Basal Decollement Research Articles

Formation of the Yakuno ophiolite; accretionary subduction under medium-pressure-type metamorphic conditions

The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during the Permian evolution of Japan on the Yakuno eruptive sequence, association of hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening of sequences terrigenous sandstone and conglomerate, and mildly deformed Permian and Triassic forearc basin formations. The most important indicator, however, is the seaward imbrication and repetition observed in both the Maizuru Group and the ophiolite itself. D1 deformation structures include axial–planar foliations (pressure-solution cleavage for the Maizuru Group and granulite–amphibolite metamorphic layering in the ophiolite), flattening type strain, symmetric pressure shadows and fringes, and isoclinal folds showing axial–planar foliations and thrust faulting at their overturned limb. The exceptional asymmetry observed indicates seaward-directed shearing near the thrust, while D1 structures in the Maizuru zone are explained by off-scraping, above the basal decollement. The later Jurassic D2 kink fold structure includes a first-order asymmetric kink with a brittle thrust at its overturned limb, more-or-less coeval with M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism in the Yakuno ophiolite produced layering of granulite and amphibolite, and in the Maizuru Group, formation of illite along pressure-solution cleavage of mudstones. The metamorphic grade is controlled by the stratigraphic relationships and appears typical of that in ocean floor regions. However, there was only one episode of M1 prograde metamorphism which occurred contemporaneously with D1 off-scraping. Given that subduction zones are normally characterized by high P/ T metamorphic regimes, the observed P/ T history appears to reflect relatively unusual conditions. Such high thermal gradients may plausibly reflect the approach of a young, hot oceanic plate which continued subducting beneath the Japanese arc. Accordingly, the Yakuno ophiolite was probably formed at the trench–trench–ridge triple junction.

Seismogenic structures along continental convergent zones: from oblique subduction to mature collision

We summarize seismogenic structures in four regions of active convergence, each at a different stage of the collision process, with particular emphases on unusual, deep-seated seismogenic zones that were recently discovered. Along the eastern Hellenic arc near Crete, an additional seismogenic zone seems to occur below the seismogenic portion of the interplate thrust zone—a configuration found in several other oblique subduction zones that terminate laterally against collision belts. The unusual earthquakes show lateral compression, probably reflecting convergence between the subducting lithosphere's flank and the collision zone nearby. Along oblique zones of recent collision, the equivalence between space and time reveals the transition from subduction to full collision. In particular, intense seismicity beneath western Taiwan indicates that along the incipient zone of arc–continent collision, major earthquakes occur along high-angle reverse faults that reach deep into the crust or even the uppermost mantle. The seismogenic structures are likely to be reactivated normal faults on the passive continental margin of southeastern China. Since high-angle faults are ineffective in accommodating horizontal motion, it is not surprising that in the developed portion of the central Taiwan orogen (<5 Ma), seismogenic faulting occurs mainly along moderate-dipping (20–30°) thrusts. This is probably the only well-documented case of concurrent earthquake faulting on two major thrust faults, with the second seismogenic zone reaching down to depths of 30 km. Furthermore, the dual thrusts are out-of-sequence, being active in the hinterland of the deformation front. Along the mature Himalayan collision zone, where collision initiated about 50 Ma ago, current data are insufficient to distinguish whether most earthquakes occurred along multiple, out-of-sequence thrusts or along a major ramp thrust. Intriguingly, a very active seismic zone, including a large ( M w=6.7) earthquake in 1988, occurs at depths near 50 km beneath the foreland. Such a configuration may indicate the onset of a crustal nappe, involving the entire cratonic crust. In all cases of collision discussed here, the basal decollement, a key feature in the critical taper model of mountain building, appears to be aseismic. It seems that right at the onset of collision, earthquakes reflect reactivation of high-angle faults. For mature collision belts, earthquake faulting on moderate-dipping thrust accommodates a significant portion of convergence—a process involving the bulk of crust and possibly the uppermost mantle.

The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)

The Bandar Abbas syntaxis marks the transition between the Zagros collisional belt and the Makran subduction-related prism and constitutes a major structural feature within the Alpine–Himalayan mountain belt. Within the foreland part of the Zagros (the Zagros Fold Thrust Belt, ZFTB), the transition is marked by a sudden change in structural trends and style and a rapid decrease in seismicity and salt diapirism from west to east of the syntaxis. The depth to decollement and lithology affect the geometry and size of the folds on each side of the syntaxis, while different mechanical coupling along the basal decollement affects the degree of internal rock deformation. Thus the short and broad folds of the western side of the syntaxis are decoupled along the 8-km deep Hormuz evaporites, visible at surface in numerous salt diapirs. Strong seismicity in this area suggests that the basement is involved in the deformation. By contrast, the long and narrow folds of the eastern side of the syntaxis appear to be controlled by frictional slip along a 6-km deep detachment. The absence of seismicity suggests that the basement is not involved in the deformation in this area. We propose that the basal decollement for these structures corresponds to the eroded top of the NNE–SSW trending Oman Mesozoic nappes resulting from Late Cretaceous obduction of Tethys on the Arabian margin. These structures are visible onshore in the northern tip of the Oman peninsula and seen in offshore seismic profiles continuing northward beneath the Strait of Hormuz. Finally, map view observations show that the Main Zagros Thrust and the Zendan Fault—separating the ZFTB from the internal zones of the orogen—are shallow low angle thrust faults rather than steeply dipping faults.

Variation of structural style and basin evolution in the central Zagros (Izeh zone and Dezful Embayment), Iran

Structural style, and its relationship to sedimentary facies and the evolution of sedimentary depocenters since Late Cretaceous times have been studied, on the basis of one regional balanced transect and several updated isopach maps, in the Izeh zone and the Dezful Embayment, central Zagros. This study relies on fieldwork data, existing geological maps, seismic data and well information. A new structural classification for part of the Zagros sedimentary cover is presented to highlight the different mechanical behavior of the formations in the stratigraphic column. It shows the existence of several local decollement levels activated during folding. These decollement levels separate lithotectonic units, which accommodate shortening in different ways. The Lower Paleozoic is the basal decollement level throughout the studied area. Triassic evaporites, Albian shales, Eocene marls and Miocene evaporites can act as intermediate decollement levels, and present variable facies in the Central Zagros. Lateral facies and thickness variations, the sedimentary overburden and the close relationship with inherited fault patterns influenced the wavelength, amplitude and style of folding in the study area. Furthermore, surface structures do not necessarily coincide with deeper objectives where these disharmonic levels are involved in folding. The evolution of sedimentary depocenters from the Late Cretaceous (obduction episode) until early Miocene collision is described, based on updated isopach maps and tectonic subsidence curves. This analysis shows continuous compression and movements along N–S and NW–SE trending faults in the Zagros basement which predate the Neogene Zagros folding and influence sedimentation history. It also indicates a southwestward migration of the depocenters through time, in addition to the basement involvement below some folds during the Zagros orogeny.

Formation of decollement upwarps during thrusting

AbstractGeometric and kinematic models reveal that rotation of layers during thrusting or development of drag folds produced by movement along listric thrust faults results in the formation of dilation space at the junction of the thrust ramp with the basal decollement. This is accompanied by upwarping of the basement to occupy the available space thereby producing an antiformal structure, which restricts the thrust displacement. The structure is termed a decollement upwarp and examples are cited from the foothill belt of the Himalaya.

Tectonic evolution of the Lachlan Fold Belt, southeastern Australia: constraints from coupled numerical models of crustal deformation and surface erosion driven by subduction of the underlying mantle

We have used a coupled thermo-mechanical finite-element (FE) model of crustal deformation driven by mantle/oceanic subduction to demonstrate that the tectonic evolution of the Lachlan Fold Belt (LFB) during the Mid-Palaeozoic (Late Ordovician to Early Carboniferous) can be linked to continuous subduction along a single subduction zone. This contrasts with most models proposed to date which assume that separate subduction zones were active beneath the western, central and eastern sections of the Lachlan Orogen. We demonstrate how the existing data on the structural, volcanic and erosional evolution of the Lachlan Fold Belt can be accounted for by our model. We focus particularly on the timing of fault movement in the various sectors of the orogen. We demonstrate that the presence of the weak basal decollement on which most of the Lachlan Fold Belt is constructed effectively decouples crustal structures from those in the underlying mantle. The patterns of faulting in the upper crust appears therefore to be controlled by lateral strength contrasts inherited from previous orogenic events rather than the location of one or several subduction zones. The model also predicts that the uplift and deep exhumation of the Wagga–Omeo Metamorphic Belt (WOMB) is associated with the advection of this terrane above the subduction point and is the only tectonic event that gives us direct constraints on the location of the subduction zone. We also discuss the implications of our model for the nature of the basement underlying the present-day orogen.

Growth and erosion of fold‐and‐thrust belts with an application to the Aconcagua fold‐and‐thrust belt, Argentina

The development of topography within and erosional removal of material from an orogen exerts a primary control on its structure. We develop a model that describes the temporal development of a frontally accreting, critically growing Coulomb wedge whose topography is largely limited by bedrock fluvial incision. We present general results for arbitrary initial critical wedge geometries and investigate the temporal development of a critical wedge with no initial topography. Increasing rock erodibility and/or precipitation, decreasing mass flux accreting to the wedge front, increasing wedge sole‐out depth, decreasing wedge and basal decollement overpressure, and increasing basal decollement friction lead to narrow wedges. Large power law exponent values cause the wedge geometry to quickly reach a condition in which all material accreted to the front of the wedge is removed by erosion. We apply our model to the Aconcagua fold‐and‐thrust belt in the central Andes of Argentina where wedge development over time is well constrained. We solve for the erosional coefficient K that is required to recreate the field‐constrained wedge growth history, and these values are within the range of independently determined values in analogous rock types. Using qualitative observations of rock erodibilities within the wedge, we speculate that power law exponents of 1/3 ≤ m ≤ 0.4 and 2/3 ≤ n ≤ 1 characterize the erosional growth of the Aconcagua fold‐and‐thrust belt. This general model may be used to understand the development of mountain belts where orogenic wedges grow as they deform at their Coulomb failure limit.

Steady state erosion of critical Coulomb wedges with applications to Taiwan and the Himalaya

Orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion, respectively. We developed a coupled erosion‐deformation model for orogenic wedges that are in erosional steady state and deform at their Coulomb failure limit. Erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock. We found that the ultimate form of a wedge is controlled by the wedge mechanical properties, sole‐out depth of the basal decollement, erosional exponents, basin geometry, and the ratio of the added material flux to the erosional constant. As this latter ratio is increased, wedge width and surface slopes increase. We applied these models to the Taiwan and Himalayan orogenic wedges and found that despite a higher flux of material entering the former, the inferred ratio was larger for the latter. Calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the Himalaya relative to Taiwan. These values are consistent with the lower precipitation regime in the Himalaya relative to Taiwan and the exposure of crystalline rocks within the Himalayan orogenic wedge. Independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high‐grade metamorphic rocks exposed within Taiwan and the Himalaya, respectively. Therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts.

Modeling of thermo-rheological structure of lithosphere under the foreland basin and mountain belt of Taiwan

The system of Taiwan mountain belt and the West Taiwan foreland basin is a manifestation of the collision between eastern Asian continental margin and the Philippine volcanic arc. It provides an ideal place for the study of the flexure behavior of lithosphere under the mountain–basin system. The paper presents results of thermal and rheological modeling of the system along Profile B, which extends 200 km in a NW–SE direction from the Taiwan Strait across the foreland basin and mountain range to the Longitudinal Valley. Along the profile, the crustal structure is constrained by wide-angle seismic and gravity data as well as P-wave tomography, while the structure of the foreland basin is constrained by multichannel seismic and drilling. Assuming that a steady-state deformation has proceeded in Taiwan since the collision of the Luzon Arc with the SE Eurasian margin at 6.5 Ma, the present-day thermal and rheological structures of the lithosphere are modeled by finite element analysis. The base of lithosphere is assumed to be isothermal, with a temperature of 1300 °C. The interior heat source consists of radiogenic heating, frictional heating on thrust faults and basal decollement, and body heating by internal friction within thrust sheets. In addition to conduction, the heat convection is carried out by the subduction of the Eurasian margin, the exhumation and erosion in the mountain range, and the sedimentation in the foreland basin. Thermal parameters are carefully selected and updated based on available data on surface heat flow, downhole thermal gradient, and thermal conductivity from Taiwan and nearby regions. The mechanical parameters are determined based on regional geology. The resulting thermal model predicts well the elevated surface heat flow in the mountain range and the depressed surface heat flow in the thrust front. A high-temperature core of >500 °C appears in the bottom of thrust sheets and the uppermost upper crust. Sensitivity analysis indicates that this elevated temperature is mainly due to the frictional heating along thrust faults and basal decollement, and to the heat carried up by the exhumation of the deeper rocks and subsequent erosion. The depth-dependent strength envelopes were computed based on the thermal model and commonly used rheological parameters. Rheological stratifiction of the lithosphere along the profile is depicted. Although the estimation of the thickness of the rheological strata is affected by the uncertainty in lithosphere thickness, the modeling results indicate clearly that the rheological structure of the lithosphere under the mountain range is remarkably different from that in the foreland basin. While the foreland basin has a typical thermal and rheological structure of the rifted continental margin with three brittle layers above three ductile layers, the raised temperature under the mountain range has weakened the lithosphere dramatically. Under compressional stress, the lithosphere beneath the mountain range becomes ductile almost entirely, except a thin (about 6 km in the central part) brittle layer near the surface and perhaps a thin brittle layer in the uppermost mantle. Such a significant weakening of the lithosphere in the mountain range should not be overlooked while discussing the flexure behavior of the lithosphere in the area.

Stratigraphic and structural expression of the lateral growth of thrust fault‐propagation folds: results and implications from kinematic modelling

AbstractIn order to better understand the development of thrust fault‐related folds, a 3D forward numerical model has been developed to investigate the effects that lateral slip distribution and propagation rate have on the fold geometry of pre‐ and syn‐tectonic strata. We consider a fault‐propagation fold in which the fault propagates upwards from a basal decollement and along‐strike normal to transport direction. Over a 1 Ma runtime, the fault reaches a maximum length of 10 km and accumulates a maximum displacement of 1 km. Deformation ahead of the propagating fault tip is modelled using trishear kinematics while backlimb deformation is modelled using kink‐band migration.The applicability of two different lateral slip distributions, namely linear‐taper and block‐taper, are firstly tested using a constant lateral propagation rate. A block‐taper slip distribution replicates the geometry of natural fold‐thrusts better and is then used to test the sensitivity of thrust‐fold morphology to varied propagation rates in a set of fault‐propagation folds that have identical final displacement to length (Dmax/Lmax) ratios.Two stratigraphic settings are considered: a model in which background sedimentation rates are high and no topography develops, and a model in which a topographic high develops above the growing fold and local erosion, transport and deposition occur. If the lateral propagation rate is rapid (or geologically instantaneous), the fault tips quickly become pinned as the fault reaches its maximum lateral extent (10 km), after which displacement accumulates. In both stratigraphic settings, this leads to strike‐parallel rotation of the syn‐tectonic strata near the fault tips; high sedimentation rates relative to rates of uplift result in along‐strike thinning over the structural high, while low sedimentation rates result in pinchout against it. In contrast, slower lateral propagation rates (i.e. up to one order of magnitude greater than slip rate) lead to the development of along‐strike growth triangles when sedimentation rates are high, whereas when sedimentation rates are low, offflap geometries result. Overall we find that the most rapid lateral propagation rates produce the most realistic geometries. In both settings, time‐equivalent units display both nongrowth and growth stratal geometries along‐strike and the transition from growth to nongrowth has the potential to delineate the time of fault/fold growth at a given location. This work highlights the importance of lateral fault‐propagation and fault tip pinning on fault and fold growth in three dimensions and the complex syn‐tectonic geometries that can result.

Coupled magma chamber inflation and sector collapse slip observed with synthetic aperture radar interferometry on Mt. Etna volcano

Volcanoes deform dynamically due to changes in both their magmatic system and instability of their edifice. Mt. Etna features vigorous and almost continuous eruptive activity from its summit craters and periodic flank eruptions. Even though its shape is that of a large stratovolcano, its structure features two rift systems and a flank collapse structure similar to Hawaiian shield volcanoes. We analyze European remote sensing (ERS) satellite differential interferometric synthetic aperture radar (InSAR) data (1993–1996) for Mt. Etna spanning its quiescence from 1993 through the initiation of renewed eruptive activity in late 1995. We use synthetic aperture radar (SAR) data from both ascending and descending ERS satellite tracks. Comparison of independent interferograms covering the first 2 years of the inflationary period shows a pattern consistent with inflation of the volcano. Calculation of the tropospheric path delay based on meteorological data does not change this interpretation. Interferograms from late summer 1995–1996 show no significant deformation. Joint inversion of interferograms from ascending and descending satellite tracks require both inflation from a spheroidal magmatic source located beneath the summit at 5 km below sea level, and displacement of the east flank of Etna along a basal decollement. Both sources of deformation were contemporaneous within the resolution of our data and suggest that inflation of the central magma chamber acted to trigger slip of Etna's eastern flank. These results demonstrate that flank instability and recharge of a volcano's magma system must both be considered toward understanding how volcanoes work and in their hazard evaluation.

Deformation history of the NW salient of the Eastern Ghats Mobile Belt, India

The Eastern Ghats Mobile Belt of India displays fold-thrust belt structure with a prominent salient on its NW margin. The salient consists of Lathore nappe and Turekela klippe that have overthrust the craton to NW. The rocks of the nappes have undergone granulite facies metamorphism and multiple phases of folding corresponding to the Eastern Ghats orogeny, prior to thrusting. As a result the granulites along the thrust are retrograded to amphibolites and the axial traces of the folds have been truncated against the thrust plane. The basal decollement of the fold-thrust belt is represented by the Terrane Boundary Shear Zone that defines the tectonic margin between the craton and the mobile belt. It occurs as a ductile thrust affecting the cratonic basement as well as the mobile belt suggesting that the basement did not behave as a rigid body during thrusting. Thus the study area is comparable with the Caledonide fold-thrust belt. Further, two large lateral ramps namely Khariar and Paikamal lateral ramps have been developed on the decollement at the lithological contact between tonalite–trondhjemite gneisses/granite gneisses and the latetectonic potassic granites of the craton. Fault-bend folds associate with these ramps too. It is suggested that the salient structure of the fold-thrust belt is the combined result of (1) lateral ramps on the decollement and (2) differential displacement along the sole thrust due to lateral variation in detachment strength.

Passive-roof thrust model for the emplacement of the Pelona-Orocopia Schist in southern California, United States

The early Tertiary Orocopia-Chocolate fault in southern California was originally interpreted as a southwest-directed thrust assisting tectonic underplating of the Orocopia Schist beneath North America during subduction. However, the predicted thrusting direction is opposite to that observed for the fault. Although this problem appeared to be resolved by interpreting the Orocopia-Chocolate fault as a normal fault, this alternative is inconsistent with the fact that neither an early Tertiary breakaway nor early Tertiary upper plate extension has been observed. I suggest that the fault was a northeast-directed passive-roof thrust that linked with a southwest-directed basal decollement (the Vincent thrust) during early Tertiary flat subduction of the Farallon plate beneath North America. This model may also explain a similar structural relationship between the Main Central thrust and the South Tibet detachment fault in the Himalaya.

Lithospheric flexure under a critically tapered mountain belt: a new technique to study the evolution of the Tertiary Taiwan orogeny

A joint analysis of both the lithospheric flexure of an underlying plate and mountain-building mechanics is developed based on the critical wedge theory and the flexure response of an elastic plate over an inviscid asthenosphere. According to the analysis, the amount of deflection beneath a critically tapered mountain belt strongly depends upon the internal and basal friction coefficients of that belt and the flexural rigidity of the underlying plate rather than the horizontal stress induced by plate convergence. An increase in the internal friction of a mountain belt or a decrease in the friction along its basal decollement tends to enlarge the deflection of the underlying plate. The tendency toward an increased deflection of the underlying plate due to a decrease in flexural rigidity is suppressed by the development of a more narrowly tapered mountain belt. The newly developed technique presented here is used to analyze the evolution of the Tertiary Taiwan orogeny and shows that the optimal flexural rigidity of the Eurasian Plate in Taiwan is about 3×10 22 Nm. It also demonstrates that the collision between the Eurasian Plate and the Luzon Arc might have initiated 7 Myr ago, a finding that would account for the burial history of the foreland basin in south-central Taiwan.

Active spreading and regional extension at Mount Etna imaged by SAR interferometry

Large-flank instability has been proposed by most recent authors as a major process in the dynamics of Mount Etna. However, several aspects of the models are still disputed, such as the boundaries of the moving area and the driving force for the instability. In this paper we present synthetic aperture radar interferometric data which allow us to identify two main sectors of active instability where the deformational process differs. An eastern sector is bounded to the north by the Pernicana–Provenzana fault, to the west by the North Rift Zone and the South Rift Zone and to the south by the Mascalucia–Tremestieri–Trecastagni fault system. Interferograms provide new data on the activity and extent of the southern boundary, which was previously underestimated. In this sector the structural features indicate an eastward sliding, driven by E–W regional extension, which can be interpreted as a result of the retreating slab of the Ionian subduction. A second collapsing sector exists along the southern flank of Mount Etna, where the interferograms show the presence of an active anticlinal ridge. This ridge is interpreted as the result of southward gravity spreading over a basal decollement between Etna and its Plio–Quaternary basement and the Hyblean platform. The two sectors first became active in summer 1996, after the beginning of a new cycle of eruptive activity at the summit. The activity was ongoing until January 1998, with the deformation rate ranging from 4–6 mm/yr for the Mascalucia–Tremestieri–Trecastagni faults to 12 mm/yr for the anticlinal ridge.

Wide-angle Reflection Profiling with Quarry Blasts in the Eastern Tennessee Seismic Zone

This study evaluates the feasibility of using relatively small, delay-fired quarry blasts as energy sources for wide-angle reflection profiling of the middle to lower crust beneath the southern half of the eastern Tennessee seismic zone. The test recordings targeted structure on both sides of the New York–Alabama lineament, a prominent northeast-trending zone of steep magnetic gradients. The lineament marks a near-vertical boundary between zones of high and low seismic activity and also correlates with a linear boundary between blocks of high- and low-velocity crust imaged by joint hypocenter-velocity inversion of regional P and S arrival time data. Timed blasts at four crushed-stone quarries were recorded with a portable array of 19 PRS-4 recorders and three-component, 4.5-Hz geophones with stations at roughly 200 m intervals. Source-receiver offsets ranged from 16 to 67 km. Blasts were timed with a PRS-4 deployed at the quarry sites. The blasts ranged from 75 to 542 ms in duration and from 259 to 942 lbs. explosive per delay. The effects of ripple firing are clearly evident in trace autocorrelations and as coherent peaks and nodes in the amplitude spectra for each shot; the latter show signal energy at frequencies up to 45 Hz. Source wavelets estimated by minimum entropy deconvolution have durations that are consistent with the reported durations of the blasts. Unprocessed field records recorded over distances between 16 and 29 km show strong reflections from the basal decollement (depth 4.5 km) and deeper interfaces within the basement. Shot gathers and slant stacks for blasts recorded at offsets between 62 and 67 km show coherent energy with apparent slownesses and travel times consistent with precritical P and S reflections from the lower crust and Moho. Although sparsely sampled, preliminary migrated sections suggest the presence of several highly reflective structures, including a concentration of reflectors at a depth of about 25 km, near the maximum reported depth for earthquakes in the region, a zone of south-dipping reflectors at depths between 27 and 40 km, northwest of the New York–Alabama lineament, and a clustering of reflectors at depths between 40 and 50 km, possibly marking the crustmantle transition. They also show north-dipping reflectors that correlate with the top of a prominent wedge of reflective crust imaged in nearby vertical-incidence profiles. The results demonstrate that signals from relatively small delayfired blasts can be quite useful for imaging the crust.

An elastic wedge model for the development of coeval normal and thrust faulting in the Mauna Loa‐Kilauea rift system in Hawaii

A long‐standing enigma of the Mauna Loa‐Kilauea rift system in Hawaii is the coeval development of normal and thrust faults that are vertically partitioned. To address this question, we developed a simple elastic wedge model that explores plausible boundary conditions in terms of tractions for generating such a fault pattern. Analytical solutions that best simulate the observed faulting style and geodetically determined strain at the surface require that (1) the pore fluid pressure ratio within the wedge (λ) and along the basal decollement (λb,) must be exceedingly high (i.e., λ = λb= 0.90–0.95) and (2) a tensile stress of the order of 10–30 MPa must have existed in the very top part of the rift zone at the back side of the wedge‐shaped rift flank. The high pore fluid pressure within the rift flank may be induced by pumping of fluids during emplacement of magma, whereas the high pore fluid pressure along the basal decollement may be caused by compaction of water‐saturated sediments between the volcanic pile above and the oceanic floor below. Although the predicted tensile stress in the rift zone could be related to the presence of a relatively steep topographic slope, our results show that this is not a prerequisite. Therefore we attribute occurrence of tensile stress to either upward bending of the Hawaiian volcanic pile due to emplacement of magma, or inflation of a shallow magma chamber several kilometers beneath the surface. In any case, the results of our model indicate that magma emplacement in the shallow part of the rift zone may be a passive process, while the deep rift zone experiences forceful emplacement (i.e., active rifting via magma push).

A structural model for the western-central Sudetes: a deformed stack of Variscan thrust sheets

A model based on new structural and geochemical data is presented. It unifies the structural history of the Izera, Rudawy Janowickie and Kaczawa complexes with the Fore Sudetic block, despite their current separation by the Intra-Sudetic and Marginal Sudetic faults. Above the granitoid Izera, Kowary and Wa¸droże gneisses, at the base of the structural sequence, the ductile Kowary shear zone marks the basal decollement of the Świerzawa thrust sheet, comprising often highly strained metasediments associated with enriched tholeiitic and alkaline metabasites. Above it, the ductile Kaczorów shear zone, corresponding to the main mylonitic zone within the Leszczyniec shear zone in the Rudawy Janowickie Complex, marks the base of the Dobromierz thrust sheet, characterized by voluminous MORB-like meta-tholeiites and minor metasediments in the higher parts of the Rudawy Janowickie and Kaczawa complexes and the Pyszczyńska Hill area of the Fore-Sudetic Block. In the east the Sle¸za ophiolite and the Góry Sowie Block override the entire nappe stack. Kinematic fabrics in the major and related shear zones indicate D 1 compressional transport towards the northwest, followed by minor D 2 extensional movements. The thrust stack was deformed during D 3 by southwest verging folds, was subsequently intruded by post-orogenic granites, and later disrupted by the Intra-Sudetic, Marginal Sudetic and associated faults.

Actively growing anticlines beneath catania from the distal motion of Mount Etna's Decollement measured by SAR interferometry and GPS

We used SAR interferometry and GPS to measure the rise of an anticline beneath the urban area of Catania (Italy), which originates from outward thrusting above the basal decollement of Etna volcano. The anticline grows at a rate of over 0.015 m a−1 of SE thrusting and about 0.007 m a−1 of relative uplift. By relating this growth to the simultaneous extension measured at the summit of the volcano, we demonstrate the occurrence of active volcanic spreading. This process may exert a control on eruptions, earthquakes, sector collapses, and landslides.

The chi-Chi earthquake sequence: active, out-of-sequence thrust faulting in taiwan

We combined precise focal depths and fault plane solutions of more than 40 events from the 20 September 1999 Chi-Chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central Taiwan is a moderately dipping (20 degrees to 30 degrees ) thrust fault away from the deformation front. A second, subparallel seismic zone lies about 15 kilometers below the main thrust. These seismic zones differ from previous models, indicating that both the basal decollement and relic normal faults are aseismic.