Tectonic Disruption Research Articles

Some thoughts on the stability of cratonic lithosphere: Effects of buoyancy and viscosity

Continental cratons have not experienced major tectonic disruptions over a timescale of 109 years. The thickness of cratonic lithosphere also appears to have changed little over this timescale. These observations are often attributed to the presence of chemically buoyant and/or highly viscous subcratonic roots. Simple physical scaling relationships are developed to explore the buoyancy and/or viscosity conditions required to stabilize such roots against large‐scale deformation and rapid remixing into the mantle. The scalings are tested using idealized numerical simulations with good general agreement. Applied to Earth, the scalings suggest that (1) buoyancy alone is unlikely to stabilize cratonic roots and (2) if root viscosity is to provide stability into the Archean, then roots must be 103 times as viscous as the mantle. Based on available experimental data, root dehydration cannot account for the required viscosity increase. Temperature‐dependent viscosity can stabilize roots, but it does so at the expense of stagnating the entire mantle lithosphere, i.e., at the expense of sacrificing plate tectonics. This suggests that the plastic yielding properties of rocks at low temperatures will need to be more directly accounted for in future experiments exploring root stability.

Mountains and Calderas on Io: Possible Implications for Lithosphere Structure and Magma Generation

The combination of Voyager images and newly acquired Galileo images with low illumination and resolutions ranging from 2 to 6 km/pixel now allows determination of the global distribution of mountains and volcanic centers on Io. The mountains generally do not have characteristics typical of terrestrial volcanic landforms, they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes. Relative elevations, determined by shadow measurements and stereoscopy, indicate that mountains in the newly imaged area range in elevation up to at least 7.6 km. The origin of the mountains remains uncertain. Some appear to be multitiered volcanic constructs; others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes; yet others show extensive tectonic disruption. Volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes. The low latitudes have one volcanic center per 7 × 104km2, and, on average, the centers are spaced roughly 250 km apart. The global distribution of high mountains suggests that the lithosphere over most of Io is thick. Although the thickness cannot be calculated, the previously suggested 30 km appears reasonable as a lower limit. The high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere. The slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on Io, coupled with the high rate of magma production, will likely result in larger melt fractions than is typical for source regions on Earth. The variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface. This mixture will be carried downward through the lithosphere as a consequence of the 0.5–1.5 cm/yr resurfacing rates. During descent, the more volatile components will tend to be driven off early, but complete or near-complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains.

Ore depositional process in the Navan Zn-Pb deposit, Ireland

The Navan Zn-Pb deposit in Ireland is hosted by a lower Carboniferous, carbonate-dominated sedimentary sequence. In excess of 97 percent of the ore is hosted by a Courceyan, shallow-water succession termed the Pale Beds. The ore occurs as complex strata-bound, tabular lenses often dislocated by faulting and truncated by a submarine erosional slide structure. Above this surface (unconformity) lies the economically minor, but genetically important Conglomerate Group ore hosted by a complex debris flow termed the Boulder Conglomerate. Timing constraints on the mineralization can be defined by the relationship to the erosion surface and style of mineralization, suggesting a late Chadian or early Arundian age ( approximately 345 Ma). The bulk of the ore formed below the erosion surface in the semilithified to lithified Pale Beds ore.The mineralogy of the economic ores is relatively simple, being dominated by sphalerite and galena in a 5/1 ratio. Pyrite and marcasite are present in subordinate amounts in the majority of the Pale Beds ore but dominate the Conglomerate Group ore and the Pale Beds ore immediately below the erosion surface, particularly lens 2-1. Gangue minerals are dominated by calcite, dolomite, and barite.The morphology of the mineralization reflects subsea-floor emplacement. The orebody occurs as numerous strata-bound horizons, ranging from intense, high-grade massive sulfides to lower grade sulfide layers separated by barren limestone. In all cases, there is strong evidence for tectonic disruption during ore deposition. Sulfides were dominantly deposited as open-space growth and replacement of host rock. Open-space textures include dendritic-skeletal, stalactitic, internal sediment, geopetal, and coarse-bladed forms. The general absence of coarse, euhedral, cavity lining textures or substantial collapse brecciation suggests that large cavities were not present prior to mineralization. Replacement textures range from delicate pseudomorphs of biodasts to more destructive granular styles.Sulfide sulfur isotope analyses exhibit two dominant groupings of delta 34 S from around -23 to -5 and 0 to 15 per mil, respectively, and a third minor grouping around -32 to -28 per mil. At least two sources of sulfur are invoked. Values around -23 to -15 per mil are considered to be the result of bacteriogenic reduction of lower Carboniferous seawater sulfate ( approximately 20ppm), with a characteristic fractionation around 35 to 45 per mil away from sulfate; this is the most common range of delta 34 S in the deposit. Values around 8 to 15 per mil are interpreted to be hydrothermal sulfide transported with the metal-bearing fluid. Mixing of these two components is inferred from the isotopic data and trends in the mineral paragenesis. Barite has a mean of delta 34 S = 21 + or - 2 per mil, which falls within the range generally recognized for lower Carboniferous seawater sulfate. We consider that contemporaneous seawater sulfate was the dominant source of sulfur in the barite at Navan, implying that seawater accessed the Pale Beds at the time of mineralization.

Arc–arc collision in the Izu collision zone, central Japan, deduced from the Ashigara Basin and adjacent Tanzawa Mountains

The Pleistocene Ashigara Basin and adjacent Tanzawa Mountains, Izu collision zone, central Japan, are examined to better understand the development of an arc–arc orogeny, where the Izu–Bonin – Mariana (IBM) arc collides with the Honshu Arc. Three tectonic phases were identified based on the geohistory of the Ashigara Basin and the denudation history of the Tanzawa Mountains. In phase I, the IBM arc collided with the Honshu Arc along the Kannawa Fault. The Ashigara Basin formed as a trench basin, filled mainly by thin‐bedded turbidites derived from the Tanzawa Mountains together with pyroclastics. The Ashigara Basin subsided at a rate of 1.7 mm/year, and the denudation rate of the Tanzawa Mountains was 1.1 mm/year. The onset of Ashigara Basin Formation is likely to be older than 2.2 Ma, interpreted as the onset of collision along the Kannawa Fault. Significant tectonic disruption due to the arc–arc collision took place in phase II, ranging from 1.1 to 0.7 Ma in age. The Ashigara Basin subsided abruptly (4.6 mm/year) and the accumulation rate increased to approximately 10 times that of phase I. Simultaneously, the Tanzawa Mountains were abruptly uplifted. A tremendous volume of coarse‐grained detritus was provided from the Tanzawa Mountains and deposited in the Ashigara Basin as a slope‐type fan delta. In phase III, 0.7–0.5 Ma, the entire Ashigara Basin was uplifted at a rate of 3.6 mm/year. This uplift was most likely caused by isostatic rebound resulting from stacking of IBM arc crust along the Kannawa Fault which is not active as the decollement fault by this time. The evolution of the Ashigara Basin and adjacent Tanzawa Mountains shows a series of the development of the arc–arc collision; from the subduction of the IBM arc beneath the Honshu Arc to the accretion of IBM arc crust onto Honshu. Arc–arc collision is not the collision between the hard crusts (massif) like a continent–continent collision, but crustal stacking of the subducting IBM arc beneath the Honshu Arc intercalated with very thick trench fill deposits.

Evidence for multiphase deformation in the Archean basal Warrawoona Group in the Marble Bar area, East Pilbara, Western Australia

The results of a structural analysis of the Marble Bar Greenstone Belt in the McPhee Reward area of the eastern Pilbara Block in Western Australia indicate that the area has undergone a multiphase deformation on shears which approximately parallel the trace of stratigraphy (cf. primary igneous layering) around the Talga Talga Anticline. Early extensional movements were associated with the formation of the Warrawoona Group (chiefly the Duffer Formation) and indicate a WNW-ESE extensional direction similar to the syn-Warrawoona extension found in the area of the North Shaw Batholith. Extension was followed by thrusting with transport to the east. The Talga Talga anticline is a culmination which formed during this event by thrust stacking. A major shear in the area, the Duffada Shear, was reactivated during granodiorite emplacement with transport to the NE. In their present geometry, the shears have thrust movements on the south side of the Talga Talga anticline but normal movements on the north side. After the reactivation came a NW-SE shortening which was accompanied by conjugate shear activity on north-trending sinistral and west-trending dextral shears, and constrained to ∼2950 Ma in the North Shaw area. The final movements were associated with a NS shortening which resulted in an EW trending thrust, which cuts the northern flank of the Talga Talga anticline, and NW and NE trending conjugate faults. There is no evidence of radially oriented stretching lineations or slickensides which would be expected if the granitoid bodies which make up the Mount Edgar Batholith were emplaced by a solid-state diapiric process. The net result of deformation was tectonic disruption of the stratigraphic stacking within the type ara of the Talga Talga Subgroup in the Warrawoona Group. This may offer an additional explanation for anomolously young dates of the Talga Talga Subgroup with respect to the Duffer Formation, and also indicates that the type section for the Talga Talga Subgroup is a lithotectonic sequence.

Near‐bottom magnetic survey of the Mid‐Atlantic Ridge axis, 24°–24°40′N: Implications for crustal accretion at slow spreading ridges

Near‐bottom magnetic field measurements provide increased spatial resolution over sea surface magnetic data and allow a detailed analysis of the accretionary and tectonic processes at work in a slow spreading environment. Through the use of magnetic inversion methods and forward modeling, we investigate the fine‐scale magnetic structure of young oceanic crust along four near‐bottom profiles that cross the two bathymetric segments immediately north of the Kane transform on the Mid‐Atlantic Ridge. Our results show the presence of a narrow, peaked central anomaly magnetic high (CAMH) located over the zone of most recent volcanism. We hypothesize that the pattern of high magnetization at segment ends and relatively low values at their centers is largely the result of increased iron and titanium content away from segment centers. Magnetization lows, which are not observed in the sea surface data, flank the CAMH and are associated with the axial valley walls. These lows may locate areas of intense magnetic source layer disruption. Blocks of relatively high magnetization found outside the valley walls suggest a cyclic faulting process, whereby discrete blocks of relatively unaltered crust are transported out of the rift valley by adjacent zones of faulting. These observations of fine‐scale magnetic structure show that while the emplacement of crust at slow spreading ridges is highly focused, subsequent tectonic disruption and alteration at the rift valley walls degrades the crustal magnetization signal and may be the primary reason for Atlantic magnetic anomaly variability.

Structural control on sea-floor hydrothermal activity at the TAG active mound

THE TAG active mound1–13, on the Mid-Atlantic Ridge near 26° N, is one of the largest known, actively forming volcanogenic massive sulphide deposits. Construction of such a deposit requires that the upflow of hydrothermal fluids be localized at this site over tens of thousands of years, but the cause of this localization has been controversial. Two popular hypotheses propose that it results from young volcanism immediately adjacent to the mound6, or from the intersection of a 'transfer fault', orthogonally crossing the rift-valley floor, with ridge-parallel faults8. Here we present high-resolution sonar and photographic data which provide no evidence for young eruptions nearby, or for transverse, through-going faults intersecting the TAG mound. Instead, our data suggest that the localization of hydrothermal activity at the TAG mound is strongly controlled by permeable conduits at the intersection of an actively developing ridge-parallel fissure/fault complex with a newly recognized but older, oblique fault system. These observations may aid the interpretation of similar structures in ancient sea-floor sulphide deposits, where the tectonic disruption caused by emplacement on land has obscured the original geometry.

Interbasinal correlation of the Cenomanian Stage; testing the lateral continuity of sequence boundaries

Abstract Sequence stratigraphic analysis of the Aquitaine, Münster and Paris Basins has shown that many of the major discontinuities found can be traced with good biostratigraphic confidence between the basins, and through a variety of sedimentary, tectonic and palaeobiogeographic regimes. Up to five sequence boundaries appear to be synchronous between the basins when account is taken of the biostratigraphic shortcomings. An extra sequence boundary near the Lower-Middle Cenomanian boundary in the Münster Basin may be a tectonic disruption as all evidence for this relative sea-level fall is from the northern, active margin of the basin, where it is associated with mass flow deposits. Notable features are the absence of lowstand deposits in these epieric, hemipelagic settings and the expression of sequence boundaries as hardgrounds. Multiple transgressive surfaces are common in deeper water settings, and surfaces of maximum flooding are very poorly developed. However, deep water deposits do preserve more accurate evidence of sea-level change, due to increased lithological and biostratigraphical completeness and fewer controls on sedimentation.

Direct inversion of potential fields from an uneven track with application to the Mid‐Atlantic Ridge

Current methods of potential field inversion require measurements to be reduced onto a level plane, resulting in a loss of resolution. This is especially true for draped surveys in areas of extreme topography. We examine a method that bypasses this limiting step by making use of an approximately equivalent geometry from which data are directly inverted. Corrections are applied to an initial solution to account for errors generated by the transformation of geometries. A numerical model and an example data set are used to explore this method's benefits and drawbacks in relation to the conventional approach of data reduction onto a level plane prior to inversion. We show that this direct inversion method is particularly better than the conventional approach at resolving fine‐scale features, such as the narrow zone of crustal emplacement at the Mid‐Atlantic Ridge and adjacent areas of tectonic disruption and magnetic source alteration.

Origin of the Jurassic Tethyan Ophiolites in Bosnia: A Geochemical Approach to Tectonic Setting

The ophiolites from Bosnia are products of the Jurassic spreading of the Neo-Tethys, and are fragments of a long chain of ophiolites of the same origin. Their geological characteristics are the absence of a sheeted dyke complex and the tectonic disruption of the cumulate sequence. Upper crust volcanics as well as lower crust cumulates show low metamorphic changes of the prehnite/laumontite type, except for some marginal localities of large ophiolite massifs (Visegrad, Konjuh, Krivaja, Ozren) in which the metamorphic grade reaches the amphibolite facies.The characteristic features of these ophiolites are as follows:The mantle rocks are lherzolites and harzburgites, both characterized by a negative Eu-anomaly and a few of them have a negative Ce anomaly, which is explained as alteration due to contact with sea water.The cumulate rocks are gabbroic to ultramafic. Numerous amphibolites reveal a chemical character of cumulus rocks. Early crystallization of plagioclase in the magma chamber caused positive Eu anomalies in these rocks. The upper crust rocks are basalts, diabases and gabbros. Most of them reveal REE patterns which are typical for basalts from constructive plate margins. A few of them which have a slight enrichment of LREE are explained as the partial melt products of a mantle less depleted than the one which produced the majority of basalts.Chemical discrimination diagrams show that the magmatic rocks are of the mid-oceanic ridge type, i.e. that they formed at the spreading of oceanic plates without any influence from a subducted slab. A comparison with recent spreading zones shows that ophiolites of the described type (absence of a sheeted dyke complex, areas of tectonic disruption in the upper and lower crust, local amphibolitisation and eruptions of “enriched” MORB) probably have formed in areas of the intersection of spreading and a transform fault.In most of their characteristics the ophiolites from Bosnia show a close similarity to the Ligurian ophiolites which obducted in about the same period of time and which are also products of the spreading of Neo-Tethys.

Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22°–24°N)

Off-axis rock sampling in the lat 22°–24° N region of the Mid-Atlantic Ridge shows that the emplacement of mantle-derived rocks in the sea floor has been a common process there for the past few million years. We find a good correlation between domains of positive residual gravity anomalies (inferred to have a thin crust) and the distribution of ultramafic samples. We also find that thin-crust domains have a rugged topography, thought to reflect strong tectonic disruption. We propose that these thin-crust domains are made of tectonically uplifted ultramafic rocks, with gabbroic intrusions and a thin basaltic cover. We also suggest that strong tectonic disruption may be a direct consequence of the lithological and rheological heterogeneity of these thin-crust domains.

The global resurfacing of Venus

The impact cratering record on Venus is unique among the terrestrial planets. Fully 84% of the craters are in pristine condition, and only 12% are fractured. Remarkably, only 2.5% of the craters and crater‐related features are embayed by lava, although intense volcanism and tectonism have affected the entire planet. Furthermore, the spatial and hypsometric distribution of the craters is consistent with a completely random one, including stochastic variations. Monte Carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed, or unobserved nonrandom crater distributions for resurfacing areas between 0.03% and 100% of the planet's surface. These models also are not consistent with the number and nonrandom distribution of volcanoes, and the nonrandom distribution of embayed and heavily fractured craters. The constraints imposed by the cratering record strongly indicate that Venus experienced a global resurfacing event about 300 m.y. ago followed by a dramatic reduction of volcanism and tectonism. This global resurfacing event ended abruptly (<10 m.y.). The present crater population has accumulated since then and remains largely intact. Thermal history models suggest that similar global resurfacing events probably occurred episodically in the past. The tesserae statistically have the same average surface age (crater retention age) as the rest of the planet, but they probably represent older rock units deformed by earlier episodes of global resurfacing. Although they largely survived the latest global resurfacing event, their surfaces were severely deformed by it. Monte Carlo simulations indicate that only about 4%–6% of the planet has been volcanically resurfaced since the global event, and that the lava production rate has been no more than 0.01–0.15 km3/yr during this time. This rate is significantly less than the current rate of intraplate volcanism on Earth (0.33–0.5 km3/yr). Most of Venus' recent volcanism occurs in the Beta‐Atla‐Themis region, and most of the recent tectonism is associated with the major global‐scale tectonic disruption zones that lie within and connect the equatorial highlands. The approximately 33% of the planet's surface bounded by latitudes 30°N and 30°S, longitudes 60° and 300°E contains twice as many heavily fractured craters and 1.4 times more lava‐embayed craters as the planetary average. This region includes most of the major tectonic belts in the equatorial region. Because the craters are indistinguishable from a statistically random distribution, both spatially and hypsometrically, this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m.y. However, these craters are simply fractured and/or embayed, and very few have been subjected to complete tectonic disruption, complete burial, and subsequent removal from the surface, as was the case during the global resurfacing event. We show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record. The effects of recent geologic activity are much less than those of the earlier global resurfacing event, when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity. Episodic regional resurfacing events that had global effects also occurred on Earth (e.g., the mid‐Cretaceous superplume) and probably on Mars. On Mars they may have triggered the catastrophic releases of water that formed the outflow channels.

The Sudbury Structure: Constraints on its genesis from Lithoprobe results

Results of the LITHOPROBE transect provide additional corroboration of the impact model for the origin of the Sudbury Structure. High resolution seismic reflection data indicate that the Sudbury Igneous Complex (SIC) is indeed continuous under the Sudbury Basin. There is no indication of a feeder dike system or a previously assumed “hidden mafic mass”. All these observations strongly support the interpretation of the SIC as a coherent impact melt sheet. In addition, the seismic, and to a lesser extent magnetic and gravity, results point to a highly assymmetric structure to the SIC at depth, with significant tectonic disruption in the South Range. The present non‐circularity of the SIC is caused by post‐impact ductile deformation and thrusting due to northwest directed compression. Decompressing this strain pattern leads to an original diameter of the SIC between 60 and 80 km, which, in consequence, would extend the original diameter of the final crater up to a maximum of 280 km.

The petrogenesis and tectonic setting of lavas from the Baft Ophiolitic Mélange, southwest of Kerman, Iran

A Late Cretaceous ophiolite complex in the Baft area, southwest of Kerman, Iran, is characteristic of the Central Iranian Ophiolitic Mélange Belt, which wraps around the Lut Block. Despite the extensive tectonic disruption of the Baft complex, most ophiolitic lithologies are present and many original igneous contacts are preserved. A lack of cumulate gabbros within the sequence suggests that a large and continuous magma chamber did not exist beneath the Baft spreading axis. Geochemical data confirm the presence of two distinct compositional groups in the mafic lavas: (1) tholeiitic basalt and (2) transitional tholeiitic basalt. The tholeiitic lavas are similar to typical mid-ocean-ridge basalt compositions, whereas the transitional tholeiites are similar to intraplate basalts. The available data suggest that the Baft ophiolite complex formed in a small ocean basin, possibly at or near a ridge–transform intersection. Emplacement may have occurred as a result of conversion of the transform fault to a subduction zone during a change in relative plate motion. A ridge–transform setting is compatible with the intraplate character of some of the transitional basalts, which probably represent off-axis (seamount) magmatism, marked by the absence of cumulate gabbros and the presence of a serpentinite mélange cut by basaltic dykes. The ridge–transform model suggests formation of the ophiolite in a narrow ocean basin separating the Sanandaj-Sirjan microcontinent from the Central Iran Block in Late Cretaceous time.

Age of the Corodale Gneisses, South Uist

Synopsis Whole-rock Sm-Nd and Pb-Pb ages of 2770 ± 140 Ma and 2900 ± 100 Ma respectively are presented from a suite of samples from the Corodale Gneiss unit of eastern South Uist, Outer Hebrides. The initial ε Nd value of c. + 2.4 and model μ 1 value of 7.9 are typical of late-Archaean mantle source regions in the Lewisian and are similar to values obtained from other Lewisian mafic-ultramafic complexes. These data confirm that the igneous protoliths to the Corodale Gneisses belong to the earliest crust-forming episode in the Lewisian, and are not correlated with younger mafic intrusive rocks in South Harris. Although the Corodale Gneiss unit has undergone extensive tectonic disruption during formation of the Outer Isles Thrust, there has been little post-Archaean disturbance of large-ion lithophile elements: U and Th have remained immobile in most samples, and there has been limited redistribution of Rb within the Corodale Gneiss unit itself.

On the origins and tectonic significance of the charnockite suite of the Archaean Limpopo Belt, Northern Marginal Zone, Zimbabwe

The Northern Marginal Zone of the late-Archaean Limpopo Belt consists predominantly of massive to gneissic charnockites and enderbites. The general textural characteristics and the field relations of the charnockites suggests that they represent plutonic bodies that have undergone relatively little internal tectonic disruption. Mineral textures are more consistent with an origin of the charnockites as direct crystallisation products of a felsic magma under granulite-facies conditions than as metamorphosed granites. Available petrological and geochronological evidence suggests that uplift of the charnockites took place relatively rapidly after cessation of granulite-facies metamorphism. Uplift involved movement along a diffuse steeply-dipping shear zone that forms the northern boundary of the Limpopo Belt against the Zimbabwe Craton. The steep dip of this structure puts a limit to the extent of possible underthrusting by the Craton as a ‘block’ beneath the Northern Marginal Zone and implies that the charnockites are not lower-crustal rocks which have been uplifted along a post- or late-metamorphic thrust, but formed within thickened crust, and movement along the boundary structure is reflecting uplift of this abnormally thick Northern Marginal Zone crust. The constraints placed on thermal models from the inferred crustal thickness and from the P-T conditions of charnockite formation imply development of the Northern Marginal Zone in a tectonic regime with low mantle heat flux. This is inconsistent with a model of thickening of the crust through addition of mantle-derived magmas, and suggests thickening involved tectonic processes above a zone of down-welling of mantle convection.

Melange and related structures in Torlesse accretionary wedge, Wairarapa, New Zealand

Abstract In southeastern North Island, New Zealand, the middle Cretaceous Mangapokia Formation represents the youngest part of the Torlesse accretionary wedge. This accreted submarine fan turbidite sequence is weakly metamorphosed and complexly deformed. Structures including local development of melange fabric, several episodes of shear fracturing, and two fold events, were associated with Cretaceous accretion. An early, extensional fabric is overprinted by F1 folds that were later dissected by tensional fractures and veins. These early structures were observed in melange inclusions and all predate completion of regional stratal disruption and melange formation. Structures associated with regional disruption include attenuated beds, boundins, “pinch and swell” structure, tectonically elongated conglomerate clasts, lozenge (transposition) fabric, and tectonic melange. Varying states of disruption represent arbitrary stages in a progressive structural continuum, with the most deformed sequences acquiring their structural fabric via a combination of repeated movement on discontinuous and often anastomosing bedding‐parallel shears, thrust and normal faults at low angles (<45°) to bedding, and reverse and normal faults at high angles (>45°) to bedding. Tectonic disruption probably occurred through a range of consolidated to semi‐indurated physical states by a combination of intergranular flow and cataclasis. Melange units containing exotic ocean‐floor (basalt, chert, red and green argillite) lithologies occur as sheet‐like bodies 2–25 m thick. They are inferred to represent frontal imbrication thrust faults, separating packets of strata ranging from tens to hundreds of metres thick. Thus, the melanges are tectonic, rather than sheared, chaotic sedimentary deposits. Isoclinal to tight, predominantly asymmetric, mesoscopic, second generation (F2) folds developed, after the initial pervasive disruption of strata, during continuing accretion. Two late‐stage, post‐Torlesse fold events (F3 and F4) are probably responses to Miocene‐Recent subduction at the Hikurangi Margin. Mesoscopic faults at high angles to bedding overprint all fold events. The structural history of the rocks is similar to older Torlesse sequences elsewhere in New Zealand. This reflects the diachronous development of structures as the accretionary wedge evolved.

Upper Cretaceous tectonic disruptions in a placid Chalk sequence in the Anglo-Paris Basin

Syn-Upper Cretaceous tectonic movements are recognized along tectonic axes including the Portsdown-South Downs axis, Sandown pericline, Kingston anticline, the Fécamp-St Valery fault blocks and at the Bresle Valley axis. A spatial relationship is described between surface structures, chalk sedimentation and deeper level fault systems identified on seismic sections. The Ouse valley between Lewes and Newhaven, Sussex, provides the strongest evidence for this relationship in the Chalk of the Anglo-Paris Basin.. The sedimentary features that can be related to tectonic lines are local thickness changes, anomalous sequences including scour surfaces such as Strahan's Hardground, Rowe's Flintless Belt (Isle of Wight), the Downend slump sequence (Portsdown). These discordant surfaces and slumps relate to particular tectonic 'hinge lines' whilst other sedimentary features such as the Tilleul Hardgrounds appear to have formed flat-ramps over blocks. These two types of tectono-sedimentary feature are used to delineate horst-like structures from tilted surfaces or half grabens which underlie the surface periclinal fold-belts. Rejuvenation movements along tectonic axes during the Cenomanian to Campanian are discussed in relation to the currently recognized sequence stratigraphy and sea-level curves for the period.

Aeolian processes on Venus

In this pre-Magellan review of aeolian processes on Venus we show that the average rate of resurfacing is less than 2 to 4 km/Ga, based on the impact crater size frequency distribution derived from Venera observations, reasonable values of the impact flux, and the assumption of steady state conditions between crater production and obliteration. Viscous relaxation of crater topography, burial by volcanic deposits, tectonic disruption, chemical and mechanical weathering and erosion, and accumulation of windblown sediments probably all contribute to resurfacing. Based on the rate of disappearance of radar-bright haloes around impact craters, the rate of removal of blocky surfaces has been estimated to be about 10−2 km/Ga. Pioneer-Venus altimetry data show that the average relative permittivity (at 17 cm radar wavelength) of the surface is too high for exposure of soils ⩾ 10 cm deep, except for ~5% of the planet located primarily in tessarae terrains. The tectonically disrupted tessarae terrains may be sites of soil generation caused by tectonic disruption of bedrock and the presence of relatively steep slopes, or they may be terrains that serve as traps for windblown material. The overall impression is that Venus is a geologically active planet, but one dominated by volcanism and tectonism. On the other hand, theoretical considerations and experimental data on weathering and transport of surface materials suggest rather different conditions. Thermochemical arguments have been advanced that show: (1) CO2 and SO2 incorporate into weathering products at high elevation, (2) transport of weathered material by the wind to lower-elevation plains, and (3) re-equilibration of weathered material, releasing both CO2 and SO2. In addition, kinetic data suggest a rate of anhydrite formation of 1 km/Ga, a value comparable to the soil erosion rate on Mars, a planet with an active aeolian environment. Experiments and theoretical studies of aeolian processes show that measured surface winds are capable of moving sand and silt on Venus. Assuming that there is a ready sand supply, the flux could be as high as 2.5 × 10−5 g/cm/s, a value comparable to desert terrains on Earth. In an active aeolian abrasion environment, sand grains could have lifetimes <103 years. In addition, comminuted debris may be cold-welded to surfaces at the same time as abrasion is occurring. Magellan altimetry and SAR observations should allow assessment of which model for venusian surface modification (active vs. inactive surficial processes) is correct, given the global coverage, high spatial resolution, the calibrated nature of the data, and the potential during extended missions of acquiring multiple SAR views of the surface.

Paléomagnétisme dans les Hellénides internes: analyse des aimantations superposées par la méthode des cercles de réaimantation

There are very few reliable and well-dated paleomagnetic data on the internal Hellenides, which were strongly affected by successive tectonic events. We have analysed 268 samples from two separate sedimentary formations of this domain. These correspond to two important moments in the geotectonic evolution of the Hellenides: the Toarcian and the Jurassic– Cretaceous boundary. We show that all the analysed samples probably contain both detrital magnetite and secondary haematite and generally carry at least two components of superimposed magnetization. The diversity and convergence of the remagnetization circles allow us to show that these two components are antiparallel and to determine their directions. Because of tectonic disruption, the calculated declinations are only of local interest. On the other hand, inclinations remain significant for calculating paleolatitudes. These constitute important constraints on reconstructions of the Jurassic paleogeography of the Hellenic domain. In light of our results, two paleogeographic reconstructions are proposed for the Early–Middle Jurassic and Jurassic–Cretaceous boundaries.