Abstract The Ambergris Shoal at the southeast end of the Caicos Platform is approximately 25 km long and composed of ooids. Ooids with nuclei of quartz grains were found in one sample. The coatings are up to 0.05 mm thick. Quartz does not occur naturally on the surface in Caicos. Therefore, coatings on quartz nuclei probably formed in the last 500 years when humans may have transported quartz sand to the platform. The quartz-cored ooids have radially-oriented crystals in their cortex, in contrast to tangentially-oriented needles in other ooids on the shoal. An “island” in the Caicos Mid-Platform Shoals is approximately 500 m long by 100 m wide and is now at sea level. Lithified Holocene grainstone beds along the margins of the “island” dip away from the middle of the “island” similar to beach deposits. Those dipping beds were truncated at sea level, and have fibrous cements similar to beachrock and other marine cements. The middle of the island also contains lithified grainstones at and slightly below sea level; however those grainstones have equant calcite cements supporting an interpretation that they precipitated from freshwater. The presence of freshwater cements suggests that eolian dunes covered the entire island in the past. A large storm (hurricane) apparently removed poorly lithified eolian sands above the water table, leaving meteoric-cemented rocks in the middle and marine-cemented rocks around the margins of the island.

Geosynclines are surfaces of regional extent that subside considerably during formation of their included surficial rocks; they have limits in time and space. Early Paleozoic North America had a rather stable center (hedreocraton) margined by deeper sinking belts (miogeosynclines) that initially received carbonate rocks, and quartz sands from the interior; neither area had appreciable volcanism. The continental borders have distinctive volcanic flows and fragmentals, which with associated sediments show deep subsidence (eugeosynclines) and development of associated tectonic welts. Lands raised in the eugeosynclinal belts yielded sediments to the adjoining miogeosynclines; with deformation of the latter, terrigenous detritus spread into subsiding areas (exogeosynclines) in the margin of the hedreocraton. The craton periodically gained basin or trough-shaped depressions isolated from highland source areas (autogeosynclines) or receiving debris from associated intracratonal elevations (zeugogeosynclines). The stratigraphy of the rocks in Paleozoic eugeosynclines shows that they were formed in regions much like modern island arcs. Some of the ancient eugeosynclines are continuous in trend with modern volcanic archipelagoes, which lie on the continental side of the ocean basins. The eugeosynclines became consolidated by orogeny and by intrusion of plutonic rocks; ultrabasic intrusions are restricted to them, and great batholiths are predominant within their areas. Deeply-subsiding, relatively non-volcanic troughs (epieugeosynclines) developed later, and were succeeded by fault-bounded depressions (taphrogeosynclines). The belts became rather stable additions to the craton. They finally developed coastal plains passing marginally into coastal geosynclines (paraliageosynclines). It is axiomatic that nearly all kinds of surficial rocks are thickest in geosynclines; geosynclinal facies are characterized by thickness rather than kind. The better sorted sediments virtually are restricted to the cratons and adjoining geosynclines, and volcanic detritus is prevalently in eugeosynclines and the proximal parts of miogeo-synclines. Because of the mobility of lands in eugeosynclinal belts, poorly sorted detritus, such as conglomerate and graywacke, is most abundant, but carbonate and siliceous rocks prevailed over large areas for long times. Plutonic rocks are extensive in the pre-Paleozoic and volcanic rocks are widespread. Thus, by analogy, each part of the continent has in some past time been eugeosynclinal. Eugeosynclines and modern island arcs are thought to be in areas of thin sial. Sialic nuclei grew through orogenic consolidation of eugeosynclinal belts, formed at the expense of an originally universal simatic crust, of which the ocean basins, the low cratons, are relics. The nuclei coalesced into a single continental mass by earliest Paleozoic, when thin-sialic eugeosynclinal belts extended to the present continental margin. Stratigraphic evidence contradicts hypotheses that assume an original floating sialic block on surrounding and subjacent sima; the continent has developed through dynamic processes.

Abstract Precambrian high-pressure (HP) granulites provide important information for reconstructing ancient continental nuclei. Here we report granulites in Eastern Hebei, North China Craton (NCC). They experienced three metamorphic events related to Neoarchean–early Paleoproterozoic orogenesis. The garnet–clinopyroxene granulite defines the M 1 event at P–T conditions of 11–13 kbar and 780–830°C, while the two-pyroxene granulite was produced during the M 2 event at 7–9 kbar and 850–950°C. Both the garnet–clinopyroxene and two-pyroxene granulites experienced amphibolite retrogression during the M 3 event at 5–7 kbar and 710–730°C. Geochemical compositions of these granulites exhibit affinity to island-arc andesites. Zircon U–Pb dating shows that their magmatic precursors were erupted at c. 2538 Ma, and have experienced two-stage growth of zircon rims at c. 2458 and c. 2285 Ma, respectively. The c. 2458 Ma age may represent orogenic events during the amalgamation of micro-continental blocks of the eastern NCC, and the c. 2285 Ma age may be interpreted as the effect of late Paleoproterozoic magmatism. We suggest that the Neoarchean andesitic protoliths of the granulites were metamorphosed at HP granulite-facies conditions during collision of micro-continental blocks, and then exhumed to shallow levels. These early Paleoproterozoic HP granulites recorded the amalgamation of micro-continental blocks to reach the cratonization of the eastern NCC.

Abstract: In coastal areas of the SW part of the island of Rhodes, Greece, eolian oolitic sediments represent the latest depositional phase, and are presumed to have a Late Pleistocene to Early Holocene age. The ooids have sand-size nuclei and are lightly cemented by vadose meniscus cement. In a road cut, supplemented by minor sections in small, ancient stone quarries nearby, the sedimentary architecture and trace fossils are visible. The dunes have ramp morphology and contain three horizons of paleosols that divide the eolian sediments into three units. The paleosols contain rhizoliths and poorly preserved invertebrate bioturbation. Vertical sections allowed representative measurements of 79 tracks and limited horizontal surfaces supplied four measured tracks. Five size classes of tracks are distinguished. The mode of preservation of the tracks is poor, probably on account of the oolitic nature of the substrate. The three smallest size classes are probably of artiodactyls. The largest class probably was produced by proboscidians. A bedding-plane view of one track indicates that the next-largest class may be the work of camels. If this is the case, and the bedding-plane specimen is convincing, it is the first record of Pleistocene or Early Holocene camels on Rhodes. The combination of size groups of tracks differs in the three units, demonstrating differences in faunal composition in the different periods of deposition of the oolite.

Abstract Nine separate Cambrian to Carboniferous terranes are recognized in West Junggar, northwest China. They were amalgamated as part of the Central Asian Orogenic Belt which records accretion of continental, island-arc and oceanic terranes to Archaean-Proterozoic continental nuclei. Tangbale, Kekesayi, Ebinur and Mayila terranes (CambriaN–Silurian) evolved in intra-oceanic settings and docked, along a series of north-dipping subduction zones, on to the Laba terrane to their south. This southern continent was contiguous with lithosphere of the Kulumudi Ocean to the north. Devonian subduction on the northern edge of this ocean resulted in formation of a continental arc (Toli terrane) and accretionary complex (Kulumudi terrane). The Karamay terrane formed as an accretionary complex during the Carboniferous. The ophiolitic Sartuohai terrane was emplaced as mélange between Kulumudi and Karamay terranes during the Late Carboniferous. Subduction migrated southward, continuing beneath these terranes, resulting in the intrusion of I-type granites into the Toli, Kulumudi, Sartuohai and Karamay terranes. These granites are closely associated with epithermal and porphyry-style gold mineralization. Composite terranes either side of the Kulumudi Ocean collided in the Late Carboniferous, marking the final consolidation of Central Asia. Collision was accompanied by anorogenic granite and diabase dyke intrusion, followed by widespread latest Carboniferous to Permian extension, and subsequently the formation of the Junggar Basin. West Junggar has been further disrupted by Cenozoic strike-slip faulting along Junggar and Dalabute faults.

Aseismic ridges on underthrusting oceanic plates often trend into cusps or irregular indentations in the trace of the subduction zone. For example, the Hawaii-Emperor Ridge trends into the Kuril-Aleutian cusp, and the Marianas arc is bounded by the Marcus-Necker Ridge on the north and the Caroline Ridge on the south. The association between ridges and cusps is too common to be due to chance; it is proposed that the extra buoyancy of the plate with its aseismic ridge gives the plate greater resistance to sinking. This would inhibit back-arc extension and thereby produce a notch in the subduction zone. Island arcs may, therefore, acquire their curvature by additional constraints than the Earth’s curvature. The geology of about 15 such cusp areas is examined for evidence to test the hypothesis that cusps were caused by subducted aseismic ridges. This hypothesis applies only to cases where extensional basins lie behind the arcs. There also appear to be cases where the trace of the subduction zone has been modified not by inhibited back-arc spreading but by splintering of the overthrusting and possibly the underthrusting plate as well. Extremely high, massive aseismic ridges might induce arc polarity reversals and thereby assume the role of protocontinental nuclei. Seismicity and volcanism are examined where aseismic ridges are being subducted; there are several examples of reduced seismicity that cannot be explained by insufficient sampling time. By modifying the geometry of the subduction zone, the downgoing ridges necessarily affect seismicity. In addition, the plate containing the ridge may be thinner and hotter and more likely to deform by creep. There is no systematic increase or decrease in the number of andesite volcanoes where the ridges are subducted. However, lines of volcanoes and sometimes other kinds of geologic and seismic provinces may stop or start at the arc-ridge intersections. This is attributed to segmenting of the lithosphere into distinct tongues, each tongue acting more or less independently. Aseismic ridges would act as lines of weakness along which the downthrust slab becomes detached.