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John D.a. Piper - One of the best experts on this subject based on the ideXlab platform.

  • Neotectonic deformation in the transition zone between the Dead Sea Transform and the East Anatolian Fault Zone, Southern Turkey: a palaeomagnetic study of the Karasu Rift Volcanism
    Tectonophysics, 2004
    Co-Authors: Orhan Tatar, John D.a. Piper, H. Gürsoy, A. Heimann, Fikret Koçbulut
    Abstract:

    In southern Turkey ongoing differential impingement of Arabia into the weak Anatolian collisional collage resulting from subduction of the Neotethyan Ocean has produced one of the most complex crustal interactions along the Alpine–Himalayan Orogen. Several major transforms with disputed motions, including the northward extension of the Dead Sea Fault Zone (DSFZ), meet in this region. To evaluate neotectonic motion on the Amanos and East Hatay fault zones considered to be northward extensions of the DSFZ, the palaeomagnetism of volcanic fields in the Karasu Rift between these faults has been studied. Remanence carriers are low-Ti magnetites and all except 5 of 51 basalt lavas have normal polarity. Morphological, polarity and K–Ar evidence show that rift formation occurred largely during the Brunhes chron with volcanism concentrated at 0.66–0.35 Ma and a subsidiary episode at ∼0.25–0.05. Forty-four units of normal polarity yield a mean of D/I=8.8°/54.7° with inclination identical to the present-day field and declination rotated Clockwise by 8.8±4.0°. Within the ∼15-km-wide Hassa sector of the Karasu Rift, the volcanic activity is concentrated between the Amanos and East Hatay faults, both with left lateral motions, which have rotated blocks bounded by NW–SE cross faults in a Clockwise Sense as the Arabian Block has moved northwestwards. An average lava age of ∼0.5 Ma yields a minimum cumulative slip rate on the system bounding faults of 0.46 cm/year according with the rate deduced from the Africa–Arabia Euler vector and reduced rates of slip on the southern extension of the DSFZ during Plio-Quaternary times. Estimates deduced from offsets of dated lavas flows and morphological features on the Amanos Fault Zone [Tectonophysics 344 (2002) 207] are lower (0.09–0.18 cm/year) probably because they are limited to surface fault breaks and do not embrace the seismogenic crust. Results of this study suggest that most strike slip on the DSFZ is taken up by the Amanos–East Hatay–Afrin fault array in southern Turkey. Comparable estimates of Quaternary slip rate are identified on other faults meeting at an unstable FFF junction (DSFZ, East Anatolian Fault Zone, Karatas Fault Zone). A deceleration in slip rate across the DSFZ and its northward continuation during Plio-Quaternary times correlates with reorganization of the tectonic regime during the last 1–3 Ma including tectonic escape within Anatolia, establishment of the North and East Anatolian Fault Zones bounding the Anatolian collage in mid–late Pliocene times, a contemporaneous transition from transpression to transtension and concentration of all basaltic magmatism in this region within the last 1 Ma.

  • The palaeomagnetism of middle Proterozoic dyke swarms of the Gardar Province and Mesozoic dykes in SW Greenland
    Geophysical Journal International, 1995
    Co-Authors: John D.a. Piper
    Abstract:

    SUMMARY The Precambrian metamorphic basement in SW Greenland was subjected to widespread emplacement of dolerite dykes between circa 1290 and 1160 Ma; during this period the regional stress field rotated in a counter-Clockwise Sense. A regional palaeomagnetic study of 59 of these dykes and contacts is reported. Whilst contact tests generally indicate primary magnetizations, two- and three-component structures in some dykes record partial overprinting at later stages of the igneous cycle. Component directions are distributed from shallow negative westerly to steep NW positive. Over most of the outcrop the former magnetizations are found in the earlist (E-W trending) dykes. Steepening and rotation of magnetization directions correlates with rotation of dyke trends towards later (NE-SW) trends. The exception occurs where dykes are deflected in a sigmoidal way through an older alkaline complex. The quasi-continuous nature of dyke emplacement records migration of the palaeofield direction between circa 1290 and 1160 Ma and representative sequential pole positions are (A1) 215.3dE, 3.1dN (21 dykes, dp/dm= 4.8/9.6d), (A2) 220.4dE, 7.4dN (5 dykes, dp/dm= 4.3/8.5d) and (A3) 222.3dE, 33.5dN (30 dykes, dp/dm= 4.5/7.0d). Palaeomagnetic poles from the Gardar Igneous Province define the major part of a double APW loop anchored by dated poles from igneous complexes and executed between 1300 and 1140 Ma. This interval appears to have been dominated by one field polarity. Palaeomagnetic results are also reported for five dykes belonging to the Mesozoic coast parallel swarm, and palaeomagnetic results from these dykes in south and south-west Greenland are reassessed. They define a dual-polarity axis of intermediate inclination (D/I = 336/51d, 13 sites, palaeomagnetic pole (TD1) at 168.3dE, 55.6dN. dp/dm= 4.7/6.9d) plus a steeper single-polarity group (mean direction D/I = 329/69d, 14 sites, palaeomagnetic pole (TD2) at 207.7dE, 68.6dN, dp/dm= 6.6/7.8d). The TD1 pole corresponds to North American apparent polar wander at circa 165 Ma compatible with the 168-138 Ma age constraint, whilst the TD2 pole correlates with near-static APW between 129 and 88 Ma. It appears to record an episode of dyke emplacement across south Greenland during the Cretaceous Normal Superchron shortly before commencement of sea-floor spreading between Greenland and Labrador.

  • Siluro-Devonian palaeomagnetism, terrane emplacement and rotation in the Caledonides of western Ireland
    Geophysical Journal International, 1991
    Co-Authors: John D.a. Piper
    Abstract:

    SUMMARY The palaeomagnetism of a Silurian succession (Lough Mask Formation lavas and sediments) and intrusive rocks (Kilbride dolerites and andesites and microgranodiorites, Killary Harbour area) in the Connemara-Mayo segment of the Irish Caledonides is described. Magnetizations are related to the regional tectonic history with the aid of fold, conglomerate and contact tests. They describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this Caledonian inlier. The shallow components appear to pre-date Siluro-Devonian folding while the intermediate ones post-date this episode. The change in palaeofield inclination is identical to the transition described by contemporaneous deep level (Silurian) to shallow and surface level (Lower Devonian) bodies in the British and Scandinavian Caledonides but declinations are rotated consistently to the west. The Lower-Middle Silurian vector is rotated Clockwise by c. 100". This difference is progressively reduced to c. 50" by the time of the Siluro-Devonian D2 folding, showing that this fold generation (now oriented E-W) developed in parallelism with contemporaneous folds on the NE strike continuation of the Caledonides. Post-folding remanences continue to deviate in a Clockwise Sense from the remainder of the Caledonides showing that rotation continued into Middle-Upper Devonian times and finally ceased during Carboniferous times. The integrated effect of this rotation is recognized in Ordovician magnetizations from the Dalradian metamorphic terrane of south Connemara. It can be explained by block rotations within a zone of distributed deformation according to the McKenzie-Jackson model and illustrated by many neovolcanic zones. Palaeomagnetism defines a tectonic regime commencing with rotations of c. 3"Myr-' and lasting from mid-Silurian to Carboniferous times. Western Ireland is sited within a closure gap along the Iapetus suture extending from Ireland to Newfoundland, and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the Caledonian orogen.

Noel Carbajal - One of the best experts on this subject based on the ideXlab platform.

  • A note on tidal current rotation
    Ocean Dynamics, 2004
    Co-Authors: Noel Carbajal
    Abstract:

    The rotational form of the vertically averaged equations of motion is applied to derive a formula, linear friction included, which establishes a direct connection between Sense of rotation of tidal currents and features of tidal amphidromic systems. Two factors in the formula, called α and β, influence the Sense of rotation of tidal currents; the factor α involves the frequency of the tidal signal ω, the Coriolis parameter f, and the linear friction coefficient r. The sign of the cross-product of the logarithm of sea-surface elevation (ζ), and phase (ϕζ) gradients Open image in new window determines whether the factor β favors Clockwise or antiClockwise Sense of rotation. Open image in new window is a unit vector and γ is the angle between ∇lnζ and ∇ϕζ. The limits |∇ζ|→0, |∇ϕζ|→0 and γ→0 lead to a Clockwise Sense of rotation in the Northern Hemisphere. 0≤γ≤π favors antiClockwise rotation in the Northern Hemisphere. Friction and low frequencies favor an antiClockwise Sense of rotation. The theory works well in semi-enclosed regions like the North Sea. Although only linear friction and sea-surface elevation gradients were considered, there are ocean regions where the agreement between theory and observations is also good.

  • A note on tidal current rotation
    Ocean Dynamics, 2004
    Co-Authors: Noel Carbajal
    Abstract:

    The rotational form of the vertically averaged equations of motion is applied to derive a formula, linear friction included, which establishes a direct connection between Sense of rotation of tidal currents and features of tidal amphidromic systems. Two factors in the formula, called α and β, influence the Sense of rotation of tidal currents; the factor α involves the frequency of the tidal signal ω, the Coriolis parameter f , and the linear friction coefficient r . The sign of the cross-product of the logarithm of sea-surface elevation (ζ), and phase (ϕ_ζ) gradients determines whether the factor β favors Clockwise or antiClockwise Sense of rotation. is a unit vector and γ is the angle between ∇lnζ and ∇ϕ_ζ. The limits |∇ζ|→0, |∇ϕ_ζ|→0 and γ→0 lead to a Clockwise Sense of rotation in the Northern Hemisphere. 0≤γ≤π favors antiClockwise rotation in the Northern Hemisphere. Friction and low frequencies favor an antiClockwise Sense of rotation. The theory works well in semi-enclosed regions like the North Sea. Although only linear friction and sea-surface elevation gradients were considered, there are ocean regions where the agreement between theory and observations is also good.

Santanu Kumar Bhowmik - One of the best experts on this subject based on the ideXlab platform.

  • Constraining the metamorphic evolution of a cryptic hot Mesoproterozoic orogen in the Central Indian Tectonic Zone, using P–T pseudosection modelling of mafic intrusions and host reworked granulites
    Precambrian Research, 2007
    Co-Authors: Amit Basu Sarbadhikari, Santanu Kumar Bhowmik
    Abstract:

    Abstract In this study, we reconstruct the metamorphic pathways of reworking of a deep crustal granulite terrane in the Central Indian Tectonic Zone, using Archaean/Palaeoproterozoic (?) polycyclic felsic granulites and two groups of intrusive Mesoproterozoic mafic granulites, a coarse-grained noritic gabbro and a fine-grained gabbroic norite. The granulite terrane, locally referred to as the Bhandara–Balaghat granulite domain, is bounded by the South Indian Block and a Grenville-aged, younger tectonic domain of the Central Indian Tectonic Zone. The granulites and the mafic intrusions were multiply deformed and metamorphosed in the early Mesoproterozoic. Using P–T pseudosection modelling of host felsic granulites and the mafic intrusions, two distinct metamorphic events (BM2 and BM3) with contrasting P–T paths have been established. The P–T path of BM2 metamorphism has a Clockwise Sense, having an important prograde segment of heating of more than 250 °C with pressure fall, followed by cooling. The peak BM2 metamorphism has been constrained at ∼6 kbar, ∼725 °C. Emplacement of the mafic intrusions was broadly coincident with the low-pressure metamorphism, and they underwent a phase of subsolidus cooling. During subsequent metamorphism, BM3, the mafic intrusions and the host felsic granulites were re-metamorphosed along a counterClockwise P–T path. This led to tectonic burial of the mid-crust to ∼9.4 kbar, ∼760 °C, which was followed by cooling accompanying pressure decline. Although, the Clockwise P–T path is generally interpreted in terms of thermal relaxation of crust following thrusting, the scale of heating (∼250 °C) during initial decompression, documented here, is much larger than modelled for tectonically thickened crust, but consistent with magmatically active extensional zones. Based on this evidence and also the syn-metamorphic mafic intrusions, we present an alternate interpretation in terms of mid-crustal extension for the BM2 metamorphism. In contrast, the counter Clockwise P–T path during BM3, recording cooling following prograde burial is explained by tectonic thickening of a hot mid-crust. Collating available geochronological data, the two metamorphic events appear to indicate tectonic switching from lithospheric extension to contraction in the early Mesoproterozoic. The findings provide the first quantitative constraints on an early Mesoproterozoic hot orogen at the craton–mobile belt interface in the Central Indian Tectonic Zone.

  • Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P–T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone
    Journal of Petrology, 2003
    Co-Authors: Santanu Kumar Bhowmik
    Abstract:

    A suite of garnetiferous amphibolites and mafic granulites occur as small boudins within layered felsic migmatite gneiss in the northern part of the Sausar Mobile Belt (SMB), the latter constituting the southern component of the Proterozoic Central IndianTectonicZone(CITZ).Althoughthetwotypesofmetabasites are in various stages of retrogression, textural, compositional and phase equilibria studies attest to four distinct metamorphic episodes. The early prograde stage (Mo) is represented by an inclusion assemblage of hornblende1a ilmenite1a plagioclase1 quartz and growth zoning preserved in garnet. The peak assemblage (M1) consists of porphyroblastic garnet a clinopyroxene quartz rutile hornblende in mafic granulites and garneta quartza hornblende in amphibolites and stabilized at pressure‐temperature conditions of 9‐10kbar and 750‐800C and 8 kbar and 675C, respectively. This was followed by near-isothermal decompression (M2), and postdecompression cooling (M3) events. In mafic granulites, the former resulted in the development of early clinopyroxene2A‐ hornblende2A‐plagioclase2A symplectites at 8 kbar and 775C (M2A stage), synchronous with D2 and later anhydrous clinopyroxene2B‐plagioclase2B‐ilmenite2B symplectites and coronal assemblagesat7kbar,750C(M2B stage)andpost-datingD2. In amphibolites, ilmenitea plagioclasea quartz hornblende symplectites appeared during M2 at 64kbar and 700C. During M3, coronal garnet a clinopyroxene a quartz hornblende-bearing symplectites in metabasic dykes and hornblende3‐plagioclase3 symplectites embaying garnet in mafic granulites were formed. P‐T estimates show near-isobaric cooling from 7kbar and 750C to 6kbar and 650C during M3. It is argued that the decompression in the mafic granulites is not continuous, being punctuated by a distinct heating (prograde?) event. The latter is also coincident with a period of extension, marked by mafic dyke emplacement. The combined P‐T path of evolution has a Clockwise Sense and provides evidence for a major phase of early continental subduction in parts of the CITZ. This was followed by a later continent‐ continent collision event during which granulites of the first phase became tectonically interleaved with younger lithological units. This tectonothermal event, of possibly Grenvillian age, marks the final amalgamation of the North and theSouth Indian Blocks along the CITZ to produce the Indian subcontinent.

Pierre-marie Poulain - One of the best experts on this subject based on the ideXlab platform.

  • Tidal currents in the Adriatic as measured by surface drifters
    Journal of Geophysical Research: Oceans, 2013
    Co-Authors: Pierre-marie Poulain
    Abstract:

    [1] Velocities of surface drifters are analyzed to study tidal currents throughout the Adriatic Sea. Spectral and harmonic analyses indicate that the M2, S2, and K1 constituents dominate. Maps of tidal characteristics show that M2 and S2 are rectilinear currents (reversing tides) aligned with the main axis of the Adriatic basin with maximum amplitude (~7 cm/s for M2 and ~4 cm/s for S2) in the northern area off the Istrian Peninsula. Near the northern coast, semidiurnal tidal currents decrease in amplitude and rotate in the counterClockwise Sense. Near the Po River delta, M2 (S2) motions rotate in the counterClockwise (Clockwise) Sense. S2 rotation is also counterClockwise near the northeastern coast. M2 phases increase from about 130° on the eastern Croatian coast to 190° on the western Italian side. S2 phases range from 150° to 200°. In the middle and southern Adriatic, the semidiurnal tides are small (~1 cm/s). The diurnal tidal currents (K1) are strong across the basin at the levels of Monte Conero and the Gargano Peninsula with speed larger than 5 cm/s and mainly Clockwise rotation, and also in coastal areas (e.g., on the Albanian shelf and close to the Otranto Channel). Phases increase from the east to the west coasts (by as much as 150°). These new results compare satisfactorily with previous observations and numerical simulations, although tidal amplitudes are under-estimated with respect to mooring measurements. They extend for the first time the description of the Adriatic tidal currents to the entire basin based on direct velocity observations.

M. Chu - One of the best experts on this subject based on the ideXlab platform.