Geodetic evidence for passive control of a major Miocene tectonic boundary on the contemporary deformation field of Athens (Greece)

<p>A GPS-derived velocity field is presented from a dense geodetic network (~5km distance between stations) established in the broader area of Athens. It shows significant local variations of strain rates across a major inactive tectonic boundary separating metamorphic and non-metamorphic geotectonic units. The southeastern part of Athens plain displays negligible deformation rates, whereas towards the northwestern part higher strain rates are observed, indicating the control of the inactive tectonic boundary on the contemporary deformation field of the region. These findings are in agreement with previous geological observations, however, due to the dense local GPS network it was fatherly possible to localize and quantify the effect of such a major inherited tectonic feature on the deformation pattern of the area.</p>

A Late Pleistocene-Holocene natural seismograph along the Boconó Fault (Mérida Andes, Venezuela): the moraine-dammed Los Zerpa paleo-lake

The Boconó Fault system is a major active tectonic feature accommodating an important part of the dextral relative motion between the Caribbean Plate and northern South-America. The main trace follows an axial valley running SW-NE within the Mérida Andes (northwestern Venezuela), and crosscuts a series of moraines related to late Pleistocene glaciers developments and retreats, at an altitude between 2600 and 5000 m. Several lakes were generated after the last retreat (between the Late Glacial Maximum –LGM– and the Younger Dryas re-advance), dammed by lateral and frontal moraines. Among them, the Los Zerpa moraine system yielded rich outcrops ranging from an upstream very coarse torrential to deltaic fill, to a downstream clayey-silty horizontal laminated lacustrine accumulation; a fore-set-type heterogeneous “prograding” body links the two sets. The whole system, as well as the surrounding moraines, underwent successive major earthquakes during the Late Glacial/lower Holocene period as evidenced by co-seismic scarps in the moraines, migrations of the outlet, and associated sagponds. Besides active faulting affecting both the moraines and the sedimentary fill, the latter –main purpose of our detail study– exhibits various evidence of strong disturbances which we relate to seismic shaking, such as: i) successive unconformities with co-seismic slips along fractures in the coarse proximal sediments; ii) successive dip changes, discontinuities, and slumps in the foreset-like set; iii) slumps with basal liquefaction, syn-sedimentary fractures, and instantaneous re-sedimentation in the fine-grained laminated accumulation. Lateral (temporal) correlations are established between the successive disturbances detected in the three situations; in turn, these sedimentary events are correlated with seismic activity of the Boconó Fault main trace. Thus, the whole paleo-lake may be considered as a natural seismograph which worked during several thousands years, after the end of the LGM and during early Holocene.

Austrochlamys heardensis (Fleming, 1957) (Bivalvia: Pectinidae) from Central Kerguelen Plateau, Indian Ocean: palaeontology and possible tectonic significance

Austrochlamys heardensis (Fleming) is recorded from a boulder of Late Pliocene (3.62–2.5 Ma) volcaniclastic sandstone dredged 70 km east-north-east of Heard Island, the third record of the species. The collection is much larger than the original described by Fleming and includes left valves which are described for the first time. The species is compared with A. anderssoni (Hennig) from Cockburn Island and ‘Chlamys’ mawsoni Fletcher from Îles Kerguelen. The source rock accumulated in fully marine, highly current-affected conditions. The collection is dominated by right valves, possibly because left valves are more susceptible to breakage and the effects of current winnowing. The locality may have subsided some 500 m since deposition. It lies immediately north of a straight, north-east–south-west trending lineament which may mark a major tectonic feature with left-lateral displacement of approximately 50 km, and provides a natural boundary within the Central Province of Kerguelen Plateau.

Geophysical interpretation of tectonic features in Bangladesh

Bangladesh occupies the major part of the Bengal basin. It is mainly a plain fluvi-dieltaic land. Except the eastern and northeastern Tertiary hilly region, the whole country is covered with thick Quaternary sediments. Tectonic features in Bangladesh, except the eastern and northeastern hilly structures, are hidden under thick cover of sediments. Gravity anomalies dearly show the locations of major subsurface tectonic features of the country, namely a) Himalayan foredeep, b) Rangpur platform, c) Hinge zone, d) Surma basin and e) Bengal foredeep. Magnetic anomalies and the seismic sections also support the gravity results. Shape, extent and depth of the tectonic features in Bangladesh are interpreted from geophysical data, particularly from the gravity data supported by available geological and drilling information. Interpretation shows that the Rangpur platform, situated in the northwestern Bangladesh, is the shallowest subsurface tectonic feature in the country. The northern part of the Rangpur platform slopes down to the Himalayan foredeep and the southern part to the Hinge zone. The Surma Basin, containing very thick sediments, lies in north-east corner of the country; and the Bengal foredeep, the most extended tectonic feature, occupies the southern deltaic part of the country. The Fold Belt, the only exposed tectonic feature, lies in the eastern and northeastern hilly region of the country.

A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada

Alternating greenstone–granite and metasedimentary gneiss belts are a first-order tectonic feature of the southern Superior Province. The tectonic development of the Quetico metasedimentary belt is reviewed with regard to depositional, structural, and metamorphic–plutonic history. Over its 1200 km length, the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite. Polyphase deformation has resulted in a steep easterly-striking foliation and regional, gently east-plunging stretching lineation. Metamorphic grade varies in a low-P facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite – intrusive granite zone. Mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 MPa (2.5 kbar) near the Canada – United States border to 600 MPa (6 kbar) in granulites adjacent to the Kapuskasing structural zone.Geochronology suggests that sediments were deposited at approximately the same time as active volcanism in adjacent volcanic belts, although evidence of volcanic–sedimentary stratigraphic contiguity is weak as a result of later transcurrent movement parallel to major lithological boundaries. Adjacent belts are inferred to have been contiguous since common D2 deformation, 2689–2684 Ma ago. Major plutonism and associated metamorphism occurred in the Quetico Belt approximately 2670–2650 Ma ago, significantly later than major plutonism in the adjacent volcanic belts.The linear disposition of greywacke-rich sediments over 1200 km invites an analogy with modern accretionary prisms. However, the high-temperature, low-pressure metamorphism of the Quetico Belt is inconsistent with such a low-heat-flow environment, and a change in tectonic regime would be required to account for the metamorphism and intracrustal plutonism. Simple cessation of subduction beneath the thick sedimentary prism could have led to restoration of isotherms, with possible attendant crustal melting and isostatic recovery.

A cored Jurassic sequence from north Lincolnshire, England: stratigraphy, facies analysis and regional context

SummaryA virtually complete Jurassic sequence recently drilled in north Lincolnshire is considered in a regional context. Liassic thicknesses and facies are largely unexceptional. The Frodingham and Pecten Ironstones are relatively thin and sandy, but the Marlstone Rock Bed is thicker and more ferruginous than elsewhere in the region. The Inferior Oolite is exceptionally thick and exhibits strong deltaic influence; equivalents of the Gristhorpe Member and Scarborough Formation of Yorkshire may be present. The Great Oolite was deposited in coastal-lacustrine-deltaic environments, more proximal than to the south or east. Thin Kellaways Clay separates the argillaceous Cornbrash from the Kellaways Sand. The organic-rich Lower Oxford Clay is thin owing to a basal non-sequence whilst the Upper Oxford Clay is condensed and less organic than in the East Midlands. The regressive West Walton Beds are followed by very thick Ampthill and Kimmeridge Clay sections beneath the Spilsby Sandstone. Regional thickness and facies analysis records the behaviour of the Market Weighton Block and reveals the influence of the Pennine High as both a positive tectonic feature and an occasionally active sediment source.

Microearthquake surveys in the Central and Northern Philippines

abstract Microearthquake surveys were carried out in three sections of Central and Northern Philippines during 1975-1976 for a period of 5 months. A 4-month survey of Bataan Peninsula identified a major tectonic feature near Manila Bay which could not have been postulated from examination of seismicity maps. This feature appears to be situated near the southern end of ultramafic rocks of West Central Luzon and West Luzon Trough and trends W-SW from east of Corregidor Island toward Manila Trench for a distance of about 100 km. This survey also showed no microearthquake activity beneath two presently inactive volcanoes on Bataan Peninsula. The rate of activity in Bataan Peninsula region was found to be very low (8.4 events/1000 km2/yr). A short-duration survey (16 days) of the Philippine Fault in North Central Luzon revealed no microearthquake activity on the fault. During a third short-duration survey (16 days), the Verde Island Passage area between Luzon and Mindoro was found to be as highly active at the microearthquake level as it is for large earthquakes.