Laschamp Excursion at Mono Lake?

The Laschamp Geomagnetic Excursion (ca. 41 ka) and a related increase of cosmogenic nuclides provides a global tie point among sedimentary and ice core records. In the Wilson Creek Formation, Mono Lake, California, the Laschamp Excursion has not been recognized although the so-called Mono Lake excursion was found in the section with an estimated age of about 28 14C ka. However, our reevaluation of the age of the Mono Lake excursion at its type locality using new 14C dates on carbonates and 40Ar/39Ar sanidine dates on ash layers yields an estimate of 38-41 ka. This chronology and the absence of a second excursion in the Wilson Creek Formation suggest that the distinct paleomagnetic feature with negative inclinations at Mono Lake is correlative with the Laschamp Excursion

A 19 to 17 Ma amagmatic extension event at the Mid-Atlantic Ridge: Ultramafic mylonites from the Vema Lithospheric Section

A >300 km long lithospheric section (Vema Lithospheric Section or VLS) is exposed south of the Vema transform at 11°N in the Atlantic. It is oriented along a seafloor spreading flow line and represents ∼26 Ma of accretion at a single 80 km long segment (EMAR) of the Mid-Atlantic Ridge. The basal part of the VLS exposes a mantle unit made mostly of relatively undeformed coarse-grained/porphyroclastic peridotites that were sampled at close intervals. Strongly deformed mylonitic peridotites were found at 14 contiguous sites within a ∼80 km stretch (∼4.7 Ma interval); they are dominant in a time interval of 1.4 Ma, from crustal ages of 16.8 to 18.2 Ma (mylonitic stretch). Some of the mylonites are "dry," showing anhydrous high-T deformation, but most contain amphibole. The mylonitic peridotites tend to be less depleted than the porphyroclastic peridotites on the basis of mineral major and trace elements composition, suggesting that the mylonites parent was a subridge mantle that underwent a relatively low degree of melting. The Sr, Nd, and O isotopic composition of the amphiboles is MORB-like and suggests either that seawater did not contribute to their isotopic signature or that their isotopic ratios re-equilibrated during fluid circulation in the upper mantle. Four 40Ar/39Ar ages, on three amphiboles separated from the peridotites, are close to crustal ages predicted from magnetic anomalies, confirming that the amphiboles formed close to ridge axis. We propose that crustal accretion at the EMAR segment has been mostly symmetrical for the 26 Ma of its recorded history, except for the ∼1.4 Ma interval of prevalent ultramafic mylonites (mylonitic stretch) that may record a period of quasi-amagmatic asymmetric accretion of oceanic lithosphere close to the ridge–Vema transform intersection, possibly with development of detachment faults. This interval may correspond to a thermal minimum of the subridge upwelling mantle, marking the transition from a period of decreasing to one of increasing mantle melting below the EMAR segment

Paleomagnetic and rock magnetic properties of ODP Leg 173 sites, west Iberia continental margin

We present detailed paleomagnetic and rock magnetic results of rock samples recovered during Leg 173. The Leg 173 cores display a multicomponent magnetization nature. Variations in magnetic properties correlate with changes in lithology that result from differences in the abundance and size of magnetic minerals. The combined investigation suggests that the magnetic properties of the "fresher" peridotite samples from Site 1070 are controlled mainly by titanomagnetite, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of titanomagnetites. These results are in excellent agreement with thermomagnetic characteristics where titanomagnetites with Curie temperature ~580°C were identified from the "fresher" peridotites. In contrast to the magnetic properties observed from the "fresher" peridotites, the low-temperature curves for the "altered" peridotites did not show any Verwey transition. Thermomagnetic analysis using the high-temperature vibrating sample magnetometer also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at ~420°C, probably maghemite. The field- and frequency-dependent relationships are also directly opposite to those in the reversal zone, with no signs of titanomagnetite characteristics. Altogether, these rock magnetic data seem to be sensitive indicators of alteration and support the contention that maghemite is responsible for the magnetic signatures displayed in the altered peridotites of the upper section. The magnetic minerals of the basement rocks from Sites 1068, 1069, and 1070 are of variable particle size but fall within the pseudo-single-domain size range (0.2-14 µm). The average natural remanent magnetization (NRM) intensity of recovered serpenitinized peridotite is typically on the order of 20 mA/m for samples from Site 1068, but ~120 mA/m for samples from Site 1070. The much stronger magnetization intensity of Site 1070 is apparently in excellent agreement with the observed magnetic anomaly high. Nearly half of the NRM intensity remained after 400°C demagnetization, suggesting that the remanence can contribute significantly to the marine magnetic anomaly

i0091-7613-35-12-1087.indd

ABSTRACT We report U-Pb and The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such "off-axis" magmatism might be a common process in (ultra-) slow oceanic spreading systems, where "magmatic" and "tectonic" spreading varies in both space and time

Aglomerados volcánicos y conglomerados volcanogénicos estratificados del mioceno temprano en la base de las series cenozoicas del campo de Talampaya: Interpretación estratigráfica y signiicado geológicoAglomerados volcánicos y conglomerados volcanogénicos estratificados del mioceno temprano en la base de las series cenozoicas del campo de Talampaya: Interpretación estratigráfica y signiicado geológico

Este trabajo destaca una época de intenso volcanismo andesítico en el interior de la placa sudamericana desarrollado con anterioridadal inicio de la cuenca de antepaís de retroarco en el segmento sur de los Andes Centrales. Análisis de facies junto a unacartografía detallada del campo de Talampaya, localizado entre las Sierras Pampeanas y la Precordillera (29º30´-30º lat. S) en eloeste argentino, permiten reconstruir un estadío de volcanismo andesítico representado por una unidad volcanosedimentaria degrano grueso que ocurrió con antelación a la amplia cuenca de antepaís del Bermejo. La estratigrafía volcanosedimentaria estudiada(Formación La Moradita nov. nom.) cubre en discordancia a rocas del Cretácico y es truncada erosivamente por series bien datadasdel Mioceno medio. El análisis estratigráfico acompañado de nuevas dataciones 40Ar/39Ar permite reconstruir la evolución paleogeográficay realizar una correlación regional de este evento a través del antepaís incluyendo la faja corrida y plegada de Precordillera yel cordón de Famatina (dentro del antepaís fragmentado) y, señala un evento de magmatismo regional que ocurrió entre ca. 20-18Ma (Mioceno temprano). Este episodio habría generado sistemas volcánicos localizados dentro del retroarco andino asociados conextensión que junto a la carga litostática contribuyeron a incrementar la acomodación y favorecer la preservación de este intervalovolcánico. Este evento es más joven que el volcanismo asociado con horizontalización de la subducción ocurrido entre el Miocenotardío y Plioceno y precede a los espesos depósitos sinorogénicos que caracterizan al antepaís en este segmento de los Andes.Fil: Astini, Ricardo Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Colombi, Carina Ester. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Candiani, Juan C.. Secretaría de Industria y Minería. Servicio Geológico Minero Argentino; ArgentinaFil: Kent, Dennis. Rutgers University; Estados UnidosFil: Swisher, Carl. U.s. Geological Survey; Estados UnidosFil: Turrin, Brent D.. U.s. Geological Survey; Estados Unido