Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet

Field, structural, and metamorphic petrology investigations of Mabja gneiss dome, southern Tibet, suggest that contractional, extensional, and diapiric processes contributed to the structural evolution and formation of the domal geometry. The dome is cored by migmatites overlain by sillimanite-zone metasedimentary rocks and orthogneiss; metamorphic grade diminishes upsection and is defined by a series of concentric isograds. Evidence for three major deformational events, two older penetrative contractional and extensional events and a younger doming event, is preserved. Metamorphism, migmitization, and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid-crustal levels. Metamorphic temperatures and pressures range from 500 °C and 150–450 MPa in chloritoid-zone rocks to 705±65 °C and 820±100 MPa in sillimanite-zone rocks. We suggest that adiabatic decompression during extensional collapse contributed to development of migmatites. Diapiric rise of low density migmatites was the driving force, at least in part, for the development of the domal geometry. The structural and metamorphic histories documented in Mabja Dome are similar to Kangmar Dome, suggesting widespread occurrence of these events throughout southern Tibet.     

The volcano-tectonic evolution of Concepción,Nicaragua

We describe the evolution of Concepción volcano by integrating regional geology, eruptive activity, morphology, stratigraphy, petrology, structure and active deformation data. This Nicaraguan volcano is set close to the back limb of the northwest-trending Tertiary Rivas anticline, a regional structure that bounds the southwest side of Lake Nicaragua. Concepción rises 1,600 m above a 1-km-thick sequence of Quaternary lacustrine mud-stones. There is no record of volcanism in the lake prior to Concepción. In addition, the only nearby volcano, Maderas volcano, has not deposited material on Concepción because of the trade winds. Thus, Concepción (and Made ras, too) can be considered as pristine volcanic environments, unaffected by other centres. A topographic rise forms an annulus 20 km in diameter around the cone. The rise is created by thrust-related folds at the western base, where the trade winds have accumulated a thick sequence of tephra, and by mud diapirs at the eastern base where only lake mudstones are present. Four magmatic-eruptive episodes exist in the stratigraphic record. The first begins with primitive low-alumina basalt and subsequently evolves to dacitic compositions. The following three episodes begin with high-alumina basalts and evolve only to silicic andesites. The occurrence of the high-alumina basalt after the first episode is indicative of crystal fractionation at lower crustal depths. The first episode may be associated with a compressive phase of volcano evolution. In this phase, the edifice load compresses substrata, allowing a longer magma residence time and differentiation in a shallow reservoir (possibly located at the density contrast between the lake sediments and the Tertiary flysch). During the next three episodes the weak sediments below the volcano started to rupture and yield under its increasing load, beginning a thrusting/diapiring phase of volcano evolution. Because of outward thrusting, vertical and horizontal stresses above the chamber were reduced, allowing magma to erupt more easily and to reach a lesser degree of evolution. If we consider the future evolution of Concepción, the differentiation in the shallow reservoir has probably generated a cumulitic complex, which eventually will start to deform and spread, beginning another, this time plutonic, spreading phase. This phase, which may be beginning now, could allow less evolved magmas to be erupted again. Four components influence the phases of volcano evolution: (1) the regional geology that is the boundary condition of the environment, (2) the substrata rheology that controls deformation, (3) the load of the volcanic edifice and (4) the magma, which provides the input of mass and energy. Our model of volcanic evolution suggests that Concepción is a complex geologic environment. The volcanic activity, tectonics and hazards can only be constrained through a complete knowledge of the many components of this environment. Published online: 20 February 2003 Editorial responsibility: R. Cioni     

Reconstructions of the Mediterranean Outflow Water during the quaternary based on the study of changes in buried mounded drift stacking pattern in the Gulf of Cadiz

Contourite deposits in the central sector of the middle slope of the Gulf of Cadiz have been studied using a comprehensive acoustic, seismic and core database. Buried, mounded, elongated and separated drifts developed under the influence of the lower core of the Mediterranean Outflow Water are preserved in the sedimentary record. These are characterised by depositional features in an area where strong tectonic and erosive processes are now dominant. The general stacking pattern of the depositional system is mainly influenced by climatic changes through the Quaternary, whereas changes in the depositional style observed in two, buried, mounded drifts, the Guadalquivir and Huelva Drifts, are evidence of a tectonic control. In the western Guadalquivir Drift, the onset of the sheeted drift construction (aggrading QII unit) above a mounded drift (prograding QI unit) resulted from a new Lower Mediterranean Core Water hydrodynamic regime. This change is correlated with a tectonic event coeval with the Mid Pleistocene Revolution (MPR) discontinuity that produced new irregularities of the seafloor during the Mid- to Late-Pleistocene. Changes in the Huelva Drift from a mounded to a sheeted drift geometry during the Late-Pleistocene, and from a prograding drift (QI and most part of QII) to an aggrading one (upper seismic unit of QII), highlight a new change in oceanographic conditions. This depositional and then oceanographic change is associated with a tectonic event, coeval with the Marine Isotope Stage (MIS) 6 discontinuity, in which a redistribution of the diapiric ridges led to the development of new local gateways, three principal branches of the Mediterranean Lower Core Water, and associated contourite channels. As a result, these buried contourite drifts hold a key palaeoceanographic record of the evolution of Mediterranean Lower Core Water, influenced by both neotectonic activity and climatic changes during the Quaternary. This study is an example of how contourite deposits and erosive elements in the marine environment can provide evidence for the reconstruction of palaeoceanographic and recent tectonic changes.     

岩石密度测量在黑龙江伊春地区花岗岩体就位机制研究中的应用

通过对伊春地区花岗岩体的密度特征的研究,认为该区晚奥陶世花岗岩体的侵位以构造扩展机制为主,气球膨胀作用为辅,最后发生了顶蚀作用.晚石炭世花岗岩是以强力旋转上升定位的气球膨胀作用机制为主,构造扩展机制为辅.晚三叠世-早侏罗世二长、正长花岗岩体就位过程中构造扩展机制起主导作用,岩浆底辟作用也较强;早侏罗世碱性花岗岩体岩浆就位过程中以强力的气球膨胀作用为主,构造扩展机制为辅.     

Cycles of passive versus active diapirism recorded along an exposed salt wall

Although it has long been recognised that passive salt diapirism may encompass sub-ordinate cycles of active diapirism, where sedimentary overburden is periodically shed off the roof of the rising salt, there has been very little study of this process around exposed salt (halite) diapirs. However, the Late Miocene-Pliocene Sedom salt wall, on the western side of the Dead Sea Basin, presents an opportunity for detailed outcrop analysis of diapiric salt and the associated depositional and deformational record of its movement during both passive and active phases of diapirism. The sub-seismic scale record of diapirism includes sedimentary breccia horizons interpreted to reflect sediments being shed off the crest of the growing salt wall, together with exceptional preservation of rotated unconformities and growth faults. Areas of more pronounced dips directed towards the salt wall are capped by unconformities, and interpreted to represent withdrawal basins within the overburden that extend for at least 1500 m from the salt margin. Elsewhere, broad areas of upturn directed away from the salt extend for up to 1250 m and are marked by a sequence of rotated unconformities which are interpreted to bound halokinetic sequences. The margins of the salt wall are defined by steep extensional boundary faults that cut upturned strata, and have enabled rapid and active uplift of the salt since the Holocene. The Sedom salt wall therefore charts the transition from passive growth marked by withdrawal basins, growth faults and unconformities, to more active intrusion associated with major boundary faults that enable the rapid uplift of overburden deposited on top of the salt to ∼100 m above regional elevations in the past 43 ka. Individual cycles of passive and active diapirism occur over timescales of <30 ka, which is up to an order of magnitude less than typically suggested for other settings, and highlights the dynamic interplay between salt tectonics and sedimentation in an environment undergoing rapid fluctuations in water level.