Dominant Style Of Deformation Research Articles

Neotectonic evidence for Late Quaternary reverse faulting in the northern Chile outer forearc (22.5°S-23°S): Implications for seismic hazard

In the northern Chile outer forearc (19°S to 23°S), extension is the dominant deformation style conducted by crustal faults during the Neogene-Quaternary. Most of this extension has been produced by normal faulting along submeridian branches of the Atacama Fault System (AFS). During the Late Quaternary, several of these faults have built conspicuous metric-scale normal faulting scarps in alluvial deposits; the construction of these scarps has been related, in some cases, to M~7 normal faulting paleoearthquakes. On the contrary, shortening during this time span has only been locally reported. Until now, no metric-scale reverse faulting scarps have been documented along submeridian strands of the AFS. This study presents novel evidence corresponding to reverse fault scarps produced during the Late Quaternary by three NNE and NNW trending faults in the outer forearc between 22.5°S and 23°S: the El Toro (ETF), Caliche (CF), and Sierra Valenzuela (SVF) faults. These scarps, with up to 2.5 m height, developed in basement rocks, pre-Quaternary, and/or Quaternary alluvial deposits. Based on their characteristics and the evidence suggesting that similar fault scarps in the Andean forearc have a seismogenic origin, we interpret that these neotectonic landforms were produced by single reverse faulting paleoearthquakes. Considering the maximum scarp height as a proxy for the coseismic slip and fault length as a proxy for surface rupture length, we have estimated magnitudes between M6.2 and M6.9 for the causative rupture events. To discuss how reverse crustal faulting may be related to the interseismic and/or coseismic stages of the subduction cycle, we perform elastic dislocation models to calculate the horizontal displacement fields (at 0 and 20 km depth) induced by the interplate locking distribution in the area (over a seismic cycle) and three megathrust earthquakes (Antofagasta, 1995, Tocopilla, 2007, and a synthetic rupture). Modeling results suggest that the studied faults may experience reverse slip during both stages of the subduction cycle. We propose that the ETF, CF and SVF are faults capable of producing moderate-to-large earthquakes in the future. Finally, we reinforce the need to address the earthquake potential of the studied faults, and others similar to them, to acquire a complete assessment of the seismic hazard in the northern Chile outer forearc.

How does orogenic crust deform? Evidence of crustal-scale competent behaviour within the partially molten middle crust during orogenic compression

Granitic partial melts are generally thought to significantly weaken the orogenic crust and, ultimately, lead to the collapse of an orogen. Studies from different orogens have shown that the syn-melt deformation behaviour at the orogenic scale is, however, more complex. In addition, once fully crystalline, granitic material strengthens the crust. Linking the evolution from melt-present to melt-absent deformation at the scale of the orogen is a challenging but necessary task if one wishes to investigate the overall behaviour of the middle crust. In this paper, we make observations of orogen-scale deformation, developed over a period of 30–40 Ma during crustal partial melting of the middle and lower crust of a Palaeoproterozoic orogen. The crust shows a globally common pattern where coeval partial melting, compressional deformation, and transtensional structures co-exist. We demonstrate this complex interaction through an integrated approach using multiple datasets. The key observation is that of widespread, regular, orogen-scale shortening by folding of sub-horizontal anatectic granitic sheets and highly migmatized lithologies; this shortening interacted with modest orogen-parallel lateral stretch expressed as local extensional/transtensional structures. The pervasive and dominant deformation style is that of folding of the syn-kinematic granite sheets and voluminous migmatites: this demonstrates that the principal deformation style of deformation within the partially molten mid-crust is competent rather than weak. The observed evidence of weak behaviour such as strike-slip or transtensional shear zones accommodating lateral escape are localized in comparison and not necessarily associated directly with the granite sheets or the highly migmatized volumes of the crust. Moreover, the strain intensity (fold wavelengths and amplitudes) seems independent of the overall melt fraction. The implications are that, while widespread volumes of partial melt will weaken the crust overall (compared to the brittle upper crust) and while individual melt bodies can possibly persist for up to some millions of years in the middle crust, i) the relative strength between (partially) molten volumes is not primarily controlled by melt fraction; and ii) individual melt volumes may be too short-lived at the time-scale of orogenic deformation (i.e. orogenic strain rates) to significantly influence the overall deformation style.

Ascent and emplacement dynamics of obsidian lavas inferred from microlite textures

To assess the eruption and emplacement of volumetrically diverse rhyolite lavas, we measured microlite number densities and orientations from samples collected from nine lavas in Yellowstone Caldera and two from Mono Craters, USA. Microlite populations are composed of Fe-Ti oxides ± alkali feldspar ± clinopyroxene. Number densities range from 108.11 ± 0.03 to 109.45 ± 0.15 cm−3 and do not correlate with distance from the vent across individual flows and are remarkably similar between large- and small-volume lavas. Together, those observations suggest that number densities are unmodified during emplacement and that ascent rates in the conduit are similar between small domes and large lava flows. Microtextures produced by continuous decompression experiments best replicate natural textures at decompression rates of 1–2 MPa hr−1. Acicular microlites have a preferred orientation in all natural samples. Because the standard deviation of microlite orientation does not become better aligned with distance travelled, we conclude that microlites exit the conduit aligned and that strain during subaerial flow was insufficient to further align microlites. The orientations of microlite trend and plunge in near-vent samples indicate that pure shear was the dominant style of deformation in the conduit. We speculate that collapsing permeable foam(s) provides a mechanism to concurrently allow microlite formation and alignment in response to the combination of degassing and flattening by pure shear.

Paleoseismology of the Mount Narryer fault zone, Western Australia: A multistrand intraplate fault system

Our paleoseismological study of faults and fault-related folds comprising the Mount Narryer fault zone reveals a mid- to late Quaternary history of repeated morphogenic earthquakes that have influenced the planform and course of the Murchison, Roderick, and Sanford Rivers, Western Australia. The dominant style of deformation involves folding of near-surface sediments overlying discrete basement faults. Carbon-14, optically stimulated luminescence, and in situ–produced 10 Be constrain the timing of the events and late Quaternary slip rates associated with fault propagation folds in tectonically uplifted and deformed alluvial channel deposits. A flight of five inset fluvial terraces is preserved where the Murchison River flows across the Roderick River fault. These terraces, which we infer to be coseismic, are consistent with at least four late Quaternary seismic events on the order of moment magnitude (M w ) 7.1 within the last ~240 k.y. Secondary shears expressed on the folds indicate a component of dextral strike-slip displacement. Quaternary slip rates on the underlying faults range from 0.01 to 0.07 mm yr –1 , with a total slip rate for the zone between 0.04 and 0.11 mm yr –1 . These rates are intermediate to those in the adjacent Mesozoic basin (>0.1 mm yr –1 ) and Precambrian craton ( –1 ) and so provide insight into how tectonic strain is partitioned and transferred across a craton margin.

Strike-slip tectonics in the Pannonian basin based on seismic surveys at Lake Balaton

Strike-slip tectonics has been the dominant style of deformation during the neotectonic (Pliocene and Quaternary) evolution of the Pannonian basin. Main faults are exposed in the “island mountains” of the basin, but strike-slip tectonic features can be best studied in the basin fill by seismic data. Lake Balaton offers the opportunity to carry out high to ultra-high-resolution seismo-acoustic surveys to image stratigraphic and tectonic features in the central part of the Pannonian basin. Several campaigns in the lake using different acquisition techniques have resulted in more than 2000-km seismo-acoustic profiles with a range of resolutions and penetration depths. Interpretation of faults and folds shows a few kilometers wide shear zone below the lake in Late Miocene–Pliocene strata. This zone can be identified as the continuation of the Balatonfő line known onshore to the east of the lake. Mapping revealed a set of duplex structures and highlighted the importance of this shear zone in the formation of Lake Balaton. Comparison of our results to analogue clay models suggests that the observed shear zone is sinistral and the horizontal displacement is on the order of hundreds of meters. Looking at 3D industrial seismic data to the south of the lake, we suggest that the first-order Balaton line, which represents the continuation of Periadriatic line, is also sinistral and characterized by small horizontal displacement of about 1.0–1.5 km during Pliocene and Quaternary times. This indicates a 0.2–0.3 mm/year average slip rate, which is compatible with recent GPS measurements.

Triggered aseismic slip adjacent to the 6 February 2013 Mw 8.0 Santa Cruz Islands megathrust earthquake

Aseismic or slow slip events have been observed in many subduction zones, but whether they affect the occurrence of earthquakes or result from stress changes caused by nearby events is unclear. In an area lacking direct geodetic observations, inferences can be made from seismological studies of co-seismic slip, associated stress changes and the spatiotemporal nature of aftershocks. These observations indicate that the February 2013 Mw 8.0 Santa Cruz Islands earthquake may have triggered slow or aseismic slip on an adjacent section of the subduction thrust over the following hours to days. This aseismic event was equivalent to Mw 7.6, significantly larger than any earthquakes in the aftershock sequence. The aseismic slip was situated within the seismogenic portion of the subduction interface, and must have occurred to the south of the main seismic slip and most aftershocks in order to promote right-lateral faulting in the upper plate, the dominant deformation style of the aftershock sequence. This plate boundary segment can support either stable sliding (aseismic) or stick-slip (seismic) deformation in response to different driving conditions. The complete lack of aftershocks on the thrust interface implies this pair of megathrust slip episodes (seismic and aseismic) released a substantial portion of the stored strain on the northernmost section of the Vanuatu subduction zone.

Hinterland basin development and infilling through tectonic and eustatic processes: latest Messinian‐Gelasian Valdelsa Basin, Northern Apennines, Italy

AbstractThis article reports a stratigraphic and structural analysis of the Neogene‐Quaternary Valdelsa Basin (Central Italy), filled with up to 1000 m of uppermost Miocene to lower Pleistocene strata. The succession is subdivided into seven unconformity‐bounded stratigraphic units (synthems, or large‐scale depositional sequences) that include fluvio‐deltaic and shallow‐marine deposits. Structures related to basin shoulders and internal boundaries controlled the Neogene location and geometry of different depocentres. During the Tortonian‐Messinian, a buried NE‐trending high related to regional, basin‐transverse lineaments separated two adjacent sub‐basins. During the lower Pliocene, compressional displacement along NW‐trending, thrust‐related highs controlled the distribution of depocentres and dispersal of sediment. Extensional tectonics, although previously considered the dominant deformation style affecting the rear of the Northern Apennines since the late Miocene, is no longer considered a dominant control on tectono‐sedimentary development of the Valdelsa basin. Instead, the Valdelsa Basin shares features with continental hinterland basins of orogenic belts where compression, extension, and transcurrent stress fields determine a complex spatial and temporal record of accommodation and sediment supply. In the Valdelsa Basin tectonics and eustatic sea‐level fluctuations were dominant in forcing the deposition of sedimentary cycles at several scales. Zanclean and Gelasian large‐scale depositional sequences were mainly controlled by crustal shortening, whereas a eustatic signal was preferentially recorded during the Piacenzian. Smaller scale depositional sequences, common to most synthems, were controlled by orbitally forced glacio‐eustatic cycles.

A new view of an old suture zone: Evidence for sinistral transpression in the Cheyenne belt

The type locality of the Archean–Paleoproterozoic suture zone in the southern Rocky Mountains is marked by a series of subvertical shear zones collectively called the Cheyenne belt. The Cheyenne belt is a key structure for developing models for 1780–1740 Ma tectonism along the southern margin of the Archean Wyoming Province, which heralded a rapid period of continental amalgamation. This paper tests existing structural and plate-tectonic models for the Cheyenne belt with detailed geologic mapping, kinematic analyses, quartz crystallographic fabric analyses, and deformation mechanism analyses of the northern mylonite zone of the eastern Medicine Bow Mountains. Mylonites of this zone record a complex deformation history, but the main deformation phase was sinistral/northwest-side-up oblique transpression. Evidence for southeast-side-up, dip-slip motion that characterizes many other areas of the belt is confined to ultramylonites immediately adjacent to the terrane boundary. Hence, fabrics related to sinistral transpression were likely overprinted by southeast-side-up motion. Sinistral strike-slip motion is recorded in at least two other localities in the Cheyenne belt. Because synmetamorphic fabrics on both sides of the suture zone record sinistral strike-slip and northwest-side-up motion, this was probably the dominant deformation style in the field area and may have been the dominant deformation style throughout the Cheyenne belt. Based on these data and regional constraints, we interpret the Cheyenne belt as a subvertical transpressional stretching fault system that simultaneously accommodated sinistral strike-slip motion, penetrative horizontal shortening, and dip-slip motion related to differential crustal thickening between the relatively cold Wyoming Province and younger, hotter rocks to the south.

Comparing thin-sheet models with 3-D multilayer models for continental collision

Various models have been proposed to explain tectonic deformations during continent collision. A frequently applied model is the thin viscous sheet model which is however not fully 3-D and assumes a priori diffuse thickening as the dominant deformation style. We compare a fully 3-D multilayer numerical model with a corresponding thin viscous sheet numerical model for the scenario of continent indentation. In our comparison we focus on the three basic viscous deformation styles thickening, buckling (folding) and lateral crustal flow. Both numerical models are based on the finite element method (FEM) and employ either a linear or power-law viscous rheology. The 3-D model consists of four layers representing a simplified continental lithosphere: strong upper crust, weak lower crust, strong upper mantle and weak lower mantle. The effective viscosity depth-profile in the 3-D model is used to calculate the depth-averaged effective viscosity used in the thin-sheet model allowing a direct comparison of both models. We quantify the differences in the strain rate and velocity fields, and investigate the evolution of crustal thickening, buckling and crustal flow resulting from the two models for two different phases of deformation: (1) indentation with a constant velocity and (2) gravitational collapse after a decrease of the indenting velocity by a factor of 5. The results indicate that thin-sheet models approximate well the overall large-scale lithospheric deformation, especially during indentation and for a linear viscous rheology. However, in the 3-D model, additional processes such as multilayer buckling and lower crustal flow emerge, which are ignored in the thin-sheet model but dominate the deformation style in the 3-D model within a range of a few hundreds of kilometres around the collision zone and indenter corner. Differences between the 3-D and thin-sheet model are considerably larger for a power-law viscous than for a linear viscous rheology. Buckling and lower crustal flow are significant in the 3-D model with power-law viscous rheology. For example, fibre strain rates due to buckling can be several hundred per cent different to the depth-averaged strain rate and the lateral mass flow of lower crustal material can be up to six times more than the flow of upper crustal material. Our results also show that the horizontal velocity fields of the upper crust and upper mantle remain nearly identical in the 3-D model during indentation despite their mechanical decoupling due to an intermediate weak lower crust. This result questions the validity of using similarities between velocities from the surface global positioning system (GPS) and mantle shear wave splitting data as evidence for a mechanically coupled lithosphere. 3-D multilayer models provide a more complete picture of continental collision than thin-sheet models as they enable studying the timing, locality and relative importance of different processes simultaneously which is especially important for the hundreds of kilometre scale around the collision zone and indenter corners. 3-D models are, however, still computationally challenging and we, therefore, also present results of a computational performance test of several solution algorithms.

Frequency-size distribution of competent lenses in a block-in-matrix mélange: Imposed length scales of brittle deformation?

[1] This study explores the frequency-size distribution of competent lenses (phacoids) in the Chrystalls Beach Complex, a melange shear zone within the Otago Schist accretion-collision assemblage, New Zealand. The distribution of phacoids within a cleaved, pelitic matrix follows a power law distribution with exponent 1.18 < D < 2.37 (2-D) at outcrop scale, whereas distributions at thin section scale have higher D than the outcrop they were sampled from. D relates to volume fraction and distribution of competent material, where high D correlates with small aspect ratio phacoids and high matrix volume fraction. In outcrops which experienced mixed continuous/discontinuous deformation, D = 1.4 ± 0.2. It is suggested that D ≫ 1.4 indicates dominant continuous deformation, whereas D ≪ 1.4 is indicative of localized deformation on shear discontinuities. In this melange, D therefore appears a function of dominant deformation style. Actively deforming melange shear zones likely exhibit a range of seismic styles and earthquake magnitudes at length scales limited by lithological layering and shear zone thickness. A hypothesis is suggested where strongly coupled shear zone segments may be distinguished by a high volume fraction of competent material (low D), while aseismically creeping regions are characterized by a high proportion of incompetent rock (high D). As the b value in the Gutenberg-Richter frequency-magnitude relationship also tends to be high in coupled and low in creeping fault segments, it may be that D is analogous to b, and the b value is also related to the ratio of incompetent/competent material and dominant fault zone deformation style.

Seismicity and source parameters for small-scale earthquakes along the splays of the North Anatolian Fault (NAF) in the Marmara Sea

Major discrepancy between present-day fault models in the Marmara Sea is closely related to structural development of depression areas beneath the sea. Therefore, in this study, the Marmara Sea and especially its major basins are investigated by means of recent seismological observations consisting of microseismicity (1.2 ≤ Md ≤ 3.6; 2002 May–2006 May), focal mechanisms (2.4 ≤Md ≤ 5.0; 1999–2006) and stress analysis based on the focal mechanisms. In the Çınarcık Basin where either pull-apart model or negative flower structure is proposed, northeastern margin is represented by strike-slip sources having a NW alignment, probably corresponding to a cross-basin strike-slip fault developing in a matured pull-apart basin. This suggestion is directly relevant to experiments of sand box analogue models. These experiments indicate that such a cross-basin strike-slip fault is produced after the pull-apart basins attain their mature stage, eventually resulting in less vertical displacements attributed to normal faults bounding the basin. This experimental decline may be corroborated by the paucity of normal mechanisms in the basin. Extensional motion in this region is more influential outside of the basin, especially to the area where an onshore seismic activity is monitored on the land. On the other hand, another strike-slip activity in the Çınarcık Basin is distributed on a possible strand, parallel to the northern coastline of the Marmara Sea in the Armutlu Peninsula. This may indicate that one or more short strands of the northern splay extend to the west by bounding the basin from the south. In the Central Basin, the existence of a right-lateral strike-slip faulting on an east—west trending fault is supported by similar strike-slip focal mechanisms located at both extremities of the basin. To the west, dominant style of deformation gradually shifts from strike-slip faulting to transpression along the western coastline of the Marmara Sea where the northern splay joins with the offshore continuation of the Ganos Fault. Finally, focal mechanisms in the Marmara Sea are inverted for the best fitting stress tensor, characterized by a subvertical σ2-axis and two subhorizontal stress axes of σ1 and σ3, oriented NW—SE and NE—SW, respectively. Conclusively, this study infers that the current tectonics of the Marmara Sea is mainly governed by a strike-slip regime, ascribed to both submarine splays of the NAF, and partly dominated by extensional and compressional features indicating shallow processes at local scales.

Inhibition of subduction thrust earthquakes by low-temperature plastic flow in serpentine

Serpentine is considered a candidate hydrous mineral in explaining aseismic zones along faults; however, recent frictional experiments have shown a marked increase in its shear strength with increasing depth, implying its deformation via unstable sliding at a plate interface in subduction zones. Here we report the results of a series of simple-shear experiments designed to determine the dominant deformation style of low-temperature serpentine species under P– T conditions that correspond to the mantle wedge corner in cool subduction zones (ca. P = 1 GPa and T = 200 °C). We found that with increasing shear strain, the dominant flow mechanism of the serpentinite evolves from (1) inhomogeneous semi-brittle flow by strain localization into sample-scale shear bands to (2) homogeneous ductile flow by intracrystalline deformation within individual serpentine grains, with the development of a planar shape fabric and a strong crystallographic-preferred orientation. TEM observations revealed that grain size reduction in lizardite crystals occurs due to (001) interlayer glide, represented by the occurrence of tiny sheet-like sectors oriented parallel to (001) liz lattice fringes, which evolved from large, elongated sectors with long axes oriented normal to (001) liz. Given that a low-viscosity serpentinized layer upon a subducting plate interface produces strain localization within the layer and subsequent large bulk shear strains, the above results indicate that the presence of low-temperature serpentine species at the plate interface in cool subduction zones inhibits the initiation of subduction thrust earthquakes, as stress is preferentially accommodated by plastic flow. This hypothesis explains the occurrence of an anomalous non-seismic region (devoid of large thrust earthquakes) in locally hydrated forearc mantle within northeast Japan.

Present-day stress field and tectonic inversion in the Pannonian basin

This paper presents a latest compilation of data on the present-day stress pattern in the Pannonian basin, and its tectonic environment, the Alpine–Dinaric orogens. Extensional formation of the basin system commenced in the early Miocene, whereas its structural reactivation, in the form of gradual basin inversion, has been taking place since Pliocene to recent times. Reconstructed compression and associated horizontal contraction are mainly governed by the convergence between Adria and its buffer, the Alpine belt of orogens. The resulting contemporaneous stress field exhibits important lateral variation resulting in a complex pattern of ongoing tectonic activity. In the Friuli zone of the Southern Alps, where thrust faulting prevails, compression is orthogonal to the strike of the mountain belt. More to the southeast, intense contraction is combined with active strike–slip faulting constituting the dextral Dinaric transpressional corridor. Stresses are transferred far from Adria into the Pannonian basin, and the dominant style of deformation gradually changes from pure contraction through transpression to strike–slip faulting. The importance of late-stage inversion in the Pannonian basin is interpreted in a more general context of structural reactivation of back-arc basins where the sources of compression driving basin inversion are also identified and discussed. The state of recent stress and deformation in the Pannonian basin, particularly in its western and southern part, is governed by the complex interaction of plate boundary and intra-plate forces. The counterclockwise rotation and north-northeast-directed indentation of the Adriatic microplate appears to be of key importance as the dominant source of compression (“Adria-push”). Intra-plate stress sources, such as buoyancy forces associated with an elevated topography, and crustal as well as lithospheric inhomogeneities can also play essential, yet rather local role.

Late Cenozoic and modern stress fields in the western Fars (Iran): Implications for the tectonic and kinematic evolution of central Zagros

The Zagros (Iran) developed during Mio‐Pliocene times in response to Arabia‐Eurasia convergence. The western Fars highlights a major bend of the deformation front and displays a remarkable set of nearly N‐S right‐lateral strike‐slip faults (the Kazerun‐Borazjan/Karebass/Sabz‐Pushan/Sarvestan faults) oblique at high angle to the belt. The region likely plays a major kinematic role by accommodating the change in shortening modes from partitioned in the western central Zagros to nonpartitioned in the eastern Zagros. The inversion of focal mechanisms from small and moderate earthquakes shows a consistent N020°–030° compression with a low ratio between differential stresses. This regime accounts for the combination of strike‐slip and thrust‐type mechanisms through likely σ2/σ3 permutations. Fault slip analysis reveals two successive late Cenozoic regional compressional trends, NE‐SW then N020°. The latter is in good agreement with the present‐day stress. The significance of the NE‐SW compression is discussed alternatively in terms of stress deviations or block rotations in relation to the strike‐slip fault system. Fieldwork and satellite imagery suggest that these faults behave first as transfer faults during folding of the cover and later as strike‐slip faults, in agreement with the succession of stress regimes and the evolution of the dominant deformation style from thin‐skinned to thick‐skinned. The first‐order stability of the collision‐related state of stress since ∼5 Ma supports that the Arabia‐Eurasia convergence did not give rise to partitioning in the western Fars but rather was (and is still) accommodated by distributed deformation involving both shortening and strike‐slip motion throughout the cover and the basement.

Characterization of the time‐dependent strain field at seismogenic depths using first‐motion focal mechanisms: Observations of large‐scale decadal variations in stress along the San Andreas Fault system

We present a method for summing moment tensors derived from first‐motion focal mechanisms to study temporal dependence in features of the subsurface regional strain field. Time‐dependent processes are inferred by comparing mechanisms summed over differing time periods. We apply this methodology to seismogenic zones in central and southern California using focal mechanisms produced by the Northern and Southern California Seismograph Networks for events during 1980–1999. We find a consistent pattern in both the style of deformation (strike‐slip versus compressional) and seismicity rate across the entire region. If these temporal variations are causally related, it suggests a temporal change in the regional‐scale stress field. One change consistent with the observations is a rotation in the regional maximum horizontal compressive stress direction, followed by a reversal to the original direction. Depending upon the dominant style of deformation locally, this change in orientation of the regional stress will tend to either enhance or hinder deformation. The mode of enhanced deformation can range from increased microseismicity and creep to major earthquakes. We hypothesize that these temporal changes in the regional stress field are the result of subtle changes in apparent relative plate motion between the Pacific and North American plates, perhaps due to long‐range postseismic stress diffusion. Others have hypothesized that small changes in plate motion over thousands of years, and/or over decades, are responsible for changes in the style of deformation in southern California. We propose that such changes, over the course of just a few years, also affect the style of deformation.

Effect of a retreating subduction zone on deformation in simple regions of plate convergence

Plane strain, two‐dimensional, finite element models are used to calculate deformation patterns within model mountain belt systems where subduction zone retreat (the process by which space is created between the subducting (“pro”) and overriding (“retro”) lithospheres either by motion of one or both) is active. The model uses Coulomb plastic and thermally activated power law viscous rheologies. Overall deformation is determined by balancing the internal strength of the model layer against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming model. The effect of incorporating retreat in the subduction setting diffuses deformation, as the moving (retreating) subduction zone deforms a larger cross‐sectional area of material for a shorter period of time than equivalent models without subduction zone retreat. Material is deformed while within the subduction zone, but once the material has passed completely through the moving subduction zone it is passively appended onto the retrolithosphere and is no longer deformed. Thus, relative to a convergent setting without subduction zone retreat, the retreating convergent zone has an areally extensive zone of simple deformation. A deformation factor, γ, is derived which characterizes the deformation (shortening) the material experienced within the subduction zone. Variable γ is the ratio of convergence rate (rate at which material enters the subduction zone) to subduction rate (rate at which material is subducted). Variables γ and f, the fraction of the layer that is subducted, together determine the dominant style of deformation in the orogenic zone. Subduction zone retreat is shown to have significant and predictable effects on the model morphology (e.g., basin or mountain formation) and on the model geology (e.g., its presence suggests lower topography, lower metamorphic grade rocks at the surface, and simpler deformational structures), which agree with observations.

Forearc uplift and extension, southern Hawke's Bay, New Zealand: Mid‐Pleistocene to present

The Maraetotara Plateau‐Cape Kidnappers area in southern Hawke's Bay, New Zealand, lies in the forearc region of the obliquely convergent Hikurangi margin, where the Pacific Plate is presently subducting beneath the leading edge of the Australian Plate. Uplift accompanied by extensional faulting has been the dominant style of deformation in this area since mid‐Pleistocene time. Principal strain directions and strain ratios, determined from mesoscale fault arrays associated with major faults, document the regional extension. In the Cape Kidnappers area, fault arrays record plane strain with northwest horizontal extension and vertical shortening. In the Maraetotara Plateau area, fault arrays record triaxial strain with roughly equal magnitudes of horizontal extension in two principal strain directions, either east‐west and north‐south, or northwest and northeast, and vertical shortening. The morphology, seismicity, and nature and distribution of active faults of the southern Hawke's Bay region suggest that the Maraetotara Plateau‐Cape Kidnappers area is the on‐land expression of an outer‐arc high, which is developed between the accretionary wedge on the east and the forearc basin on the west. The cause of uplift and extension in the Maraetotara Plateau‐Cape Kidnappers area may be either upper plate deformation related to structural evolution of the outer‐arc high and underlying backstop, underplating on the Hikurangi subduction zone, or a combination of these processes. The uplift and horizontal extension occurring in the Maraetotara Plateau‐Cape Kidnappers area contrast sharply with active deformation in adjacent parts of the forearc, reflecting a distinct partitioning of strain in this part of the obliquely convergent plate boundary.

Active tectonics of the Adriatic Region

Seismicity and fault-plane solutions show that the active deformation in the Adriatic region is very varied. West of Messina, N–S shortening occurs with slip vectors representative of the overall Africa–Eurasia motion. Along the length of peninsular Italy, NE–SW extension on normal faults is the dominant style of deformation, but changes to N–S shortening in N. Italy. Inland central and northern Yugoslavia is deforming on strike-slip and thrust faults, and an intense belt of NE–SW shortening continues south along the coast from central Yugoslavia into Albania. South of Albania the shortening in coastal regions is in a more easterly direction. The most remarkable feature of the region is the low level of seismicity in the Adriatic Sea itself, compared with the intense activity in the high topographic belts that border it on the SW, NW and NE. The relatively rigid behaviour of the Adriatic allows its motion relative to Eurasia to be described by rotation about a pole in N. Italy. Anticlockwise rotation about this pole accounts, in a general way, for the change in style and orientation of the deformation in the circum-Adriatic belts. Historical and recent seismicity account for approximately equal rates of extension in central Italy and shortening in southern Yugoslavia of about 2 mm yr−1; however, these are uncertain by at least a factor of two, and are anyway likely to be underestimates of the true motion, because of the unknown contribution of aseismic creep. The Adriatic region resembles, in some ways, other relatively stable continental blocks, such as Central Iran and the Tarim Basin, that are caught up within the distributed deformation of the Alpine–Himalayan Belt. The Adriatic, however, is bounded on three sides by the relatively stable Eurasia plate. Its boundary with the African plate is short and ill-defined by seismicity, but is likely to be located in the Southern Adriatic, near the Strait of Otranto. The present day seismicity shows that the Adriatic, although once perhaps ‘a promontory of Africa', is no longer behaving in this way, and the motions on its boundaries do not directly reflect the Africa–Eurasia convergence.

Regional Tectonic Implications of Seismic Reflection, Gravity, Structural, and Stratigraphic Data from New Appalachian Ultradeep Core Hole (ADCOH) Site Study: ABSTRACT

The Appalachian Ultradeep Core Hole (ADCOH) site study area is located in the eastern Blue Ridge Mountains and western Piedmont of the Carolinas and northeastern Georgia. The goal of the proposed project is an ultradeep core hole that would penetrate and prove the existence of the main detachment thrust of the Blue Ridge-Piedmont (BRP) thrust sheet. Intermediate objectives include study of higher level faults and rock units within the thrust sheet, as well as investigation of the Paleozoic platform cover and basement beneath the BRP thrust sheet. Site study data have revealed that the level of the main BRP detachment may have shallowed rapidly west of the Brevard fault zone, possibly bringing lower Paleozoic carbonate platform rocks to within 3-4 km of the surface near Hayesville, North Carolina, in a large duplex structure. If so, there are implications here and farther west for possible hydrocarbon accumulations. Duplex structure appears to be a dominant style of deformation both within and beneath the BRP thrust sheet. A recently discovered shear zone on the Tallulah Falls dome may represent an imbricate from one of the blind duplexes.

Geometry of submarine slides in the Bowland Basin (Dinantian) and their relation to debris flows

Sedimentary slide deformation structures in the Lower Carboniferous (Dinantian) sediments of the Bowland basin show a spoon-shaped morphology with extension in the head region and compression in the toe. The particular structures developed depend on the rheological properties of the sediment involved. Slides in soft sediment have deformation distributed throughout the slide mass, with folding being the dominant deformation style. Slides within more lithified sediment have deformation concentrated along slip planes. Both types of deformation may be present within a particular slide, depending on diagenetic factors and the depth to which slide planes cut down. Disaggregation of slides to yield material for sediment gravity flows is explained by processes operating within the slide unit.