Tectonic Fabric Research Articles

THE CONFIGURATION OF THE BASEMENT BENEATH THE ZAGROS BASIN

It has long been recognised that the deposition and deformation of the Phanerozoic cover in the Zagros Basin (mountains plus foreland) was strongly influenced by the reactivation of old tectonic fabrics in its basement. Facies boundaries and structures trending north‐south and NW‐SE can be attributed to the reactivation of Pan‐African sutures and Najd faults which are exposed in the Nubian‐Arabian Shield. However, to the east of a projection of the Oman line SWwards into the Rhub Al Khali Basin, cover structures have a NE‐SW trend which is not seen in Arabia. This boundary may overlie a Pan‐African suture between Arabia and India (Somalia or Pakistan).Data including magnetic intensities, geothermal gradients and isopach maps are used here to distinguish old faults which were reactivated in the basement from more recent faults formed in the cover by Zagros shortening. Old faults trending NW‐SE are interpreted as having reactivated episodically since the Permo‐Triassic opening of Neo‐Tethys; perhaps more significantly, the basement faults that reactivated in the East Arabian Block since then trend north‐south. The basement configuration is clarified by extending a modified East Arabian Block across the Zagros to an “East Arabian‐Zagros block” in which the NW trend of the Zagros lies between two syntaxes. This suggests a new tectonic framework for the region. The repeated reactivation of basement faults throughout the East Arabian‐Zagros Block controlled source rocks, traps and seals for the supergiant and giant oil and gas reserves which are present at various stratigraphic levels in different areas.

Influence of along-strike pre-orogenic sedimentary tapering on the internal architecture of experimental thrust wedges

The occurrence of along-strike thickness variations in pretectonic sedimentary packages is expected to influence the structural architecture of doubly verging thrust wedges. To test this hypothesis, we used laboratory sandbox experiments. Model results show that longitudinal tapering of pretectonic sediments causes a great complexity in the internal tectonic fabric of the wedge, particularly in the pro-wedge, dominated by highly segmented, curvilinear thrusts. The along-strike variation of the mode by which the same amount of bulk contraction is accommodated in different regions of the orogen produces the obliquity of the deformation fronts in both the pro-wedge and the retro-wedge. Comparison with the overall architecture of the Lesser Antilles and Manila accretionary systems validates our experimental results.

The Gebra Slide: a submarine slide on the Trinity Peninsula Margin, Antarctica

A large submarine slide – the Gebra Slide – has been discovered on the continental margin of Trinity Peninsula, Central Bransfield Basin, Antarctic Peninsula. The slide scar is clearly expressed in the bathymetry and is cut into the toe of the glacial-period slope-prograding strata on the lower continental slope. Seismic data give evidence of an associated debris-flow deposit embedded in the interglacial-period basin-fill strata of the basin floor. The total volume of sediment involved in the mass movement is about 20 km 3. Indirect dating of the mass-wasting event, based on seismic–stratigraphic relationships of the slide scar and associated debris-flow deposit with underlying glacial-period slope units and the overlying interglacial-period basin-floor units, suggests that it took place at the transition between the last glacial period and the present-day interglacial. The initiation of the Gebra Slide is attributed to a combination of several factors, such as high sedimentation rates during the last glacial period, the unloading effect of a retreating ice sheet during deglaciation, pre-existing tectonic fabric and high seismicity in the area. This is the first recent submarine slide of this size identified on the glacial, continental margins of Antarctica. In morphology and general characteristics it is quite similar to the well-known large-scale submarine slides from the northern hemisphere glacial margins, although it is smaller. Its most striking characteristic is its lower-slope position (at 1500–2000 m of water depth), which remains up to now difficult to explain.

Sheeted intrusion of the synkinematic McDoogle pluton, Sierra Nevada, California

Field, microstructural, and geochronologic evidence indicates that the Late Cretaceous McDoogle pluton, located near the eastern margin of the Sierra Nevada batholith, was emplaced as a subvertically sheeted complex into a steep reverse-sense shear zone. Evidence of internal subvertical sheeting includes abundant concordant wall-rock inclusions and screens that separate the pluton from adjacent Jurassic plutons, preservation of a ghost tectonostratigraphy in the distribution of the inclusions, and rare interphase contacts. The solid-state tectonic fabric in the wall rocks and the magmatic, submagmatic, and weak solid-state fabrics in the McDoogle pluton all are concordant and record northeast-southwest horizontal shortening, subvertical extension, and a significant component of northeast-side-up simple shear. Intrusive contacts generally are concordant with the fabrics but, where discordances occur, the wall-rock fabric invariably is truncated by the contact. However, late synkinematic emplacement of the McDoogle pluton is indicated by synintrusive boudinage of apophyses of the pluton and by the overall concordance of pluton fabrics including magmatic lineation. Zircon U-Pb isotope dates were obtained from the quartz monzodiorite central phase of the McDoogle pluton (94 ± 4 Ma), the mafic granodiorite border phase (94.8 ± 0.6 Ma), and an older hornblende granodiorite included in the central phase (97.6 ± 0.4 Ma). Granodiorite orthogneiss in the wall rock yielded a U-Pb zircon date of 164 +2.1/–6.8 Ma, which we interpret to be the crystallization age and which is indistinguishable from a new age of 164.5 ± 2.3 Ma for the Twin Lakes pluton. A U-Pb titanite date of 94.4 ± 0.9 Ma from the 164 Ma orthogneiss sample may reflect thermal effects of the intruding McDoogle pluton, synkinematic growth of titanite in the shear zone, or perhaps both. The age of the Sawmill Lake shear zone that hosts the McDoogle pluton is bracketed between 148 Ma, the age of the Independence dike swarm, and 92 Ma, the age of the Lamarck pluton that crosscuts both the McDoogle pluton and the shear-zone fabrics. Regional evidence suggests a shear-zone age younger than 110 Ma, but older deformation is possible. The predominance of horizontal shortening over transcurrent motion and an age older than 90 Ma exclude a direct relationship to the dextral Sierra Crest shear system, which has been proposed to pass near the study area. The location, age constraints, and kinematics of the shear zone are consistent with both the later stages of movement in the East Sierran thrust system and isostatic sinking of the arc magmatic complex into its substrate.

The palaeolatitude controversy in the Silurian of Newfoundland resolved: new palaeomagnetic results from the central mobile belt

A detailed palaeomagnetic investigation of early Silurian volcanics of the Lawrenceton Formation in the Central Mobile Belt (CMB) of Newfoundland reveals primary remanence that places the sampled part of the CMB on the margin of Laurentia in low southerly latitudes in the early Silurian. The palaeomagnetic data indicate large vertical axis (net clockwise) rotations within the CMB and between the CMB and cratonic Laurentia. The volcanics were folded, faulted and in places cleaved in Silurian to Early Devonian times. Anisotropy of magnetic susceptibility measurements on lava flow samples verify field observations of a primary flow-parallel foliation and confirm field measurements of the palaeo-horizontal. A well-developed tectonic fabric was detected at one sampling site near the Reach Fault. Silurian redbeds of the Big Indian Pond Formation were also sampled for palaeomagnetic study. The sediments are folded, faulted and cleaved. A statistically sound syn-folding remanence was identified which, in a tectonically corrected state, is consistent in direction with the remanence in the Silurian volcanics. The sampled sediments are more intensely deformed than the volcanics and magnetic anisotropy measurements indicate both primary layering- and secondary cleavage-related fabrics in varying degrees from site to site. It is possible that cleavage development resulted in remanence reorientation leading to the illusion of syn-folding remanence when, in fact, the remanence was acquired prior to folding. Previously published palaeomagnetic investigations of Silurian rocks in the CMB revealed a discrepancy in palaeofield inclination between volcanic and sedimentary rock types. The sedimentary rocks yielded shallow inclinations and the volcanics intermediate inclinations. Inclination shallowing in the sediments was put forward as an explanation, as well as contamination of the sedimentary results with a shallow overprint. Others argued that the results from the volcanics were in error. The present investigation demonstrates that key volcanics carry a shallower remanence than previously thought, bringing them into approximate agreement with the majority of the results from sediments. Steep results from the Springdale volcanics and very shallow results from the King George IV lake sediments are at variance with the rest of the data. The Springdale result has large error limits that overlap with the other data. The anomalous King George IV result remains an outlier.

Remote sensing, field studies, petrography, and geochemistry of rocks in central Zambia: no evidence of a meteoritic impact in the area of the Lukanga Swamp

The Lukanga Swamp in central Zambia has previously been proposed as the site of a large (52 km diameter) impact structure on the basis of alleged observation of shock-diagnostic planar deformation features (PDFs) in quartz of breccia from the southern margin of the swamp. The southern margin of the swamp, marked by the Nyama Dislocation Zone, consists of quartzite and silicified meta-siltstone, shale, sedimentary quartz breccia, and fault breccia. Structures and textures in the meta-sediments are syn-sedimentary, whereas fault breccia displays tectonic fabrics. In thin section, quartz in sedimentary quartz breccia displays widely spaced, randomly oriented, subparallel, non-planar fluid inclusion trails, which were earlier misidentified to represent decorated PDFs formed by meteoritic impact. Siderophile element abundances (<105 ppm) in the rocks from around the swamp are normal for quartz-rich crustal rocks, and there are no relative enrichments in the breccias compared to other rocks to suggest extraterrestrial contributions. Aeromagnetic data do not reflect an impact crater signature. We analyzed regional structural and seismic data in an effort to account for the development of the swamp. From the structural synthesis it appears that reactivation of movements along the Nyama and Kapiri-Mposhi Dislocation Zones may have led to the development of the Lukanga Swamp, probably during the Cenozoic era.

Sediment subduction, subduction erosion, and strain regime in the northern South Sandwich forearc

The first swath bathymetry and side‐scan sonar imagery from the South Sandwich forearc reveals detailed seafloor morphology, tectonic fabric, and sedimentary features in an area where plate convergence is approximately normal to the trench. Simultaneously collected seismic reflection, gravity, and magnetic data provide information about the structure and composition of the forearc crust. Interpretation of these data together with the marine magnetic record of seafloor spreading in the east Scotia Sea back arc basin since 15 Ma has enabled quantitative estimation of sediment subduction and subduction erosion rates. Any accretionary prism present is constrained to be very small, extending 6 km or less from the trench. The contrast between this result and a time‐integrated estimate of potential accretionary prism volume suggests that &gt;95% of the sediment which has entered the trench since 15 Ma has been subducted. The position of the South Sandwich island arc in relation to back arc magnetic lineations implies that the arc has migrated ∼70 km westward relative to the Sandwich plate since 15 Ma. Taking account of published observations concerning the typical geometric evolution of arc‐trench systems, the average rate of forearc slope retreat during this interval is inferred to be 3.1–4.7 km Myr−1. Gravity modeling suggests that the forearc crust averages ≤10 km in thickness within 110 km of the trench. For 10‐km‐thick forearc crust the rate of slope retreat implies an average rate of subduction erosion of forearc crust of 31–47 km3 km−1 Myr−1. At the present convergence rate of 74 km Myr−1 this erosion rate requires a 420–635 m thick layer of subducting material derived from the forearc crust. A time‐integrated estimate of the potential volume of forearc‐derived trench fill sediments subducted since 15 Ma implies that frontal erosion can only account for &lt;40% of total subduction erosion. Therefore basal erosion must significantly exceed frontal erosion. The new data also provide insights concerning forearc strain regime and forearc basin evolution in the area studied. Extensional faulting near the trench slope break is probably caused by gravitational instability of the steep lower forearc slope. A lack of extensional strain indicators in the upper forearc and arc may be a consequence of the fact that the east Scotia Sea is a mature back arc basin contributing significant “ridge push” in the vicinity of the arc. The data reveal no evidence of arc‐parallel extension or of large serpentinite seamounts, as found in the Mariana forearc, and a causal link between these two observations is proposed. Seismic profiles across the forearc basin suggest that the balance between sediment accumulation and erosion is sensitive to changes in the elevation of the trench slope break. It is hypothesized that the basin is a dynamic feature which goes through repeated cycles of growth and destruction controlled by cyclic uplift and collapse of the trench slope break.

Artemis: Surface expression of a deep mantle plume on Venus

Artemis, a unique 2600-km-diameter circular feature on Venus, defies geomorphic classification as a corona, crustal plateau, or volcanic rise. Artemis is similar in size to plateaus and rises, yet topographically resembles a corona. Geologic mapping using correlated digital remote data sets including NASA Magellan C1-, C2-, and F-scale synthetic aperture radar (SAR) imagery, altimetry, and synthetic stereo has led to a determination of the geologic history of Artemis. Artemis comprises a large topographic welt that includes a paired circular ∼1-km-deep trough (Artemis Chasma) and ∼1-km-high outer rise; thus, Artemis is divisible into chasma (trough), interior region, and exterior region. The chasma hosts trough-normal faults and folds. The interior includes five ∼350-km-diameter coronae (quasi-circular features marked by radial and/or concentric fractures) that record rich tectono-volcanic histories; radial extension fractures and trough-concentric wrinkle ridges dominate the exterior tectonic fabric. Artemis formed as a coherent entity; the coronae, chasma, chasma structures, radial fractures, and wrinkle ridges are all consistent with a deep plume model for Artemis formation. Rising and flattening of the plume head led to early uplift, doming, and radial fracturing as the plume head collapsed vertically and spread laterally, likely causing outward migration of the trough, as well as fracturing and wrinkle-ridge formation outboard of the trough. Within the trough, material was pulled downward, forming normal faults and folds. The plume continued to spread laterally outboard of the trough, resulting in continued radial fracturing and wrinkle-ridge formation. Small-scale interior convection cells or compositional diapirs resulted in coronae with radial fractures, and/or concentric fractures and/or folds, and associated volcanism. Artemis is akin in its formation to crustal plateaus and volcanic rises and likely formed during the transition from globally thin to thick lithosphere.

Titanohematite lattice-preferred orientation and magnetic anisotropy in high-temperature mylonites

Magnetic and crystallographic fabric studies were performed in mylonitic granulites from a km-wide strike-slip shear zone in the Ribeira Belt (southeastern Brazil). In these mylonites, a strong compositional layering underlines the tectonic foliation and the elongation of titanohematite, amphibole and orthopyroxene crystals defines a mineral stretching lineation. Magnetic fabric deduced from anisotropy of magnetic susceptibility (AMS) measurements and tectonic fabric compare favorably. Rock-magnetic studies show that both paramagnetic and ferromagnetic minerals can be carriers of AMS. The anisotropy of isothermal remanent magnetization, which is due to the shape-preferred orientation of magnetite grains, is coaxial with AMS. Lattice-preferred orientation (LPO) measurements using the electron backscattered diffraction technique show that orthopyroxene, amphibole, biotite and titanohematite have a strong LPO tightly related to the tectonic fabric. Among these four minerals, titanohematite is the only mineral present in relatively large proportions (>1.5%) in all studied samples. Titanohematite LPO is characterized by a strong concentration of (0001) poles ( c-axes) sub-perpendicular to the foliation and by a distribution of the poles of the (2 11 0) and (10 1 0) prism planes within the foliation with a maximum close to the lineation. This characteristic LPO is interpreted as resulting from dislocation creep during the mylonitization. Magnetic fabrics and titanohematite LPO fit well: the axis of minimum susceptibility is aligned with the c-axis maximum and the axis of maximum susceptibility coincides with the maximum concentration of poles of the prism planes. Titanohematite LPO may provide a valuable constraint for the kinematic interpretation of the magnetic fabrics.

Syndepositional deformation of the Permian Capitan reef carbonate platform, Guadalupe Mountains, New Mexico, USA

Permian shelf strata equivalent to the Capitan reef are cut by at least 13 closely spaced syndepositional dip-slip faults in Slaughter Canyon, Guadalupe Mountains, New Mexico, USA. The control of five of these faults on platform development, architecture, stratigraphy and diagenesis is revealed by an integrated stratigraphic, sedimentological and structural framework within a 500-m-wide and 200-m-high outcrop window. Here, faulting and fault-related deformation acted as a primary control on changes in thickness, facies and stratal geometry, and resulted in the local steepening, shallowing and even the reversal of dip in shelf strata. The role of primary depositional relief in controlling changes in thickness and stratal geometry was of secondary importance. The relationship between geopetal fabrics (average dip=12° towards 141°) and bedding shows that during deposition of the Yates Formation fault growth was concurrent with down-to-the-basin tilting (and rotation?) of at least 6–8°. This tilting appears to have been the main control on the down-dip expansion of shelf strata towards the basin. A further 4–6° of basinward tilt occurred after fault growth, during deposition of the Tansill Formation and later. The syndepositional faults reported here have a maximum displacement of 24 m. Most tip-out below asymmetric growth folds and have high displacement-distance gradients typical of growth faults and faults cutting unlithified strata. The average rate of fault displacement (0.021 m/ka) and the maximum rates of fault propagation (0.088–0.123 m/ka) were normally less than the platform accumulation rates (0.053–0.336 m/ka). Thus, the faults were normally blind and rarely broke the platform top so that slumps and fault-scarp degradation breccias are rare. The fault zones were substantially modified by diagenesis during platform development. They are up to 9 m wide, taper both up and downward, have irregular margins and complex fills mainly of sedimentary origin. Their margins and fill were subject to extensive modification by karstic(?)/mixing zone dissolution, gravitational collapse and dolomitising fluids. Consequently, tectonic fabrics and kinematic indicators are rare. Preserved tectonic fabrics consistently indicate a normal and reverse dip-slip sense of movement. Previously, these faults were mistaken for ‘neptunian’ dykes and fissures, so that the Seven Rivers and Yates 1–2 HFS shelf stratigraphy has been miscorrelated across them. It is apparent that the stratal relationships exposed in the Guadalupe Mountains do not simply preserve the original depositional morphology of the Capitan-equivalent shelf. The subsidence history, stratigraphy and development of the platform succession is more complex than previously thought. The study has important implications for many aspects of the Capitan system, including: (i) shelf-reef correlations, (ii) the controls on platform architecture and development, (iii) Capitan reef palaeobathymetry, (iv) diagenesis, and (v) the amplitude of sea-level changes affecting the platform's stratigraphic development.

Evolution of Precambrian continental lithosphere in Western Canada: results from Lithoprobe studies in Alberta and beyond

The Precambrian lithosphere of western Canada was assembled into the present crustal configuration between ca. 2.0–1.78 Ga by plate collisions, sometimes accompanied by arc magmatism, with subsequent cooling of the lithosphere since ca. 1.7 Ga. Collisional processes inferred along preserved plate sutures include (1) subduction of oceanic lithosphere and accretion of Proterozoic arc crust to the western Rae Province; (2) marginal basin consumption and tectonic entrapment of the Hearne Province between coeval subduction–collision zones; and (3) amagmatic marginal basin closure, perhaps analogous to the roots of small collisional orogens, such as the Pyrenees. Seismic reflection profiles acquired during the Lithoprobe Alberta Basement Transect have captured images of syn- to post-collisional structures along these sutures and evidence for crustal-scale thrust imbrication and rigid body accretion of Archean crust with preservation of precollisional tectonic fabric. The degree to which lithospheric mantle beneath Archean crustal blocks was preserved during these collisions is unknown, although tectonic geometries imply significant thermal and (or) mechanical interaction. Post-collisional, intrusive mafic magmatism is imaged widely in both seismic reflection and refraction surveys. These magmatic events are demonstrably Proterozoic, based on crosscutting relationships seen on seismic reflection profiles and geochronology of lower crustal xenoliths, and are comparable in scale to Phanerozoic igneous provinces (e.g., large igneous provinces) but have little preserved surface manifestation. Reactivation of Precambrian basement structures is limited or very subtle, reflecting strength control by the mantle on stress transmission and crustal failure. Long-wavelength elastic deformation of the crust during the Phanerozoic occurred in regions associated with, or adjacent to, Proterozoic mafic magmatism, suggesting local rheologic control of anomalous Phanerozoic paleotopography.

Deep structure of the Mid Black Sea High (offshore Turkey) imaged by multi-channel seismic survey (BLACKSIS cruise) 1,2

The Black Sea is considered to be a Mesozoic–Early Cenozoic marginal basin related to the north-dipping subduction of Tethys beneath Europe. However deformation of this basin during the successive Eocene to Neogene collisions of African-derived continental fragments (Kirshehir and Arabian micro plates) remains poorly understood. A multi-channel seismic survey conducted in the central part of the Black Sea has shed light on the superimposed tectonic fabrics of the Central Ridge (the Mid Black Sea High, MBSH) in response to these successive collisions. The MBSH is formed by a series of large NW–SE trending anticlines and synclines, and possible northeast-verging thrusts were identified at the boundary with the deep East Black Sea Basin northeast of the ridge. These buried folds and thrusts, blanketed by 3 to 8 s TWT of undeformed sediments, are interpreted as the offshore extension of the Early Cenozoic tectonic belt resulting from the collision between the Pontides in the north and the Kirshehir block to the south. The offshore part of the belt forming the ridge could have then collapsed when collision ended. Neogene structures also affect the MBSH. A recent graben (the Sinop Trough) extends between this central high and mainland Turkey. This graben could have been formed during the late Miocene incipient dextral strike slip motion of the North Anatolian Fault that was initiated during extrusion of the Anatolian microplate. Active tectonic inversion of deep-seated normal faults present along the Pontides passive margin was also observed along the northeastern flank of the Eastern Pontides. This deformation is the westernmost extension of the Lesser Caucasus front that outlines the suturing of the Eastern Black Sea Basin in easternmost Turkey and in Georgia.

Shear wave anisotropy in the upper mantle beneath the Aegean related to internal deformation

Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present‐day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present‐day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.

Satellite-derived gravity having an impact on marine exploration

Converting sea-surface height variations, derived from satellite altimetry, to free air gravity is not new. In the early 1980s William Haxby (Lamont Doherty Geological Observatory) produced the first global marine gravity map from SeaSat satellite altimeter data using interorbital track spacing of about 180 km. Haxby's map had a significant impact on plate tectonic theory because marine free-air gravity data were able for the first time to uniformly image the tectonic fabric of the earth's oceanic crust. Since that time, much effort has been applied to improving satellite-derived gravity resolution. A major advance occurred in 1995, when the altimeter data from Geodetic Missions (GM) of GeoSat and ERS-1 satellites were released. Table 1 gives details of these satellites.

Excess argon incorporation in phengite of the Mulhacén Complex: submicroscopic illitization and fluid ingress during late Miocene extension in the Betic Zone, south-eastern Spain

40Ar/ 39Ar induction furnace and laser step-heating of well-crystallised post-tectonic phengitic mica single grains from gneisses of the Mulhacén Complex with an early Alpine tectonic fabric has resulted in: (1) highly scattered integrated ages, (2) an abnormally high atmospheric contamination and (3) often anomalously old apparent ages during early 39Ar release that is associated with a high 36Ar AIR and 37Ar Ca contamination. This low-temperature excess argon ( 40Ar XS) component is probably released from carbonate formed during slight alteration of the mica. More than 50% of the samples yielded plateau ages ranging from 15.8±0.4 to 90.1±1.0 Ma. Samples taken only a few metres apart may differ in age by as much as 50 Ma; a grain that was split over the basal plane yielded plateau ages for each half that differ by 12%. The age variation on these different scales is explained by heterogeneous 40Ar XS incorporation during a period with a high transient partial argon pressure in the metamorphic fluid, resulting from a late stage reheating event. The very swift cooling of 50–100°C/Ma during exhumation of the Mulhacén Complex concomitant with late Miocene extension may have prevented the equilibration of different 40Ar XS levels in the mica. HRTEM images of the oldest and youngest phengite specimens show that at least 20% of the lattice is affected by submicroscopic illitisation, which is concentrated in several micrometer wide zones and veins that cross-cut the basal cleavage. These are made up of aggregates of 0.07–0.30 μm thick crystallites of three illitic micas types, which are chemically and structurally progressively closer to pure illite and occur in different textures. The oldest specimen is affected most severely as the veins contain newly formed (pseudo)illite that does not inherit its crystallographic orientation and chemistry from the host mica, in contrast to the youngest sample. HRTEM–AEM analyses revealed that phengite and the different illitic micas may be depleted in K. The oldest sample is derived from a coarse-grained augen gneiss with extensively developed hydraulic cracks, which are lacking in the youngest sample, a fine-grained mylonitic gneiss. Fluid–rock interaction and consequently sub-microscopic illitization were therefore more intense in the coarser-grained rocks. Growth of the illitic micas in equilibrium with a high partial 40Ar pressure could account for 40Ar XS incorporation in K-vacancies and other lattice imperfections. Variation in illitisation and associated textural dissimilarities between the oldest and youngest mica permit the different levels of 40Ar XS incorporation that account for the observed age discordance. The finding of 40Ar XS plateau ages, despite the degassing of intimately intergrown micaceous minerals, is interpreted by gas release invoked by in-vacuo chemical and structural changes that led to a joint collapse of the lattices of phengite and the illitic micas between 800°C and 1000°C.

Tectonic origin of the shape of the Broken Hill lodes supported by their structural setting in a high‐grade shear zone

Recent studies have claimed structural support for a syngenetic model of ore formation at Broken Hill. The structural features of the Line of Lode—foliation, lineation, boudinage, folding and shearing—are re‐evaluated and new data presented from several locations in and around the Line of Lode, including the Kintore Opencut and Readymix Quarries. Although deformation partitioning preserves areas of low strain, especially in the hangingwall, that exhibit primary features, the deformation history described shows a history of high‐strain on the Line of Lode. Gneisses in the wall rocks of the orebody show extreme extension in places, with destruction of primary layering and imposition of transposed tectonic fabrics. Sulfide bodies would have been softer than the wall rocks during deformation and any layering in the lodes is likely to be a result of tectonic processes rather than preserved bedding. The geometry of the orebody is described and its setting is revealed as a low‐strain site, a minor fold pair, that developed early in a major high‐grade shear zone. The orebody probably acquired its linear shape first as a result of mass transport of sulfides to this structural site and then by extension within the shear zone, an epigenetic process. Previous fold models for the Line of Lode are rejected, along with the application of regional stratigraphic units to the orebody footwall. Deformation of the Line of Lode before peak metamorphism is obscured by recrystallisation. Subsequent minor deformation occurred at both high metamorphic grade and under retrograde conditions to produce the range of features previously quoted in support of syngenesis. Sulfides were remobilised during both the post‐peak metamorphic high‐grade, and later low‐grade, deformation events.

Did the East Pacific rise subduct beneath the North America plate (western Mexico)?

New data collected between the northernmost tip of the East Pacific rise (18°05′N, 105°35′W) and the Middle America trench provide evidence that the seafloor, which lacks significant sedimentary cover, has a typical spreading-derived abyssal hill topography. The tectonic fabric of this seafloor is concave to the west, as it is today at the tip of the East Pacific rise. Farther to the east, at the outer wall of the trench, the seafloor topography exhibits a north-south trending fabric. We suggest that this fabric originated along the East Pacific rise, as it reached the trench and possibly subducted beneath the North America plate prior to the development of the complex connection of the East Pacific rise with the Rivera transform.

Continental rifting parallel to ancient collisional belts: an effect of the mechanical anisotropy of the lithospheric mantle

Abstract Analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process. Rift propagation is not random, but tends to follow the trend of the orogenic fabric of the plates, systematically reactivating ancient lithospheric structures. Continental rifts often display a clear component of strike–slip deformation, in particular in the early rifting stage. Moreover, although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process, there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts. These observations suggest that the deformation of the lithosphere, especially during rifting, is controlled by its preexisting structure. On the other hand, (1) the plasticity anisotropy of olivine single crystal and aggregates, (2) the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs, and (3) seismic anisotropy data, which require a tectonic fabric in the upper mantle coherent over large areas, suggest that preservation within the lithospheric mantle of a lattice preferred orientation (LPO) of olivine crystals may induce a large-scale mechanical anisotropy of the lithospheric mantle. We use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process. We assess the deformation of an anisotropic continental lithosphere in response to an axi-symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head. Model results show that a LPO-induced mechanical anisotropy of the lithospheric mantle may result in directional softening, leading to heterogeneous deformation. During continental rifting, this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique (30–55°) to the preexisting mantle fabric. This directional softening associated with olivine LPO frozen in the lithospheric mantle may also guide the propagation of the initial instability, that will follow the preexisting structural trend. The preexisting mantle fabric also controls the deformation regime, imposing a strong strike–slip shear component. A LPO-induced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting (structural inheritance), the plume–rift paradox, and the onset of transtension within continental rifts.

Neotectonic structure and evolution of Lefkada island

In this paper, the formations that participate in the geological structure of Lefkada island are described in detail, as well as two alpine tectonic phases that affected the island, together with the criteria for their evaluation. The tectonic fabric is then described and the mapped fault systems are analysed kinematically and geometrically through the time. The current geodynamic regime has found its expression on the island through a dense fault network that has overprinted the pre-existing thrust-related features and has been dominant throughout the Plioquaternary. The dense fault network has broken up the island in a multitude of independent fault blocks which are described; an attempt is also made to determine the relative movements among these fault-blocks. The recent geological history of Lefkada is characterized by basic periods of predominant compression, interrupted by brief extensional episodes, where gravitational movements are favoured.

Propagation of continental break-up in the southwestern South China Sea

Abstract We present new bathymetric, seismic and gravity data on the southwestern tip of the South China Sea oceanic basin, where propagation of continental break-up occurred before c. 15 Ma. The oceanic domain has a V-shape typical of oceanic propagating rifts. The tectonic fabric of its margins shows that the main stretching direction was slightly oblique to that of the rift axis. A 2D gravity anomaly inversion, corrected for the thermal effect, is used to estimate the crustal structure. At the continent-ocean boundary, the continental crust is stretched by a factor of about four, rapidly decreasing to about two over a few tens of kilometres, a distance corresponding to just over 1 Ma of break-up propagation. Thus, strain localization occurs at the tip of the propagating oceanic crust just before break-up. The along-axis variation in continental crustal stretching is in good agreement with the kinematics of the oceanic crust derived from magnetic anomalies. This analysis suggests that break-up propagates toward the pole of relative rotation and is primarily controlled by the amount of stretching of the continental crust before oceanization.