Western Termination Research Articles

Decoupling and its relation to strain partitioning in continental lithosphere: insight from the Periadriatic fault system (European Alps)

Abstract The Periadriatic fault system (PFS) is an array of late orogenic faults (35-15 Ma) in the retro-wedge of the Alpine orogen that accommodated dextral transpression during oblique indentation by the southern Alpine crust. Decoupling along the leading edges of the southern Alpine indenter occurred where inherited lithological and rheological contrasts were accentuated by lateral thermal gradients during emplacement of the warm orogenic retro-wedge next to the cold indenter. In contrast, decoupling within the core and retro-wedge of the orogen occurred in a network of folds and mylonitic faults. In the Eastern Alps, this network comprises conjugate sets of upright, constrictional folds, strike-slip faults and low-angle normal faults that accommodated nearly coaxial NNE-SSW shortening and E-W extensional exhumation of the Tauern thermal dome. The dextral shear component of oblique convergence was taken up by a discrete, brittle fault parallel to the indenter surface. In the Central and Western Alps, a steep mylonitic backthrust, upright folds, and low-angle normal faults effected transpressional exhumation of the Lepontine thermal dome. Mylonitic thrusting and dextral strike-slip shearing along the steep indenter surface are transitional along strike to low-angle normal faults that accommodated extension at the western termination of the PFS. The areal distribution of poles to mylonitic foliation and stretching lineation of these networked structures is related to the local shape and orientation of the southern Alpine indenter surface, supporting the interpretation of this surface as the macroscopic shearing plane for all mylonitic segments of the PFS. We propose that mylonitic faults nucleate as viscous instabilities induced by cooling, or more often, by folding and progressive rotation of pre-existing foliations into orientations that are optimal for simple shearing parallel to the eigenvectors of flow. The mechanical anisotropy of the viscous continental crust makes it a preferred site of decoupling and weakening. Networking of folds and mylonitic fault zones allow the viscous crust to maintain strain compatibility between the stronger brittle crust and upper mantle, while transmitting plate forces through the lithosphere. Decoupling within the continental lithosphere is therefore governed by the symmetry and kinematics of strain partitioning at, and below, the brittle-to-viscous transition.

Columbia Gorge Gap Winds: Their Climatological Influence and Synoptic Evolution

Abstract This paper quantifies the impact of the Columbia Gorge on the weather and climate within and downstream of this mesoscale gap and examines the influence of synoptic-scale flow on gorge weather. Easterly winds occur more frequently and are stronger at stations such as Portland International Airport (KPDX) that are close to the western terminus of the gorge than at other lowland stations west of the Cascades. In the cool season, there is a strong correlation between east winds at KPDX and cooler temperatures in the Columbia Basin, within the gorge, and over the northern Willamette River valley. At least 56% of the annual snowfall, 70% of days with snowfall, and 90% of days with freezing rain at KPDX coincide with easterly gorge flow. Synoptic composites were created to identify the large-scale patterns leading to strong winds, snowfall, and freezing rain in the gorge. These composites showed that all gorge gap flow events are associated with a high-amplitude 500-mb ridge upstream of the Pacific Northwest, colder than normal 850-mb temperatures over the study region, and a substantial offshore sea level pressure gradient force between the interior and the northwest coast. However, the synoptic evolution varies for different kinds of gorge weather events. For example, the composite of the 500-mb field for freezing rain events features a split developing in the upstream ridge with zonal flow at midlatitudes, while for easterly gap winds accompanied by snowfall, there is an amplification of the ridge.

A preliminary study of crustal structure in Taiwan region using receiver function analysis

SUMMARY Selected teleseismic data observed at temporary and permanent broad-band stations have been analysed using the receiver function method in order to investigate the very complex crustal structure in Taiwan region. Very significant azimuthal variations of radial and transverse receiver function responses from broad-band stations could be attributed to, among other things, the sampling of incoming seismic waves across the nearby subduction zone, a subsurface dipping interface, or a localized anisotropic region. A mid-crust discontinuity, interpreted as the Conrad discontinuity, can be identified at 18‐20 km depth beneath TATO and TPUB stations in the Western Foothills, but is absent beneath the two nearby stations SSLB and TDCB in the Central Mountain Range. The separation of upper and lower crust beneath the Western Foothills and the steady increase in crustal velocity as a function of depth across the entire thicker crust beneath the Central Mountain Range suggest that the tectonic evolution of the crust may be significantly different for these two adjacent regions. Although a ‘thin-skinned’ model may be associated with the tectonic evolution of the upper crust of the Western Foothills and Western Coastal Plain, a ‘thick-skinned’ or ‘lithospheric deformation’ model can probably be applied to explain the crustal evolution of the Central Mountain Range. A trend of crustal thinning from east (50‐52 km) to west (28‐32 km) is in very good agreement with the results from two east‐west-trending deep seismic profiles obtained using airgun sources. The thinner crust (20‐30 km) beneath TWB1 station in northeastern Taiwan can be associated with the high-heat-flow backarc opening at the western terminus of the Okinawa trough behind the subduction of the Philippine Sea plate. The relatively simple crustal structure beneath KMNB station, offshore southeastern China, depicts typical continental crust, with the Moho depth at 28‐32 km. An apparent offset of the thickest Moho beneath NACB station from the topographic high in the central Central Mountain Range suggests that the Taiwan orogeny has probably not reached its isostatic status.

Deep structure of the West African continental margin (Congo, Zaïre, Angola), between 5°S and 8°S, from reflection/refraction seismics and gravity data

The deep structure of the West African continental margin between 5°S and 8°S was investigated using vertical reflection and wide-angle reflection/refraction techniques, during the ZaïAngo project, a joint programme conducted in 2000 April by Ifremer and TotalFinaElf. To penetrate below the salt layer, a non-conventional, low-frequency seismic source was used in the ‘single-bubble’ mode, together with ocean bottom instruments (hydrophones and seismometers) and a 4.5 km long streamer that recorded multichannel seismic reflection (MCS). The data show that the continental crust thins abruptly over a lateral distance of less than 50 km, from 30 km thick below the continental platform (based on gravity data), to less than 4 km thick below the Lower Congo Basin that formed prior to the Aptian salt deposition. This subsalt sedimentary basin (180 km wide, 4 km thick, with velocities varying from 4.7 km s-1 to 5.8 km s-1 at the bottom) is located between the foot of the continental slope and the oceanic domain. It is underlain by crust of an intermediary or transitional type, between continental crust and what can be recognized as oceanic crust. In the transitional zone, a crustal upper layer is present below the pre-salt sedimentary basin, 3 to 7 km thick, with velocities increasing from 5.8 km s-1 at the top to 6.8 km s-1 at the bottom of the layer. This layer appears to thin regularly, from 6-7 km thick below the depocentre of the pre-salt sedimentary basin to 3-4 km thick below the western termination of the basin. Below this upper crustal layer, an anomalous velocity layer (7.2 to 7.8 km s-1), is documented, below the eastern side of the basin, where the crustal thinning is at a maximum. The origin of this layer is unknown. Several arguments, like rifting duration (between 15 Ma and 30 Ma) or the absence of seaward-dipping reflectors, precludes the hypothesis of underplated mantle material, but other hypotheses (such as serpentinized material or high-grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks) are plausible. Near the ocean termination of the basin, the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles (‘7+11’ and 14) having a dense ocean bottom seismometer/hydrophone (OBS/OBH) spacing. On these profiles, an anomalous velocity layer is present in the westernmost part of the transitional zone (below the basement ridge) and in the oceanic domain. This layer, absent on profile 3, may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process.

The source process of the 2001 July 26 Skyros Island (Greece) earthquake

SUMMARY The spatial and temporal distribution of slip during the 2001 July 26 Skyros (Greece) earthquake Moment magnitude (M 6.5) is investigated using broadband data recorded at regional distances. The applied method involves estimation of the source time functions of the examined event through an empirical Green's function approach and inversion of their shapes to estimate kinematic source parameters. Our test inversions to statistically identify the fault plane, together with the distribution of aftershocks clearly indicate sinistral strike-slip faulting. In view of the fact that the Skyros epicentre lies near the western termination of the dextral strike-slip North Anatolian Fault (NAF) into the Aegean Sea, this sinistral strike-slip motion, for the first time instrumentally identified, has great tectonic significance. The best values searched through the inversion are 0.7 s for the rise time, and 2.4 km s−1 for the rupture velocity. Most of the slip appears to be concentrated in a relatively small area around the hypocentre, while a smaller slip patch was found at relatively large depth (18–24 km). At least two of the large aftershocks following the main event also occurred at the deeper part of the fault. Smaller amounts of slip are distributed in a wider area with dimensions similar to those inferred from the aftershock distribution studies and the empirical relations applicable to Greece.

Fossil hot spot‐ridge interaction in the Musicians Seamount Province: Geophysical investigations of hot spot volcanism at volcanic elongated ridges

The Musicians Seamount Province is a group of volcanic elongated ridges (VERs) and single seamounts located north of the Hawaiian Chain. A 327° trending seamount chain defines the western part of the province and has been interpreted as the expression of a Cretaceous hot spot beneath the northward moving Pacific Plate. To the east, elongated E‐W striking ridges dominate the morphology. In 1999, wide‐angle seismic data were collected across two 400 km long VERs. We present tomographic images of the volcanic edifices, which indicate that crustal thickening occurs in oceanic layer 2 rather than in layer 3. This extrusive style of volcanism appears to strongly contrast with the formation processes of aseismic ridges, where crustal thickening is mostly accommodated by intrusive underplating. High‐resolution bathymetry was also collected, which yields a detailed image of the morphology of the VERs. From the occurrence of flat‐top guyots and from the unique geomorphologic setting, two independent age constraints for the Pacific crust during the Cretaceous “quiet” zone are obtained, allowing a tectonic reconstruction for the formation of the Musicians VERs. Hot spot‐ridge interaction leads to asthenosphere channeling from the plume to the nearby spreading center over a maximum distance of 400 km. The Musicians VERs were formed by mainly extrusive volcanism on top of this melt‐generating channel. The proposed formation model may be applicable to a number of observed volcanic ridges in the Pacific, including the Tuamotu Isles, the eastern portion of the Foundation chain, and the western termination of the Salas y Gomez seamount chain.

SH-Wave Seismic Reflection Images of Anomalous Foundation Conditions at the Mississinewa Dam, Indiana

Geologic features (i.e., solution cavities, active∕inactive faults, fractures, and filled∕open joints) often go undetected by traditional geotechnical exploration methods, but can have long-term adverse affects on the foundation conditions of large engineered structures. These geologic features can be relatively small and nevertheless have significant consequences for foundation integrity when high hydraulic heads are present. A noninvasive, horizontally polarized shear-wave, seismic reflection survey was performed at a large flood-control facility in northern Indiana, U.S.A., in order to identify a possible geologic cause for irregular settlement along the crest of a compacted earth-filled dam. The objectives included acquisition of a high-resolution bedrock surface image beneath the structure for remedial design considerations, as well as a search for signal anomalies that may be indicative of unfavorable geologic foundation conditions. The resultant SH-wave reflection profile collected coincident with die dam’s centerline successfully imaged a coherent, relatively continuous signal consistent with known points of top-of-rock elevation. Depth to bedrock along the profile varied from [Formula: see text] near the right abutment, to [Formula: see text] near the outlet works at the survey’s western terminus. A distinct, isolated amplitude anomaly in the bedrock reflector directly below the crest settlement area was imaged and interpreted as an enlarged (open) joint or collapsed karst feature. Subsequent geotechnical drilling of the anomaly found a [Formula: see text] discrepancy in the expected top-of-rock elevation, as well as soft foundation fill, thus confirming the geophysical interpretation.

Arc parallel extension and localization of volcanic complexes in Guadeloupe, Lesser Antilles

Subduction of Atlantic seafloor under the Caribbean plate causes shallow earthquakes within the Lesser Antilles volcanic arc. Such earthquakes, above the subduction interface, show strike‐slip or normal fault plane solutions, the latter with ∼E‐W striking nodal planes. To better assess seismic hazard and the coupling between volcanism and tectonics, we investigated faulting related to overriding‐plate deformation in the Guadeloupe archipelago. Using aerial photographs, satellite SPOT images, and topographic maps (1/25000 scale), we mapped active and middle to late Pleistocene fissures and normal fault systems that cut the uplifted coral platforms Grande‐Terre and Marie‐Galante and the volcanic rocks of Basse‐Terre. The available marine geophysical data show that the faults extend offshore to bound submarine rifts. The E‐W striking, 1500 m deep, V‐shaped Marie‐Galante rift separates the two islands of Marie‐Galante and Grande‐Terre. Normal faults in the north of Grande‐Terre appear to mark the similarly V‐shaped, western termination of the 5000 m deep, N°50E to N130°E striking Desirade graben. Three shallow, M ∼ 5.5 earthquakes (6 May 1851, 29 April 1897, 3 August 1992) appear to have ruptured segments of the Marie‐Galante rift boundary faults. The young “La Grande Découverte” volcanic complex of Basse‐Terre, including the 1440 A.D. Soufrière dome, lies within the western termination of the Marie‐Galante rift. The ancient volcanic shoulders of the rift buttress the active dome to the north and south, which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the Caribbean Sea, or southeastward into the Atlantic Ocean. The Marie‐Galante rift is typical of other troughs transverse to the northeastern edge of the Caribbean plate. We interpret such troughs, which are roughly orthogonal to the arc, to result from slip‐partitioning and extension perpendicular to plate convergence. That they disappear southward implies that they result from interaction between the Caribbean and North American plates.

Role of lateral thickness variations on the development of oblique structures at the Western end of the South Pyrenean Central Unit

The Montsec unit is one of the most important detached South-verging nappes within the South Pyrenean Central Unit (SPCU, Southern Pyrenees). A N–S cross-section of its Western sector, based on seismic reflection profiles, shows a hangingwall ramp geometry in Mesozoic strata, overlain by a syntectonic series of Lower Eocene sediments with growth geometry. The geometry of growth strata constrains the age of its movement between the Paleocene and the Middle Eocene. The geometry of the Western, oblique ramp of the South Pyrenean Central Unit is defined by a series of N–S folds, in some cases associated with underlying West-verging thrusts, as indicated by seismic reflection profiles and field data. In this paper, we propose that the geometry of the thrust wedge of Mesozoic units, progressively thinning from East to West, strongly contributed to constrain the location and geometry of the Western termination of the Montsec thrust. The hypothesis proposed is checked by a series of experimental wedges developed in a sandpack with lateral and three-dimensional thickness variations. Oblique structures form as thrusting progresses at the tip of the sand wedge.

Joint Inversion of InSAR, GPS, Teleseismic, and Strong-Motion Data for the Spatial and Temporal Distribution of Earthquake Slip: Application to the 1999 Izmit Mainshock

The space-time distribution of slip of the 17 August 1999 Izmit earthquake is investigated by inverting synthetic aperture radar (SAR) interferometry and Global Positioning System (GPS) data, together with teleseismic broadband and near-field strong-motion records. Surface offsets are used as an added constraint. Special emphasis is given to analysis of the resolution of the different data sets. We use a four-segment finite fault model and a nonlinear inversion scheme, allowing slip to vary in amplitude, direction, and duration, as well as variable rupture velocity. From the inversion of synthetic data, we find that the best spatial resolution can be expected in the upper half of the fault model (above 12 km), where coverage of the interferometric SAR data is good (western half of the rupture), and near the GPS and strong-motion stations. Teleseismic data are found to have a lower resolution that is more evenly distributed over the fault model. The joint inversion of all the data sets has an increased resolving power compared with the separate inversions and gives a more robust description of the space and time distribution of slip. Our study shows the importance of resolution tests in evaluating the reliability of earthquake kinematic models, and it confirms that an excellent fit of a single kind of data does not necessarily imply a good retrieval of the kinematic properties of an earthquake. The Izmit rupture, which is almost pure right-lateral strike-slip faulting, is dominated by the bilateral breaking of a central asperity located between 29.7° E (about 10 km west of the city of Golcuk) and 30.4° E (eastern margin of Sapanka Lake), with slip reaching 6-8 m in the depth range 6-12 km. The western termination of the rupture is found near the city of Yalova, but large slip ends around 29.7° E (about 10 km east of Hersek Delta). A second area of large slip is required by all the data sets further east toward the city of Duzce, between 30.7° E and 31.1° E (Karadere and Duzce faults). This eastern slip zone, which is separated from the main central asperity by an area of greatly reduced slip, is less well constrained by the data. However, a strong-motion station near the city of Duzce helps to locate a high-slip patch near 31.1° E in the depth range 6-12 km. The total seismic moment resulting from the joint inversion is 2.4 × 1027 dyne cm. Most of the energy release occurred in a short time, less than 15 sec, corresponding to the bilateral breaking of the central asperity. Rupture propagation is relatively uniform and fast toward the west, with a rupture velocity close to 3.5 km/sec. Propagation of large slip toward the east is initially slower, but it accelerates during a short time interval about 10 sec after rupture nucleation. Eastward progression then slows down to less than 2 km/sec after 15 sec, and rupture almost vanishes in amplitude ca. 20 sec after initiation. Rupture propagation then proceeds on the easternmost Karadere and Duzce fault segments, east of 30.7° E, from 22 to ca. 50 sec. Supershear rupture propagation is not required for modeling the waveforms considered in this study. The hypocenter of the Duzce earthquake, which occurred 3 months later (12 November 1999, M w 7.2), is located in the immediate vicinity of the easternmost slip patch of the Izmit earthquake. Manuscript received 28 July 2000.

Deep lenses of circumpolar water in the Argentine Basin

Three deep anticyclonic eddies of a species only reported once before [Gordon and Greengrove, 1986] were intersected by hydrographic lines of the World Ocean Circulation Experiment (WOCE) and South Atlantic Ventilation Experiment (SAVE) programs in the Argentine Basin. The vortices are centered near 3500 m depth at the interface between North Atlantic Deep Water and Bottom Water. They have ∼1500‐m‐thick cores containing Lower Circumpolar Deep Water and a dynamic influence that may span up to two thirds of the water column. As one eddy was observed just downstream of the western termination of the Falkland Escarpment, a destabilization of the deep boundary current by the sudden slope relaxation is suggested as a potential cause of eddy formation. Besides isopycnal interleaving at the eddy perimeters, strongly eroded core properties in the upper parts of the lenses, associated with low density ratios, hint at double diffusion at the top of the structures as another major decay mechanism. The presence of an eddy in the northern Argentine Basin shows the possibility for a northward drift of the vortices, in this basin at least. Deep events in recent current measurements from the Vema Channel are presented that raise the question of further equatorward motion to the Brazil Basin.

Alpine deformation at the western termination of the axial zone, Southern Pyrenees

Detailed structural and sedimentological research has been conducted in order to unravel the local tectono-sedimentary history of an area located at the western termination of the axial zone, in the Southern Pyrenees. Two cross-sections have been constructed perpendicular to the axis of the orogen, situated above the westward dipping axial zone antiform and the northern part of the south Pyrenean zone, striking perpendicular to the axis of the orogen. The sediments of this area date from Late Cretaceous to Middle Eocene age and display syntectonic characteristics. The cross-sections, together with detailed analysis of key outcrops, reveal that there have been two main phases of deformation. The first phase (D1) is related to the movement along the Lakhoura basement thrust and can be subdivided into three sub-phases, related to activity of two main thrusts, which splay of the basement thrust. The first sub-phase is related to activity along the Lakhoura thrust (D1a), the second sub-phase to the Larra thrust (D1b) and the third sub-phase to reactivation of the Lakhoura thrust (D1c). The second phase (D2) is related to the Gavarnie basement thrust, which resulted in the formation of the axial zone antiform. D1 and D2 show their distinct type of deformation. The first phase is characterised by south vergent ramp-flat thrusting and fault propagation folding, whereas the second phase is characterised by upright to overturned folding and steep reverse faulting. One main structure, the Urzainqui fault propagation fold, shows a synsedimentary relationship and its activity can be dated at Early and/or Middle Lutetian. The total amount of shortening has been estimated at ~ 16.5 km, which represents ~ 49% of the undeformed length. The Lakhoura thrust (D1a, D1c) accounts for ~ 3.5 km of the total amount of shortening. Approximately 8.0 km of shortening can be attributed to the Larra thrust (D1b), whereas the remaining 5.0 km of shortening can be related to the Gavarnie thrust (D2).

Reconstruction of the evolution of the Alpine-Himalayan orogen - an introduction

The Alpine-Himalayan belt is a young orogen suturing Gondwana-derived fragments with Eurasian continental crust. Its general structure, as a continuous belt of Mesozoic and Cenozoic deformation from Spain to Southeast Asia is described in the pioneering work of Argand (1924)(Figure 1). Further southeast, in the southwest Pacific, the orogenic belt is partly submerged but probably stretched as far as New Zealand (Figure 2). The advent of plate tectonics thirty-five years ago provided an explanation for the large-scale plate kinematics associated with Alpine-Himalayan orogeny. However, when it comes to details, plate tectonics has proved to be insufficient to reconstruct this complex system. In reality, the Alpine-Himalayan orogen has been subjected to numerous processes involving motions of smaller continental ribbons, retreating subduction systems, back-arc extension and accretion of magmatic arcs. In order to reconstruct the Alpine-Himalayan system, we need to analyse data derived from multidisciplinary sources and develop sets of spatio-temporal constraints. We can then test these constraints against a set of possible alternative reconstructions. Examples are the works of Dercourt et al. (1986; 1993; 2000) and Stampfli et al. (2001), which are extremely valuable to the reconstruction community. This volume includes ten contributions on the reconstruction of the tectonic evolution of the AlpineHimalayan orogen (see locations of study areas in Figure 3). The first paper by Martinez-Martinez & Azanon presents results of their study in the westernmost part of the Alpine-Himalayan orogen, the Gibraltar Arc. The authors provide evidence for two episodes of orthogonal extension in this region, and they discuss the significance of such events in terms of the regional kinematics and the evolution of the Alboran Sea. In the same region, Sanchez-Go mez et al. present structural and metamorphic analysis related to the emplacement of mantle peridotites within the Rif-Betic orogen. The authors argue that these peridotites were emplaced as a single slab during an Oligocene/Early Miocene suturing. Two contributions deal with the Pyrenean Mountains that formed a collisional plate boundary between Iberia and Europe until the Oligocene (Srivastava et al., 1990). Schellart presents results of a structural and sedimentological study of the western termination of the axial zone in the southern Pyrenees. These results provide the means for the construction of balanced cross sections and for crustal shortening calculations. Verge s et al. provide a comprehensive review of the tectonism in the Pyrenees since the Cretaceous. The authors analyse stratigraphical cross sections and lithospheric-scale transects, and discuss transitions between crustal shortening and extensional regimes. A plate reconstruction of western Tethys is presented by Stampfli et al., with an emphasis on orogeny in the western Alps. The latter is probably the most studied segment of the Alpine-Himalayan belt, but is still controversial as far as tectonic reconstruction is concerned.

Extension active perpendiculaire à la subduction dans l'arc des Petites Antilles (Guadeloupe, Antilles françaises)

Abstract Active normal faults cut the uplifted reef platforms of Grande-Terre and Marie-Galante and the volcanic rocks of Basse-Terre in the Lesser Antilles arc. New marine geophysical data shows that such faults extend offshore, forming two distinct sets. One set bounds graben perpendicular to the arc, attesting to ∼ north–south extension. The ‘en echelon’ faults of the other set, roughly along the volcanic arc, accommodate a component of sinistral motion. The active Soufriere volcano lies inside the western termination of the Marie-Galante graben. Historical and instrumental earthquakes with magnitude ⩾5.5 may have ruptured the Marie-Galante graben bounding faults.

Combination of Single-Grain Fission-Track Chronology and Morphological Analysis of Detrital Zircon Crystals in Provenance Studies: Sources of the Macigno Formation (Apennines, Italy)

Fission track (FT) analyses on unannealed detrital min- erals provide a powerful tool both for refining provenance models de- rived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies con- sistently identify the uplifting western Central Alps as the main source region for the Macigno Formation. Most zircon grains fall into a young age cluster ( ; 40-30 Ma), de- rived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distin- guished at 30 and 40 Ma. In the younger subgroup, the zircon mor- phology supports the presence of two main populations: (1) from ig- neous rocks (S-type euhedral zircons), which appear to be partly de- rived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission- track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and mig- matitic rocks affected by an Eocene high-temperature phase. A Late Cretaceous age cluster ( ; 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively. Finally, an ill-defined Jurassic age cluster, with a mean in Late Ju- rassic times, is related to rift-shoulder heating of the Austroalpine/ South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neo- gene erosion is suggested.

Structure sismique de la plate-forme Atlantique au large d'Agadir (Maroc sud-occidental)

Offshore seismic reflection and well data have been used to establish the structural style of the offshore of Agadir and to extend the Atlas fabric into the deep-water domain. The obtained structural subsurface map exhibits a complex architecture resulting from the coexistence of normal, inverse, wrench faults and folds cored by Triassic salt. The seismic data indicate a structural style characterised by narrow, detached folding above Triassic salt layers. The strong similarities in the style of deformation between the Agadir offshore and the Cap-Tafelney fold belt, suggest that both of them form one continuous belt, which corresponds to the western termination of the Atlas alpine system.

The western termination of the Jebilet–High Atlas system (Offshore Essaouira Basin, Morocco)

Industrial seismic profiles from the offshore Essaouira Basin reveal the previously unknown Cap Tafelney folded belt as the offshore termination of the Jebilet–High Atlas system of Morocco. This evaporite-based décollement folded belt involves a dramatically southward-thickening Mesozoic basinal sequence. The Cretaceous of this sequence was deposited in a flexural basin that responded to tectonic loading by the nascent Atlas Mountains to the south. The NE–SW striking Cap Tafelney folds are formed over a basal décollement which towards the east ramps down into the underlying Paleozoic continental basement. This, over 100 km wide, lateral ramp system is associated with a pronounced Cretaceous⧸Tertiary shelf margin flexure (SMF) which is superimposed on the Jurassic⧸Lower Cretaceous carbonate platform margin. The deformation of the Cap Tafelney folded belt began in the Mid-Cenomanian, and lasted into the Tertiary.

Growth folding and active thrusting in the Montello region, Veneto, northern Italy

The Montello is an elongated hill about 15 km long and 5 km wide located south of the Venetian Alps front and ∼100 km southwest of Gemona, site of the destructive Ms ∼6, 1976 earthquake sequence. Mio‐Pliocene strata in the core of the hill are folded. Seven Quaternary terraces across the western termination of the anticline have also been folded and uplifted. The terraces flank the abandoned Biadene valley, a former course of the Piave river which now flows eastwards along the north side of the hill. Topographic profiles along and transverse to the valley and terraces are used to measure the progressive development of the anticline. Fossil remains and archaeological sites dated with 14C suggest that the Biadene paleovalley was abandoned between 14 and 8 ka (11±3 ka). The successive terraces appear to have been emplaced at the onset of interglacials and interstadials, since about 350 ka. The best fitting terrace ages suggest vertical uplift rates of about 0.5 mm/yr before 172 ka and of about 1 mm/yr after 121 ka. The Montello thus appears to be a growing ramp anticline on top of an active, north dipping thrust that has migrated south of the mountain into the foreland. Modeling the deformation of the terraces as a result of motion on such a thrust ramp requires that it propagated both south and upwards with time but with a constant slip rate (1.8–2 mm/yr). For at least 300 kyr the lateral growth of the anticline kept pushing the course of the Piave river southwestwards, at a rate at first of 10 mm/yr, and then 20 mm/yr. Though the growth rate doubled more than 120 kyr ago, the anticline kept a constant height/length growth ratio (≃20) implying self‐similar depth/length growth of the thrust underneath. The clustering of historical earthquakes north of Treviso suggests that the thrust responsible for ongoing folding of the Montello slipped seismically three times (778, 1268, 1859 A.D.; intensity I ≥ VIII) in the last 2000 years, with events of maximum magnitude close to 6 and with average recurrence time between 500 and 1000 years. NW shortening on NE‐SW trending thrusts along the Venetian Alps front is compatible with the direction of convergence between Africa and Europe but does not suffice to absorb this convergence.

The evolution of the Main Mantle Thrust in the Western Syntaxis, Northern Pakistan

Abstract Neogene events in the Nanga Parbat-Haramosh massif have obscured much of its earlier evolution. However, structural mapping of the eastern margin reveals a ductile contact zone preserving many features of the original Main Mantle Thrust that emplaced the Ladakh island arc over the Indian margin in the late Cretaceous. The sequence of ductile deformation was controlled both by the contrasting rheologies of the Ladakh island arc and the Main Mantle Thrust footwall, and the changing thermal regime during subduction, collision and burial. Preliminary P-T estimates indicate conditions during southward thrusting on the Main Mantle Thrust of c. 650 °C and 9.5 kbar, with later deformation (post-dating garnet growth) in some units at c. 500 °C and 7.4 kbar. The concordant fabrics and lithological boundaries on either side of the contact are only disrupted by a NW-vergent, brittle thrust south of the village of Subsar (Indus gorge) which cross-cuts the steepened Main Mantle Thrust Zone. This thrust is related to the neotectonic Liachar Thrust on the western margin of the massif, and is an expression of the regional tectonics at the western termination of the Himalayan arc. This late thrusting followed formation of the syntaxial antiform in Neogene times.

Microseismicity and focal mechanisms at the western termination of the North Anatolian Fault and their implications for continental tectonics

For seven weeks, a temporary network of 68 seismological stations was operated in Central Greece, in the region of Thessaly and Evia, located at the western termination of the North Anatolian Fault system. We recorded 510 earthquakes and computed 80 focal mechanisms. Seismic activity is associated with the NE–SW dextral North Aegean Fault, or with very young E–W-striking normal faults that are located around the Gulf of Volos and the Gulf of Lamia. The important NW–SE-striking faults bounding the Pilion, or the basins of Larissa and Karditsa, are not seismically active, suggesting that it is easier to break continental crust, creating new faults perpendicular to the principal stresses, than to reactivate faults that strike obliquely to the principal stress axes