To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within <±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad-scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first-order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate-driving forces. These buoyancy forces are just one of several categories of “second-order” stresses, or local perturbations, that can be identified once the first-order stress patterns are recognized. These second-order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second-order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two-dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations of SHmax orientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin-normal extension derived from sediment-loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first-order (plate boundary force-derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlee's law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlee's law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first-order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude stress differences in the upper, brittle part of the lithosphere.

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First‐ and second‐order patterns of stress in the lithosphere: The World Stress Map Project

After thirty years of plate-tectonic theory, the reasons why supercontinents disintegrate and disperse to form smaller continental plates remain enigmatic. Possible causes range from abnormally hot mantle upwellings, or plumes, to changes in plate-boundary driving forces. The breakup of the Gondwanaland super-continent, which started about 180 million years ago, provides an excellent case history against which to test models.

The role of mantle plumes in continental breakup: case histories from Gondwanaland

Patagonian origin of glacial dust deposited in East Antarctica (Vostok and Dome C) during glacial stages 2, 4 and 6

The bend in the Hawaiian-Emperor seamount chain is a prominent feature usually attributed to a change in Pacific plate motion ∼47 Myr ago. However, global plate motion reconstructions fail to predict the bend. Here we show how the geometry of the Hawaiian-Emperor chain and other hotspot tracks can be explained when we combine global plate motions with intraplate deformation and movement of hotspot plumes through distortion by global mantle flow. Global mantle flow models predict a southward motion of the Hawaiian hotspot. This, in combination with a plate motion reconstruction connecting Pacific and African plates through Antarctica, predicts the Hawaiian track correctly since the date of the bend, but predicts the chain to be too far west before it. But if a reconstruction through Australia and Lord Howe rise is used instead, the track is predicted correctly back to 65 Myr ago, including the bend. The difference between the two predictions indicates the effect of intraplate deformation not yet recognized or else not recorded on the ocean floor. The remaining misfit before 65 Myr ago can be attributed to additional intraplate deformation of similar magnitude.

Prediction of Emperor-Hawaii seamount locations from a revised model of global plate motion and mantle flow

The role of lunar volcanism in the history of the moon is documented using lunar-orbit and earth-based data along with characterizations derived from Apollo and Luna sample-return missions. Characteristics of mare and highland volcanic features are described, Apollo and Luna results are discussed, and the characteristics of other mare deposits and of other highland features of possible volcanic origin are summarized. Major conclusions are that: (1) there is little unequivocal morphologic evidence for highland volcanism, (2) lunar mare lavas appear to have originated from depths of 100 to 500 km, (3) impact melting does not appear to have been a factor in the generation of mare lavas, (4) mare volcanism was characterized by massive outpourings of very fluid volatile-poor lava analogous to terrestrial flood basalts, (5) mare volcanism took place from 3.83 to about 2.5 billion years ago, (6) the preferential occurrence of mare deposits in large impact basins appears to be generically unrelated to basin formation, and (7) a thicker farside crust may be responsible for the distinctive nearside-farside asymmetry of mare deposits.

Lunar volcanism in space and time.

Volcanoes of the Antarctic plate and southern oceans

The West Antarctic rift system extends over a 3000 × 750 km, largely ice covered area from the Ross Sea to the base of the Antarctic Peninsula, comparable in area to the Basin and Range and the East African rift system. A spectacular rift shoulder scarp along which peaks reach 4–5 km maximum elevation marks one flank and extends from northern Victoria Land-Queen Maud Mountains to the Ellsworth-Whitmore-Horlick Mountains. The rift shoulder has maximum present physiographic relief of 5 km in the Ross Embayment and 7 km in the Ellsworth Mountains-Byrd Subglacial Basin area. The Transantarctic Mountains part of the rift shoulder (and probably the entire shoulder) has been interpreted as rising since about 60 Ma, at episodic rates of ∼1 km/m.y., most recently since mid-Pliocene time, rather than continuously at the mean rate of 100 m/m.y. The rift system is characterized by bimodal alkaline volcanic rocks ranging from at least Oligocene to the present. These are exposed asymmetrically along the rift flanks and at the south end of the Antarctic Peninsula. The trend of the Jurassic tholeiites (Ferrar dolerites, Kirkpatric basalts) marking the Jurassic Transantarctic rift is coincident with exposures of the late Cenozoic volcanic rocks along the section of the Transantarctic Mountains from northern Victoria Land to the Horlick Mountains. The Cenozoic rift shoulder diverges here from the Jurassic tholeiite trend, and the tholeiites are exposed continuously (including the Dufek intrusion) along the lower- elevation (1–2 km) section of Transantarctic Mountains to the Weddell Sea. Widely spaced aeromagnetic profiles in West Antarctica indicate the absence of Cenozoic volcanic rocks in the ice covered part of the Whitmore-Ellsworth-Mountain block and suggest their widespread occurrence beneath the western part of the ice sheet overlying the Byrd Subglacial Basin. A German Federal Institute for Geosciences and Natural Resources (BGR)-U.S. Geological Survey (USGS) aeromagnetic survey over the Ross Sea continental shelf indicates rift fabric and suggests numerous submarine volcanoes along discrete NNW trending zones. A Bouguer anomaly range of approximately 200 (+50 to −150) mGal having 4–7 mGal/km gradients where measured in places marks the rift shoulder from northern Victoria Land possibly to the Ellsworth Mountains (where data are too sparse to determine maximum amplitude and gradient). The steepest gravity gradients across the rift shoulder require high density (mafic or ultramafic?) rock within the crust as well as at least 12 km of thinner crust beneath the West Antarctic rift system in contrast to East Antarctica. Sparse land seismic data reported along the rift shoulder, where velocities are greater than 7 km/s, and marine data indicating velocities above 7 km/s beneath the Ross Sea continental shelf support this interpretation. The maximum Bouguer gravity range in the Pensacola Mountains area of the Transantarctic Mountains is only about 130 mGal with a maximum 2 mGal/km gradient, which can be explained solely by 8 km of crustal thickening. Large offset seismic profiles over the Ross Sea shelf collected by the German Antarctic North Victoria Land Expedition V (GANOVEX V) combined with earlier USGS and other results indicate 17–21 km thickness for the crust beneath the Ross Sea shelf which we interpret as evidence of extended rifted continental crust. A regional positive Bouguer anomaly (0 to +50 mGal), the width of the rift, extends from the Ross Sea continental shelf throughout the Ross Embayment and Byrd Subglacial Basin area of the West Antarctic rift system and indicates that the Moho is approximately 20 km deep tied to the seismic results (probably coincident with the top of the asthenosphere) rather than the 30 km reported in earlier interpretations. The interpretation of horst and graben structures in the Ross Sea, made from marine seismic reflection data, probably can be extended throughout the rift (i.e., the Ross Ice shelf and the Byrd Subglacial Basin areas). The near absence of earthquakes in the West Antarctic rift system probably results from a combination of primarily sparse seismograph coverage and, secondarily, suppression of earthquakes by the ice sheet (e.g., Johnston, 1987) and very high seismicity shortly after deglaciation in the Ross Embayment followed by abnormally low seismicity at present (e.g., Muir Wood, 1989). The evidence of high temperatures at shallow depth beneath the Ross Sea continental shelf and adjacent Transantarctic Mountains is supportive of thermal uplift of the mountains associated with lateral heat conduction from the rift and can possibly also explain the volcanism, rifting, and high elevation of the entire rift shoulder to the Ellsworth-Horlick-Whitmore Mountains. We infer that the Gondwana breakup and the West Antarctic rift are part of a continuously propagating rift that started in the Jurassic when Africa separated from East Antarctica (including the failed Jurassic Transantarctic rift). Rifting proceeded clockwise around East Antarctica to the separation of New Zealand and the Campbell Plateau about 85–95 Ma and has continued (with a spreading center jump) to its present location in the Ross Embayment and West Antarctica. The Cenozoic activity of the West Antarctic rift system appears to be continuous in time with rifting in the same area that began only in the late Mesozoic. Although the mechanism for rifting is not completely explained, we suggest a combination of the flexural rigidity model (Stem and ten Brink, 1989) proposed for the Ross Embayment and the thermal plume or hot spot concepts. The propagating rift may have been “captured” by the thermal plume.

Geophysical studies of the West Antarctic Rift System

Mantle-plume activity has been proposed to explain Neogene and mid-Cretaceous magmatic events, as well as associated tectonism, in West Antarctica; but the arrival time and dimensions of plume influence have been hard to define and are still a subject of debate. Two low-relief erosion surfaces, one in West Antarctica and the other in New Zealand (herein named the Waipounamu erosion surface), provide a way of assessing plume activity by measuring vertical displacements associated with these events. Both surfaces bevel mid-Cretaceous rocks, and both represent prolonged intervals of erosional leveling in a stable tectonic environment. Overlying strata in New Zealand indicate that leveling was near completion in coastal regions by ca. 75 Ma and therefore must have begun around 85 Ma, when New Zealand was beginning to break away from West Antarctica. Fluvial erosion followed by subsidence and marine planation are clearly recorded by these strata, and a similar history seems likely for West Antarctica, accounting for isostatically corrected ice-free bedrock elevations that are well below sea level over much of the region. The absence of uplift at the time of breakup seems incompatible with a plume mechanism for continental breakup. By contrast, the present elevation of the West Antarctic erosion surface records an estimated maximum of ≈ 3 km of tectonic uplift, associated with alkalic volcanism, beginning at ca. 28–30 Ma. We suggest that this event marks the inception of plume activity in West Antarctica. The resulting structure, the Marie Byrd Land dome, defines an area of plume influence that is smaller than the area defined by geochemistry, but is similar in scale to the Yellowstone plume.

Mantle-plume activity recorded by low-relief erosion surfaces in West Antarctica and New Zealand

Cenozoic, mafic alkaline volcanic rocks throughout West Antarctica (WA) occupy diverse tectonic environments. On the Antarctic Peninsula (AP), late Miocene-Pleistocene (7 to <1 Ma) alkaline basaltic rocks were erupted <1 to 45 million years after subduction ceased along the Pacific margin of the AP. In Marie Byrd Land (MBL), by contrast, alkaline basaltic volcanism has been semi-continuous from 25–30 Ma to the present, and occurs in the West Antarctic rift system. Together, these Antarctic tectono-magmatic associations are analogous to the Basin and Range, Sierran, and Coast Range batholith provinces. Unlike the western US, however, basaltic rocks throughout WA have uniform geochemical characteristics, with especially narrow ranges in initial87Sr/86Sr (0.7026–0.7035),143Nd/144Nd (0.51286–0.51299), and La/Nb (0.6–1.4) ratios, suggesting very limited liput from “old” subcontinental lithosphere or crustal sources during magma genesis. However, there are significant differences in the relative and absolute abundances of the LILE (large-ionlithophile elements), and these divide WA into two provinces. Basalts from the AP region have unusually high K/Ba and K/Rb ratios (50–140 and 500–1500 respectively) and marked Ba depletion (Ba/Nb=2.5–8.0; Ba ppm 66–320) relative to MBL basalts, which have LILE distributions within the range for OIB (ocean-island basalt) (K/Ba <50, Ba/Nb 5–20). This geochemical contrast is accompanied by a three-fold increase in the age range of volcanic activity and a three orders of magnitude increase in the volume of eruptive products, within MBL. The regional differences in geochemistry, and in the volume and duration of volcanic activity, are best explained by a plume-related origin for MBL basalts, whereas alkaline magmatism in the AP is causally related to slab window formation following the cessation of subduction. Plume activity has alreadybeen proposed to explain tectonic doming and associated spatial patterns of volcanism in MBL. Most MBL geochemical traits are shared by the volcanic rocks of the western Ross Sea, suggesting that a large plume head underlies the West Antarctic rift system. The uniformity of basalt compositions throughout WA and the entire rift system suggest uniformly minimal extension throughout this region during late Cenozoic time. Differences in crustal thicknesses can be explained by early Cenozoic or pre-Cenozoic extension, but restraint on extension is suggested by the size of the region and the implied size of the plume. The c. 95% encirclement of the Antarctic plate by mid-ocean ridges and transforms restrains extension on a regional scale, leading to nonadiabatic plume rise and correspondingly little decompression melting.

Wesley E. LeMasurier

Papers

Volcanoes of the Antarctic plate and southern oceans

Geophysical studies of the West Antarctic Rift System

Mantle-plume activity recorded by low-relief erosion surfaces in West Antarctica and New Zealand

Tectonic controls on the geochemical composition of Cenozoic, mafic alkaline volcanic rocks from West Antarctica

Hobbs Coast Cenozoic volcanism: Implications for the West Antarctic rift system