Period Of Tectonic Activity Research Articles

Multistage Evolution of Proterozoic Crust of East Antarctica by the Example of the Filla Terrane (Rauer Islands): New Geological and Isotope Data

Abstract —The paper presents new data on the Rauer Islands, one of the unique objects of the East Antarctic Shield. The interest in this area is triggered by its complex geologic structure, including both Archean and Proterozoic fragments of the Earth’s crust, and by its multiphase formation. A detailed scheme of the geologic structure of the area is proposed, new petrologic complexes are revealed, and the stages of tectonomagmatic activity at ~1400–1320 Ma and 1150 Ma are reliably dated. This serves as a factual basis for comparison the study area with other regions of East Antarctica. Based on the geological and isotope data obtained, the Meso–Neoproterozoic Filla Terrane in the area of the Rauer Islands is recognized. It is composed of metamorphic and primarily intrusive rocks, whose protoliths formed in the time interval 1400–950 Ma. Three periods of tectonothermal activity have been established in the Filla Terrane: Mid-Mesoproterozoic (1400–1320 Ma), Meso–Neoproterozoic (1150–886 Ma), and early Cambrian (536–504 Ma). The first period is the formation time of Mesoproterozoic crust, and it is time-correlated with the tectonogenesis phase in the adjacent Rayner province. The second period corresponds to the later phase of tectonothermal activity in the Rayner province. In the Filla Terrane, this period can be divided into two intervals, 1150–1100 Ma and 1010–886 Ma. The former interval is treated as intense crustal growth in the course of granitoid and mantle magmatism. The latter interval is a period of tectonothermal processes accompanied by intense deformations, high-temperature metamorphism, and intrusion of porphyritic granitoids. Apparently, the gap between the first and the second intervals is the time of deposition of the sedimentary protolith of paragneisses, which, together with the surrounding rocks, underwent high-temperature metamorphism and deformations at 950–914 Ma. The synchronous evolution of the Archean block and the Filla Terrane began at least within 1100–1000 Ma. The youngest, early Cambrian period of tectonic activity coincides with the development of local low-temperature mylonite zones and the intrusion of synkinematic pegmatite veins. Thus, the tectonothermal evolution of the Filla Terrane includes almost the same main phases of crustal growth and transformation as the Rayner province. This indicates that the Filla Terrane is a fragment of the Rayner province, which accreted to the Archean terrane at least in the late Mesoproterozoic.

Early post-rift depositional systems of the Central Atlantic: Lower and Middle Jurassic of the Essaouira-Agadir Basin, Morocco

Passive margins are traditionally regarded as tectonically quiescent, however the increasing recognition of significant post-rift tectonic uplift along their flanks offers an important control on sediment delivery. The most extensive record of the early post-rift succession of the Central Atlantic Margin (CAM) is found in the Lower and Middle Jurassic outcrops of the Essaouira-Agadir Basin (EAB). This important succession is characterised by alternating deposition of marine carbonates and paralic siliciclastics that correlate with periods of tectonic activity along the margin, rejuvenating sediment input to the basin. Field observations, well data and petrographic analysis are integrated into a coherent sedimentological model, correlated across the basin within a sequence stratigraphic framework. Comparison is drawn with equivalent dated units in the Central High Atlas, which allows a constraint on the regional versus local tectonostratigraphic evolution.In the EAB, Upper Sinemurian to Lower Pliensbachian open marine ramp carbonates record an initial transgression. They are only preserved locally in the north of the basin, below a major fluvial erosion surface that is regionally traceable across the basin and incisive into the Pliensbachian, CAMP basalts or Triassic sediments. In the Central High Atlas (CHA), the correlative fluvial erosive event has been dated as Toarcian in age. This influx of siliciclastic sediments is interpreted to have been sourced from the Meseta and/or the Anti-Atlas, supporting recent apatite-fission track thermochronology that indicates erosional exhumation at this time.During the Upper Toarcian, a regional carbonate platform, dominated by peritidal deposits, developed across the EAB in response to renewed marine transgression. Facies include oolitic and bioclastic grainstones, crystalline dolomite, stromatolites and dissolution breccias or evaporites. Overlying Middle Jurassic shallow-marine and fluvial siliciclastics encroached from south of the basin (possibly related to a potential source area in the Anti-Atlas), while to the north shallow marine carbonates dominated. These observations evidence the role of tectonic movements of the hinterland during a passive margin phase as a mechanism to trigger forced regressions, compensating the effect of eustasy.

An active deformation structure in the Po Plain (N Italy): The Romanengo anticline

Field surveys, well data and a detailed interpretation of a depth-converted seismic line, reveal that the Romanengo High, an isolated low relief that interrupts the morphological continuity of the Po Plain (N Italy) west of the Oglio River, coincides with a partially buried anticline, still active in recent times (Romanengo anticline). The Pliocene to Holocene evolution of the structure has been reconstructed by sequential restoration based on combined decompaction and unfolding methods, from the early Pliocene to Holocene. The analysis shows that from the beginning of the Pliocene onward, periods of tectonic activity have alternated with periods of quiescence. The more significant tectonic uplift rates are recorded during the late Pliocene (0.33 ± 0.06 mm/yr) and the beginning of Calabrian (0.284 ± 0.12 mm/yr), while at the end of Calabrian the structure was virtually inactive. During the early Pleistocene the reversal in the vergence of the structure indicates the onset of interaction with the Apennines deformational front in the area. From middle Pleistocene to Holocene, a marked increase in the tectonic uplift has been recorded, with an uplift rate of about 0.12 ± 0.05 mm/yr. A preliminary evaluation of the seismic hazards connected to blind thrusts affecting this active frontal sector of the Alpine chain, where it collides with the Apennines buried deformation fronts, is finally discussed.

From orogen to passive margin: constraints from fission track and (U–Th)/He analyses on Mesozoic uplift and fault reactivation in SW Norway

Abstract The post-Caledonian tectonic history and landscape evolution of southwestern Norway are poorly understood, primarily owing to the lack of onshore post-Devonian sediments. To bridge this knowledge gap, low-temperature thermochronological techniques were applied to investigate vertical movements in the upper crust. New apatite fission track and apatite and zircon (U–Th)/He analyses on samples from southwestern Norway yielded Permian to Jurassic, Triassic to Cretaceous and Carboniferous to Triassic ages, respectively. Thermal history modelling indicates relatively high cooling rates (2–3 °C Ma −1 ) throughout Permian to early Jurassic times. Since the Jurassic, samples from coastal areas have remained close to the surface and were reheated to 30–50 °C during sedimentary burial in the Cretaceous. Inland samples experienced lesser amounts of Permo-Triassic exhumation, continued to cool slowly (<1 °C Ma −1 ) throughout the Jurassic–Cretaceous and did not reach the surface until the Cenozoic. Both fission track and (U–Th)/He ages are offset across faults, highlighting the importance of fault activity throughout the Mesozoic. In combination with previously published results, the new data suggest that the geomorphological evolution of southwestern Norway is closely connected to rift- and post-rift tectonics related to North Sea and North Atlantic rifting. The topographic relief was most likely repeatedly rejuvenated during periods of tectonic activity.

Neogene and Quaternary development of the Turiec Basin and landscape in its catchment: a tentative mass balance model

Neogene and Quaternary development of the Turiec Basin and landscape in its catchment: a tentative mass balance modelThe development of the Turiec Basin and landscape evolution in its catchment has been reconstructed by methods of geological research (structural geology, sedimentology, paleoecology, and geochronological data) as well as by geophysics and geomorphology. The basin and its surrounding mountains were a subject of a mass balance study during periods of tectonic activity, accompanied by considerable altitudinal differentiation of relief and also during quiet periods, characterized by a development of planation surfaces in the mountains. The coarse clastic alluvial fans deposited beneath the offshore pelitic sediments document the rapid Middle Miocene uplift of mountains on the margin of the Turiec Basin. The Late Miocene finegrained sedimentation represents the main fill of this basin and its origin was associated with the formation of planation surfaces in the surrounding mountains. The rapid uplift of the western and northern parts of the catchment area during the latest Miocene and Early Pliocene times further generated the deposition of coarse-grained alluvial fans. The Late Pliocene basin inversion, due to uplift of the whole Western Carpathians mountain chain, was associated with the formation of the Early Quaternary pediment and ultimately with the formation of the Turiec river terrace systems.

Integration of DEM, ETM+, Geologic, and Magnetic Data for Geological Investigations in the Jifara Plain, Libya

We used an integrated approach to constrain the geological structure of the Jifara Plain in northwest Libya. The analysis of surface data, including a digital elevation model (DEM), Landsat Enhanced Thematic Mapper Plus images, and geologic maps, was combined with subsurface data, including well logs and magnetic data. The DEM data were used for the identification of geological lineaments in the study area. The interpretation and analysis of the lineaments indicate that the Jifara Plain is controlled by three main fault systems, trending northwest-southeast, east-west, and northeast-southwest. The three trends represent the remnants of reactivated structures that formed under the stress regimes generated during the tectonic evolution of North Africa. The magnetic data reveal three northwest- and northeast-trending sedimentary basins in the study area. The depth of the basement inside the main basin ranges from 1 to 5 km. The results indicate that the Jifara Basin is shallower than the surrounding basins. The integration of the results reflects different periods of tectonic activity in the Jifara Plain and the adjacent Jabal Nafusah.

Late Pleistocene to Holocene tectonic activity along the Nesher fault, Mount Carmel, Israel

ABSTRACT Zilberman, E., Greenbaum, N., Nahmias, Y., Porat, N., and Ashqar, L. 2008. Late Pleistocene to Holocene tectonic activity along the Nesher fault, Mount Carmel, Israel. Isr. J. Earth Sci. 57: 87-100. A paleoseismic analysis of the Nesher fault, a branch of the Yagur fault (the western seg­ment of the Carmel tectonic system), was conducted east of Nesher, at a site where the fault trace crosses a small, abandoned alluvial fan. A 30-m-Iong and 2-4 m-deep trench excavated in the alluvial fan across the fault trace exposed a steeply northeastward-tilted late Paleocene to Early Eocene chalky-marly sequence on the northern side of the fault and a thick (more than 4 m) alluvial-colluvial sequence on its southern side. Two periods of tectonic activity accompanied by surface deformation were identi­fied along the Nesher fault: I. Subsidence of a small basin south of the main fault manifested by colluvium that accumulated south of the fault trace and clay, unit I that accumulated in a small depression further to the south. The OSL age of unit 1 is 178

Oligocene–Miocene alluvial sedimentation in the northern Ebro Basin, NE Spain: Tectonic control and palaeogeographical evolution

The Oligocene–Early Miocene rocks cropping out in the northern Ebro Basin (central sector) comprise conglomerates at the margin of the basin gradually passing into sandstones and mudstones towards more central areas. These rocks belong to the Peraltilla and Sariñena Formations and originated in both small monogenic-conglomerate alluvial fans that fed from the South Pyrenean Sierras directly, and large polygenic-conglomerate fans which drained more internal areas of the Pyrenees. Their lithological, textural and geometrical features, in combination with sedimentary structures, reveal the existence of several lithofacies associations whose lateral and vertical relationships, and distribution throughout time have been used to establish the main features for each type of depositional system. Near the margin of the basin, the Tertiary series is folded and thrusted and several unconformities can be identified. Three tecto-sedimentary units (T3 to T5) have been characterised for the whole area. At the margin, they are bounded by angular or syntectonic unconformities. In more central areas, their correlative conformities can be registered as changes from coarsening- to fining-upward general trends or as sudden shifts in the vertical trend. Fining-upward trends are related to onlaps and record the retrogradational stages of the alluvial fans. In contrast, coarsening-upward trends record the progradation of the fans. Stratigraphic and sedimentological studies permit to establish the palaeogeographical evolution of the area during Oligocene and Early Miocene times. Also, it can be demonstrated that during periods of tectonic activity in the Pyrenees the alluvial fans prograded whereas during the stages of relative tectonic inactivity the fans retrograded.

A plume tectonics model for the Tharsis province, Mars

Morphological and structural data from the whole Tharsis province suggest that a number of shallow grabens radially oriented about the Tharsis bulge on Mars are underlain by dykes, which define giant radiating swarms similar to, e.g. the Mackenzie dyke swarm of the Canadian shield. Mechanisms for graben formation are proposed, and the depth, width, and height of the associated dykes are estimated. Structural mapping leads to define successive stages of dyke emplacement, and provide stress-trajectory maps that indicate a steady source of the regional stress during the whole history of the Tharsis province. A new tectonic model of Tharsis is presented, based on an analogy with dyke swarms on the Earth that form inside hot spots. This model successfully matches the following features: (1) the geometry of the South Tharsis Ridge Belt, which may have been a consequence of the compressional stress field at the boundary between the uplifted and non-uplifted areas in the upper part of the lithosphere at the onset of hot spot activity; (2) extensive lava flooding, interpreted as a consequence of the high thermal anomaly at the onset of plume (hot spot) activity; (3) wrinkle ridge geometry in the Tharsis hemisphere, the formation of which is interpreted as a consequence of buoyant subsidence of the brittle crust in response to the lava load; (4) Valles Marineris limited stretching by preliminary passive rifting, and uplift, viewed as a necessary consequence of adiabatic mantle decompression induced by stretching. The geometrical analysis of dyke swarms suggests the existence of a large, Tharsis-independent extensional state of stress during all the period of tectonic activity, in which the minimum compressive stress is roughly NS oriented. Although magmatism must have loaded the lithosphere significantly after the plume activity ceased and be responsible for additional surface deformations, there is no requirement for further loading stress to explain surficial features. Comparison with succession of magmatic and tectonic events related to hot spots on the Earth suggests that the total time required to produce all the surface deformation observed in the Tharsis province over the last 3.8 Ga does probably not exceed 10 or 15 Ma.

Breccias of the Adnet Formation: indicators of a Mid-Liassic tectonic event in the Northern Calcareous Alps (Salzburg/Austria)

Stratigraphy, lithology and depositional structures of Liassic red limestone-breccias of the Adnet Formation, including the ‘Adnet Scheck’, were studied at several outcrops of the Northern Calcareous Alps (NCA) south-east of Salzburg. A four-fold lithostratigraphic division is proposed for the Adnet Formation of the Osterhorn Mountains: the hemipelagic Schmiedwirt (Sinemurian) and Kehlbach (Carixian) members are separated from the pelagic Saubach Member (Toarcian) by a layer of amalgamated breccias (Scheck Member, probably Domerian to early Toarcian). Several other breccia beds occur locally from the base of the Kehlbach Member up to the lower Saubach Member. Although the sediments overlying the Scheck Member breccias are of coeval age, the ages of the underlying strata are very different. This can be explained by submarine Liassic erosion during a period of resedimentation from the middle Carixian until the early Toarcian. At least 10–15 m of partly lithified sediments were eroded by gravity flows. The entire Kehlbach Member and up to two-thirds of the Schmiedwirt Member were removed at Adnet. The breccias originated from submarine debris flows. Repeated flows over a long period and the depositional setting exclude a triggering by sea-level fluctuations. Most probably they arose from tectonically triggered slumps and slides of superficial sediments. The ‘Scheck’ was initiated on the steep upper slope of the drowned Triassic Adnet reef and flowed to the north-east. The Pliensbachian to early Toarcian period of tectonic activity indicated by the breccias was the most important during the Liassic in the Osterhorn Mountains and other parts of the NCA. From the large-scale regional distribution of the breccias and in accord with published data, a roughly northeast trending strike-slip fault zone is proposed, crossing the NCA south of the Osterhorn block, with a peaking activity during the Pliensbachian to early Toarcian as the cause of the tectonic movements.

Computer modelling of the influence of tectonics on sequence architecture of coarse-grained fan deltas

Fan delta deposition and extensional faulting are combined in a computer model using a general tectonosedimentary forward modelling equation. Fan delta forms and architectures resulting from tectonic activity are investigated using this model. Fan deltas formed during periods of tectonic quiescence are shown to have a sheet-like form and grow mainly as a result of progradation. Topset development is limited, resulting in the development of oblique to sigmoidal clinoforms. Fan deltas formed during periods of tectonic activity have a more wedge-shaped form and are restricted to a region adjacent to the controlling fault. These fan deltas grow as a result of both progradation and aggradation, resulting in the development of sigmoidal clinoforms. Under conditions of constant tectonic subsidence and sediment supply, fan deltas are shown to undergo a brief period of progradation followed by aggradation and a long period of retrogradation. This is a consequence of an increasing amount of sediment being deposited as topset material as the delta progrades and the position of the offlap break being displaced basinward during hangingwall subsidence. Other controls on fan delta architecture, such as variations in sediment supply and sea/lake level, are also considered. Pleistocene-Recent, Gilbert-type fan deltas from the southern margin of the Gulf of Corinth rift, Greece contain many of the features described above and modelling is used to investigate whether it is possible to identify the controls on their sequence architectures.

The geology of the north-eastern corner of Greenland - photogeological studies and 1993 field work

Late Archaean and Early Proterozoic crust-forming events in North-East and eastern North Greenland were succeeded by Middle Proterozoic sedimentation and volcanic activity; Late Proterozoic through Tertiary sedimentation was interrupted by several periods of tectonic activity, including the Caledonian orogeny in East Greenland and the Mesozoic deformation of the Wandel Hav mobile belt. Photogeological studies helped pinpoint areas of special interest which were investigated during the short 1993 field season. Insights gained during field work include: the nature of the crystalline basement terrain in the Caledonian fold belt, redefinition of the upper boundary of the Upper Proterozoic Rivieradal sandstones, revision of Caledonian nappe terminology, and the northern extension of the Caledonian Storstrømmen shear zone.

Postcollisional cooling of the Penokean orogen in east-central Minnesota

New 40Ar/39Ar thermochronologic data from the internal zone of the Penokean orogen in east-central Minnesota indicate rapid cooling from mid-crustal temperatures. Early Proterozoic metamorphosed cover rocks yield concordant hornblende and biotite plateau ages of ~ 1755 Ma. Underlying Archean basement gneiss yields concordant muscovite and biotite plateau ages of ~ 1705 Ma. These are some of the oldest cooling ages recorded in rocks buried during the 1830–1860 Ma Penokean collisional orogeny. Cooling was coeval with regional rhyolitic volcanism and granite emplacement, suggesting a period of tectonic activity in this region during mid to late 1700 Ma. Based on similarities with Phanerozoic extensional regimes, we suggest, preliminarily, that these data might be explained by a previously unrecognized major extensional episode in the Lake Superior region.

Geometry and Evolution of a Syntectonic Alluvial Fan, Southern Pyrenees

Syntectonic alluvial fans formed on the northern margin of the Ebro Foreland Basin along the South Pyrenean thrust front during late orogenic thrust movements in the late Oligocene/early Miocene. The present-day geometry, structural relations and sedimentology of one of these fans, the Aguero fan in the province of Huesca, Spain, were studied. Field observations of the architecture of depositional facies and the geometries of syn-tectonic folds and unconformities indicate that the Aguero fan formed as the result of several phases of sedimentation which were primarily controlled by periods of tectonic activity and quiescence. The syntectonic unconformities and growth folds in the fan deposits provide a detailed record of the evolution of a fan adjacent to an active thrust front. Using a computer program to simulate sedimentation and deformation of an alluvial fan it is possible to constrain rates of both sedimentary and tectonic processes by modeling the evolution of the fan body. A facies model for the fan phases indicates that the facies change from proximal (coarse-grained, amalgamated) to distal (finger grained, stacked fining up cycles) in less than 1 km across a fan of radius estimated to be about 2 km.

On the waterfront: Quaternary environments and the formative occupation of Southwestern Ecuador

AbstractThe occurrence of tectonic uplift in coastal South American prehistory is discussed and evidence of emerged shoreline features is presented for the area of the Santa Elena Peninsula in Ecuador. It is hypothesized that this activity, once thought to be exclusively Pleistocene in timing, continued on into the Holocene. Given this perspective and field data, the role of the Early Formative Period Valdivia phase (3500‐1500 B.C.) site of Real Alto is examined, as is the economic structure of Real Alto society. the pattern of site distribution and data from geological units indicate a long period of tectonic activity that restructured the Ecuadorian coastline and affected northern Andean prehistory. the implications for the origins of Valdivia and New World pottery are reconsidered in light of new information on the Quaternary environments of Ecuador.

Modeling periodic megaplume emission by black smoker systems

Two huge bodies of anomalously warm water, megaplumes, have been discovered above the normal black smoker plumes on the southern Juan de Fuca Ridge. Their chemistry indicates that they contain high‐temperature mature hydrothermal fluid mixed with entrained seawater. Their excess heat content (estimated at about 1017 J) is equivalent to that in about 0.1 km3 of black smoker fluid. Their buoyancy flux, estimated from the height of rise of the plume, indicates flow rates of up to about 20,000 kg s−1. The size, buoyancy flux, and hydrothermal characteristics of megaplumes strongly suggest that they are the result of a massive, catastrophic emission from a black smoker system. Such flow rates require a high recharge permeability (10−12 m2 or higher). An abrupt increase in discharge permeability is essential to allow black smoker flow rates to increase by orders of magnitude. This can be achieved in two ways. The most obvious way is a tectonic stretching event, such as those observed in Iceland, where some spreading segments undergo periodic extension with episodes of activity lasting a few years. This would result in a fundamental change in the hydraulic properties of the system. Megaplume emission would be associated with periods of tectonic activity, and it is estimated that several tectonically induced megaplumes might occur each year globally. We investigate hydrofracturing as an alternative mechanism for spontaneous megaplume discharge. This relies on the presence of a clogged cap to the discharge and a clogged cylindrical shell around the discharge pipe, both the result of subseafloor precipitation of sulfides and quartz through mixing of hydrothermal fluid with cool recharge water. With this structure, decrease in fluid density can lead to pressures sufficient to fracture the clogged cap and release a megaplume. Fluid density can be decreased either by injection of magmatic CO2, in which case megaplumes would be related to magmatic evolution or, if heat transport in a system is less than the rate of heat supply, by increase of fluid temperature to about 400°C, where there is rapid, nonlinear expansion of seawater at subseafloor pressures. We model hydrofracturing by increasing temperature and show that it can occur under geologically realistic conditions. We calculate the change in temperature, buoyancy pressure, and flow rate before and after initial fracture and show that megaplume flow rates can be generated by hydrofracturing. Periodic emission of megaplumes from a normal black smoker system is possible if the hydrofractures are clogged during the latter stages of megaplume activity. Sites of hydrofracture‐induced megaplumes should be distinguished by mounds of fractured and recemented sulfides surrounding hydrothermal vent areas.

Sand waves, Echinocardium traces and their bathyal depositional setting (Monte Torre Palaeostrait, Plio‐Pleistocene, southern Italy).

ABSTRACT Stacked cross‐sets, up to 2.5 m thick, produced by sand wave migration and meniscate trace fossils produced by Echinocardium cordatum, both considered in the literature as typical of shallow‐water marine depositional settings, commonly occur in the bathyal Plio‐Pleistocene deposits of Monte Torre (Calabria, southern Italy).The Plio‐Pleistocene sediments form two coarsening‐upward depositional sequences, separated by an unconformity and by a palaeobathymetric gap of at least 300 m. The lower sequence passes upwards from hemipelagic marls and thin‐bedded turbidites to thick‐bedded sandy turbidites, then to sand wave deposits alternated with sandy turbidites, and finally to base‐of‐slope megabreccias. Facies characteristics and relationships, and the occurrence of deep‐sea faunal associations, indicate deposition in the bathyal zone. The facies of the upper sequence reflect a fan‐delta environment, no deeper than a few tens of metres.The depositional setting of the lower sequence, where the sand wave deposits and meniscate trace fossils occur, appears to have been a tectonically controlled seaway, connecting the Tyrrhenian and Ionian Seas. This seaway became progressively narrower with time, evolving into a strait. The overall coarsening‐upward trend reflects the upward transition from a low to a high‐energy environment, possibly caused by the tectonic narrowing of the seaway. Deposition and erosion from high‐concentration turbidity currents and from tidal bottom currents were important processes. Periods of tectonic activity, producing first the uplift of the seaway margins and culminating with the uplift of the strait sequence itself, are marked by‐scattered rockfall deposits.The strait setting, causing the development of powerful, oxygenated bottom currents, produced optimal conditions in the bathyal zone for the colonization of sandy bottoms by a single infaunal r‐selected species, Echinocardium sp.

The Fenwei rift and its recent periodic activity

The Fenwei rift on the southern sector of the Jin—Shaan rift system of China is marked by a crescent-shaped valley 600 km in length and 30–90 km in width depressed up to 10 km and filled with about 7000 m of Cenozoic deposits, bounded on both northern and southern sides by majestic mountain ranges. The geometry of the rift valley is characterized by six branch depressions and five intervening swells extending east-northeastward in a dextral en-echelon pattern and bounded on both sides by abrupt topographic slopes reflecting the underlying faults. These are typically a system of growth faults having downthrows ranging from 800 m to 10 km and dipping toward the centre of the valley forming an asymmetric graben structure. The geometry, kinematics and evolution of these faults have had controlling influences on the neotectonic movement of the rift and its recent periodic activity as the present overall form of the rift valley. Estimates of the amount of extension across the rift for various recent geological periods were obtained from calculations made on the fault separation of corresponding stratigraphie horizons. The total amount of extension in response to tensile stresses, acting in a direction varying from 25° NW on the west to 70° NW on the northeast is estimated to be 9065 m, since the beginning of the rift formation in the Eocene whereas the rate of extension in the Recent is 4.5 mm/yr and in modern times it is 8–24 mm/yr. The amount of left-lateral displacement across the rift during various stages of its development was also calculated from the observed effects of strike-slip movement on the drainage system. The left-lateral offset since the mid-Pleistocene is approximately 7170 m and the offset rate in modern times is 6 mm/yr. These estimates suggest that the Fenwei rift has been a place of intense neotectonic activity. Details of more recent activity of the rift were investigated in terms of the various rift-related phenomena such as seismic events, ground fissuring, epeirogenic movement, shifting of streams or lakes and climatic changes which have occurred in the historic period. Seven highly periodic cycles of seismicity are recognized from available historic records of the earthquakes that have occurred in the rift region during the last 4000 yrs. Each cycle appears to begin with a period of intense seismicity consisting of a series of violent shocks chiefly of M6–8 and lasting approximately 200 yrs, and then passes on to a prolonged period of quiescence of about 600 yrs during which only minor seismic events occur. The 22 events of ground-fissuring recorded in the historic annals are also concentrated in several distinctive periods of intense activity which coincide with seven corresponding periods of active seismicity, suggesting that seismic events and ground fissures resulted from the same cause, both being indications of periodic tectonic activity of the rift. Moreover, periodic and coincident with periods of tectonic activity are the occurrences of the natural phenomena related to rifting such as the appearance and disappearance of lakes, the shifting of streams and changes of river water from clear to turbid.

Tectonique et quaternaire moyen en Méditerranée (Tectonics and the middle quaternary in the mediterranean)

Abstract. - For the mediterranean regions, the Middle Quaternary is a period of tectonic activity on all the landscape's scales. Large bulges and buckles involve a relief, emphasized by the deep valleys and the terraced landforms. On a local scale, tectonic faulting, accompanied or not by sismic processes and forms, oppose the evolution of occidental and oriental mediterranean basins. Then different dynamical processes fashion the relief. For the more unstable regions, the processus of the "rhexistasie" continue, unaffected by the climatological rhythm of this quaternary period.

Review of evidence for late Tertiary shorelines occurring on South Atlantic coasts

Recognition of (late) Tertiary shorelines on continental coasts is becoming increasingly common. It is argued that the elevations of such features are central to their approximate dating and the demonstration of contemporaneity with similar features elsewhere. South Atlantic coasts have not had a long history of investigation, yet there exist many diffuse observations, in both time and space, which are here drawn together into a preliminary synthesis and the case for widespread occurrences of late Tertiary shorelines in the region reviewed. This allows comparison with other areas from which Tertiary shorelines are known, the Atlantic seaboards of the Southeastern United States and Northwest Europe, for example. The coasts of oceanic islands are probably the best places to examine the legacy of late Cenozoic sea-level changes. It is suggested that, since many of the cliffed shores of these islands appear to be the result of a rapid emergence, coastal features predating this event might be preserved on the cliff-tops. Possible late Tertiary shorelines from South Atlantic islands are described, as are those which have been positively dated to this period, in the Eastern Canary Islands, for instance. Sedimentary and morphological indicators of Tertiary high sea-levels are described from Antarctic coasts. South American and African Atlantic continental margins. Evidence from the latter two areas is most suspect, owing to their generally more complex Quaternary tectonic histories. A summary of the evidence for Tertiary shorelines on South Atlantic coasts is tabulated. Methods which have been or could be used to date late Tertiary shorelines are described. Minimum age can be deduced from that of deposits resting on an erosional surface, maximum age from that of the youngest formation across which a surface is cut. More precise age can be estimated where a marine surface is sandwiched between datable non-marine formations or where periods of tectonic activity (responsible for differential surface deformation) can be dated. Elevations of surfaces can be used to estimate ages. Problems in both recognising and dating Tertiary shorelines are discussed. Recognition of their origin is made more difficult by their degraded state and, if they occur within a Quaternary erosional sequence, their antiquity may not be realised. It is suggested that fluctuations in the Antarctic ice sheet during the late Tertiary induced substantial glacio-eustatic changes of sea-level which may have been most marked in adjacent areas. This is considered to be the mechanism through which late Tertiary shorelines in the area were formed yet, considering the paucity of the morphological and sedimentary data, these should not be used as the basis for a eustatic chronology.