Subsidence Structures Research Articles

Relationship between Karstification and Tectonism in the Upper Jurassic Evaporite Formation: A Case Study of the Lalla Fatna Escarpment Safi, Morocco

The Upper Jurassic Evaporite Formation accumulated up to 200 m of interbedded gypsum-carbonates layers across the coastal cliffed region of Safi (Morocco) have been affected by karstification process, resulting in karst collapse and subsidence structures associated with faulting, fracturing and different types of instabilities, which are among the main causes of natural hazard risk, impacting the socio-economic and environmental issues. Surface investigation (physiognomic approach) allowed the characterization and identification of deep-seated and subjacent gypsum interstratal stages and showed that the karstic features are controlled by orientations of faults and fractures. To highlight the relationship between tectonics and karstification, in this area, fracture orientations were studied, and the result of analysis of diagrams showed that, in most cases, orientations of karst landforms are controlled by regional tectonic features.

Unique Objects in the Cross-Section of the Eastern Black Sea Found Using 3D Seismic Data

Unique vertical subsidence structures in Upper Cretaceous–Lower Miocene deposits were discovered and described in a section of the eastern Black Sea basin and the Caucasus continental slope, according to 3D seismic data. One interesting feature of these structures is a shape that is close to isometric on a map. Approximately 40 objects have been identified in an area of more than 1000 m2. “Bulls-eye” (“multiphase”) paleo-pockmarks (Andresen and Huuse, 2011), and karst collapse (Zuo et al., 2009) are proposed as analogues with similar wave patterns. The corresponding genesis models are considered, but none of the theories allows definite conclusions to be drawn. The discovered objects are unique and require further investigation.

Sinkhole investigation in an urban area by trenching in combination with GPR, ERT and high-precision leveling. Mantled evaporite karst of Zaragoza city, NE Spain

Sinkhole risks are becoming particularly severe in urban areas that lack careful planning and where karst depressions are frequently filled and developed. These built-up areas are the most challenging for sinkhole investigation; they are the ones that require the highest priority and the application of novel approaches aimed at reducing the epistemic uncertainties associated with hazard assessments. This work illustrates the strengths of the rarely used trenching technique for the investigation of sinkholes in mantled karst areas. The approach has been tested on a buried sinkhole in the evaporite karst of Zaragoza city, analysing and comparing data gathered by trenching, Ground Penetrating Radar (GPR) using shielded and unshielded antennas with different frequencies, Electrical Resistivity Tomography (ERT) and high-precision leveling. The sinkhole is located beneath a street and has led to the demolition of buildings, creating a risk situation that requires careful assessment and management. Trenching provided the largest amount of objective data required for hazard assessment: (1) Precise location of the sinkhole edges. (2) Internal structure, cumulative displacement, subsidence mechanisms and their contribution. The trench exposed an asymmetric structure with a minimum vertical displacement of 6.2m, comprising a complex collapse with different generations of faults and two marginal zones affected by gentle inward tilting. Collapse faulting accommodates at least 70% of the subsidence. (3) Age of the sinkhole and average subsidence rate. The sinkhole was initiated sometime before or within 337–435calyr AD, indicating a long-term subsidence rate of ca. 3.7–4.2mm/yr. (4) Kinematic style, characterised by episodic collapse events, as supported by leveling data. The retrodeformation analysis, together with geochronological data, indicates four collapse events with a recurrence of around 400years and suggests enhanced activity since ca. 1950 AD. The performance of GPR was highly variable, depending mainly on the type of antenna and the timing of data acquisition. The best-quality data were obtained with shielded antennas and during relatively drier conditions. The rather poor performance of ERT, commonly an adequate method for resolving subsidence structures at depth, is attributed to the complex structure of the collapse sinkhole, with spatially-dense and abrupt resistivity changes in resistivity. This case study strongly substantiates the implementation of the relatively inexpensive and highly objective trenching technique in sinkhole investigations.

Reconstructing the internal structure and long-term evolution of hazardous sinkholes combining trenching, electrical resistivity imaging (ERI) and ground penetrating radar (GPR)

The approaches aimed at characterising specific damaging sinkholes have received limited attention compared with other ground instability phenomena (e.g. landslides). Moreover, the practicality of the trenching technique in combination with numerical dating and retro-deformation analysis for sinkhole site-investigations has been barely explored. This work illustrates the advantages of combining geomorphic mapping, electrical resistivity imaging (ERI), ground penetrating radar (GPR) and trenching for sinkhole characterisation and shows how the trenching technique contributes to fill significant gaps that neither geomorphic nor geophysical methods can address. Two large sinkholes (>200m long) related to the interstratal karstification of evaporites and generated by contrasting subsidence mechanisms (sagging, collapse) were investigated in the Fluvia Valley, NE Spain. Although GPR data may provide high resolution information on subsidence-related stratigraphic and structural features at shallow depth, the profiles acquired in the investigated sites with 100MHz shielded and 40MHz unshielded antennae provided limited insight into the internal geometry of the sinkholes due to reduced signal penetration related to the presence of conductive clayey material. The ERI sections satisfactorily imaged the general geometry of the sagging and collapse subsidence structures up to depths higher than 100m and clearly captured the basal contact of the low-resistivity sinkhole fill in the sections with adequate layout and resolution. The trenches, despite their limited depth (ca. 5m) allowed us to obtain valuable objective information on several key aspects of the subsidence phenomenon: (1) mechanisms (deformation style) and kinematics (progressive versus episodic); (2) limits of ground deformation; (3) temporal evolution (expansion versus contraction); (4) chronology and timing of most recent deformation phase; (5) rates of subsidence and sedimentation; and (6) the role played by subsidence in the development of lacustrine environments and the associated sedimentation patterns.

Complex circular subsidence structures in tephra deposited on large blocks of ice: Varða tuff cone, Öræfajökull, Iceland.

Several broadly circular structures up to 16 m in diameter, into which higher strata have sagged and locally collapsed, are present in a tephra outcrop on southwest Öræfajökull, southern Iceland. The tephra was sourced in a nearby basaltic tuff cone at Varða. The structures have not previously been described in tuff cones, and they probably formed by the melting out of large buried blocks of ice emplaced during a preceding jökulhlaup that may have been triggered by a subglacial eruption within the Öræfajökull ice cap. They are named ice-melt subsidence structures, and they are analogous to kettle holes that are commonly found in proglacial sandurs and some lahars sourced in ice-clad volcanoes. The internal structure is better exposed in the Varða examples because of an absence of fluvial infilling and reworking, and erosion of the outcrop to reveal the deeper geometry. The ice-melt subsidence structures at Varða are a proxy for buried ice. They are the only known evidence for a subglacial eruption and associated jökulhlaup that created the ice blocks. The recognition of such structures elsewhere will be useful in reconstructing more complete regional volcanic histories as well as for identifying ice-proximal settings during palaeoenvironmental investigations.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-016-1048-x) contains supplementary material, which is available to authorized users.

Core evidence of paleoseismic events in Paleogene deposits of the Shulu Sag in the Bohai Bay Basin, east China, and their petroleum geologic significance

The Shulu Sag, located in the southwestern corner of the Jizhong Depression, Bohai Bay Basin of east China, is a NE–SW trending, elongate Cenozoic half-graben basin. The lowermost part of the third member of the Shahejie Formation in this basin is characterized by continental rudstone and calcilutite to calcisiltite facies. Based on core observation and regional geologic analysis, seismites are recognized in these lacustrine deposits, which include soft-sediment deformation structures (sedimentary dikes, hydraulic shattering, diapir structures, convolute lamination, load-flame structures, ball-and-pillow structures, loop bedding, and subsidence structures), synsedimentary faults, and seismoturbidites. In addition, mixed-source rudstones, consisting of the Paleozoic carbonate clasts and in situ calcilutite clasts in the lowermost submember of Shahejie 3, appear in the seismites, suggesting an earthquake origin. A complete representative vertical sequence in the lowermost part of the third member found in well ST1H located in the central part of the Shulu Sag shows, from the base to the top: underlying undeformed layers, synsedimentary faults, liquefied carbonate rocks, allogenetic seismoturbidites, and overlying undeformed layers. Seismites are widely distributed around this well and there are multiple sets of stacked seismites separated by undeformed sediment. The nearby NW-trending Taijiazhuang fault whose fault growth index is from 1.1 to 1.8 and the NNE-trending Xinhe fault with a fault growth index of 1.3–1.9 may be the source of the instability to create the seismites. These deformed sedimentary layers are favorable for the accumulation of oil and gas; for example, sedimentary dikes can cut through many layers and serve as conduits for fluid migration. Sedimentary faults and fractures induced by earthquakes can act as oil and gas migration channels or store petroleum products as well. Seismoturbidites and mixed-source rudstones are excellent reservoirs due to their abundant primary or dissolved pores.

Evaluation of trenching, ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for sinkhole characterization

ABSTRACTThis paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three‐dimensional (3D) internal geometry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above‐surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd.

Large depressions, thickened terraces, and gravitational deformation in the Ebro River valley (Zaragoza area, NE Spain): Evidence of glauberite and halite interstratal karstification

In the studied reach of the Ebro Valley, the terrace and pediment sediments deposited over glauberite- and halite-bearing evaporites show local thickenings (>50m) recording dissolution-induced synsedimentary subsidence. Recent data on the lithostratigraphy of the evaporite sequence allow relating the alluvium thickenings with either halite or glauberite dissolution. The alluvium-filled dissolution basin underlying the youngest terraces (T8–T11) is ascribed to halite karstification; the top of a halite unit approximately 75m thick is situated 40–15m below the valley bottom. The thickenings of terrace (T1–T7) and pediment sediments are attributed to interstratal glauberite karstification: (1) Coincidence between the elevation range of the terraces and that of the glauberite-rich unit. Glauberite beds reach 30 and 100m in single-bed and cumulative thickness, respectively. (2) The exposed bedrock underlying thickened alluvium shows abundant subsidence features indicative of interstratal karstification. The most common structure corresponds to hectometer-scale sag basins with superimposed collapses in the central sector of each basin. The subsided bedrock is frequently transformed into dissolution-collapse breccias showing a complete textural gradation, from crackle packbreccias to chaotic floatbreccias and karstic residues. (3) Paleokarst exposures show evidence of karstification confined to specific beds made up of secondary gypsum after precursory glauberite, partly dissolved and partly replaced. Despite the magnitude of the subsidence recorded by the thickened alluvium and unlike nearby tributaries, the terraces show a continuous and parallel arrangement indicating that the fluvial system was able to counterbalance subsidence by aggradation. A number of kilometer-size flat-bottom depressions have been developed in the valley margin, typically next to and inset into thickened terrace and pediment deposits. The subsidence structures exposed in artificial excavations excavated in the bottom of some depressions and the correlation between the altitudinal distribution of these basins and that of the glauberite-rich unit reveal that subsidence related to interstratal glauberite karstification is the main process involved in their genesis. This research ascribes for the first time the thickenings and deformation of specific terrace levels and pediment levels and the development of large karstic depressions to interstratal karstification of exceptionally thick glauberite units.

Caldera collapse: Perspectives from comparing Galápagos volcanoes, nuclear-test sinks, sandbox models, and volcanoes on Mars

The 1968 trapdoor collapse (1.5 km 3 ) of Fernandina caldera in the Galapagos Islands developed the same kinds of structures as found in small sandbox-collapse models and in concentrically zoned sinks formed in desert alluvium by fault subsidence into underground nuclear-explosion cavities. Fernandina’s collapse developed through shear failure in which the roof above the evacuating chamber was lowered mostly intact. This coherent subsidence contrasts to chaotic piecemeal collapse at small, rocky pit craters, underscoring the role of rock strength relative to subsidence size. The zoning at Fernandina implies that the deflated magma chamber underlay a central basin and a bordering inward-dipping monocline, which separates a blind inner reverse fault from an outer zone of normal faulting. Similar concentric zoning patterns can be recognized in coherent subsidence structures ranging over 16 orders of magnitude in size, from sandbox experiments to the giant Olympus Mons caldera on Mars.

Seismic reflection characteristics of naturally-induced subsidence affecting transportation

High-resolution seismic reflections have been used effectively to investigate sinkholes formed from the dissolution of a bedded salt unit found throughout most of Central Kansas. Surface subsidence can have devastating effects on transportation structures. Roads, rails, bridges, and pipelines can even be dramatically affected by minor ground instability. Areas susceptible to surface subsidence can put public safety at risk. Subsurface expressions significantly larger than surface depressions are consistently observed on seismic images recorded over sinkholes in Kansas. Until subsidence reaches the ground surface, failure appears to be controlled by compressional forces evidenced by faults with reverse orientation. Once a surface depression forms or dissolution of the salt slows or stops, subsidence structures are consistent with a tensional stress environment with prevalent normal faults. Detecting areas of rapid subsidence potential, prior to surface failure, is the ultimate goal of any geotechnical survey where the ground surface is susceptible to settling. Seismic reflection images have helped correlate active subsidence to dormant paleofeatures, project horizontal growth of active sinkholes based on subsurface structures, and appraise the risk of catastrophic failure.

Explosive volcanism (VEI 6) without caldera formation: insight from Huaynaputina volcano, southern Peru

Through examination of the vent region of Volcan Huaynaputina, Peru, we address why some major explosive eruptions do not produce an equivalent caldera at the eruption site. Here, in 1600, more than 11 km3 DRE (VEI 6) were erupted in three stages without developing a volumetrically equivalent caldera. Fieldwork and analysis of aerial photographs reveal evidence for cryptic collapse in the form of two small subsidence structures. The first is a small non-coherent collapse that is superimposed on a cored-out vent. This structure is delimited by a partial ring of steep faults estimated at 0.85 by 0.95 km. Collapse was non-coherent with an inwardly tilted terrace in the north and a southern sector broken up along a pre-existing local fault. Displacement was variable along this fault, but subsidence of approximately 70 m was found and caused the formation of restricted extensional gashes in the periphery. The second subsidence structure developed at the margin of a dome; the structure has a diameter of 0.56 km and crosscuts the non-coherent collapse structure. Subsidence of the dome occurred along a series of up to seven concentric listric faults that together accommodate approximately 14 m of subsidence. Both subsidence structures total 0.043 km3 in volume, and are much smaller than the 11 km3 of erupted magma. Crosscutting relationships show that subsidence occurred during stages II and III when ∼2 km3 was erupted and not during the main plinian eruption of stage I (8.8 km3).

Subsidence hazards due to evaporite dissolution in the United States

Evaporites, including gypsum (or anhydrite) and salt, are the most soluble of common rocks; they are dissolved readily to form the same type of karst features that typically are found in limestones and dolomites, and their dissolution can locally result in major subsidence structures. The four basic requirements for evaporite dissolution to occur are: (1) a deposit of gypsum or salt; (2) water, unsaturated with CaSO4 or NaCl; (3) an outlet for escape of dissolving water; and (4) energy to cause water to flow through the system. Evaporites are present in 32 of the 48 contiguous states of the United States, and they underlie about 35–40% of the land area. Karst is known at least locally (and sometimes quite extensively) in almost all areas underlain by evaporites, and some of these karst features involve significant subsidence. The most widespread and pronounced examples of both gypsum and salt karst and subsidence are in the Permian basin of the southwestern United States, but many other areas also are significant. Human activities have caused some evaporite–subsidence development, primarily in salt deposits. Boreholes may enable (either intentionally or inadvertently) unsaturated water to flow through or against salt deposits, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures above the cavity can cause land subsidence or catastrophic collapse.

Origin of the salt valleys in the Canyonlands section of the Colorado Plateau: Evaporite-dissolution collapse versus tectonic subsidence

The salt valleys over the axis of the salt-cored anticlines in the Paradox fold and fault belt (Canyonlands, Utah and Colorado) are created by subsidence of the anticline crests. Traditionally, the collapse of the anticlinal crests was attributed to dissolution of the salt walls (diapirs) forming the anticline cores. Recent studies based on scaled physical models and field observations propose that the salt valleys are a result of regional extension and that salt dissolution had only a minor influence in the development of the axial depressions. This paper presents several arguments and lines of evidence that refute the tectonic model and support the salt dissolution subsidence interpretation. The development of contractional structures in salt dissolution experiments led the advocates of the tectonic interpretation to reject the dissolution-induced subsidence explanation. However, these salt dissolution models do not reproduce the karstification of salt walls in a realistic way, since their analog involves removal of salt from the base of the diapirs during the experiments. Additionally, numerous field examples and laboratory models conducted by other authors indicate that brittle subsidence in karst settings is commonly controlled by subvertical gravity faults. Field evidence against the regional extension model includes (1) a thick cap rock at the top of the salt walls, (2) the concentration of subsidence deformation structures along the crest of the anticlines (salt walls), (3) deformational structures not consistent with the proposed NNE extension, like crestal synforms and NE–SW grabens, (4) dissolution-induced subsidence structures controlled by ring faulting, revealing deep-seated dissolution, (5) large blocks foundered several hundred meters into the salt wall, (6) evidence of recent and active dissolution subsidence, and (7) the aseismic nature of the recently active collapse faults. Although underground salt dissolution seems to be the main cause for the generation of the salt valleys, this phenomenon may have been favored by regional extension tectonics that enhance the circulation of groundwater and salt dissolution.

Paleosubsidence and active subsidence due to evaporite dissolution in Spain

Evaporite formations crop out or are at shallow depth present in an extensive area of Spain. These soluble sediments occur in diverse geological domains and were deposited over a long time span, from the Triassic up to the present day. Broadly, the Mesozoic and Paleogene formations (Alpine cycle) are affected by compressional structures, wheras the Neogene (post-orogenic) sediments remain underformed. Subsidence caused by subsurface dissolution of evaporites (subjacent karst) takes place in three main types of stratigraphic settings: a) subsidence affecting evaporite-bearing Mesozoic and Tertiary successions (interstratal karst); b) subsidence in Quaternary alluvial deposits related to the exorheic evolution of present-day fluvial systems (alluvial or mantled karst); and c) subsidence in exposed evaporites (uncovered karst). These types may be represented by paleosubsidence phenomena (synsedimentary and/or postsedimentary) recognizable in the stratigraphic record, or by equivalent, currently active or modern examples which have a surface expression. Interstratal karstification of Mesozoic marine evaporites, and the consequent subsidence of overlying strata, is revealed by stratiform collapse breccias and wedge outs of the evaporites grading into unsoluble residues. In several Tertiary basins, the sediments overlying evaporites locally show synsedimentary and/or postsedimentary subsidence structures. Dissolution-induced subsidence coeval with sedimentation is accompanied by local thicknening of strata in basin-like structures with convergent dips and cumulative wedge-out systems. This sinking process controls the generation of depositional environments and lithofacies distribution. Postsedimentary subsidence produces a great variety of gravitational deformations in Tertiary supra-evaporitic units, including both ductile and brittle structures (flexures, synforms, fractures, collapse, and brecciation). Quaternary fluvial terrace deposits overlying evaporites show anomalous thickenings (>150m) caused by a dissolution-induced subsidence process in the alluvial plain, which is balanced by alluvial aggradation. The complex evolution (in time and space) of paleosubsidence leads to intricate and chaotic structures in the alluvium, which may be erroneously interpreted as pure tectonic deformations. The current subsidence and generation of sinkholes due to suballuvial karstification constitutes a geohazard which affects large, densely populated areas, and thus endangers human safety and poses limitations on development. An outstanding example can be seen in Calatayud, an important historical city where subsidence has severely damaged highly valuable monuments. Subsidence resulting from the underground karstification of evaporites has caused or influenced the generation of some important modern lacustrine basins, such as Gallocanta, Fuente de Piedra, and Banyoles Lakes. The sudden formation of sinkholes due to collapse of cave roofs is fairly frequent in some evaporite outcrops. Very harmful and spectacular subsidence activity is currently occurring in the Cardona salt diapir, where subsidence has been dramatically exacerbated by mining practices.

Deep karst conduits, flooding, and sinkholes: lessons for the aggregates industry

Limestone aggregate quarries in deeply penetrating karst terrain are often at considerable risk of artesian inflow from groundwater or surface water channeled through the karstic aquifer. The inflow occurs through what are likely to be complex conduits that penetrate hundreds of feet into bedrock. Rates of inflow can exceed the operation's pumping capabilities proving to be uneconomic to manage over the long term. Over time, inflow rates can increase dramatically as turbulent flow through the conduit erodes its soft residual clay-rich fill. One recent investigation observed an inflow rate of more than 40,000 gpm from a surface water source. Floodwater persistently laden with sediment is an indicator of conduit washout and implies increasing inflow rates over time. Conduits carrying floodwater can exist in a variety of forms: along deeply penetrating geologic faults, joints, or following the path of preferentially eroded bedding. Preferential structural deformation along faults or bedding can enhance dissolution during subsequent interaction with groundwater. The resulting conduit may be a complex combination of many geological features, making the exploration and remediation of the pathway difficult. Sinkholes at the site can occur within several contexts. Pre-existing subsidence structures can reactivate and subside further, forming new collapse sinkholes within soil directly overlying the conduit. Cover-collapse sinkhole development can be a direct result of increasing downward groundwater velocities and subsurface erosion associated with the enlargement of a conduit. Normal operation events such as a quarry blast can also provide a significant new linkage between the groundwater and the quarry, allowing rapid drainage of the groundwater reservoir. With such drainage and erosion of karst-fill, sinkholes will develop over localized water table depressions, most significantly over enhanced permeability zones associated with fractures. Paradoxically, although the rise in quarry water level will lead to regional reduction in the hydraulic gradients, on local scales, drainage of the groundwater reservoir increases gradients and leads to the development of cover-collapse sinkholes. Recommended methods for preliminary site investigation can include a detailed review of geological literature and drilling logs to compile a conceptual model of the site. A fracture trace analysis with EM geophysics can confirm the locations of major faults and fractures. Fingerprinting of the various water sources to the quarry and the water in the quarry is an inexpensive and effective means of identifying the source and likely direction of the groundwater and surface water flow. Automated geophysical equipment on the market for performing rapid resistivity and microgravity surveys speeds up the site screening process during reconnaissance exploration for deep structure. It is recommended that mine planning fully incorporate this information so that quarry operators can take proactive measures to avoid catastrophic and costly flooding events.

Growth of plutons by floor subsidence: implications for rates of emplacement, intrusion spacing and melt-extraction mechanisms

Geophysical and field-based studies indicate that granitic plutons occur as either tabular (disk) or wedge (funnel) shapes whose length (L) to thickness (T) ratio is controlled by the empirical power law, T = 0.6(±0.15)L0.6(±0.1). The dimensions of plutons are self-similar to other natural subsidence phenomena (calderas, ice cauldrons, sinkholes, ice pits) and it is proposed that they grow in a similar fashion by withdrawal of material (melt) from an underlying source, which is then transferred to the growing pluton within the crust. Experimental studies show that growth of subsidence structures occurs by vertical inflation ⪢ horizontal elongation of an initial depression with L ≈ width of the source region. If pluton growth is modelled in the same way, the empirical power law relating T and L defines limits for pluton growth that are imposed by the width, thickness and degree of partial melting from a lower crustal source. Several growth modes that predict testable internal structural patterns are identified for plutons, depending on whether they are tabular or wedge-shaped, grow by continuous or pulsed magma delivery and whether magma is accreted from bottom to top, or vice versa. Rates of pluton growth are geologically fast (hundreds to hundreds of thousands of years) if magma supply is effectively continuous, but can also take millions of years if the time between magma delivery events is much longer than magma injection events. Plutons formed by melt extraction from an area directly beneath require large degrees of partial melting and or very thick sources. Lower degrees of partial melting and thinner sources are permitted when melt extraction occurs over a larger region, which can lead to the formation of spaced plutons. Tabular pluton growth will tend to favour widely spaced plutons, unless degrees of partial melting in the source are high. Wedge-shaped plutons can form much closer together and require lower degrees of partial melting. These results are in general agreement with current geophysical, petrological and experimental estimates of partial melting in the lower continental crust.

Quaternary subsidence zones in Albania: some case studies

The Neotectonic evolution of Albania, from the Middle Pleistocene to the present day, is characterised by a general uplift that began after the Pliocene. Subsidence took place locally and led to the formation of graben-shaped Quaternary lakes and plains. During this period, graben lakes were formed at Shkodra, Ohrid, Prespa and Butrinti, whereas at Korca, Elbasani, Zadrima, Tirana, Myzeqe, etc., graben and graben-like plains were created. The Quaternary subsidence structures can be classified into two groups: graben structures created by local or regional extension and graben-like structures formed under a compressional regime. Some case studies are presented to illustrate the formation of these structures and their main engineering geological and hydrogeological characteristics.

Subsidence Structures Associated with Subaerial Desiccation-Crack Piping and Their Role in Drainage Evolution on a Drained Proglacial Lake Bed: Hagavatn, Iceland

ABSTRACT A range of subsidence structures associated with desiccation-crack piping are described from the drained proglacial lake bed of Hagavatn, Iceland. Subsurface drainage along desiccation cracks leads to the formation of small-scale subsidence structures. These subsidence structures include linear fissures, located along the limbs of desiccation cracks, and crater-like depressions that form at crack intersections. The controls on subsurface piping are discussed and the role of these subsidence structures in determining the pattern of surface drainage on the former lake bed is identified. Preservation potential of these sedimentary structure is considered to be high, given renewed lacustrine sedimentation. The potential for similar structures to be found in the geological record is considered likely, and consequently these structures may be of importance in the interpretation of crack networks in the geological record as being of subaerial as opposed to subaqueous origin.

The Carstairs Kames (Lanarkshire, Scotland): morphology, sedimentology and formation

The Carstairs Kames have been the subject of debate for over 150 yr. A synthesis of previous work, along with new sedimentological data, is used to reappraise this classic British esker locality. Three morphological elements are recognised: (i) large, sinuous and anastomosing ridges; (ii) large, irregular, linear mounds; and (iii) small, low and irregular mounds. The main ridge system is composed of a core of boulder gravel with sand/pebble gravel above and adjacent to the core. The large, linear mounds contain a varied facies range which include: glaciolacustrine sediments (deltaic sequences, laminites and dropstones); debris flows; and glaciofluvial channel systems. All the deposits, both in the main ridges and in the adjacent linear mounds, contain syn-and post-depositional subsidence structures, including normal faults, synclinal folds and growth faults. A range of potential origins for these landforms is reviewed and it is suggested that glaciofluvial sedimentation in a supraglacial environment, controlled by a topography of ice-cored ridges, is the most appropriate depositional model. © 1997 John Wiley & Sons, Ltd.

The Grand Brule exploration drilling: New data on the deep framework of the Piton de la Fournaise volcano. Part 1: Lithostratigraphic units and volcanostructural implications

The sinking of a deep drill hole (3003.5 m) for geothermal exploration in the eastern part of the Piton de la Fournaise volcano on Réunion Island has revealed the underlying lithostratigraphy. Six principal sequences have been distinguished: (a) 0–440 m, subaerial basaltic flows; (b) 440–556 m, a transitional series characterized by an alternation of subaerial basalts and submarine deposits (vitric basalt and hyaloclastic breccia); (c) 556–616 m, submarine basalts (lavas and hyaloclastites); (d) 616–890 m, subaerial basalts; (e) 890–1010 m, submarine basaltic deposits with abundant argillized hyaloclastites; (f) 1010–3003.5 m a layered intrusive complex comprising: 1010–1129 m, sheets of gabbro injected into the lavas and hyaloclastites; 1129–1920 m, a zoned intrusion of olivine gabbro and dolerite; 1920–3003.5 m, stratified cumulate gabbro-wehrlite-dunite.The occurrence of the first submarine deposits at 440 m (268 m below present sea level) and the intercalation with them of subaerial basalts down to 890 m (728 m below present sea level) tend to support a hypothesis of sliding into the sea during subsidence of the eastern flank of the Fournaise massif.The intrusive complex intersected from 1010 m down is regarded as a former shallow magma chamber. A large positive gravimetric anomaly (40–50 mgal) centered over Grand Brûlé and attributed to the intrusive complex indicates that its roof forms an ellipse 3 × 20 km, elongated N-S.Stratigraphic and morphological-structural evidence (on land and on the sea-floor) and the results of the mineralogical study of this very large magma chamber suggest that it belongs to an ancient edifice (proto-Fournaise) whose activity preceded the construction of the present Fournaise and ended at about 0.6 Ma. On land, the evidence for this earlier edifice is to be found in the zeolitised basalts and differentiated lavas (hawaiites) forming the basement to the Phase 1 lava flows ( < 0.35 Ma) of the present Fournaise and found in the deepest valleys cut into the volcano. On the sea floor, an E-W elongated topographic high may be a vestige of the ancient edifice.Certain subsidence structures on the present Fournaise are interpreted as resulting from settling of the proto-Fournaise volcano. The destabilization of the east flank (Grand Brûlé area) of the currently active volcano and the location of the NE and SE rift zones could be attributed to the same phenomenon.