Geomorphologic assessment of relative tectonic activity in the Maharlou Lake Basin,Zagros Mountains of Iran

Spatial differences of Quaternary deformation and intensity of tectonic activity are assessed through a detailed quantitative geomorphic study of the fault‐generated mountain fronts and alluvial/fluvial systems around the Maharlou Lake Basin in the Zagros Fold–Thrust Belt of Iran. The Maharlou Lake Basin is defined as an approximately northwest–southeast trending, linear, topographic depression located in the central Zagros Mountains of Iran. The lake is located in a tectonically active area delineated by the Ghareh and Maharlou faults. Combined geomorphic and morphometric data reveal differences between the Ghareh and Maharlou mountain front faults indicating different levels of tectonic activity along each mountain front. Geomorphic indices show a relatively high degree of tectonic activity along the Ghareh Mountain Front in the southwest, in contrast with less tectonic activity along the Ahmadi Mountain Front northeast of the lake which is consistent with field evidence and seismotectonic data for the study area. A ramp valley tectonic setting is proposed to explain the tectonosedimentary evolution of the lake. Copyright © 2011 John Wiley & Sons, Ltd.     

Role of fluvial and structural processes in the formation of the Wahiba Sands, Oman: A remote sensing perspective

The Wahiba Sands in northeastern Oman are bordered on the north, south and west by highlands. Remote sensing data are used to characterize the region between 19–23.5°N and 56.5–60°E by mapping surface and near-surface drainage, faults and fractures and aeolian features. It is suggested that the sands were originally deposited with surface runoff from the principal wadis and fluvially reworked fault zones, which define the northeastern and southwestern margins. These fluvial processes resulted in the accumulation of the vast groundwater resources now stored there. During dry climates, wind became the principal modification regime and it began to sort and shape the sediments into the dune forms that characterize today's Wahiba region. The thickness of the sands reflects the depth of the basin in which they lie. The center of the basin is filled with the thickest sand (the High Sands) and contains the highest groundwater concentrations. Presently, aeolian reworking dominates in the Wahiba region, although the Low and the Peripheral Sands continue to experience some fluvial action from occasional, seasonal rainfall. Even though dry conditions dominate today, it is clear that similar to the Sahara of North Africa, the surface sands of the Wahiba basin are indicators of groundwater occurrence.     

Neoproterozoic nappes and superposed folding of the Itremo Group, west-central Madagascar

The Precambrian geology of west-central Madagascar is reviewed and re-interpreted in light of new field observations, Landsat Thematic Mapper image analysis, and U–Pb geochronology. The bedrock of the area consists of: (1) late Archean (to Paleoproterozoic) migmatite gneiss and schist; (2) Mesoproterozoic stratified rocks (Itremo, Amborompotsy, and Malakialina Groups) perhaps deposited unconformably on the older metamorphic rocks (1, above); (3) Proterozoic ( 1000 Ma–720 Ma) plutonic rocks emplaced into both units above (1 and 2), and; (4) latest Neoproterozoic to middle Cambrian ( 570–520 Ma) granitoids emplaced as regionally discordant and weakly foliated plutons throughout the regions.

The effects of Neoproterozoic orogenic processes are widespread throughout the region and our observations and isotopic measurements provide important constraints on the tectonic history of the region: (i) Archean gneisses and Mesoproterozoic stratified rocks are the crystalline basement and platformal sedimentary cover, respectively, of a continental fragment of undetermined tectonic affinity (East or West Gondwanan, or neither). (ii) This continental fragment (both basement and cover) was extensively invaded by subduction-related plutons in the period from  1000 Ma to  720 Ma that were emplaced prior to the onset of regional metamorphism and deformation. (iii) Continental collision related to Gondwana's amalgamation began after  720 Ma and before  570 Ma. Collision related deformation and metamorphism continued throughout the rest of the Neoproterozoic with thermal effects that lasted until  520 Ma. The oldest structures produced during continental collision were km-scale fold- and thrust-nappes with east or southeast-directed vergence (present-day direction). They resulted in the inversion and repetition of Archean and Proterozoic rocks throughout the region. During this early phase of convergence warm rocks were thrust over cool rocks thereby producing the present distribution of regional metamorphic isograds. The vergence of the nappes and the distribution of metamorphic rocks are consistent with their formation within a zone of west or northwest-dipping continental convergence (present-day direction). (iv) Later upright folding of the nappes (and related folds and thrusts) produced km-scale interference fold patterns. The geometry and orientation of these younger upright folds is consistent with E–W horizontal shortening (present-day direction) within a sinistral transpressive regime. We relate this final phase of deformation to motion along the Ranotsara and related shear zones of south Madagascar, and to the initial phases of lower crustal exhumation and extensional tectonics within greater Gondwana.     

Relationship between salt diapirism and faulting in the central structural belt of the Dongying sag, Bohai Gulf basin, China

1 Introduction D iapiric structures are an im portant andw idespread structural style in m any tectonic settings(Ism ail-Zadeh etal.,2001),including m agm a diapirs,salt diapirs, m ud diapirs, and serpentinite diapirs.C om pared w ith the form ertw o,m ud diapirism in sedi-m ents is rarely reported and studied although it hasbeen know n for a long tim e both on land and underthe sea.H ow ever,w ith the geologicalinvestigations ofO D P and related on-land studies of accretionaryprism s, m ud…     

A critical examination of evidence for a Quaternary glaciation in Mt. Laoshan,Eastern China

Accurately determining how far south Quaternary glaciers existed in eastern China has important implications for understanding global Quaternary climate. In this study, we provide evidence for a colder paleoclimate in the late Pleistocene of the Laoshan range on the eastern Shandong Peninsula at 36°05′–36°19′N latitude, through documentation of erosional and depositional landforms that were possibly formed by glaciation. We integrate field observations documenting and mapping the existence of typical glacial cirques, glacial horns, moraines, arêtes, tarn lakes, and other paleoglacial features in Mt. Laoshan, with a detailed digital elevation model (DEM) created in an ArcGIS environment, merged with Landsat ETM + imagery to document the spatial relations of these features. Other features that may or may not be glacial in origin include large boulder fields (that could be moraines or debris avalanches), and potholes (that could have formed beneath glaciers or in river systems). Additionally, some features previously interpreted as glacial features are here re-interpreted as weathering-related phenomena. Although some of these features alone provide only weak to moderate evidence for glaciation, together the suite of indicators provides strong evidence for sea-level glaciation at 36°N in the late Pleistocene of eastern China. We further integrate the evidence of glaciation with the late Pleistocene paleogeography and climate, and suggest that the glaciers formed as a result of orographic effects from moist monsoonal breezes crossing a paleolake developed in North China then rising over Mt. Laoshan and dropping their moisture there, with a wide desert formed on the leeward side of the mountains.     

A Late Archean foreland fold and thrust belt in the North China Craton: Implications for early collisional tectonics

The Archean North China craton is divided into the Western and Eastern blocks along the Central Orogenic belt. A 1600 km long Archean foreland basin and thrust belt fringes the eastern side of the Central Orogenic belt. Rocks in the orogen form tectonically-stacked east-vergent fold and thrust sheets including foreland basin sediments, 2.50 Ga ophiolitic mélange, and an island arc complex. Foreland basin sediments overlie a passive margin sequence, and include a 2.50 Ga deep-water turbidite sequence that grades upward and westward into shallow-water molasse, now disposed in structurally imbricated east-verging thrusts and asymmetric folds that gradually migrated craton-ward with deformation, uplift, and erosion of the orogen. There is a strong linked relationship of the formation of the foreland basin to collision of the east and west blocks of the North China craton along the Central Orogenic belt at 2.50 Ga. The Qinglong foreland basin and Central Orogenic belt of the North China craton represents one of the best-preserved Archean orogen-to-craton transitions in the world. Its classic internal to external zonation, and flexural response to loading, demonstrate that convergent tectonics in the Archean were broadly similar to Phanerozoic convergent margin processes.     

Gushan magnetite–apatite deposit in the Ningwu basin, Lower Yangtze River Valley, SE China: Hydrothermal or Kiruna-type?

The Gushan deposit is one of the typical magnetite–apatite deposits associated with dioritic porphyries in the Lower Yangtze River Valley belt of the eastern Yangtze craton. The origin of this deposit is still uncertain and remains a controversial issue. Divergent opinions are centered on whether the iron deposits are magmatic or hydrothermal in origin. However, our field observations and mineralogical studies, combined with previous published petrological and geochemical features strongly suggest that the main ore bodies in the Gushan magnetite–apatite deposit are magmatic. Specific evidence includes the existence of gas bubbles, tubes, and miarolitic and amygdaloidal structures, melt flow banding structure and the presence of “ore breccia”. New electron microprobe analyses of the pyroxene phenocrysts of the dioritic porphyry genetically associated with the Gushan magnetite–apatite deposit show that the Fe contents in the evolving magma dramatically decrease, and then gradually increase. Because there is no evidence of mafic magma recharge, this scenario (decreasing Fe) could be plausibly interpreted by Fe-rich melts separated from Fe-poor silicate melts, i.e., liquid immiscibility was triggered by minor addition of phosphorus by crustal contamination. The occurrence of massive iron ore bodies can be satisfactorily explained by the immiscible Fe-rich melt with enormous volatile contents was driven to the top of the magma chamber due to the low density. The hot and volatile-rich iron ore magma was injected along fractures and spaces between the dioritic intrusions and wall-rocks, and led to an explosion near the surface, resulting in the immediate fragmentation of the roof of the intrusion and wall-rocks, forming brecciated ores. Moreover, other types of ores can be considered as a result of post-magmatic hydrothermal activities. Our proposed metallogenic model involving the Kiruna-type mineralization is consistent with the observed phenomenon in the Gushan deposit.     

World's largest known Precambrian fossil black smoker chimneys and associated microbial vent communities, North China: Implications for early life

Black smoker chimneys and biological vent communities have been identified at many sites on the deep seafloor, particularly along oceanic spreading centers. We report the largest and oldest known, microbe-rich sub-meter-sized black smoker chimneys and mounds from a 1.43 billion-year old sulfide deposit in a continental graben in northern China. These chimneys are especially well preserved, with characteristic morphology, internal textures and internal cylindrical mineralogical zonation. Four main types of chimneys are distinguished on textural and mineralogical criteria, exhibiting either Zn–Fe-sulfide or Pb–Zn–Fe-sulfide internal cylindrical mineralogical zones. The chimneys mark vent sites in submarine grabens indicating focused flow-venting processes. The fossil chimneys have mineralogical and geological evolutionary features similar to their counterparts on the modern seafloor and other submarine hydrothermal vents. Black smoker vent fluids and seafloor tectonism played important roles for formation of the massive sulfide deposits in the Mesoproterozoic.We also report the first known, remarkably diverse assemblage of fossil microbialites from around and inside Precambrian vent chimneys, demonstrating that Proterozoic life flourished around submarine hot vents and deep within the chimney vent passages. Filamentous, spherical, rod, and coccus-shaped fossil microbes are preserved preferentially on sulfide precipitates. Based on the depth and setting of the fossil biota, the organisms that produced the microbialites were likely sulfate-reducing chemosynthetic and thermophyllic microbes. Textural and mineralogical evidence shows that biomineralization processes enhanced chimney growth and sulfide precipitation.Close association of microorganisms with sulfide chimneys in modern deep-sea hydrothermal vents and younger ophiolites has sparked speculation about whether life may have originated at similar vents. However, little is known about fossil equivalents of vent microfossils and black smoker chimneys from Earth's early evolution. The fossilized microorganisms from the Gaobanhe black smoker chimney sulfide deposits include thread-like filaments with branching and twisted forms and preserved organic carbon, representing fossilized remnants of microbial mats metabolized at high temperatures characteristic of venting fluids. The preservation of fossil microorganisms provides evidence that microbial populations were closely associated with black smoker chimneys in Earth's early history. The microbial population clearly constitutes the site for mediating mineral formation. These ancient microbial fossils lead to a much better understanding of early life on the deep seafloor. The discovery of the Mesoproterozoic microfossils within black-smoker hydrothermal chimneys indicates that hydrothermal activity around sea-floor vents supported dense microbial communities, and supports speculation that vent sites may have hosted the origin of life.     

The Pongola structure of southeastern Africa: The world's oldest preserved rift?

Rocks of the Pongola Supergroup form an elongate belt in the Archean Kaapvaal Craton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, including rapid lateral variations in thickness and character of sediments, volcanic rocks that are bimodal in silica content, coarse, basement derived conglomerates and thick sequences of shallow water sedimentary facies associations, we suggest that the Pongola Supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in earth history reveals striking similarities, suggesting that the processes that formed this rift were not significantly different from those that form continental rifts today.     

Plate tectonics in relation to mantle temperatures and metamorphic properties

When plate tectonics emerged and how it has evolved over Earth history are two of the most fundamental challenges in Earth Sciences. These questions are tackled using a holistic approach to analyze tectonic styles in the history of Earth, giving rise to the interpretation of two styles of plate tectonics since the Archean. In these interpretations, there are different styles of deformation and metamorphism between early times dominated by warm subduction, and later times preferring cold subduction.The two styles of plate tectonics are recorded by different properties of regional metamorphism at convergent plate boundaries,which are linked to the differences in mantle temperature between the Archean and Phanerozoic. A transition to modern plate tectonics is recorded by the signature of blueschist facies metamorphism developed in the Neoproterozoic. This is consistent with geological evidence for the operation of ancient plate tectonics since the early Archean. The temporal cooling of the mantle explains the geochemical trends of mantle-derived melts, the likely change from numerous small plates to fewer but larger plates,changes in thickness and preservation of oceanic crust and lithosphere in accretionary and collisional orogens, and led to the oxygenation of the surface environment providing the environments needed to foster life.