Abstract The Halaha River–Chaoer River (HC) volcanic field in the Greater Hinggan Mountain Range (NE China) consists of at least 41 monogenetic basaltic volcanoes. Strombolian, violent Strombolian and phreatomagmatic eruptions, as well as the transitional eruptions, generated simple volcanic cones (single vent) and composite volcanic cones (multiple vents). The simple elongated cone is the most abundant geomorphology type. By analysing the elongated crater and coalescent aligned circular crater, cone breaching and depression, and aligned vents, we identified a number of magma-feeding fissures. The majority of these fissures strike NE–ENE. Accordingly, we infer that the regional stress field affected volcanism in the HC field. The lavas in this field are alkali basalts that are enriched in light rare earth elements (La/Yb N = 7.9–24.5). Their ocean island basalt-like rare earth element and spider-diagram patterns, high Nb/U ratios and high TiO 2 contents (>2 wt%) indicate that the basalts were derived from the asthenosphere mantle. Both the asthenosphere upwelling and the tectonic forces are the key controlling factors of the volcanism in the HC field.

Abstract During the late Cenozoic, the extensional tectonic setting in northeastern China caused large-scale block uplifts and depressions, and thus a large amount of magma erupted along structural fractures in the eastern Inner Mongolia, NE China. The Abaga, Beilike, Dalinor and Wulanhada (ABDW) volcanic rocks along the Daxing'anling–Taihangshan Gravity Lineament in the southern section of the Daxing'anling are characterized by their extensive distribution, numerous volcanic cones and various eruption types. Each volcanic group has distinctive volcanic landforms and geochemical characteristics. The geochronological data have revealed that the volcanism spanned Miocene to late Pleistocene. The ABDW volcanic rocks contain primary alkaline basalts and subordinate tholeiites. The trace element curve pattern is similar to that of ocean island basalt, but completely different from that of mid ocean ridge basalt, while the light rare earth elements are more enriched than the heavy rare earth elements. The geochemical features of the volcanic rocks and the entrained mantle xenoliths reveal the broad heterogeneities of the lithospheric mantle and varieties of the volcanic rock evolution in the south Daxing'anling. The Cenozoic volcanism in eastern Inner Mongolia, and even within the east Asian plate, is attributed to the westward subduction and rolling backward of the Pacific slab, as well as the trench retreat.

ABSTRACT The mid-Cenozoic succession in the northeast limb of the Mount Diablo anticline records the evolution of plate interactions at the leading edge of the North America plate. Subduction of the Kula plate and later Farallon plate beneath the North America plate created a marine forearc basin that existed from late Mesozoic to mid-Cenozoic times. In the early Cenozoic, extension on north-south faults formed a graben depocenter on the west side of the basin. Deposition of the Markley Formation of middle to late? Eocene age took place in the late stages of the marine forearc basin. In the Oligocene, the marine forearc basin changed to a primarily nonmarine basin, and the depocenter of the basin shifted eastward of the Midland fault to a south-central location for the remainder of the Cenozoic. The causes of these changes may have included slowing in the rate of subduction, resulting in slowing subsidence, and they might also have been related to the initiation of transform motion far to the south. Two unconformities in the mid-Cenozoic succession record the changing events on the plate boundary. The first hiatus is between the Markley Formation and the overlying Kirker Formation of Oligocene age. The succession above the unconformity records the widespread appearance of nonmarine rocks and the first abundant appearance of silicic volcanic detritus due to slab rollback, which reversed the northeastward migration of the volcanic arc to a more proximal location. A second regional unconformity separates the Kirker/Valley Springs formations from the overlying Cierbo/Mehrten formations of late Miocene age. This late Miocene unconformity may reflect readjustment of stresses in the North America plate that occurred when subduction was replaced by transform motion at the plate boundary. The Cierbo and Neroly formations above the unconformity contain abundant andesitic detritus due to proto-Cascade volcanism. In the late Cenozoic, the northward-migrating triple junction produced volcanic eruptive centers in the Coast Ranges. Tephra from these local sources produced time markers in the late Cenozoic succession.

ABSTRACT We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.