington and Webster Drill- Craddock, J.P., and Relle, M., 2003, Fold axis-parallel rota- program for the calcite strain gauge technique: Journal ing and Enterprises Ltd. 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