Isotopic and thermochronologic evidence of extremely cold lithosphere associated with a slab flattening in the Central Andes of Argentina

We present mineralogic, isotopic and thermochronologic analyses on psammopelitic and tuffaceous levels from the Bermejo and Vinchina basins – both foreland depocentres of the Central Andes of Argentina – that define a low‐temperature regime for the crust akin to a slab shallowing and flattening process. The contents of illite in illite/smectite interstratified (I/S) show a progressive illitization into the deeper parts of both basins. The distribution of I/S is compatible with theoretical simulations and predicted heat flow values of ca. 26 mW m^?2 in the 8–3.4 Ma interval for the Vinchina Basin and ca. 42 mW m^?2 since 9 Ma for the Bermejo Basin. The latter shows heat flow values that are comparable to those reported by magnetotelluric analysis (36–40 mW m^?2) in agreement with previously published heat flow calculations along the modern Andean foreland. The Rb–Sr isochrones in psammopelites (<2 μm fractions) show ages between 125 and 165 Ma, whereas the K–Ar ages decrease as the grain size is smaller (136–224 Ma for 1–2 μm, 112–159 Ma for 0.2–1 μm, 76–116 Ma for <0.2 μ and 39.3–42 Ma for <0.1 μm). These ages are significantly older than the sedimentation in the basins (ca. 16 Ma for the Vinchina Basin; U–Pb age), and can be explained by the presence of a significant amount of detrital components, mainly illite, even in the finer fractions. The preservation of detrital ages is consistent with the shallow diagenesis related to a low‐temperature regime, proposed here for the basins. Younger K–Ar ages (21.3–12 Ma) were obtained for a basal tuffaceous level. Clay mineralogy and R0 ordering in the deepest part of the Vinchina Basin, together with the evolution model of I/S with depth, suggest that the burial temperatures would have not exceeded ca. 100°C in agreement with (U–Th)/He analyses performed on apatite extracted from two tuffaceous units. Thermal indicators from both studied basins confirm the existence of a low‐temperature regime during flat subduction.     

欧亚大陆草原地上生物量的空间格局及其与环境因子的关系

本文以欧亚大陆草原为研究对象,收集和整合分析了该区域采用收获法实测的地上生物量数据,探讨了地上生物量的空间分布规律,地上生物量空间格局与环境因子之间的关系。主要结论为：① 欧亚大陆草原地上生物量的空间格局具有复杂的水平和垂直地带性分布规律,具体表现为：随着纬度的升高,海拔的升高,地上生物量都呈开口向下的抛物线变化趋势;随着经度的增大,呈开口向上的抛物线变化趋势。② 欧亚大陆草原地上生物量的空间分布虽然与年平均温度、年总太阳辐射、表层土壤（0~30 cm）的砾石含量、pH和有机碳含量有一定的相关关系,但是,主要受到年总降水量空间变异的影响。③ 欧亚大陆草原地上生物量的空间格局与年总降水量空间变异之间呈现为高斯函数关系,这主要是由青藏高原独特的草原分布与环境因素空间格局特征决定的。     

Long‐term pockmark maintenance by fluid seepage and subsurface sediment mobilization – sedimentological investigations in Lake Neuchâtel

Pockmarks and mud volcanoes from marine and lacustrine environments are thought to be the surface expression of focused fluid flow (gas and/or water). However, the control fluid flow exerts on the sediment dynamics and rates of activity of such features, especially the maintenance and growth of pockmarks, is not well understood. This study suggests that variable fluid flow is the driving process that has maintained two lacustrine pockmarks over thousands of years. In Lake Neuchâtel (western Switzerland), the currently active Chez‐le‐Bart Pockmark (diameter ca 160 m, depth ca 10 m) and the Treytel Pockmark (diameter ca 100 m, depth ca 4 m) indicate ‘quiescent’ fluid flow as well as past, ‘eruptive’, events of subsurface sediment mobilization. This study aims to test the hypothesis that phases of increased fluid flow through the pockmarks have led to the remobilization and spilling of sediment over the pockmark rims, and that different modes of activity phases are responsible for their maintenance and growth. So termed ‘subsurface sediment mobilization deposits’ are visible in seismic profiles and correlate to specific, sedimentary intervals in Kullenberg‐type long piston cores. In a detailed analysis, different modes of transport are recognized, which are attributed to high‐density flows that correspond to multiple pulses of activity. The pockmark morphology, seismic stratigraphy and core correlation with pre‐existing data reveals that the two pockmarks have been maintained throughout the Holocene and underwent several switches between ‘quiescent’ and ‘eruptive’ mode activity.     

On the formation and stability of Uranus' rings

We discuss the possibility of outlining a global model of formation and evolution for the Uranian rings with the minimum allowance for arbitrary assumptions. A condition for the stability of the -ring against differential precession is derived and compared with the similar results by Goldreich and Tremaine (1979). We discuss the problem of the long-term stability of the rings against collisional broadening, showing that the assumption of a confinement due only to self-gravitation requires the presence of a large amount of mass in the rings (10²²–10²³ g for the -ring). This latter hypothesis seems difficult to accept both for observational and for theoretical reasons, so that we suggest that some other confining mechanism (even if more peculiar) should work for this system. Finally we describe a possible scenario for the formation of the rings based on the idea of the break-up at the Roche limit of a tidally evolved satellite: we assume a sequence of events which appears, at least qualitatively, adequate to account for the observational evidence and endowed with a low degree of arbitrarity.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Tidal amplification and along‐strike process variability in a mixed‐energy paralic system prograding onto a low accommodation shelf,Edgeøya,Svalbard

The study describes the depositional development and sediment partitioning in a prograding paralic Triassic succession. The deposits are associated with the advance of large prism‐scale clinoforms across a shallower platform area. Approaching the platform, the limited accommodation and associated relative higher rates of deposition generated straighter clinoforms with lower foreset angles. The vertical restriction across the platform is interpreted to have amplified the tidal signature. Sediment was redistributed from the coast into increasingly sandy delta‐front deposits, compared to offshore equivalents. The deposits comprise extensive compound dune fields of amalgamated and increasingly clean sandbodies up‐section. Rapid deposition of significant amounts of sand led to differential subsidence and growth‐faulting in the delta front, with downthrown fault blocks further amplifying the tidal energy through funnelling. A mixed‐energy environment created along‐strike variability along the delta front with sedimentation governing process‐regime. Areas of lower sedimentation were reworked by wave and storm‐action, whereas high sedimentation rates preserved fluvially dominated mouth bars. A major transgression, however, favoured tidally dominated deposits also in these areas, attributed to increasing rugosity of the coastline. Formation of an extensive subaqueous platform between the coast and delta front dampened incoming wave energy, and tidally dominated deposits dominate the near‐shore successions. Meanwhile formation of wave‐built sand‐bars atop the platform attest to continued wave influence. The strong tidal regime led to the development of a heterolithic near‐shore tidally dominated channel system, and sandier fluvial channels up‐river. The highly meandering tidal channels incising the subaqueous platform form kilometre wide successions of inclined heterolithic stratification. The fluvially dominated channels which govern deposition on the delta plain are narrower and slightly less deep, straighter, generally symmetric and filled with cleaner sands. This study provides important insight into tidal amplification and sand redistribution during shallowing on a wide shelf, along with along‐strike process‐regime variability resulting from variations in sediment influx.     

Alluvial deposits from the strike-slip fault Lo River Basin (Oligocene/Miocene), Red River Fault Zone, north-western Vietnam

The Lo River Basin (LRB) is one of several narrow sedimentary basins associated with the main faults of the Red River Fault Zone separating the South China and Indochina microplates. The basin is located on the NE boundary of the high-grade metamorphic Con Voi Massif and the sedimentary and metasedimentary Viet Bac fold zone in north-eastern Vietnam.The LRB is filled with over 6000 m of Oligocene/Miocene alluvial deposits. The source area was probably located on the NE margin of the basin and was composed mostly of low-grade metamorphic rocks with a minor component of sedimentary rocks. Three alluvial systems are recognised. The oldest system was a proximal braided river system, with the minor occurrence of alluvial fans. The younger systems record changes in clast composition and lithofacies, which suggests a transition from a distal braided river to a distal braidplain system. The LRB fill shows a range of features characteristic of strike-slip fault basins. The origin of the LRB is correlated with the left-lateral transtensional regime. The present shape of the basin is a result of post-sedimentation tectonic activity.