Basin Lithosphere Research Articles

Geochemical Evidence for the Tectonic Setting of Late Proterozoic Volcanic Suites in Central England

Summary The late Proterozoic basement of central England is largely concealed by a cover of Phanerozoic rocks. Proterozoic volcanic rocks are exposed as a result of localized uplift/folding in the eastern Midlands (the Charnian Supergroup) and adjacent to major crustal lineaments which include the Church Stretton and Pontesford-Linley Fault Zones (the Uriconian volcanic group) and the Malvern lineament (the Warren House volcanic group). The geochemistry of these volcanic associations has been examined through midocean ridge basalt (MORB)-normalized multi-element diagrams which emphasize the contrasted and variable relationships between large-ion lithophile (LIL) elements and high field strength (HFS) elements within/between these volcanic groups. The Charnian Supergroup comprises over 2.5 km of bedded volcaniclastic and sedimentary rocks locally with interbedded lavas and high-level intrusions (termed ‘porphyroids’) of andesitic to rhyodacitic composition. The more mafic lavas are characterized by enrichment of the LIL elements K, Rb, Ba and Th, while HFS elements (Nb, Ta, Zr and Y) are depleted relative to MORB, and suggest that the Charnian lavas resulted from calcalkaline volcanism in an island arc. The Charnwood diorites , a minor intrusive suite emplaced into the bedded series, reflect increasing maturity of this same arc. The Uriconian volcanic group comprises a bimodal association of volcanic and pyroclastic rocks ranging from basalt to rhyolite in composition. They are enriched in HFS elements relative to MORB, which may indicate a within-plate component to the source, although their high LIL/HFS element ratios imply a subduction-related environment. The tectonic environment of eruption may have been a post-subduction, ensialic faulted basin between the Church Stretton and Pontesford-Linley Fault Zones. The Warren House group is in tectonic contact with the Malvernian (calcalkaline) plutonic complex and has LIL-element concentrations enriched in relation to MORB, but shows slight depletion of HFS elements similar to that exhibited by some marginal basin basalts and island arc tholeiites. These lavas may represent relics of marginal basin lithosphere trapped and uplifted in a Proterozoic suture between the Uriconian and Charnian terranes. These new geochemical data suggest that the late Proterozoic basement of central England comprises a heterogeneous mosaic of geochemically-distinct volcanic terranes, formed in separate volcanic arc/marginal basin environments and juxtaposed by subsequent accretionary tectonism in the Avalonian/Cadomian Orogeny (cf. Thorpe et al. 1984).

Tectonically controlled origin of three unusual rock suites in the Woodlark Basin

We propose alternative mechanisms for the origin of three unusual rock suites, high‐Mg andesites, NaTi basalts, and arclike rocks, that have been dredged from the Woodlark basin, southwest Pacific Ocean. We show that the high‐Mg andesites and NaTi basalts are associated with an unusually cool ridge environment. The cooling is due to increased hydrothermal circulation, stimulated by an unusually high crustal permeability. The high permeability is mainly due to cracking of the Woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone, although local processes, such as faulting in fracture zones, may also make some contribution. This increased convective cooling affects magma dynamics and chemistry at the ridge crest. The high‐Mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust, causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite. The NaTi basalts are also the indirect result of increased crustal cooling, which causes anomalously low degrees of partial melting of their depleted mantle source. The arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m.y. ago by a now inactive subduction zone to the north of the currently active one. Simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active. As the old plate loses volatiles, they rise into the mantle convection cell which feeds the Woodlark basin ridge crest. Because ridge subduction increases the likelihood of ophiolite obduction, these observations and explanations of Woodlark basin tectonics are potentially important for ophiolites.

A satellite altimetric geoid in the Philippine Sea

A detailed geoidal map of the Philippine Sea, constructed from GEOS 3 altimeter data, revealed that the average geoidal height in the basins of the Philippine Sea decreases as the age of the basins increases. The geoidal height–age relationship is comparable with that of mid-oceanic ridges, suggesting that the cooling of the marginal sea basin's lithosphere after its formation is similar to that of the oceanic lithosphere created at the mid-oceanic ridges. The data appear to be consistent with the back-arc opening model. However, the corner flow model is also compatible with the smoothly varying geoid over the entire Philippine Sea.

Subduction-obduction: a possible north-south transition along the west flank of the Three Kings Ridge

The Three Kings Ridge has been reinterpreted as a west-facing island arc under which a significant amount of Norfolk Basin lithosphere may have been subducted. Examination of additional seismic reflection profiles adds credence to this interpretation and suggests the presence of a north-south transition from subduction under the northern half of the ridge, evidenced by well-preserved island-arc morphology, to obduction along the southern half of the ridge. This obduction probably obliterated the trench, resulting in overthrusting and severe deformation of the forearc basin as well as intense faulting of the volcanic arc.

Metallogenesis in SE Asia

Southeast Asia may be subdivided into three distinct metallogenic provinces— ( a ) peripheral Cenozoic volcanic arc, ( b ) Mesozoic Sundaland core, and ( c ) cratonic China north of the Red River Suture: a is a major producer of copper with minor gold and silver; b is the World's foremost producer of tin with subordinate tungsten and antimony; c is the World's premier tungsten and antimony province, with subordinate tin and mercury. Ophiolites, obducted since the Palaeocene from Pacific and marginal basin lithosphere, yield substantial chromite, increasing amounts of nickel from residual laterite, and massive Cyprus-type sulphides. Palaeocene to Miocene dioritic stocks, within the ensimatic arcs of the Philippines and north Sulawesi, contain important porphyry coppers. Gold and silver are important associates, but molybdenum is rare. Kuroko-type sulphides and mesothermal copper-gold vein deposits are important in the Philippines and Taiwan. Epithermal gold-silver telluride vein deposits are more widespread throughout the Cenozoic volcanic arc. Tin mineralization, spatially related to acid granitoids of Permo–Triassic age in Indonesia and the Malay Peninsula and of Cretaceous age in Phuket and Tenasserim, is widespread in province b. Important lode deposits are confined to Billiton and the East Coast Belt of the Peninsula, emplaced within the zone of fracture around high level granitoids. Some stratiform iron-tin deposits characterize this Eastern Belt. Other tin fields, which yield the bulk of the region's production, contain few worthwhile lode deposits. Huge amounts of tin have been produced from the contact zones of large deep-seated batholiths in Bangka, along the Main Range of the Malay Peninsula, and in the Phuket region of Thailand. The tin has been concentrated in Quaternary placers by the favourable combination of climate and topography. In the Southern Shan States and north Thailand, the deposits are associated with Triassic granites and have yielded more tungsten than tin, as well as significant amounts of antimony. Major tungsten and less important tin deposits are associated with Mesozoic granitoids in the Caledonian foldbelt of southeastern China. The important antimony and mercury occurrences lie on the continental side of the tungsten-tin belt. Gold, antimony and mercury mineralization is associated with Tertiary igneous rocks in West Borneo, and mercury occurs in Palawan island. Iron contact-metasomatic and sedimentary ore deposits are associated with the Mesozoic igneous rocks in China and Sundaland, and with Cenozoic dioritic rocks in the Philippines. The metallogenic zonation of China may be related to subduction of oceanic material beneath a continental margin, persistent since the early Mesozoic. In SE Asia, more complex patterns of subduction have resulted in less well defined metal zonation in the continental regions. Cenozoic mineralization in the Philippines and Indonesia resulted from convergent tectonics of great complexity involving both continental and oceanic crust of diverse origins.

Rigid plate accretion in an inter-arc basin: Mariana trough.

. It is verified that an oblique rigid opening results in good fit of the rifted portions of arc on each side of the Mariana trough. The good fit suggests that there has been rigid plate accretion and that a plate 200km wide and 1500 km long can stay undeformed between active consuming and accreting plate boundaries. Small ocean basins, which often exist behind island arcs, have been called basins. KARIG (1 970, 197 1 b) showed that the Lau-Havre trough between the Tonga-Kermadec ridge (frontal arc) and the Lau-Colville ridge (third arc), and the Mariana trough between the Mariana ridge (frontal arc) and the West Mariana ridge (third arc) are the sites of active extension. Using Karig's terminology, a new inter-arc basin is formed by relative migration of the third arc (an inactive rifted portion of the frontal arc) away from the frontal arc-trench system. Some authors such as VAN DER LINDEN (1969) and MILSOM (1970) have implied that the extension results from rigid plate accretion, as on mid-ocean ridges. However, it is now often assumed, following KARIG (1971a) that the process of rifting and creation of new lithosphere in marginal basins is different from the process of rigid plate accretion at mid-ocean ridge crests (PACKHAM and FALVEY, 1971). The actual process of opening is not clearly defined but is assumed by Karig to result from the rise of a hot diapir from the asthenosphere. It is also assumed that, during the opening of the marginal basin, the frontal arc did not remain rigid but was deformed, probably through block faulting, along a series of faults both parallel and perpendicular to the arc (KARIG, 1970; 1971a and b) Westward from the Mariana trench, there is a poorly developed midslope basement-high on the eastern flank of the frontal arc (Mariana ridge) which carries the raised limestone islands: Guam, Saipan (Fig. 1). The back of the frontal arc is defined by a westward facing scarp with 2-3 km of relief t Contribution no 271 du Departement Scientifique du Centre Oceanologique de Bre

Stresses in the lithosphere caused by crustal thickness inhomogeneities

A physical analysis is given of the state of stresses in the earth's lithosphere floating on the asthenosphere. Crustal thickness strongly varies in a horizontal direction and thus creates large variations in the potential energy stored in the crust. As a result, the crustal material tends to spread and to reduce crustal thickness inhomogeneities. A similar situation occurs where the masses of low-density mantle are located under the crust. Large nonhydrostatic stresses arise in the lithosphere from that tendency, even if the rate of spreading is very low. The main stresses are caused by regional and global crustal thickness inhomogeneities. Their magnitudes are of the order of the additional loads associated with the regional and global relief of the crustal surface. The stresses inherent in the variations in the thickness of crustal and low-density mantle layers form a global system. Especially high stresses of the order of thousands of bars exist in the regions of high uplifts on the continents. They may be both tensile and compressive, depending on the position of uplifts with respect to the boundaries between the lithospheric plates. They are responsible for rock deformations and earthquakes. The state of stress in the oceanic lithosphere is mainly defined by the low-velocity mantle distribution beneath the mid-ocean ridges. Compressive stresses of the order of a few hundred bars exist in the lithosphere in the oceanic basins. They may be responsible for sea floor spreading and continental drift. The stresses in the lithosphere produced by other mechanisms such as thermal convection and the lithosphere's sinking into the mantle are also considered. Crustal and low-density mantle thickness inhomogeneities represent the main factor determining the stresses in the lithosphere, and the plate motions are mainly caused by the spreading of low-density mantle material rising below the oceanic ridges. It is shown that the pressure of the oceanic lithosphere on the continents caused by sea floor spreading cannot produce folded mountains on the continents.