Central Subsidence Research Articles

Prediction Parameters for Mining Subsidence Based on Interferometric Synthetic Aperture Radar and Unmanned Aerial Vehicle Collaborative Monitoring

Coal mining induces surface subsidence, making rapid and precise monitoring of this subsidence a key area of current research. To address the limitations of D-InSAR technology in capturing large-gradient deformations in the central subsidence basin and the challenges facing UAVs in accurately monitoring small deformations at the basin’s edge, we propose a method for inverting the expected parameters of surface subsidence by synergistically integrating InSAR and UAV monitoring. We determined the cumulative subsidence of monitoring points along the dip and strike observation line of the Banji 110,801 working face between 10 April 2021 and 28 June 2022, employing D-InSAR and UAV techniques. By leveraging the complementary strengths of both monitoring techniques, we fused the two types of monitoring data and verified the error of the fusion data to be within 10 cm through leveling data verification. Simulation experiments utilizing the probability integration method and the Broyden–Fletcher–Goldfarb–Shanno (BFGS) optimization algorithm confirmed that the 10 cm data source error remains within the required limits for probability integration parameter inversion. Finally, the BFGS algorithm was employed to invert the parameters of the probability integration method based on the fusion data results. Subsequently, these inversion parameters were used to predict the subsidence at the monitoring point and were compared with the level measured data. The results demonstrate that the use of collaborative InSAR and UAV monitoring technology for inverting the expected parameters of surface subsidence in the mining area yields superior results, aligning with the actual patterns of ground surface movement and deformation. This study addresses the global need for unmanned monitoring of mining-related subsidence. It employs InSAR and UAV technologies in a synergistic approach to monitor surface subsidence in mining regions. This approach harnesses the strengths of multiple data sources and presents a novel concept for the unmanned monitoring of surface subsidence in mining areas, contributing to environmental protection efforts.

Experimental study on the evolution law of mining‐induced fractures and deformation rock mass structure of gently inclined layered slope

AbstractLandslide induced by underground mining is the main type of geological disaster in Guizhou Province. The spatial position of the mined‐out area is the key factor affecting the deformation evolution of mining landslides. Three typical mining‐induced slopes in western Guizhou Province are selected to establish the engineering geological model. The orthogonal experimental design is carried out with the position of the mined‐out area relative to the slope shoulder (PM) and the slope height (SH) as variables, and the discrete element simulation test is carried out. The influence of the two variables on the evolution of overlying strata fractures and the structure of slope surface deformed rock mass is analyzed by gray relational method. The results show that under the effect of mining, the overlying strata rock on the mined‐out area gradually collapses and produces cracks through the surface. The force chain arch gradually develops from the lower part to the surface rock layer, and the slope surface rock layer is transformed from a cantilever beam embedded at one end to a multisegment articulated beam. Depending on the number of segments of the articulated beam structure, it can be divided into three categories, and the rock mass where the slope collapse occurs is mainly concentrated at the end of the multisegment articulated beam. The central subsidence of the subsidence basin, the length of the end of the articulated beam, the position of the fulcrum, and the rotation angle of the beam structure are the key indicators affecting the stability of the multisegment articulated beam. Gray relational analysis shows that PM is the primary factor to control the output of these indicators. The multiple linear regression model of gray relational degree between PM and SH was constructed, and the prediction formula of gray relational grade was obtained, which realized the multiobjective prediction of the overlying strata rock structure model under the same slope parameters. The results provide the theoretical basis for disaster prevention and mitigation of gently inclined layered mining slopes.

Research on Surface Deformation of Ordos Coal Mining Area by Integrating Multitemporal D‐InSAR and Offset Tracking Technology

Underground mining in coal mining areas will induce large‐scale, large‐gradient surface deformation, threatening the safety of people’s lives and property in nearby areas. Due to mining‐related subsidence is characterized by fast displacement and high nonlinearity, monitoring this process by using traditional and single interferometric synthetic aperture radar (InSAR) technology is very challenging, and it cannot accurately and quantitatively calculate the deformation of the mining area. In this paper, we proposed a new method that combines both multitemporal consecutive D‐InSAR and offset tracking technology to construct a complete deformation field of the coal mining area. Taking into account the accuracy of multitemporal consecutive D‐InSAR in calculating small deformation areas and the ability of offset tracking to measure large deformation areas, we utilized their respective advantages to extract the surface influence range and applied an adaptive spatial filtering method to integrate their respective results for inversion of the deformation field. 12 ascending high‐resolution TerraSAR‐X images (2 m) from September 3, 2018, to October 26, 2019, and 39 descending Sentinel‐1 TOPS SAR images from August 5, 2018, to November 4, 2019, in the Ordos Coalfield located at Inner Mongolia, China, were utilized to obtain the whole subsidence field of the working faces F6211 and F6207 during the 454‐day mining period. The GPS monitoring station located in the direction of the mining surface is used to verify the accuracy of the above method; at the same time, to a certain extent, the difference between the unmanned aerial vehicle’s DSM data acquired after coal mining and the Shuttle Radar Topography Mission (STRM) DEM can qualitatively verify the accuracy of the results. Our results show that the results of TerraSAR are basically consistent with the deformation trend of GPS data, and that of Sentinel‐1 have large errors compared with GPS. The maximum central subsidence reaches ~12 m in the working face F6211 and ~4 m in the working face F6207. In the working face F6207, the good agreement between GPS and TerraSAR results indicated that the method above using high‐resolution SAR data could be reliable for monitoring the large deformation area in the mining field.

Passive continental margin segmentation of the marginal seas and its effect on hydrocarbon accumulation: A case study of the northern continental margin in South China Sea

Global continental marginal basins are often segmented and significantly impact hydrocarbon accumulation. The northern South China Sea is a marginal sea, in which several passive continental marginal basins have been developed. It is represented by the Pearl River Mouth Basin in the eastern part and the Qiongdongnan Basin in the western part, with rich oil and gas resources that exhibit significant differences in hydrocarbon accumulation. However, the impacts on hydrocarbon accumulations due to various segmentations have rarely been discussed. Based on the systematic interpretation of seismic data, paleo-biological data, detailed thickness mappings, sedimentary facies mappings, analyses of organic geochemical characteristics of source rocks, reservoir formation characteristics, and hydrocarbon accumulation patterns, the tectonic segmentation and its effect on hydrocarbon accumulation in the two basins are discussed. The tectonic evolution in the northern part of the South China Sea was characterized earlier in east segmented and later in west segmented; this same pattern is always displayed, wherein the east segmentation occurs one phase earlier than the west segmentation. The tectonic segmentations caused differences in the source rocks, resulting in the formation of two sets of source rocks that were mainly developed in the east basin, namely: the lacustrine source rocks and marine–continental transitional facies coal-series source rocks. Meanwhile, only one set of the marine-continent transitional facies coal-series source rocks and terrestrial marine facies source rocks was developed in the west basin. The segmentations also yielded differences in the reservoir characteristics, even though both basins developed marine–continental transitional facies as well as delta and deep-water gravity flow sandstone reservoirs; meanwhile, the eastern basin developed continental fluvial reservoirs. The differences in hydrocarbon accumulation were controlled by segmentation; at the east basin, structural ridges in shallow layers, deep lithologic–stratigraphic traps, and bulges in sags affected the accumulations of oil and gas. Meanwhile, at the west Qiongdongnan Basin, coal-bearing delta systems produced the gas distributions along the margins of the central subsidence that was adjacent to the source rocks.

Inversion and Analysis of Mining Subsidence by Integrating DInSAR, Offset Tracking, and PIM Technology

High-intensity underground mining generates considerable surface subsidence in mining areas, including ground cracks and collapse pits on roads and farmland, threatening the safety of buildings. Large-amplitude subsidence (e.g., >2 m) is usually characterized by a large phase gradient in interferograms, leading to severe phase decorrelation and unwrapping errors. Therefore, the subsidence on the surface cannot be well derived simply using conventional differential interferometric synthetic aperture radar (DInSAR) or other geodetic measurements. We propose a new method that combines both DInSAR and subpixel offset-tracking technology to improve mine subsidence monitoring over large areas. We utilize their respective advantages to extract both the spatial boundaries and the amplitude of displacements. Using high-resolution RADARSAT-2 SAR images (5 m) acquired on February 13, 2012, and November 27, 2012, in the Shendong Coalfield located at the border between Shaanxi Province and Inner Mongolia Province, China, we obtain the subcentimetre-level subsidence of the mine boundary by DInSAR and resolve the metre-level mine subsidence centre based on subpixel offset tracking. The whole subsidence field is obtained by combining and analyzing the subcentimetre-level and the metre-level subsidence. We use the probability integral method (PIM) function model to fit the boundary and central mine subsidence to reconstruct the spatial distribution of the mine subsidence. Our results show that the maximum central subsidence reaches ~4.0 m (beyond the monitoring capabilities of DInSAR), which is generally in agreement with the maximum subsidence of ~4.0-5.0 m from field investigation. We also model the boundary and the central subsidence (the final fitting coefficient is 0.978). Our findings indicate that the offset-tracking method can compensate for the deficiency of DInSAR in large-amplitude subsidence extraction, and the inclusion of the PIM technique helps reconstruct the whole subsidence field in mining areas.

Barrier effect of improved porcine small intestinal submucosa absorbable membrane on early healing of mandibular defects in rabbits

Objective To evaluate the barrier effect of an absorbable barrier membrane made by small intestinal submucosa (SIS) compared with Bio-Gide collagen membrane. Methods 12 healthy New Zealand male white rabbits were randomly assigned. A or B round bone defects with a depth of 2 mm and a diameter of 5 mm or 8 mm was made in each rabbit's mandibular. The following treatments were given respectively: covered with SIS membrane (S), covered with Bio-Gide membrane (G) and blank control (O). Then we got six groups: AS, AG, AO, BS, BG, and BO (n=4). After 4 weeks, the rabbits were sacrificed. The specimens were examined by naked-eye observation, new bone percentage (BV/TV) and bone mineralized density (BMD), which were measured and analyzed by Micro-CT. The data were analyzed with one-way ANOVA. Results After 4 weeks, Bio-Gide membranes were fused with the surrounding tissue while SIS membranes held the form with no significant degradation. In the AS, BS and AG groups, the absorbable membranes smoothly covered on the new bone. While in the BG group, Bio-Gide membranes collapsed to the center of the bone defects. The 3D reconstruction of Micro-CT showed that a large number of newly formed trabeculae were found in the four groups of AS, BS, AG, and BG. In the central subsidence area of the BG group, the newly formed trabeculae were sparse. However only a small amount of new bone trabecula appeared at the bottom of the defects in groups AO and BO. Micro-CT quantitative results showed that BV/TV (39.10%±0.79%) and BMD [(517.73±11.22) mg/cm3] of AS group were significantly higher than those of AO group [26.67%±1.12%, (319.81±8.00) mg/cm3] (P 0.05). BV/TV (34.90%±1.35%) and BMD [(409.09±8.14) mg/cm3] of BS group were significantly higher than those of BO group [23.63%±2.07%, (171.00±16.24) mg/cm3] (P 0.05). HE staining analysis revealed that significant bone formation was achieved in the AS, AG, BS and BG groups, and trabecular bone of AS and AG groups were thicker and denser. In AO and BO group, there were scattered new bone tissues in edges of host bone, and no coarse trabecular bone formed. Conclusion In the early healing of two sizes bone defects in rabbit mandibular, SIS membrane and Bio-Gide membrane have a similar barrier effect in guided bone regeneration. And SIS membrane's ability to maintain space for bone regeneration seems to be better.

Coeval Uplift and Subsidence Reveal Magma Recharging Near Rome (Italy)

AbstractUplift and subsidence alternate throughout the life cycle of a volcano mirroring magma accumulation, migration and/or gas pressurization, and magma cooling or depressurization, respectively. The unusual occurrence of coeval inflation and deflation is difficult to reconcile with the recharging or cooling of a single magma chamber, or hydrological processes. Here we show a persistent flank uplift and central subsidence at Colli Albani volcano near Rome (Italy), by mapping about 20 years of deformation by InSAR data. The magmatic helium signature increases in correspondence with N‐S faults along the western slope of Colli Albani, which are therefore interpreted as deep‐seated structures directly or indirectly connected with an underlying magma reservoir. Deformation modeling shows that these faults are pathways for fresh magma intrusion, whereas a residual magma is cooling below the deflating caldera. Therefore, magma recharge at depth is controlled by major faults along which the vents of the most recent activity (<200 ka ago) concentrate in the western side of the volcano and not by the caldera structures. We demonstrate that the Colli Albani magmatic system is slowly rejuvenating, posing a volcanic threat for Rome.

Seismic and inter-seismic ground surface deformations of the Murono mud volcano (central Japan): a laser scanning approach

A small mud volcano in Murono, Niigata Prefecture, north-central Japan, shows active ground surface displacements, not only when large earthquakes occur in the region but also during quiescent periods between earthquake events. The site recently underwent abrupt deformations due to strong regional earthquakes in 2004, 2007, 2011, and 2014, while gradual surface deformations were reported during quiescent periods between the earthquakes. To detect the spatial distribution of the changes in the mud volcano’s ground surface elevation, we carried out multi-temporal terrestrial laser scanning. Point cloud datasets were registered at different times by minimizing the distance between the closest points in different clouds for stable ground features, which revealed centimeter- to decimeter-scale deformations around the domain of the conspicuous uplift. The spatial distribution of the deformation triggered by the earthquakes, including both central uplift and peripheral subsidence, exhibits an elliptical pattern, on which open crack fractures, associated with the earthquake-triggered uplift, were formed. The displacement and stress fields for the earthquakes were modeled numerically, and anomalously high pressure and/or weakening of the surficial materials was expected for the formation of fractures in the local domain. In contrast, continuous uplift was observed during the inter-seismic quiescent periods, the domain of which seems to have changed after the strong earthquake in 2014. In the coming years, further measurements will be necessary to unravel the physical subsurface mechanics of the mud volcano.

TERRACE SEQUENCES OF THE SIKHOTE-ALIN RIVER VALLEYS (CONDITIONS OF FORMATION; AGE; CORRELATION)

The main factors of river terraces formation are determined. The regional types of river terraces express total rising of the Central Sikhote-Alin (with amplitude up to 300 m) and subsidence of its margins. The local types of terrace sequences have been formed in the coastal zone (under the impact of the sea level changes) within the basalt plateaus and at the places of the river system rearrangement. Tectonic asymmetry of the Sikhote-Alin has resulted in greater erosion cut on the eastern (steep) slope, than on the western (low-angle) one. Relics of Late Miocene river system were revealed among the river valley landforms. Pliocene-Quaternary terrace sequence consists of 7 terraces, which are characterized by subsidence towards Ussury-Khanka depression and coastal zone of the Sea of Japan.

Circumferential graben and the structural evolution of Alba Mons, Mars

Alba Mons is a unique, very extensive but shallow volcanotectonic construct in northern Tharsis, Mars. Numerous models have been presented to explain the formation of Alba Mons and its most characteristic feature, a wristwatch-like pattern of radial and circumferential graben. We used a wide selection of topographic datasets to characterize the fault throw variation on nine topographic transects across the circumferential graben in order to provide observational constraints for the different formation models, and to gain further insight into the evolution of Alba Mons. In most of the transects, summed throws from outwards-facing (away from the center of the volcano) faults are larger than from the inwards-facing (towards the center) ones. Only the very gently sloping western transects show the opposite, emphasizing the east–west-asymmetry of Alba Mons. 10–40% of the observed topographic relief of Alba Mons along the nine transects can be accounted for by this throw difference between the inwards- and outwards-facing faults. These results are consistent with predictions of models suggesting an uplift mechanism to explain the formation of the circumferential graben, but not with models invoking central subsidence. Horizontal extensional strain along the transects varies between 0.5% and 2%, consistent with strain predictions of the late-stage sill complex inflation model of McGovern et al. (McGovern, P.J., Solomon, S.C., Head J.W. III, Smith, D.E., Zuber M.T., Neumann, G.A. [2001]. J. Geophys. Res. 106(E10), 23769–23809).

An ice flow modeling perspective on bedrock adjustment patterns of the Greenland ice sheet

Abstract. Since the launch in 2002 of the Gravity Recovery and Climate Experiment (GRACE) satellites, several estimates of the mass balance of the Greenland ice sheet (GrIS) have been produced. To obtain ice mass changes, the GRACE data need to be corrected for the effect of deformation changes of the Earth's crust. Recently, a new method has been proposed where ice mass changes and bedrock changes are simultaneously solved. Results show bedrock subsidence over almost the entirety of Greenland in combination with ice mass loss which is only half of the currently standing estimates. This subsidence can be an elastic response, but it may however also be a delayed response to past changes. In this study we test whether these subsidence patterns are consistent with ice dynamical modeling results. We use a 3-D ice sheet–bedrock model with a surface mass balance forcing based on a mass balance gradient approach to study the pattern and magnitude of bedrock changes in Greenland. Different mass balance forcings are used. Simulations since the Last Glacial Maximum yield a bedrock delay with respect to the mass balance forcing of nearly 3000 yr and an average uplift at present of 0.3 mm yr−1. The spatial pattern of bedrock changes shows a small central subsidence as well as more intense uplift in the south. These results are not compatible with the gravity based reconstructions showing a subsidence with a maximum in central Greenland, thereby questioning whether the claim of halving of the ice mass change is justified.

The Bushveld Complex, South Africa: formation of platinum–palladium, chrome- and vanadium-rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large, relatively slowly cooling, subsiding magma chamber

Platinum-group element (PGE) deposits in the Bushveld Complex and other layered intrusions form when large, incompletely solidified magma chambers undergo central subsidence in response to crustal loading, resulting in slumping of semi-consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides, oxides, olivine and pyroxene and less dense layers enriched in plagioclase. The most economic PGE, Cr and V reefs form in large, multiple-replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification. The depth of emplacement has to be relatively shallow as, otherwise, ductile crust would not be able to flex and collapse. In smaller intrusions, cooling rates are faster, subsidence is less pronounced and, where it occurs, the cumulate may be largely solidified, resulting in insignificant mush mobility and mineral sorting. Layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower, but the reefs can be relatively thicker. An additional factor controlling the PGE, Cr and V prospectivity of intrusions is their location within cratons. Intra-cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large, layered igneous bodies. Furthermore, intrusions sited within cratons are more readily preserved because cratons are underlain by thick, buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust.

Dynamic models of continental rifting with melt generation

Active or passive continental rifting is associated with thinning of the lithosphere, ascent of the asthenosphere, and decompressional melting. This melt may percolate within the partially molten source region, accumulate and be extracted. Two-dimensional numerical models of extension of the continental lithosphere–asthenosphere system are carried out using an Eulerian visco-plastic formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component (crust–mantle) and two-phase (solid–melt) system. Temperature-, pressure-, and stress-dependent rheologies based on laboratory data for granite, pyroxenite and olivine are used for the upper and lower crust, and mantle, respectively. Rifting is modelled by externally prescribing a constant rate of widening with velocities between 2.5 and 40 mm/yr. A typical extension experiment is characterized by 3 phases: 1) distributed extension, with superimposed pinch and swell instability, 2) lithospheric necking, 3) continental break up, followed by oceanization. The timing of the transition from stages 1) to 2) depends on the presence and magnitude of a localized perturbation, and occurs typically after 100–150 km of total extension for the lithospheric system studied here. This necking phase is associated with a pronounced negative topography (“rift valley”) and a few 100 m of rift flanks. The dynamic part of this topography amounts to about 1 km positive topography. This means, if rifting stops (e.g. due to a drop of external forces), immediate additional subsidence by this amount is predicted. Solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks, often observed at volcanic margins. After continental break up, a second time-dependent upwelling event off the rift axis beneath the continental margins is found, producing further volcanics. Melting has almost no or only a small accelerating effect on the local extension value ( β-value) for a constant external extension rate. Melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift. This upwelling velocity is only weakly dependent on the rifting velocity. The melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks. Melting increases the topography of the flanks by 100–200 m due to depletion buoyancy. Another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension/subsidence. Modelled magma amounts are smaller than observed for East African Rift System. Increasing the mantle temperature, as would be the case for a large scale plume head, better fits the observed magma volumes. If extension stops before a new ocean is formed, melt remains present, and convection remains active for 50–100 Myr, and further subsidence is significant.

Magmatic Layering and Magnetic Fabrics in the Palaeogene Carlingford Later Gabbros, Co. Louth, Ireland

A detailed magnetic fabric study of the annular partially layered Palaeogene Carlingford 'Later Gabbros', Co. Louth, is combined with field observations to constrain existing emplacement models for the intrusion. The Anisotropy of Magnetic Susceptibility (AMS) fabrics are generally quite inconsistently oriented. Only in three of seven traverses of the intrusion is consistency in orientation observed, suggesting that little can be drawn from the data by way of interpretation. However, the traverse in the north of the intrusion, at Ballinteskin, corresponds to an area of considerable syn-magmatic deformation of layering. Magnetic fabrics in this area exhibit consistently oriented inward dipping and inward plunging magnetic foliations and lineations respectively. Comparison of this data with a recent AMS study of the Great Eucrite of Ardnamurchan, NW Scotland, suggests that though central subsidence may have occurred in the Later Gabbros too, it was not as important as in Ardnamurchan. This is supported to some degree by the presence of a shallowly dipping (-20?) intrusive contact between the Later Gabbros and an extensive granophyre intrusion, which also suggests that the granophyre is the older intrusion. This conclusion is inconsistent with previous interpretations of the granophyre as the youngest intrusion in the Carlingford igneous centre. Introduction and geological setting The Carlingford Igneous Complex is the oldest of three closely associated Palaeogene intrusive centres in NE Ireland dated (U-Pb zircon age) at 61.4 ? 0.8 Ma; the others are the Slieve Gul lion Complex and the Mourne Mountains Com plex (Gamble et al. 1999; Cooper and Johnston 2004 and references therein; Fig. 1). An early series of basalt lavas exposed on the southern margin of the complex is believed to mark the earliest Palaeogene activity in the area. Two separate phases of gabbro are believed to have intruded into these basalts, the earlier repre sented by several isolated areas which crop out in the SW of the complex around Trumpet Hill (Fig. 1). The later suite of gabbros (the Car lingford 'Later Gabbros') form the prominent Irish Journal of Earth Sciences 24 (2006), 37-50. ? Royal Irish Academy 37 This content downloaded from 207.46.13.129 on Sun, 26 Jun 2016 06:11:25 UTC All use subject to http://about.jstor.org/terms 38 Irish Journal of Earth Sciences (2006)

Magmatic Layering and Magnetic Fabrics in the Palaeogene Carlingford Later Gabbros, Co. Louth, Ireland

A detailed magnetic fabric study of the annular partially layered Palaeogene Carlingford 'Later Gabbros', Co. Louth, is combined with field observations to constrain existing emplacement models for the intrusion. The Anisotropy of Magnetic Susceptibility (AMS) fabrics are generally quite inconsistently oriented. Only in three of seven traverses of the intrusion is consistency in orientation observed, suggesting that little can be drawn from the data by way of interpretation. However, the traverse in the north of the intrusion, at Ballinteskin, corresponds to an area of considerable syn-magmatic deformation of layering. Magnetic fabrics in this area exhibit consistently oriented inward dipping and inward plunging magnetic foliations and lineations respectively. Comparison of this data with a recent AMS study of the Great Eucrite of Ardnamurchan, NW Scotland, suggests that though central subsidence may have occurred in the Later Gabbros too, it was not as important as in Ardnamurchan. This is supported to some degree by the presence of a shallowly dipping (∼20°) intrusive contact between the Later Gabbros and an extensive granophyre intrusion, which also suggests that the granophyre is the older intrusion. This conclusion is inconsistent with previous interpretations of the granophyre as the youngest intrusion in the Carlingford igneous centre.

Plate boundary deformation and continuing deflation of the Askja volcano, North Iceland, determined with GPS, 1987–1993

GPS geodetic measurements were conducted around the Askja central volcano located at the divergent plate boundary in north Iceland in 1987, 1990, 1992 and 1993. The accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components; the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane. Regional deformation in the Askja region is dominated by extension. Points located outside a 30–45 km wide plate boundary deformation zone indicate a displacement of 2.4±0.5 cm/a in the direction N 99°E±12° of the Eurasian plate relative to the North American plate in the period 1987–1990. Within the plate boundary deformation zone extensional strain accumulates at a rate of ∼0.8 μstrain/a. Displacement of control points next to Askja (>7 km from the caldera center) in the periods 1990–1993 and 1992–1993 show deflation and contraction towards the caldera. These results are in accordance with the results obtained by other geodetic methods in the area, which indicate that the deflation at Askja occurs in response to a pressure decrease at about 2.8 km depth, located close to the center of the main Askja caldera. A Mogi point source was fixed at this location and the GPS data used to solve for the source strength. A central subsidence of 11±2.5 cm in the period 1990–1993 is indicated, and 5.5±1.5 cm in the period 1992–1993. The maximum tensional strain rate, according to the point source model, occurs at a horizontal distance of 2.5–6 km from the source, at the same location as the main caldera boundary. Discrepancies between the observed displacements and predicted displacements from the Mogi model near the Askja caldera can be attributed to the regional eastwest extension that occurs at Askja.

Dynamic topography in rift zones: implications for lithospheric heating [extended abstract

Extension of the lithosphere in rift zones results in heating of the crust and mantle beneath the rift. Based on simple stretching models (McKenzie 1978), the subsidence history of rifts has been attributed to crustal thinning during extension and subsequent conductive cooling of the lithosphere beneath the rift. The amount of subsidence caused by cooling should provide an estimate of the amount of heat added to a column of lithosphere as it extends. Models of this type, which assume local isostatic compensation and uniform extension over the whole thickness of the lithosphere, underpredict the amount of post-rift thermal subsidence (Royden & Keen 1980) and do not explain the flanking uplifts associated with rifts. Predictions of central subsidence from models that do not explain flanking uplifts should be suspect since the same mechanisms producing the uplift may also affect the central subsidence. The origin of flanking uplifts may have important implications for the amount and horizontal distribution of lithospheric heating beneath a rift. In a study of the Gulf of Suez rift, Steckler (1985) proposes that flanking uplifts are isostatically supported by thermal expansion due to heating. Because horizontal heat conduction occurs too slowly to explain the amplitude and width of flanking uplifts, convection due to stretching-induced horizontal temperature variations beneath the rift (Buck 1986) has been suggested as a heat transfer mechanism.

The age and structural setting of limestones and basalts on the Main Ring Fault in southeast Rhum

AbstractIn southeast Rhum, a Mesozoic/Tertiary sequence is preserved as a fault-bounded and rotated wedge. This is juxtaposed between Precambrian rocks (Torridonian sediments and Lewisian gneiss) and caught up in the complex structure of the Tertiary Main Ring Fault (MRF), which shows three distinct phases of movement. The Mesozoic rocks comprise fossiliferous limestone, sandstone and shale, which show differing degrees of thermal metamorphism depending on their relationships to the Layered Complex. On the basis of faunal and lithological evidence the Mesozoic sediments have been correlated with the Lower Liassic Broadford Beds. The Rhum sediments are overlain by sheared Tertiary basalts, the contact between them probably representing the original landscape unconformity. The presence of these younger strata caught up along the MRF provides crucial evidence for a major phase of central subsidence in the early history of the Rhum Tertiary volcanic centre.

On the structure of Archean greenstone belts

Abstract Archean cratons comprise two principal elements: low-grade volcano-sedimentary greenstone belts enveloped in high-grade tonalitic gneisses/plutons and amphibolites. Younger linear mobile belts generally deform and frame present cratonic margins. The high-grade gneisses are believed to consist of polycyclic sialic crust which was finally stabilized and subjected to weathering prior to 3,800 Ma as extensive, relatively thick crustal segments. The greenstone belts were deposited on sialic crust in the interval between 3,500 and 2,500 Ma as sheet-like shield volcanoes. Subsequent deformation was dominantly gravitational resulting from the density inversion of mafic volcanic rocks overlying lighter sial. A pro posed model of deformation (based on Ramberg's model experiments) can be described in two steps: 1. (a) Marginal subsidence of the greenstones with concomitant parautochtonous basement uplift (as nappes?) compressing the volcanic rocks and resulting in thrusting which repeats the greenstone succession and inverts metamorphic isograds. Repetition of the successions can lead to discrepancies between the stratigraphic thickness and gravitational reponse of the greenstones. 2. (b) Central subsidence of the greenstones assumes the shape of an inverted “mushroom” with basement remobilization as “gregarious batholiths”. Deformation of the greenstones is compressive: tight isoclinal folding, high-angle thrust faulting and longitudinal strike-slip faulting. Partial melting of the subsiding amphibolites produces the so-called calc-alkaline Archean volcanism. Examples of the structures expected within the greenstone belts from the model are reported and suspected in the greenstone belts of Rhodesia, South Africa, Australia, and Canada. It is suggested that discrepancies between the measured stratigraphic thicknesses of greenstone successions, their gravitational response, and their grades of metamorphism (depths of burial) may indicate that there has been tectonic thickening of the greenstones by thrusting.

Moisture distribution and water vapour flux over the Arabian Sea during an active and weak spell of southwest monsoon, 1973

The radiosonde data collected during the Indo-Soviet Monsoon Experiment, 1973 have been utilised to study some aspects of the moisture distribution over the Arabian Sea during an active and weak spell of southwest monsoon in that year, The study shows that there is stratification of moisture field over the Arabian Sea during the weak monsoon except the equatorial region, particularly the eastern parts, During the active monsoon, the stratification is broken to the north of about 10°N and east of 60°E, resulting in the building up of a deep moist layer over this area from surface to 400 mb, The equatorial region is conspicuously dry in the middle layers -during the active spell. The moisture build up in the middle layers during both the spells is mainly brought about by large-scale vertical motion associated with synoptic scale disturbances rather than by advective processes. During active monsoon, large, scale upward motion is envisaged over north and east central Arabian Sea and subsidence near the equator.
 
 The moisture flux computations show that there is a large net flux divergence over the Arabian Sea during the active monsoon and small net convergence during the weak monsoon.