Abstract Early evolutionary stages of salt-bearing margins (e.g., South Atlantic or eastern Gulf of Mexico) often include one or more episodes of raft tectonism, during which large overburden blocks, separated by normal fault zones, are translated basinward. Because boundary conditions prevailing during onset of deformation partly control the trend and spacing of normal faults, they also determine the shape of rafted blocks in map view. In addition, because boundary conditions in the downdip part of the margin determine available accommodation space into which the rafted blocks are translated, they can influence the direction of raft movement (parallel, convergent, or divergent). The Canyonlands National Park area of Utah ( Fig. 1 ) provided an excellent mesoscale field example of thin-skinned extension, where local boundary conditions influenced the shape and translation direction of rafted blocks. Rafting initiated when the Colorado River incised the Cutler-Hermosa formations (overburden) down to the top of the mobile, evaporitic Paradox Formation (source layer). The overburden spread gravitationally toward the river valley, forming long rafts, that moved parallel (toward the valley), and that were separated by grabens whose traces are concave toward the valley. Using physical models, we demonstrate that fault curvature results from shear stresses between the rafted area and fixed, lateral regions. In contrast, smaller areas bordering river meanders underwent divergent spreading. Ironically, valley curvature there led to formation of linear, rather than arcuate, faults arranged in two orthogonal sets. Figure 1. Map patterns of rafted blocks in the Canyonlands area.

ABSTRACT Many passive margins have deep-water, contractional fold belts that formed above salt or shale. Margin failure, accommodated by proximal extension and distal shortening, is caused by some combination of gravity gliding above a basinward-dipping detachment and gravity spreading of a sedimentary wedge with a seaward-dipping bathymetric surface. Gravitational failure is inherently self-limiting, and sedimentation patterns provide fundamental control of deformation. Continued shortening is driven primarily by shelf and upper-slope deposition, which maintains the bathymetric slope and the gravity potential, and by increased basin-ward tilting. Deformation is retarded or halted by distal thickening of the overburden caused by the folding itself or by lower-slope and abyssal sedimentation. Net shortening amounts and deformation rates are lower than in collisional/accretionary fold belts, because the driving forces are weaker than those induced by lithospheric plate motions. Structural styles vary but depend largely on the nature of the děcollements layer, not the driving forces. Fold belts detached on shale typically comprise basinward-vergent thrust imbricates and associated folds because of the relative strength and frictional behavior of the plastic shale. Deformation does not occur until there is sufficient overburden, and it is facilitated by high fluid pressures. In contrast, salt is a viscous material with essentially no strength, which leads to symmetrical detachment folds and early deformation beneath only a thin overburden. Moreover, the surface slope can be reduced by proximal subsidence into salt and distal inflation of salt, and much of the shortening can be accommodated by lateral squeezing of diapirs and salt massifs and by extrusion of salt nappes.

Abstract Salt along a passive margin facilitates and accommodates gravitational failure of the margin in several key ways. First, it serves as the basal detachment for a linked system of updip extension and downdip contraction that develops due to a combination of gravity gliding and gravity spreading of the sediment carapace. Second, proximal subsidence into salt and distal inflation of salt reduces the bathymetric slope and the associated gravity potential. Third, preexisting salt diapirs and massifs accommodate basinward translation of the overburden by lateral squeezing and the consequent extrusion of allochthonous salt. Fourth, allochthonous canopies can serve as additional detachment levels for gravitational failure. Deep water environments are where most of the shortening occurs, which is manifested as salt-cored folds, reverse faults, squeezed diapirs, inflated massifs, and extrusion of allochthonous salt. Extensional and strike-slip faults and associated salt deformation are also present, as are loading-induced features such as turtle structures and passive diapirs.

Abstract It is probable that granitic magma ascent does not result from the intrinsic properties of the magmas. Within the uppermost crust, neither the reduced viscosity nor the density contrast between magma and surroundings are themselves sufficient to induce either low-inertia flow (diapirism) or fracture-induced magma propagation (dyking). Igneous diapirism is intrinsically restricted to the lower, ductile crust. Dyking is therefore the most probable ascent mechanism for granitic magmas that reach shallow crustal levels. A neutral buoyancy level in the crust, at which magma ascent should stall, is never observed. This is demonstrated by coeval emplacement of magmas with different compositions and densities, and the negative gravity anomalies measured over many granitic plutons. We suggest that deformation, through strain partitioning, is necessary to magma ascent. Pluton formation is controlled by local structures and rock types rather than by intrinsic magma properties. As a result of its intermittent character, deformation (both local and regional) induces magma pulses, and this may have important consequences for the chemical homogeneity of intruded magmas.

Abstract Granite intrusion in transtensional regime is modelled by injecting a Newtonian fluid into a sand pack containing a ductile layer. The transtensional regime is obtained using two plastic sheets sliding along two rigid horizontal plates, and diverging from two narrow spaces (two fixed velocity discontinuities). The injection tube is located in a central space between these plates. Both symmetric experiments (when the two sheets were displaced with equal and opposite velocity vectors) and asymmetric experiments (in which only one sheet was displaced) were performed. Transtension was applied with a systematic variation (every 15°) of the divergence angle (α), between 15° and 90°. Experiments showed that: intrusions localize strain from the first stages of deformation; intrusions result in partially conformable laccoliths with bowler-hat geometry in cross-section; intrusions show an important offset towards the mobile basal plate for asymmetric transtensional regime, and are more symmetric and centred on the injection point for symmetric transtensional regime; the geometry of intrusions is controlled by the faults developed in the overburden. The significance of this control depends upon the angle of divergence α. Examples of the Hombreiro and Los Pedroches granites of the Variscan belt of Spain have been addressed to test the applicability of these experimental results.

Abstract The process by which magma ascends into and deforms a volcanic edifice is studied by analogue modelling. A control experiment is conducted with a wooden piston moving vertically into a sand cone. This reveals a well-defined fault pattern that makes it possible to draw the main compressive stress trajectory within the cone during the ascent of the piston. This makes it possible to show that the deformational process is that of indentation of the cone by the rigid piston. Experiments with an indenter that is viscous, as in nature, show that the motion of the viscous body is controlled by the first fault created in the cone. This fault serves as a structural guide, making the viscous body deviate from the vertical and resulting in deformation of the flank of the cone, which bulges out. Other major shear faults that were observed in the control experiment are then inhibited and do not form. This result emphasizes that the structural evolution of an indentation process within a brittle cone and at low rate depends on the rheology of the indenter.

Abstract The post-orogenic extensional processes that affected the inner sector of the Northern Apennine orogenic wedge (i.e. the Northern Tyrrhenian region) were accompanied by the emplacement of chiefly anatectic intrusive rocks of Late Miocene to Mid-Pleistocene age. In this paper, we compare geological and structural data from Messinian-Pliocene monzogranitic intrusions located both offshore (Monte Capanne, Porto Azzurro, Montecristo and Giglio) and onshore (Gavorrano and Botro ai Marmi) in the Northern Tyrrhenian region to constrain modes of pluton emplacement. Offshore, eastward non-coaxial extensional shear zones active both in ductile and brittle conditions accompanied the emplacement of the monzogranitic intrusions, and accommodated extension oriented E-W to WNW-ESE. Onshore, N-S dextral strike-slip faulting was active both during and after the late stage of emplacement of both Botro ai Marmi and Gavorrano plutons, and controlled their rise in coincidence with releasing bends. In our interpretation, the N-S, Late Miocene-Pliocene strike-slip faulting constitutes a secondary shear feature in a context of generalized post-orogenic extension, accommodating in the brittle upper crust the non-coaxial ductile extension in the lower crust. In this framework, N-S strike-slip faults localized the rise of early anatectic magma, generated during regional post-orogenic extension and residing at the base of the extending crust.

Abstract Analysis of first arrivals of P waves from 113 earthquakes in the Dead Sea region and the calculation of a tomographic model indicates anomalous distribution of seismic velocities in the crust and the upper mantle. The seismic tomography model was applied to the lower and intermediate crust and the uppermost mantle there, at depths greater than the detection limits of seismic reflection surveys. At these depths the inversion shows two deep layers, at depths of 10–22 km and 22–32 km. The deeper layer shows average seismic velocity of 7.7 km s −1 , and the shallower one 6.5 km s −1 . The model suggests therefore that the Moho under the central Dead Sea is found at depth of 22 km, and the seismic velocity in the upper lithospheric mantle is anomalously low. The velocity of the layer overlying the Moho suggests a modelled layer of composite mineralogy, but high velocity anomalies were encountered in the 10–22 km layer under the boundary faults of the Dead Sea Rift, and a low velocity anomaly under the central Dead Sea is plausible. The probable interpretation of these anomalies is that the mantle under the central Dead Sea is shallow and anomalously hot, and that the lower and intermediate crust under the axial zone of the central and northern Dead Sea is also anomalously hot. Furthermore, it seems probable that magmatic diapirs ascend along the boundary faults of the rift into the intermediate crust. Structural similarity of the upper mantle and the lower crust between the Dead Sea and the northern Red Sea suggests an analogous tectonic regime for these two regions.

Abstract Kuh-e-Jahani is one of the largest extrusions of salt currently active in the Zagros mountains. Salt rises from about 4 km below sea level to nearly 1.5 km, above, where, unable to support its own weight, it spreads over the surrounding scenery in a process responsible for present and past allochthonous salt sheets elsewhere. We report vertical movements and apparent horizontal displacements of 43 markers dispersed over this mountain of salt for 4.5 years in three consecutive intervals, the first of 18 months and two others of 12 months. The geometry and inferred flow rate of the salt changed between surveys emphasizing that the gravity spreading is not steady. Our field readings of the dimensions and velocities of the salt at Kuh-e-Jahani are used to tune a simple numerical model and constrain the viscosity of the salt to between 10 16 and 10 17 Pa s −1 , its rate of surface dissolution to 2–3 cm a −1 , and its rate of rise out of its orifice at 2–3 m a −1 for c. 55 ka. These results imply that vigorous extrusion of salt at Kuh-e-Jahani is probably close to evacuating its deep source and that this mountain will soon begin to waste as salt dissolution overtakes extrusion. This progress report is warranted because our results have significant implications for sophisticated engineering in salt.

Abstract The recent PRISMED II geophysical survey has documented various styles of salt tectonics in and around the Nile deep-sea fan (Eastern Mediterranean Sea). The first main type of salt-related structures comprises listric normal growth faults and grabens, trending roughly perpendicular to the slope line of the Nile Cone. These faults and associated salt structures result from thin-skinned extension, driven by gravity gliding and spreading as a result of sediment loading of the Plio-Quaternary overburden above the Messinian evaporites, which acted as a décollement layer. The second major type of salt structures consists of lineaments that obliquely intersect the continental slope of the Nile deep-sea fan. These structures may have had some strike-slip movement, and salt diapirs grew reactively or were deformed by fault-block movement. In the western distal part of the Nile deep-sea fan, compressional tectonics of the adjacent Mediterranean Ridge caused the formation of a series of salt-cored folds and reverse faults above the Messinian evaporites. In the eastern distal part of the Nile Cone, sediment progradation progressively expelled salt northward, first forming small folds and tight diapirs, then a scarp of 400 m height around the Eratosthenes Seamount, corresponding to the basinward limit of salt deformation.

Abstract The East Carpathians Bend area has a very complex structure characterized by the presence of nappes, their post-tectonic cover and salt diapirs. The salt forming the studied diapirs is Early Miocene (Burdigalian) in age. After its accumulation the salt was more or less continuously involved in alternating extensional and compressional stages that deformed it from its original tabular position to the present-day diapir. Five stages of salt deformation have been established: initial, pre-nappe emplacement, nappe emplacement, post-nappe emplacement and Wallachian. During all of these stages the salt was configured into different shapes: it formed a truncated cone during the initial stage, a mushroom head during the prenappe emplacement stage, and an increasingly more tapered shape with nappe emplacement and during the post-nappe emplacement stages. Finally, it was squeezed out and refashioned by strike-slip faulting during the Wallachian compressional stage of Pleistocene age.

Abstract In the southern Pyrenees foreland, the Cardona salt diapir has a 250 m high stem and a small bulb, which is partially exposed. The internal structure of the diapir consists of a main sheath fold divided by a shear zone. This shear zone crosses two salt units with different initial properties. In this paper, a qualitative analysis of the fabric both units crossed by the shear zone and a 3D quantitative morphological and textural analysis of the unit with small grain size were carried out. In this unit, the fabric is characterized by a grain size similar to that of the initial fabric, disappearance of primary structures, a poor grain orientation with the long axis statistically at 20° of the shear boundary, and a strong {100} crystallographic preferred orientation. The unit composed of coarser grains shows a disappearance of primary structures. The fabric changes in both units and the strong decrease in water content during deformation suggest that fluid-assisted synkinematic recrystallization was dominant. The different behaviour of the two studied units on shear deformation also suggests that the initial grain size and water content were the main factors controlling fabric changes.

Abstract Drilling of two submarine mud domes situated in the Olimpi field on the northern flank of the Mediterranean Ridge accretionary complex has documented episodic eruptive activity over the last 1 to >1.5 Ma. Mud extrusion is related to plate convergence between Africa and Eurasia, having caused backthrust faulting of accreted strata containing overpressured mud at depth. The domes mainly consist of mud breccia with up to 65% of polymictic clasts embedded in a clayey matrix. On the basis of modifications of Poiseuille’s and Stokes’ laws, mud extrusion rates were calculated for Milano and Napoli mud domes. Mud ascent velocities are estimated to be up to 60–300 km a −1 , and are comparable with those of silicate magmas. Using physical property, structural and flux data of the mud breccias, and compiled data from mud domes on land, the diameter of the feeder channel and the depth of origin for the overpressured muds could be reliably estimated for the first time. Feeder channels are likely to be only a few metres wide. Gas efflux estimates constrain a source depth to c. 1700 ± 50 m below sea floor in the Olimpi field, which is considerably shallower than estimations made from the thermal maturity of solid organic carbon in the mud breccias. The efflux data suggest that the overpressured muds were not mobilized at décollement depth, but at a shallower level within the accretionary prism. A comparison of mud ascent rates (as determined from Poiseuille flow) and the total volumes of mud extruded indicate that only a fraction of the time span constrained from biostratigraphic data ( c. 1 Ma) is needed to build up the Milano and Napoli mud domes. Durations of 12–58 ka of extrusive activity suggest mud volcanism here to have been a highly episodic phenomenon.

Abstract The ten articles in this book describe the mode of emplacement of various types of intrusions (salt diapirs, mud volcanoes and magmatic bodies) by means of theoretical reasoning, analogue and analytical modelling, interpretation of seismic and field data, and geodetic surveying. All the articles emphasize the role of regional tectonics in driving or controlling the emplacement of the intrusions. The selection of articles includes examples from Spain, Romania, onshore and offshore Italy, the Eastern Mediterranean, Israel and iran. Better understanding of the mode of emplacement of these intrusions has applications in hydrocarbon exploration (e.g., where salt structures or mud diapirs are present) and in the mining industry (where mineralization is related to the emplacement of batholiths).

Abstract Seismic analysis of salt structures in the Nordkapp Basin, a deep salt basin in the southern Barents Sea, combined with experimental modeling suggests that regional tectonics closely controlled diapiric growth. Diapirs formed in the Early Triassic during basement-involved regional extension. The diapirs then rose rapidly by passive growth and exhausted their source layer. Regional extension in the Middle-Late Triassic triggered down-to-the-basin gravity gliding, which laterally shortened the diapirs. This squeezed salt out of diapir stems, forcing diapirs to rise, extrude, and form diapir overhangs. After burial under more than 1000 m of Upper Triassic-Lower Cretaceous sediments, the diapirs were rejuvenated by a Late Cretaceous episode of regional extension and gravity gliding, which deformed their thick roofs. After extension, diapirs stopped rising and were buried under 1500 m of lower Tertiary sediments. Regional compression of the Barents Sea region in the middle Tertiary caused one more episode of diapiric rise. Diapirs in the Nordkapp Basin are now extinct.

Compilation of rheological data (Weijermars and others 1993) shows that sedimentary overburden during salt tectonics deforms by frictional slip along fault planes, rather than by viscous flow, as is usually assumed in most salt diapir models. By contrast, rock salt deforms as a viscous or power-law fluid, with a viscosity ranging between 10 17 and 10 19 Pa·s, depending on grain size, temperature and strain rate (Van Keken and others in press; Carter and others 1992; Spiers and others 1990; Urai and others 1986). Kinematically, salt and overburden rocks also deform in a drastically different manner: salt deforms by ductile flow, whereas deformation of the brittle sediment overburden is more localized along fault planes. The deformation style of a salt-overburden system is controlled by the response of the strongest layer. Where salt is stronger, the system deforms by ductile, pervasive strain with numerous overburden faults. Whereas deformation is controlled by overburden block-faulting where the overburden is stronger. This article illustrates how the ratios between forces and stresses in the source layer (viscous and pressure stresses or forces) and the overburden (frictional strength) can be related to simple geologic parameters and can be used to predict the deformation style.

ABSTRACT We use natural examples and mechanical and geometric reasoning to demonstrate that lateral contraction can trigger episodic rise of previously extinct salt diapirs, even after their source layer has been depleted. Contraction preferentially shortens diapirs, forcing salt upward, rather than deforms the stronger, adjacent sediment overburden. Contraction may be induced by regional compression, tectonic inversion, or by downslope gravity gliding along basin margins during basement subsidence. Contraction often remains cryptic; emergent diapirs rejuvenated by contraction rise with the apparent geometry of passive diapirs, whereas rejuvenated buried diapirs rise with the apparent geometry of active diapirs. Structural clues that interpreters can use to elucidate whether diapir rise was driven by contraction include thick, deformed diapir roofs; pinched-off diapir stems; salt pedestals; diapir rise through flatlying sediment strata; and folds, thrusts, or wrench blocks along the strike or dip of the diapirs.