Masafumi Sudo

The characteristics and chronology of the earliest Acheulean at Konso, Ethiopia

The Acheulean technological tradition, characterized by a large (>10 cm) flake-based component, represents a significant technological advance over the Oldowan. Although stone tool assemblages attributed to the Acheulean have been reported from as early as circa 1.6–1.75 Ma, the characteristics of these earliest occurrences and comparisons with later assemblages have not been reported in detail. Here, we provide a newly established chronometric calibration for the Acheulean assemblages of the Konso Formation, southern Ethiopia, which span the time period ∼1.75 to <1.0 Ma. The earliest Konso Acheulean is chronologically indistinguishable from the assemblage recently published as the world’s earliest with an age of ∼1.75 Ma at Kokiselei, west of Lake Turkana, Kenya. This Konso assemblage is characterized by a combination of large picks and crude bifaces/unifaces made predominantly on large flake blanks. An increase in the number of flake scars was observed within the Konso Formation handaxe assemblages through time, but this was less so with picks. The Konso evidence suggests that both picks and handaxes were essential components of the Acheulean from its initial stages and that the two probably differed in function. The temporal refinement seen, especially in the handaxe forms at Konso, implies enhanced function through time, perhaps in processing carcasses with long and stable cutting edges. The documentation of the earliest Acheulean at ∼1.75 Ma in both northern Kenya and southern Ethiopia suggests that behavioral novelties were being established in a regional scale at that time, paralleling the emergence of Homo erectus-like hominid morphology.

Apatite triple dating and white mica 40Ar/39Ar thermochronology of syntectonic detritus in the Central Andes: A multiphase tectonothermal history

We applied apatite U-Pb, fission track, and (U-Th)/He triple dating and white mica 40 Ar/ 39 Ar thermochronology to syntectonic sedimentary rocks from the central Andean Puna plateau in order to determine the source-area geochronology and source sedimentary basin thermal histories, and ultimately the timing of multiple tectonothermal events in the Central Andes. Apatite triple dating of samples from the Eocene Geste Formation in the Salar de Pastos Grandes basin shows late Precambrian–Devonian apatite U-Pb crystallization ages, Eocene apatite fission track (AFT), and Eocene–Miocene (U-Th)/He (ca. 8–47 Ma) cooling ages. Double dating of cobbles from equivalent strata in the Arizaro basin documents early Eocene (46.2 ± 3.9 Ma) and Cretaceous (107.6 ± 7.6, 109.5 ± 7.7 Ma) AFT and Eocene–Oligocene (ca. 55–30 Ma) (U-Th)/He ages. Thermal modeling suggests relatively rapid cooling between ca. 80 and 50 Ma and reheating and subsequent diachronous basin exhumation between ca. 30 Ma and 5 Ma. The 40 Ar/ 39 Ar white mica ages from the same samples in the Salar de Pastos Grandes area are mainly 400–350 Ma, younger than apatite U-Pb ages, suggesting source-terrane cooling and exhumation during the Devonian–early Carboniferous. Together these data reveal multiple phases of mountain building in the Paleozoic and Cenozoic. Basin burial temperatures within the plateau were limited to

Noble gas partitioning between metal and silicate under high pressures

Measurements of noble gas (helium, neon, argon, krypton, and xenon) partitioning between silicate melt and iron melt under pressures up to 100 kilobars indicate that the partition coefficients are much less than unity and that they decrease systematically with increasing pressure. The results suggest that the Earths core contains only negligible amounts of noble gases if core separation took place under equilibrium conditions.

The multistage exhumation history of the Kaghan Valley UHP series, NW Himalaya, Pakistan from U-Pb and 40Ar/39Ar ages

Amphibole and mica 40Ar/39Ar ages as well as zircon, rutile and titanite U-Pb geochronology of eclogites and associated host rocks from the Higher Himalayan Crystalline Nappes (Indian Plate) in the Upper Kaghan Valley, Pakistan allow distinction of a multistage exhumation history. An Eocene age for peak-pressure metamorphism has been obtained by phengite 40Ar/39Ar (47.3 ± 0.3 Ma) and zircon U-Pb (47.3 ± 0.4 and 47.4 ± 0.3 Ma) ages from cover and basement gneisses. A very short-lived metamorphic peak and rapid cooling is documented by an amphibole 40Ar/39Ar age of 46.6 ± 0.5 Ma and a rutile U-Pb age of 44.1 ± 1.3 Ma from eclogites. Phengite and biotite ages from cover and basement sequences metamorphosed during the Himalayan orogeny are 34.5 ± 0.2 to 28.1 ± 0.2 Ma whereas youngest biotites, yielding 23.6 ± 0.1 and 21.7 ± 0.2 Ma, probably reflect argon partial resetting. The amphibole age, together with those derived from phengite and zircon demonstrate a rate of initial exhumation of 86–143 mm/a i.e . an extremely rapid transport of the Indian Plate continental crust from ultra-high pressure (UH P ) conditions back to crustal levels (47–46 Ma for transport from 140 to 40 km depth). Subsequent exhumation (46–41 Ma, 40–35 km) slowed to about 1 mm/a at the base of the continental crust but increased again later towards slightly higher exhumation rates of ca . 2 mm/a (41–34 Ma, 35–20 km). This indicates a change from buoyancy-driven exhumation at mantle depths to compression forces related to continent-continent collision and accompanied crustal folding, thrusting and stacking that finally exposed the former deeply-buried rocks.

Initiation and propagation of subduction along the Philippine Trench: evidence from the temporal and spatial distribution of volcanoes

Abstract K–Ar ages of 37 samples collected from the Bicol peninsula, the Luzon island, Philippines, were determined by the unspiked sensitivity method in order to constrain the timing of initiation of subduction along the Philippine Trench. The measured K–Ar ages range from 0 to 7 Ma with two old outliers of 27 and 43 Ma. Together with K–Ar ages previously reported on volcanics in Leyte and eastern Mindanao, subduction volcanism has likely propagated from north to south: ∼6.6 Ma in Bicol and ∼3.5 Ma in Leyte and its vicinity. The temporal and spatial distribution suggests that the subduction volcanism started earlier in the north than in the south. This is consistent with the southern propagation of subduction along the Philippine Trench from ∼8 Ma.

Pliocene orographic barrier uplift in the southern Central Andes

Sedimentary basin fills along the windward flanks of orogenic plateaus are valuable archives of paleoenvironmental change with the potential to resolve the history of surface uplift and orographic barrier formation. The intermontane basins of the southern Central Andes contain thick successions of sedimentary material that are commonly interbedded with datable volcanic ashes. We relate variations in the hydrogen isotopic composition of hydrated volcanic glass (delta D-g) of Neogene to Quaternary fills in the semiarid intermontane Humahuaca Basin (Eastern Cordillera, northwest Argentina) to spatiotemporal changes in topography and associated orographic effects. delta D values from volcanic glass in the basin strata (-117 parts per thousand to -98 parts per thousand) show two main trends that accompany observed tectonosedimentary events in the study area. Between 6.0 and 3.5 Ma, delta D-g values decrease by similar to 17 parts per thousand; this is associated with surface uplift in the catchment area. After 3.5 Ma, delta D-g values show abrupt deuterium enrichment, which we associate with (1) the attainment of threshold elevations for blocking moisture transport in the basin-bounding ranges to the east, and (2) the onset of semiarid conditions in the basin. Such orographic barriers throughout the eastern flanks of the Central Andes have impeded moisture transport into the orogen interior; this has likely helped maintain aridity and internal drainage conditions on the adjacent Andean Plateau.

P–T evolution and timing of a late Palaeozoic fore-arc system and its heterogeneous Mesozoic overprint in north-central Chile (latitudes 31–32°S)

In the late Palaeozoic fore-arc system of north-central Chile at latitudes 31–32°S (from the west to the east) three lithotectonic units are telescoped within a short distance by a Mesozoic strike-slip event (derived peak P–T conditions in brackets): (1) the basally accreted Choapa Metamorphic Complex (CMC; 350–430°C, 6–9 kbar), (2) the frontally accreted Arrayan Formation (AF; 280–320°C, 4–6 kbar) and (3) the retrowedge basin of the Huentelauquen Formation (HF; 280–320°C, 3–4 kbar). In the CMC, Ar–Ar spot ages locally date white-mica formation at peak P–T conditions and during early exhumation at 279–242 Ma. In a local garnet mica-schist intercalation (570–585°C, 11–13 kbar) Ar–Ar spot ages refer to the ascent from the subduction channel at 307–274 Ma. Portions of the CMC were isobarically heated to 510–580°C at 6.6–8.5 kbar. The age of peak P–T conditions in the AF can only vaguely be approximated at ≥ 310 Ma by relict fission-track ages consistent with the observation that frontal accretion occurred prior to basal accretion. Zircon fission-track dating indicates cooling below ~ 280°C at ~ 248 Ma in the CMC and the AF, when a regional unconformity also formed. Ar–Ar white-mica spot ages in parts of the CMC and within the entire AF and HF point to heterogeneous resetting during Mesozoic extensional and shortening events at ~ 245–240 Ma, ~ 210–200 Ma, ~ 174–159 Ma and ~ 142–127 Ma. The zircon fission-track ages are locally reset at 109–96 Ma. All resetting of Ar–Ar white-mica ages is proposed to have occurred by in situ dissolution/precipitation at low temperature in the presence of locally penetrating hydrous fluids. Hence syn- and postaccretionary events in the fore-arc system can still be distinguished and dated in spite of its complex heterogeneous postaccretional overprint.

Magmatism during Gondwana break-up: new geochronological data from Westland, New Zealand

Newly determined Late Cretaceous 40Ar/39Ar ages on megacrystic kaersutite from four lamprophyre dikes, and a U–Pb zircon age on a trachyte, from central and north Westland (New Zealand) are presented. These ages suggest that the intrusion of mafic dikes (88–86 and 69 Ma) was not necessarily restricted to the previously established narrow age range of 80–92 Ma. The younger lamprophyre and trachyte dikes (c. 68–70 Ma) imply that tensional stresses in the Western Province were either renewed at this time, or that extension and related magmatism continued during opening of the Tasman Sea. Extension-related magmatism in the region not only preceded Tasman seafloor spreading initiation (starting at c. 83 Ma, lasting to c. 53 Ma), but may have sporadically continued for up to 15 Ma after continental break-up.

New K-Ar ages of the Society Islands, French Polynesia, and implications for the Society hotspot feature

We report 52 new K-Ar age determinations of volcanic rocks from the Society Islands in French Polynesia. The newly obtained ages range from 0.51 to 4.61 Ma, which are consistent with previously reported ones. The magnetostratigraphy based on 45 data sets combining K-Ar ages and paleomagnetic polarity agrees with the geomagnetic polarity time scale at the 2s level, except for a few age data. Assuming the absolute motion of the Pacific plate, paleopositions of the dated rocks are estimated from the K-Ar ages, showing a narrow distribution in the western part of the easternmost volcano of the active hotspot region. This suggests that a hotspot of a few tens of kilometers in diameter might have been fixed nearly at the eastern margin of the presently active region in the Society archipelago and that the volcanism of each island might have persisted for about 1 myr.

Effects of fluid flow, cooling and deformation as recorded by 40Ar/39Ar, Rb–Sr and zircon fission track ages in very low- to low-grade metamorphic rocks in Avalonian SE Cape Breton Island (Nova Scotia, Canada)

40Ar/39Ar in situ UV laser ablation of white mica, Rb–Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low- to low-grade metamorphic processes at 3.5±0.4 kbar, 280±30°C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by 40Ar/39Ar spot age peaks between 396±3 and 363±14 Ma. Rb–Sr systematics of minerals and mineral aggregates yielded two isochrons at 389±7 Ma and 365±8 Ma, corroborating equilibrium conditions during very low- to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle–Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The 40Ar/39Ar spot age spectra also display relict age peaks at 477–465 Ma, 439 Ma and 420–428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician–Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242±18 and 225±21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean.