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In 2007, the Antarctic Geological Drilling\ud Program (ANDRILL) drilled 1138.54 m of\ud strata ~10 km off the East Antarctic coast,\ud includ ing an expanded early to middle Miocene\ud succession not previously recovered\ud from the Antarctic continental shelf. Here,\ud we pre sent a facies model, distribution, and\ud paleoclimatic interpretation for the AND-2A\ud drill hole, which enable us, for the fi rst time,\ud to reconstruct periods of early and middle\ud Miocene glacial advance and retreat and\ud paleo environmental changes at an ice-proximal\ud site. Three types of facies associations\ud can be recognized that imply signifi cantly\ud different paleoclimatic interpretations. (1) A\ud diamictite-dominated facies association represents\ud glacially dominated depositional environments,\ud including subglacial environments,\ud with only brief intervals where ice-free coasts\ud existed, and periods when the ice sheet was\ud periodically larger than the modern ice sheet.\ud (2) A stratifi ed diamictite and mudstone facies\ud association includes facies characteristic of\ud open-marine to iceberg-infl uenced depositional\ud environments and is more consistent\ud with a very dynamic ice sheet, with a grounding\ud line south of the modern position. (3) A\ud mudstone-dominated facies association generally\ud lacks diamictites and was produced in a\ud glacially infl uenced hemipelagic depositional\ud environment. Based on the distribution of\ud these facies associations, we can conclude that\ud the Antarctic ice sheets were dynamic, with\ud grounding lines south of the modern location\ud at ca. 20.1–19.6 Ma and ca. 19.3–18.7 Ma and during the Miocene climatic optimum, ca.\ud 17.6–15.4 Ma, with ice-sheet and sea-ice minima\ud at ca. 16.5–16.3 Ma and ca. 15.7–15.6 Ma.\ud While glacial minima at ca. 20.1–19.6 Ma\ud and ca. 19.3–18.7 Ma were characterized by\ud temperate margins, an increased abundance\ud of gravelly facies and diatomaceous siltstone\ud and a lack of meltwater plume deposits suggest\ud a cooler and drier climate with polythermal\ud conditions for the Miocene climatic\ud optimum (ca. 17.6–15.4 Ma). Several periods\ud of major ice growth with a grounding line traversing\ud the drill site are recognized between\ud ca. 20.2 and 17.6 Ma, and after ca. 15.4 Ma,\ud with evidence of cold polar glaciers with ice\ud shelves. The AND-2A core provides proximal\ud evidence that during the middle Miocene climate\ud transition, an ice sheet larger than the\ud modern ice sheet was already present by ca.\ud 14.7 Ma, ~1 m.y. earlier than generally inferred\ud from deep-sea oxygen isotope records.\ud These fi ndings highlight the importance of\ud high-latitude ice-proximal records for the interpretation\ud of far-fi eld proxies across major\ud climate transitions

We use seismic reflection and refraction data to determine crustal structure, to map a fore‐arc basin containing 12 km of sediment, and to image the subduction thrust at 35 km depth. Seismic reflection megasequences within the basin are correlated with onshore geology: megasequence X, Late Cretaceous and Paleogene marine passive margin sediments; megasequence Y, a ∼10,000 km3 submarine landslide emplaced during subduction initiation at 22 Ma; and megasequence Z, a Neogene subduction margin megasequence. The Moho lies at 17 km beneath the basin center and at 35 km at the southern margin. Beneath the western basin margin, we interpret reflective units as deformed Gondwana fore‐arc sediment that was thrust in Cretaceous time over oceanic crust 7 km thick. Raukumara Basin has normal faults at its western margin and is uplifted along its eastern and southern margins. Raukumara Basin represents a rigid fore‐arc block >150 km long, which contrasts with widespread faulting and large Neogene vertical axis rotations farther south. Taper of the western edge of allochthonous unit Y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward, as proposed for the correlative onshore allochthon. Spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between Moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore‐arc crustal thickness. The trench slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of >1 km of sediment from the downgoing plate. The lack of volcanic intrusion east of the active arc, and stratigraphic evidence for the broadening of East Cape Ridge with time, suggests net fore‐arc accretion since 22 Ma. We propose a cyclical fore‐arc kinematic: rock moves down a subduction channel to near the base of the crust, where underplating drives rock uplift, oversteepens the trench slope, and causes collapse toward the trench and subduction channel. Cyclical rock particle paths led to persistent trench slope subsidence during net accretion. Existing global estimates of fore‐arc loss are systematically too high because they assume vertical particle paths.

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