Abstract Nineteen large (2348–4285 m above sea level) central polygenetic alkaline shield-like composite volcanoes and numerous smaller volcanoes in Marie Byrd Land (MBL) and western Ellsworth Land rise above the West Antarctic Ice Sheet (WAIS) and comprise the MBL Volcanic Group (MBLVG). Earliest MBLVG volcanism dates to the latest Eocene (36.6 Ma). Polygenetic volcanism began by the middle Miocene (13.4 Ma) and has continued into the Holocene without major interruptions, producing the central volcanoes with 24 large (2–10 km-diameter) summit calderas and abundant evidence for explosive eruptions in caldera-rim deposits. Rock lithofacies are dominated by basanite and trachyte/phonolite lava and breccia, deposited in both subaerial and ice-contact environments. The chronology of MBLVG volcanism is well constrained by 330 age analyses, including 52 new 40 Ar/ 39 Ar ages. A volcanic lithofacies record of glaciation provides evidence of local ice-cap glaciation at 29–27 Ma and of widespread WAIS glaciation by 9 Ma. Late Quaternary glaciovolcanic records document WAIS expansions that correlate to eustatic sea-level lowstands (MIS 16, 4 and 2): the WAIS was +500 m at 609 ka at coastal Mount Murphy, and +400 m at 64.7 ka, +400 m at 21.2 ka and +575 m at 17.5 ka at inland Mount Takahe.

Abstract In Marie Byrd Land and Ellsworth Land 19 large polygenetic volcanoes and numerous smaller centres are exposed above the West Antarctic Ice Sheet along the northern flank of the West Antarctic Rift System. The Cenozoic (36.7 Ma to active) volcanism of the Marie Byrd Land Volcanic Group (MBLVG) encompasses the full spectrum of alkaline series compositions ranging from basalt to intermediate (e.g. mugearite, benmoreite) to phonolite, peralkaline trachyte, rhyolite and rare pantellerite. Differentiation from basalt is described by progressive fractional crystallization; however, to produce silica-oversaturated compositions two mechanisms are proposed: (1) polybaric fractionation with early-stage removal of amphibole at high pressures; and (2) assimilation–fractional crystallization to explain elevated 87 Sr/ 86 Sr i ratios. Most basalts are silica-undersaturated and enriched in incompatible trace elements (e.g. La/Yb N >10), indicating small degrees of partial melting of a garnet-bearing mantle. Mildly silica-undersaturated and rare silica-saturated basalts, including tholeiites, are less enriched (La/Yb N <10), a result of higher degrees of melting. Trace elements and isotopes (Sr, Nd, Pb) reveal a regional gradient explained by mixing between two mantle components, subduction-modified lithosphere and HIMU-like plume ( 206 Pb/ 204 Pb >20) materials. Geophysical studies indicate a deep thermal anomaly beneath central Marie Byrd Land, suggesting a plume influence on volcanism and tectonism.

Abstract Two volcanoes in Marie Byrd Land, Mount Berlin and Mount Takahe, can be considered active, and a third, Mount Waesche, may be as well; although the chronology of activity is less well constrained. The records of explosive activity of these three volcanoes is well represented through deposits on the volcano flanks and tephra layers found in blue ice areas, as well as by the presence of cryptotephra layers found in West and East Antarctic ice cores. Records of effusive volcanism are found on the volcano flanks but some deposits may be obscured by pervasive glacerization of the edifices. Based on a compilation of tephra depths–ages in ice cores, the activity patterns of Mount Takahe and Mount Berlin are dramatically different. Mount Takahe has erupted infrequently over the past 100 kyr. Mount Berlin, by contrast, has erupted episodically during this time interval, with the number of eruptions being dramatically higher in the time interval between c. 32 and 18 ka. Integration of the Mount Berlin tephra record from ice cores and blue ice areas over a 500 kyr time span reveals a pattern of geochemical evolution related to small batches of partial melt being progressively removed from a single source underlying Mount Berlin.

Detailed stratal tilt information for radioisotopically dated Tertiary rocks in the Lemitar Mountains of the central Rio Grande rift allows discrimination of two episodes of extension in the late Oligocene (largely 28.6 to 27.4 Ma) and middle to late Miocene (16 to 10 Ma). Rapid deformation in the late Oligocene (at least 25% extension) produced narrow, wedge-shaped accumulations of ash-flow tuffs and mafic lavas within half grabens defined by closely spaced (2 to 5 km) planar-rotational normal faults. Volcanic accumulation rates during this episode were sufficient to bury the developing fault-block topography, resulting in preservation of only minor amounts of intercalated sedimentary rocks. Following a period of weak extension in the early Miocene, rates of extensional strain again began to increase in the early middle Miocene (about 16 Ma) and continued to increase until about 10 Ma, when a more moderate rate ensued. During the time interval of 16 to 10 Ma, the Lemitar Mountains area was extended about 30% along widely spaced (5 to 15 km) planar-normal faults. Increased stratal-tilt rates in the middle Miocene caused the development of accommodation space to locally exceed sediment supply on the down-thrown side of a major intrabasinal fault (Silver Creek fault), resulting in topographic closure and the development of playa deposits within the lower Santa Fe Group (Popotosa Formation). As stratal-tilt rates further increased in the late Miocene, fluvial-lacustrine sedimentation offlapped toward narrow zones of maximum subsidence along active faults, causing the carving of widespread unconformities on older sediments on hanging-wall dip slopes. As tectonism waned in the latest Miocene-early Pliocene, sediment supply exceeded the development of accommodation space, resulting in onlap and aggradation of the upper Santa Fe Group (Sierra Ladrones Formation) that enabled the eventual integration of an axial-fluvial system (ancestral Rio Grande) between the longitudinally arrayed half grabens to the north and south of the Socorro Basin. The late Oligocene episode of rapid extension appears to be related to strain focusing by local thermal weakening due to voluminous concurrent magmatism in the area. In contrast, the late Miocene extensional event may be of regional scale, as shown by evidence for contemporaneous tectonism in other areas and the development of widespread unconformities between Miocene and Pliocene deposits of the Santa Fe Group in many basins of the Rio Grande rift.