Showing papers by "Paul R. Renne published in 2009"
TL;DR: In this paper, the authors obtained an age of 129.7 ± 0.5 ǫ for a basaltic lava from the bottom of the Yixian Formation and a tuff from the lowermost part of the overlying Jiufotang Formation.
183 citations
TL;DR: G Geological data presented here, along with floral, invertebrate, and vertebrate paleontological and taphonomic evidence associated with the hominids, suggest that they occupied a wooded biotope over the western three-fourths of the paleotransect.
Abstract: Sediments containing Ardipithecus ramidus were deposited 4.4 million years ago on an alluvial floodplain in Ethiopia’s western Afar rift. The Lower Aramis Member hominid-bearing unit, now exposed across a >9-kilometer structural arc, is sandwiched between two volcanic tuffs that have nearly identical 40Ar/39Ar ages. Geological data presented here, along with floral, invertebrate, and vertebrate paleontological and taphonomic evidence associated with the hominids, suggest that they occupied a wooded biotope over the western three-fourths of the paleotransect. Phytoliths and oxygen and carbon stable isotopes of pedogenic carbonates provide evidence of humid cool woodlands with a grassy substrate.
166 citations
TL;DR: In this article, the authors present a comprehensive stratigraphic and geochronologic study of the Tuscolano-Artemisio phase of activity, along with new textural and petrographic data.
137 citations
TL;DR: A review of the age data for the 25 short-listed structures suggests that 11 ages are accurate, 12 are at best ambiguous and should not be reported with any uncertainty, and 2 are not well characterized at all as mentioned in this paper.
132 citations
TL;DR: Lee et al. as discussed by the authors derived the relative abundances of the isotopes of the carbon, nitrogen, oxygen, argon, and potassium of atmospheric Ar. This was the first refinement since the work of Nier [1950] and showed that 40Ar:38Ar:36Ar proportions imply < 1% adjustments to 40Ar/39Ar ages in all but exceptional cases of very young and/or K-poor and or Ca-rich samples.
112 citations
TL;DR: In this article, the authors presented 13 new statistically robust plateau, mini-plateau and isochron ages obtained on plagioclase and sericite separates from lava flows from the Fundy (n = 10; Nova Scotia, Canada), Hartford and Deerfield (n= 3; U.S.A.) basins.
112 citations
TL;DR: In this paper, the authors investigated the relationship between the emplacement of CAMP basalts and the Triassic-Jurassic boundary in the Fundy Basin (Nova Scotia, Canada) and found that CAMP volcanism at least in Nova Scotia is entirely of Triassic age, occurred in a very short time span, and may have triggered the T-J boundary biotic and environmental crisis.
108 citations
University of California, Berkeley1, Berkeley Geochronology Center2, Auburn University3, New Mexico Institute of Mining and Technology4, Lamont–Doherty Earth Observatory5, Arizona State University6, Oregon State University7, Scottish Enterprise8, University of Melbourne9, University of Wisconsin-Madison10, Rutgers University11, University of Bern12, Geological Survey of Canada13, VU University Amsterdam14
TL;DR: In this article, an enumeration of the data that should be reported in all 40Ar/39Ar studies, including a discussion in support of these requirements, is presented, and the minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation.
103 citations
TL;DR: In this paper, an analytical technique that generates high-precision 40 Ar/39 Ar data and quantifies Ar diffusion kinetics of low-K minerals is described. But plagioclase has rarely been used for thermochronometry, largely due to difficulties in simultaneously acquiring precise geochronologic data and Quantifying argon diffusion kinetic from a mineral with low-k concentration.
100 citations
TL;DR: In this paper, the authors presented robust high-precision 40Ar/39Ar data for two tuffs from the lowest part of the Lanqi Formation near the main outcrop of floral fossils in Beipiao City, Liaoning, China.
83 citations
TL;DR: In this article, the mass-dependent kinetic isotope fractionation during or subsequent to quenching of volcanic glasses was used to determine the initial 40Ar:39Ar:36Ar (and therefore little spread on isochrons).
TL;DR: The Tore-Madeira Rise is a seamount chain located 300 km off the Portugal and Morocco coasts attributed to hotspot activity as discussed by the authors, where the authors performed 40 Ar/39 Ar measurements to better understand the geodynamic history of the Toremadeira Rise.
Abstract: The Tore–Madeira Rise is a seamount chain located 300 km off the Portugal and Morocco coasts attributed to hotspot activity. U–Pb ages of lavas from the northern and central Tore–Madeira Rise range between 103 and 80.5 Ma whereas 40 Ar/ 39 Ar ages from the central and southern Tore–Madeira Rise yield ages ranging from 94.5 to 0.5 Ma. We performed new 40 Ar/ 39 Ar measurements to better understand the geodynamic history of the Tore–Madeira Rise. Plagioclase ages from the Bikini Bottom and Torillon seamounts suggest ages of >90 Ma and ≥60 Ma, respectively. Amphiboles from the Seine seamount yield an age of 24.0 ± 0.8 Ma. Biotites from lavas of the Ashton seamount give ages of 97.4 ± 1.1 Ma and 97.8 ± 1.1 Ma. The geochronological database available on the Tore–Madeira Rise has been filtered on statistical criteria to eliminate unreliable ages. The resulting database reveals three pulses of alkaline magmatism on the Tore–Madeira Rise at 103–80.5 Ma, at c . 68 Ma and between 30 Ma and the present. The magmatism was continuous from 103 Ma until c . 68 Ma and from c . 30 Ma until the present on the Tore–Madeira Rise, the surrounding seamounts and the Portugal coast. We suggest that the space–time distribution of this magmatism results from the interaction between a wide thermal anomaly emitting magmatic pulses and the complex motion of the Iberian plate. Supplementary material: A detailed Ar measurements dataset is available at http://www.geolsoc.org.uk/SUP18359.
TL;DR: In this article, the authors studied three Permian-Triassic (PT) localities from China as part of a combined magnetostratigraphic, 40Ar/39Ar and U-Pb radioisotopic, and biostrigraphic study aimed at resolving the temporal relations between terrestrial and marine records across the Permo-triassic boundary, as well as the rate of the biotic recovery in the Early Triassic.
TL;DR: In this article, 14 C ages were found to be stratigraphically inconsistent and highly scattered, and were systematically younger than the 40 Ar/39 Ar ages, ranging from 35-ka to 3-k. The results suggest that multiple isotopic systems anchored to a robust stratigraphic framework may need to be employed to determine accurately the geochronology of the CAVD as well as other volcanic districts.
04 Feb 2009
TL;DR: The authors used ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings to track the range of magma-production rates over the past 650 ka at Coso.
Abstract: We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ≤230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ~85 ka rhyolites yielded ages between ~100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ~200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (≤10’s to 100’s ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies.
TL;DR: In this article, geologic mapping, geochronological, and geochemical data on Tertiary volcanic, volcaniclastic, and intrusive rocks were used to investigate the volcanic, stratigraphic, and structural evolution of the Carson Pass region south of Lake Tahoe in the central Sierra Nevada.
Abstract: We use new geologic mapping, geochronological, and geochemical data on Tertiary volcanic, volcaniclastic, and intrusive rocks to investigate the volcanic, stratigraphic, and structural evolution of the Carson Pass region south of Lake Tahoe in the central Sierra Nevada. Volcanic and volcaniclastic rocks were deposited in east–west-trending palaeocanyons carved into Mesozoic granitic and metamorphic basement rocks; sediments were transported westwards towards the present-day Central Valley of California. In the Carson Pass–Hope Valley area, two palaeotributaries are preserved in faulted terrane east of the present-day Sierran crest (Hope Valley area); these merge at the crest to form one large, >7 km wide palaeocanyon that is undisrupted by faults west of the crest (Carson Pass–Kirkwood area). This single, large palaeocanyon roughly coincides with the present-day Mokelumne River drainage. New 40Ar/39Ar dates and stratigraphic data east of the crest, integrated with previously published data west of the cre...
TL;DR: Paleomagnetic analyses of the Xalnene Tuff, and lavas from the volcano from which it erupted, yield fully reversed magnetic polarities, indicating that the tuff was deposited prior to the last geomagnetic reversal (the BrunhesMatuyama ca. 790 ka) as discussed by the authors.
Abstract: Impressions in a basaltic tuff located around Valsequillo Reservoir near Puebla, Mexico, have been interpreted as human and animal footprints along an ancient lakeshore, and are cited as evidence of the presence of humans in North America at 40 ka B.P. In this paper, we present new data that challenge this interpretation. Paleomagnetic analyses of the Xalnene Tuff, and lavas from the volcano from which it erupted, yield fully reversed magnetic polarities, indicating that the tuff was deposited prior to the last geomagnetic reversal (the BrunhesMatuyama ca. 790 ka). 40 Ar/ 39 Ar dating of Xalnene lapilli and lava from the source volcano yields indistinguishable ages of ~1.3 Ma, consistent with a period of reversed magnetic polarity (C1r.2r). Additional paleomagnetic measurements of individual millimeter-size lapilli indicate that the pyroclastic grains within the Xalnene Tuff have not been disturbed or rotated since their initial deposition, thereby ruling out the possibility that the tuff was reworked by wave action along the shores of an ancient lacustrine environment. This and other evidence indicate that the marks observed in the stone quarry site are not human ichnofossils.
TL;DR: In this paper, a tephra horizon was recognized in the high-Agri valley, which is a Quaternary basin located along the hinge of the Southern Apennines fold-and-thrustbelt.
Abstract: The High Agri River Valley is a Quaternary Basin located along the hinge of the Southern Apennines fold-andthrust
belt. The inner margin of the orogen has been affected by intense transtensional and normal faulting, which
accompanied vigorous volcanism during the Quaternary. Marker tephra layers are distributed across the whole of
Southern Italy and provide a powerful tool to constrain both the size of eruptions and the regional activity of extensional
faults controlling basin evolution. Paleoseismological trenching within the Monti della Maddalena
range, that borders the Agri River Valley to the south-west, has exposed a faulted stratigraphic sequence and recovered
a 10 cm thick tephra layer involved in deformation. This is the first tephra horizon recognized in the high
Agri Valley, which, based on the stratigraphic study of the trench, lies in a primary position. 40Ar/39Ar dating
constrain its age to 266 ka and provide an important marker for the Middle Pleistocene tephrochronology of the
region. Together with dating, geochemical analysis suggests a possible volcanic source in the Campanian region.
01 Jan 2009
Abstract: The junction of the western Mexican Volcanic Belt and the Gulf of California represents a superposition of subduction and continental rifting tectonic regimes in the late Cenozoic. Subduction of the oceanic Rivera plate has caused late Cenozoic uplift of western Mexico, forming the Jalisco Block. This paper addresses three questions: Where is the northern boundary of the Jalisco Block; how is the continental fracture system related to that offshore; and what is the spatial distribution and composition of subduction-related lavas? Ar/Ar dates on Cretaceous to Paleogene silicic ash flows show that the northern boundary of the Jalisco Block may be defined by the abrupt change in basement age from Cretaceous to Miocene. Ar/Ar and K-Ar dates on faulted lavas from the Nayarit region indicate that extension at the edge of the Jalisco Block has occurred since ca. 4.2 Ma. The least principal stress (s3) direction associated with these faults has had two different orientations from 4.2 Ma to the present: Several Pliocene to Holocene (4.20, 1.05, and 0.65 Ma) lava flows, and aligned cinder cones have a N45&W associated least principal stress direction, whereas three Pliocene (3.36, 3.38, and 3.11 Ma) lava flows are cut by faults indicating a north to north-northeast least principal stress direction. The two different stress directions may arise either from structural features in the basement of the arc or from changes in offshore plate boundaries (e.g., 2.5 Ma when the Rivera and Mathematicians plates were locked together). Whole-rock major and trace element analyses of lavas from the coastal Nayarit region reveal three different lava types: an alkali basalt series, basaltic andesite, and andesite. The alkali basalts show FeO and TiO2 enrichment and have low Ba/Zr and Ba/La ratios, consistent with derivation from an oceanic-island-type mantle. The basaltic andesites and andesites are both alkaline and calc-alkaline, contain hydrous phenocrysts, and have high Sr/Zr and La/Nb ratios, consistent with a subductionrelated source. Among these lavas there is a correlation between Sr/Zr, oxygen fugacity, and H2O contents, suggesting that an oxidized, hydrous fluid is involved in the genesis of the subduction-related lavas. The distribution of subduction-related volcanic centers in the coastal Nayarit region, and areas southeast within the Jalisco Block, defines a volcanic front that parallels theMiddle America Trench and is consistent with a 45& dip on the subducted Rivera plate.