Andean evolution at the Guañacos and Chos Malal fold and thrust belts (36°30′–37°s)

The Andes between 36°30′ and 37°S represent a Cretaceous fold and thrust belt strongly reactivated in the late Miocene. Most of the features that absorbed Neogene shortening were already uplifted in the late Cretaceous, as revealed by field mapping and confirmed by previous fission track analysis. This Andean section is formed by two sectors: a western-inner sector generated by the closure of the upper Oligocene-lower Miocene intra-arc Cura Mallín basin between the middle and late Miocene (Guañacos fold and thrust belt), and an eastern-outer sector, where late Triassic-early Jurassic extensional depocenters were exhumed in two discrete phases of contraction, in the latest early Cretaceous and late Miocene to the Present, respectively (Chos Malal fold and thrust belt). Late Miocene deformation has not homogeneously reactivated Cretaceous compressive structures, being minimal south of 37°30′S through the eastern-outer sector (southern continuation of the Chos Malal fold and thrust belt). The reason for such an inhomogeneous deformational evolution seems to be related to the development of a late Miocene shallow subduction regime between 34°30′ and 37°45′S, as it was proposed in previous studies. This shallow subduction zone is evidenced by the eastward expansion of the arc that was accompanied by the eastern displacement of the orogenic front at these latitudes. As a result, the Cretaceous fold and thrust belt were strongly reactivated north of 37°30′S producing the major topographic break along the Southern Central Andes.     

Seismic vulnerability of Kuwait and other Arabian Gulf countries: information base and research needs

This paper points to the need for seismic risk and vulnerability assessment of infrastructure systems, most notably tall structures and coastal facilities, in Kuwait and other Arabian Gulf countries. Building codes in the region currently lack seismic provisions, despite evidence of a potential threat from large-magnitude earthquakes originating from the southern part of the Zagros fold belt. The historical record of Iranian earthquakes that may have caused significant ground motion in the Gulf region is examined, as are reports of coastal damage from tsunamis. Various specific tasks, expected to constitute research priorities of a Joint Center for Risk Research, a cooperative research program involving Princeton and Kuwait Universities, are outlined.     

Shear-zone controlled basins in the Blouberg area, Northern Province, South Africa: syn- and post-tectonic sedimentation relating to ca. 2.0 Ga reactivation of the Limpopo Belt

The extent of the deposition and of the preservation of the Blouberg Formation and Waterberg Group was at least partially controlled by brittle reactivation along the Palala Shear Zone. The Palala Shear Zone in the Blouberg area (Northern Province, South Africa) is characterised by granulite-grade gneiss, and formed by sinistral transpressional collision between the Southern Marginal Zone (Kaapvaal Craton) and the Central Zone of the Limpopo Belt. The Limpopo collision is thought to have occurred either at 2.0 Ga or at 2.7 Ga with reactivation at 2.0 Ga. Deposition of the Blouberg Formation was characterised by syn-sedimentary tectonism, which is reflected by a sudden upward coarsening in sedimentary rocks, and by the presence of a strongly folded and thrusted lower member. Bedding orientations and slickenside lineation orientations suggest that vergence was towards the south, and such a tectonism can be inferred to have produced a highland area to the north, bound on the southern margin by the southern strand of the Melinda Fault. The presence of an inferred northerly upland area is supported by palaeocurrent directions and the preservational extent of the Setlaole and Makgabeng Formations of the Waterberg Group (post-Blouberg Formation). The extent and stratigraphy of the overlying Mogalakwena Formation suggests that these strata onlapped northwards over the denuding highlands. Younger Sibasa basalts of the Soutpansberg Group have been dated at ca. 1.85 Ga. Blouberg and Waterberg strata can therefore be interpreted as syn- and post-tectonic sedimentary rocks, respectively, following a ca. 2.0 reactivation event along the Palala Shear Zone. It is difficult to reconcile the succession of geological events at Blouberg with a ca. 2.0 Ga Limpopo orogeny, and thus sedimentary strata in the study area support a 2.7 Ga date for Limpopo collision, with syn-Blouberg tectonism relating to ca. 2.0 reactivation within the previously assembled Limpopo Belt.     

Quantitative assessment of future development of cooper/silver resources in the Kootenai National Forest,Idaho/Montana: Part I—Estimation of the copper and silver endowments

Faced with an ever-increasing diversity of demand for the use of public lands, managers and planners are turning more often to a multiple-use approach to meet those demands. This approach requires the uses to be mutually compatible and to utilize the more valuable attributes or resource values of the land. Therefore, it is imperative that planners be provided with all available information on attribute and resource values in a timely fashion and in a format that facilitates a comparative evaluation.The Kootenai National Forest administration enlisted the U.S. Geological Survey and U.S. Bureau of Mines to perform a quantitative assessment of future copper/silver production potential within the forest from sediment-hosted copper deposits in the Revett Formation that are similar to those being mined at the Troy Mine near Spar Lake. The U.S. Geological Survey employed a quantitative assessment technique that compared the favorable host terrane in the Kootenai area with worldwide examples of known sediment-hosted copper deposits. The assessment produced probabilistic estimates of the number of undiscovered deposits that may be present in the area and of the copper and silver endowment that might be contained in them.Results of the assessment suggest that the copper/silver deposit potential is highest in the southwestern one-third of the forest. In this area there is an estimated 50 percent probability of at least 50 additional deposits occurring mostly within approximately 260,000 acres where the Revett Formation is thought to be present in the subsurface at depths of less than 1,500 meters. A Monte Carlo type simulation using data on the grade and tonnage characteristics of other known silver-rich, sediment-hosted copper deposits predicts a 50 percent probability that these undiscovered deposits will contain at least 19 million tonnes of copper and 100,000 tonnes of silver. Combined with endowments estimated for identified, but not thoroughly explored deposits, and deposits that might also occur in the remaining area of the forest, the endowment potential increases to 23 million tonnes of copper and 190,000 tonnes of silver.     

Thermo-tectonic history of Ryoke Basement in Hohi volcanic zone, northeast Kyushu, Japan: Constraints from fission track thermochronology

Abstract Apatite and zircon fission track ages from Ryoke Belt basement in northeast Kyushu show late Cretaceous, middle to late Eocene, middle Miocene and Quaternary groupings. The basement cooled through 240 ± 25°C, the closure temperature for fission tracks in zircon, mainly during the interval 74-90 Ma as a result of uplift and denudation, the pattern being uniform across northeast Kyushu. In combination with published K-Ar ages and the Turonian-Santonian age of sedimentation in the Onogawa Basin, active suturing along the Median Tectonic Line from 100-80 Ma, at least, is inferred. Ryoke Belt rocks along the northern margin of Hohi volcanic zone (HVZ) cooled rapidly through ∼100°C to less than 50°C during the middle Eocene to Oligocene, associated with 2.5-3.5 km of denudation. The timing of this cooling follows peak heating in the Eocene-Oligocene part (Murotohanto subbelt) of the Shimanto Belt in Muroto Peninsula (Shikoku) inferred previously, and coincides with the 43 Ma change in convergence direction of the Pacific-Eurasian plate and the demise of the Kula-Pacific spreading centre. Ryoke Belt rocks along the southern margin of HVZ have weighted mean apatite fission track ages of 15.3 ± 3.1 Ma. These reset ages are attributed to an increase in geothermal gradient in the middle Miocene combined with rapid denudation and uplift of at least 1.4 km. These ages indicate that heating of the overriding plate associated with the middle Miocene start of subduction of hot Shikoku Basin lithosphere extended into the Ryoke Belt in northeast Kyushu. Pleistocene apatite fission track ages from Ryoke Belt granites at depth in the centre of HVZ are due to modern annealing in a geothermal environment.     

南北地震带应力场演变的数值模拟研究

应用有限元方法,考虑主要地质构造情况,就粘弹介质状态下南北地震带及其相邻区域的应力场时空演变过程进行了数值模拟研究．计算了10万零10年累积载荷作用下模型的应力场演变,发现在周边板块不断推挤作用下,当整个南北带的应力松弛趋于稳定后,带上应力场的总体扰动大小取决于印度-澳大利亚板块、菲律宾板块和太平洋板块的共同作用．南北地震带上呈现出的“地震南北呼应”现象,就是区域应力场达到平衡态后,整个南北带对外载荷(板块边界力)作用的总体反应．     

The Pindos Fold-and-thrust belt (Greece): inversion kinematics of a passive continental margin

Continuous exposure in the Pindos mountain chain (Greece) and the detailed stratigraphic measurements in the area enable us to construct eight balanced cross sections across the Pindos Fold-and-thrust belt (PFTB) and to approach quantitatively some parameters which controlled foreland evolution. The 160-km-wide passive continental margin of the Apulian continent in Greece was progressively shortened from east to west at rates of 6 mm/year between the Early Oligocene and Late Eocene. From the rear to the frontal part of the wedge, fault-bend folds, duplexes and imbricates were formed, while strain was partitioned into faulting (~34%), layer parallel shortening (~23%) and buckling (~9%). Foreland subsidence and internal deformation of the orogenic wedge are strongly affected by two parameters of equal importance: the thrust load of the overthrusted microcontinents and the rigidity of the underthrusted Apulian passive margin. Changes in the thickness of the pre-orogenic sediments and reactivated transform faults induced salients. During the Lower Miocene, the orogenic wedge in the Peloponnese suffered additional uplift and westward gravitational gliding induced by the intracontinental subduction of the Palaeozoic rift zone of the Phyllite-Quartzite Series, which was reactivated and returned to the earths surface during the Hellenic orogeny.     

Colors and collision rates within the Kuiper belt: problems with the collisional resurfacing scenario

We present a numerical check of the collisional resurfacing (CR) hypothesis proposed to explain the observed color diversity within the Kuiper Belt (where surface reddening due to space weathering is counteracted by regular resurfacing of neutral material after mutual collisions). Deterministic simulations are performed in order to estimate the relative spatial distribution of kinetic energy received by collisions, , for a population of target Kuiper Belt objects (KBOs) embedded in a swarm of impactors distributed within the belt. Four different impactor disks have been considered, depending on the excitation and the external limit of the belt and the density of the scattered KBOs (SKBOs) population. The obtained results are compared to the relative color index distribution within the observed Kuiper Belt, in order to derive possible similarities between the high vs low objects spatial distribution in our simulations and the bluer vs redder KBOs distribution in the “real” Kuiper Belt. Such similarities are found for several important features, in particular the general correlations between highly impacted objects and high rms excitation and low perihelion q values that are in good agreement with equivalent correlations found for the bluest objects of the observed belt. Nevertheless, simulations disagree with observations on two crucial points. (1) The plutinos are significantly more collisionally affected than the rest of our test KBO population, whereas there is no observed tendency toward bluer plutinos. (2) There is always a much stronger correlation between and eccentricities than inclinations, whereas observations show just the opposite feature. The presence of numerous SKBO impactors could significantly damp these problematic features, but cannot erase them. Whether these contradictions invalidate the whole CR scenario or not remains yet uncertain, since the physical processes at play are still far from being fully understood and the sample of available observational data is still relatively limited. But it seems nevertheless that the scenario might not hold in its simple present form.     

早期碰撞造山过程与板块构造：前寒武纪地质研究的机遇和挑战

普遍认为修正后的板块构造模式仍适用于新太古代地质研究，但是早期板块构造过程与后期有明显差异，包括陆块规模、造山带类型、碰撞造山过程等。典型碰撞造山带在地史上的初次形成具有划时代的构造演化意义，涉及典型板块构造初始发生过程、最早超级大陆拼合、威尔逊旋回及板块碰撞造山过程等方面。华北中部保留一条近南北向的碰撞造山带（2 600~2 500 Ma BP），保留特征的巨型复式褶皱、不同层次推覆构造、蛇绿岩混杂带等。围绕华北中部造山带及其25亿年重大构造热事件的研究，对认识华北早期构造演化及其超大陆再造具有重要意义，也是早期板块构造研究的关键突破口之一，开展其不同地壳层次构造变形及其前陆盆地的研究，将深化早期板块边界及其造山过程的科学认识。     

Mineralogical and geochemical study of a newly discovered mafic granulite, northwest China: Implications for tectonic evolution of the Altay Orogenic Belt

Abstract A mafic granulite body was newly discovered in the Altay Orogenic Belt, northwest China. The rocks comprise a suite of coarse‐grained and fine‐grained granulites. Orthopyroxenes (hypersthenes) in the rocks have high X_Mg and low Al₂O₃ contents, whereas clinopyroxenes have low TiO₂ and Al₂O₃ contents. Amphiboles and biotites have a high Mg/(Mg + Fe²⁺) ratio and low contents of F and Cl. The peak metamorphic pressure–temperature (P–T) conditions are estimated as 750–780°C and 6–7 kbar, and retrograde P–T conditions are in the range of 590–620°C and 2.3–3.7 kbar, indicating significant decompression. Metamorphic reactions and P–T estimates define a clockwise P–T path. Geochemically, the rocks are high in Mg/(Mg + Fe) and Al₂O₃, depleted in U, Th, K and Rb, and characterized by light rare earth element enrichment and a weak positive Eu anomaly. The Altay mafic granulite shows depleted Nb, P and Ti contents in the mid‐oceanic ridge basalt normalized spider diagram. The geochemical characteristics suggest that the protolith of the Altay mafic granulite was calc‐alkaline basalt and andesite with an island‐arc affinity. The rock has a high ¹⁴³Nd/¹⁴⁴Nd ratio with ?Nd(0) > 0, indicating derivation from a mantle‐depleted source. In the present study, a two‐stage model for the evolution of the Altay mafic granulite is proposed: an early stage in which calc‐alkaline basalt and andesite with island‐arc affinity were subducted into a deeper level of the crust and subjected to granulite‐facies metamorphism generating the mafic granulite, followed by the later stage exhumation of the system into the upper crust by the late Paleozoic thrusting.