Marginal Wedge Research Articles

Joint inversion of multichannel seismic reflection and wide‐angle seismic data: Improved imaging and refined velocity model of the crustal structure of the north Ecuador–south Colombia convergent margin

Improving seismic imaging of the crust is essential for understanding the structural factors controlling subduction zones processes. We developed a processing work flow based on the combined analysis of multichannel seismic reflection (MCS) and wide angle (WA) reflection/refraction data to derive both shallow and deep velocities suitable for prestack depth migration and to construct a blocky velocity model integrating all identifiable seismic phases contained in MCS and WA data. We apply this strategy to the study of the north Ecuador–SW Colombia subduction margin to improve the imaging and geostructural interpretation of a splay fault and surrounding outer and inner margin wedges. Results show improvements over tomographic inversion of WA data only, such as (1) sediment velocity variation across the trench and margin slope that correlates with lateral lithologic changes, tectonic compaction and effect of mass wasting processes; (2) a two‐layer velocity structure of the inner wedge basement that is consistent with the crust of an oceanic plateau; (3) a complex velocity structure of the outer wedge basement that consists of a deep, high‐velocity (5.0–5.5 km s−1) core and a low‐velocity zone (3.8–5.0 km s−1) associated with the major splay fault; (4) a ∼1.3‐km‐thick, low‐velocity (3.5–4.0 km s−1) subduction channel that extends beneath the margin outer wedge. Both the splay fault and subduction channel are expected to direct fluid flows; and (5) downdip velocity increase (5–6 km s−1) in the subducting oceanic crust associated with a low (7.8 km s−1) upper mantle velocity, possibly reflecting changes in rock nature or properties.

TAPHONOMIC FEATURES OF LENTICULINA AS A TOOL FOR PALEOENVIRONMENTAL INTERPRETATION OF MIDSHELF DEPOSITS OF THE UPPER JURASSIC (PREBETIC ZONE, SOUTHERN SPAIN)

Taphonomic analysis of Lenticulina from the Upper Jurassic foraminiferal assemblages of midshelf environments was carried out in a marl-limestone rhythmite of the Prebetic. Size, fragmentation-corrasion, microboring, encrustations, recrystallization, mineralization, sedimentary infillings, and cementation of tests were analyzed by thin section. Encrustations, microborings, and dominance of micritic infillings in the tests are indicative of long exposure on the sea bottom related to low-sedimentation rates. In contrast, the frequency of sparitic cements and reelaborated tests is indicative of higher-sedimentation rates, which are also increased by the input of sediment winnowed from shallower areas. The stratigraphic distribution of these taphonomic features reveals trends in sedimentation rates and sediment input that may be interpreted according to the sequence stratigraphy proposals for the Prebetic. During transgression, increasing relative distance from shore produces a lower-sedimentation rate and prolonged exposure of the fossil remains. This results in increased amounts of microborings, encrustations, and micritic infillings, while fragmentation and the proportion of the reelaborated tests of Lenticulina decrease. Rapid burial is related to increasing sediment input and sedimentation rates and can be associated with aggradation and progradation during the development of a highstand systems tract and a shelf margin wedge. A higher-sedimentation rate explains the scarcity of encrustations and microborings and the abundance of sparitic cements. A higher allochthony of Lenticulina is indicated by the locally high values of fragmentation and reelaborated tests. This taphonomic analysis is useful in monotonous epicontinental lithofacies as marl-limestone rhythmites can reveal features indicative of depositional developments that can be fitted to sequence stratigraphic models.

The development of the Carboniferous Ben‐Zireg–Zousfana Trough in the northern part of the Béchar Basin, Western Algeria: implications for its structural evolution, sequence stratigraphy and palaeogeography

AbstractThe sequence stratigraphical study of the Viséan Formations shows the importance of the Ben‐Zireg Trough for the structural development of the Béchar Basin. The sequence stratigraphical approach has allowed to decipher the systems tracts, which have filled the accommodation space of the Ben‐Zireg Trough under tectono‐eustatic control. A deep sea fan recorded by the first flysch‐type deposits transported by submarine canyons was followed by a prograding lowstand wedge recorded in the flysch with olistostromes, a transgressive system tract (TST) identified by flysch‐type deposits without olistostrome, and a highstand system tract (HST) represented by the patch reef and flyschoid deposits. The biostromal barrier and platform limestones indicate also a TST and sometimes a shelf margin wedge (SMW). Thus, these stratal patterns have been identified as Types 1 and 2 genetic sequences of third‐order eustatic cycles and are based on correlations of cross‐sections located around the Ben‐Zireg Anticline.During the Tournaisian, the Ben‐Zireg Anticline, the Djebel Ioucha, the ‘Grand Erg Occidental’ and the Erfoud areas were uplifted to form palaeohighs, whereas the Zousfana and Saoura Valley areas were situated in a trough position. There were also created sub‐basins north of Ben‐Zireg; from Grouz (Teniet El Haïdoura) to Soltane El Betoum, and southeast of the Meharez High.During the Early Viséan, an inversion of the Ben‐Zireg High occurred, resulting in a widening and deepening of the Ben‐Zireg–Zousfana Trough. In the Upper Viséan formations, peri‐reefal limestone beds appear in the current position of the Djebel Béchar. Further west in the Chebket–Mennouna–Djihani area occur small patch reefs. On the eastern edge, the bioherm formations were described previously as ‘Waulsortian‐like reefs’ of the ‘Grand Erg Occidental’. These facies recorded the TST. Thus, a carbonate platform became established at the expense of the Ben‐Zireg–Zousfana Trough. Copyright © 2008 John Wiley & Sons, Ltd.

Cenomanian–Turonian facies and sequence stratigraphy, Bahloul Formation, Tunisia

The Cenomanian–Turonian Bahloul Formation in Tunisia consists of well-bedded dark grey limestones, with common local intercalations of marls and argillaceous limestones. In the Oued Bahloul section, the Bahloul Formation (28 m in thickness) directly overlies a massive bed (4 m), consisting of conglomeratic sandy limestone showing an incised base. In terms of sequences, the Oued Bahloul section shows that the Bahloul Formation is composed of deepening upward metric cycles, each comprising three units. Each cycle starts with a laminated dark grey limestone, progressively changing to argillaceous limestone, and terminates with bioturbated marls containing microfauna of the Whiteinella archaeocretacea biozone. This deepening upward cycle package, constituting the Bahloul Formation, is interpreted to be the expression of a transgressive interval directly overlying, in the Oued Bahloul area, a shelf margin wedge formed by the conglomeratic limestones. The transgressive interval ends with a maximum flooding surface (mfs) included within the filaments-rich facies. This mfs represents the upper boundary of the Bahloul Formation, which is directly overlain by a marly unit, the Annaba Member of the Kef Formation, assigned to the Helvetoglobotruncana helvetica biozone, indicating an early Turonian age. The filament event is one of four biosedimentary event horizons identified within the late Cenomanian–early Turonian interval in the Oued Bahloul section. From base to top, these four event horizons correspond to (1) the extinction of Rotalipora, (2) the Heterohelix shift, (3) the Whiteinella proliferation, and (4) the filament event. These events are recognized also in other areas in Tunisia and abroad, and we identify them here within sections preferentially located on the platform (to the south), in the basin (to the north) and especially within the transitional platform-basin environments (in Central Tunisia). Toward the SW, in Jebel Bireno, the Bahloul Formation is thinner (6 m) than in Oued Bahloul and wholly carbonate, and it was deposited in relatively shallower conditions (probably in an outer platform environment). In fact, organic-rich facies constituting the Bahloul Formation change to the south to open platform facies, in which planktonic microfauna are associated with benthonic foraminifera and rudists. However, toward the north (in the Bazina area, for example), the Bahloul facies changes to basinal marls rich in ammonites and planktonic foraminifera. Isotope analyses show that the main δ 13C positive excursions coincide with transgressive and maximum flooding surfaces. The latter are commonly used in regional and global correlations.

Hydrothermal circulation in subducting crust reduces subduction zone temperatures

Most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment, the decollement, and wedge likely transports only a minor amount of heat. We model coupled fluid and heat transport in a subduction zone and show that hydrothermal circulation in subducting basaltic basement rocks can have a great influence on subduction zone temperatures. Fractured basaltic basement has permeability several orders of magnitude higher than a typical decollement, allowing fluid circulation to redistribute and extract heat from a subduction zone. We simulate systems with upper basaltic basement permeability ranging from 10−13 to 10−10 m2. In addition, we incorporate the effect of permeability reduction within the basaltic basement as it is subducted. The models with fluid transport show suppressed temperatures along the subducting slab relative to models with no fluid transport. With continuous sediment cover, heat is extracted from under the margin wedge to the trench. In models where faulted ocean crust exposes high-permeability basement to the ocean floor, cooling from ocean bottom water results in highly suppressed heat flow relative to conductive models. Hydrothermally cooled ocean crust also acts to slow thermally controlled diagenetic reaction progress within subducting sediment.

Tectonic framework of the mud mounds, associated BSRs and submarine landslides, offshore Nicaragua Pacific margin

The regional distribution of mounds, associated bottom-simulating reflectors (BSRs) and submarine landslides of the Pacific margin of Nicaragua suggests a genetic relationship between them. In the landslide-dominated parts of the margin, mud mounds occur in groups upslope behind the scarps and aligned parallel to the headwall. The morphotectonic features associated with the slides suggest that the slope failure could be triggered by slope oversteepening on the trailing flank of subducted seamounts. Geometric analysis of the faults triggering and controlling the mud mounds and associated BSRs also indicates that they were caused by collapses of the uplifted sea floor. Thus we propose a simple conceptual genetic model for the occurrences of the submarine landslides, surrounding mud mounds and associated BSRs in the area. Seamount subduction created locally higher fluid overpressure in the décollement. The uplift and fracturing of the margin wedge above the subducting seamount opened pathways for the overpressured fluid to escape, leading to the formation of numerous mud mounds on the sea floor and the BSR in the subsurface. The higher fluid supply locally reduced the shear strength of the sediments and facilitated failure of these sediments as landslides on the oversteepened slope caused by the subduction of the seamount.

Segmentation of the Nazca and South American plates along the Ecuador subduction zone from wide angle seismic profiles

We describe the deep structure of the south Colombian–northern Ecuador convergent margin using travel time inversion of wide-angle seismic data recently collected offshore. The margin appears segmented into three contrasting zones. In the North Zone, affected by four great subduction earthquakes during the 20th century, normal oceanic crust subducts beneath the oceanic Cretaceous substratum of the margin underlined by seismic velocities as high as 6.0–6.5 km/s. In the Central Zone the subducting oceanic crust is over-thickened beneath the Carnegie Ridge. A steeper slope and a well-developed, high velocity, Cretaceous oceanic basement characterizes the margin wedge. This area coincides with a gap in significant subduction earthquake activity. In the South Zone, the subducting oceanic crust is normal. The fore-arc is characterized by large sedimentary basins suggesting significant subsidence. Velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing. Even if the distance between the three profiles exceeds 150 km, the structural segmentation obtained along the Ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation.

Relationships between tectonics and sedimentation on the northeastern margin of the Subpyrenean trough during the late Santonian

Relationships between tectonics and sedimentation on the northeastern margin of the Subpyrenean trough during the late Santonian

Methane hydrate accumulation in “Mound 11” mud volcano, Costa Rica forearc

Shallow gas hydrate accumulation in mud volcanoes in the Costa Rica forearc was postulated before, but is now proven by a find in surface sediments at the southwestern slope of the recently discovered Mound 11, a mud volcano located 30 km arcward from the trench, on the continental slope off Costa Rica at 1000 m water depth. The gas hydrate content of the recovered core was up to 60% and consisted mainly of methane hydrate. The δ 13C (−45.2‰ to −43.3‰ PDB) and δD (−125‰ to −143‰ SMOW) values of methane from sampled hydrates indicate a deep (thermogenic) source of fossil methane generated by degradation of organic matter within the subducted slab. Near surface faults and deeply cutting faults, identified in multichannel seismic reflection profiles, provide pathways for fluid migration through the ∼6 km thick margin wedge into the ∼1 km of overlying terrigenous sediments. Mound 11 overlies a bottom simulating reflection at 340 m bsf and transport of sediment and methane-rich fluids from greater depth through the gas hydrate stability zone is suggested. The upper core segment (0–150 cm bsf) is composed of mud breccia and fluid channels, which indicates mud expulsion from Mound 11. Anaerobic methane oxidation is indicated by sulfate and methane depletion, hydrogen sulfide formation and an increase of alkalinity in the interface between the upper sediment unit and the lower laminated sediment unit where the gas hydrate is interbedded. The seawater-like sulfate and chloride concentrations and the concave up chloride profile measured in pore water of the upper core unit may rather reflect seawater influx than fluid outflow at this sampling site. The inflow is possibly driven by (episodic) mud and fluid discharge in the center of the mud mound creating shallow convective circulation cells.

Neogene stratigraphic architecture and tectonic evolution of Wanganui, King Country, and eastern Taranaki Basins, New Zealand

Analysis of the stratigraphic architecture of the fills of Wanganui, King Country, and eastern Taranaki Basins reveals the occurrence of five 2nd order Late Paleocene and Neogene sequences of tectonic origin. The oldest is the late Eocene‐Oligocene Te Kuiti Sequence, followed by the early‐early Miocene (Otaian) Mahoenui Sequence, followed by the late‐early Miocene (Altonian) Mokau Sequence, all three in King Country Basin. The fourth is the middle Miocene to early Pliocene Whangamomona Sequence, and the fifth is the middle Pliocene‐Pleistocene Rangitikei Sequence, both represented in the three basins. Higher order sequences (4th, 5th, 6th) with a eustatic origin occur particularly within the Whangamomona and Rangitikei Sequences, particularly those of 6th order with 41 000 yr periodicity. The base of each 2nd order sequence is marked by marine flooding and represents a discrete phase in basin development. The Te Kuiti Sequence accumulated in non‐marine and shelf environments, whereas the Mahoenui Sequence accumulated mostly at bathyal depths; no regressive deposits are evident, having been eroded during subsequent uplift. The Mokau Sequence comprises shoreface and non‐marine deposits including coal measures. The Whangamomona Sequence accumulated during extensive flooding of King Country Basin and collapse of the eastern margin of Taranaki Basin. It has a thin retrogressive part (Otunui Formation) and a thick progradational component (Mount Messenger to Matemateaonga Formations). The latter component represents the northward progradation of a continental margin wedge through Wanganui and King Country Basins. The Rangitikei Sequence is marked by extensive flooding at its base (Tangahoe Mudstone) and reflects the pull‐down of the main Wanganui Basin depocentre. This sequence comprises a second progradational margin wedge, which migrated on two fronts, one northward through Wanganui Basin into King Country Basin, and a second west of the Patea‐Tongaporutu High, through the Toru Trough into the Central and Northern Grabens of Taranaki Basin and on to the Western Platform as the Giant Foresets Formation, thereby building up the modern shelf and slope. All five sequences are erosionally truncated, with up to 2100 m of exhumation that affected much of central North Island. The Pliocene timing of the start of erosion, together with its pattern, point to long wavelength deformation of the crust driven from the upper mantle consequent upon migration of the late Miocene andesitic volcanic arc from northern Taranaki Basin into Taupo Volcanic Zone.

Late Miocene to early Pliocene biofacies of Wanganui and Taranaki Basins, New Zealand: Applications to paleoenvironmental and sequence stratigraphic analysis

The Matemateaonga Formation is late Miocene to early Pliocene (upper Tongaporutuan to lower Opoitian New Zealand Stages) in age. The formation comprises chiefly shellbeds, siliciclastic sandstone, and siltstone units and to a lesser extent non‐marine and shallow marine conglomerate and rare paralic facies. The Matemateaonga Formation accumulated chiefly in shelf paleoenvironments during basement onlap and progradation of a late Miocene to early Pliocene continental margin wedge in the Wanganui and Taranaki Basins. The formation is strongly cyclothemic, being characterised by recurrent vertically stacked facies successions, bounded by sequence boundaries. These facies accumulated in a range of shoreface to mid‐outer shelf paleoenvironments during conditions of successively oscillating sea level. This sequential repetition of facies and the biofacies they enclose are the result of sixth‐order glacio‐eustatic cyclicity. Macrofaunal associations have been identified from statistical analysis of macrofossil occurrences collected from multiple sequences. Each association is restricted to particular lithofacies and stratal positions and shows a consistent order and/or position within the sequences. This pattern of temporal paleoecologic change appears to be the result of lateral, facies‐related shifting of broad biofacies belts, or habitat‐tracking, in response to fluctuations of relative sea level, sediment flux, and other associated paleoenvironmental variables. The associations also show strong similarity in terms of their generic composition to biofacies identified in younger sedimentary strata and the modern marine benthic environment in New Zealand.

Are rupture zone limits of great subduction earthquakes controlled by upper plate structures? Evidence from multichannel seismic reflection data acquired across the northern Ecuador–southwest Colombia margin

Subduction of the Nazca plate beneath the Ecuador‐Colombia margin has produced four megathrust earthquakes during the last century. The 500‐km‐long rupture zone of the 1906 (Mw = 8.8) event was partially reactivated by three thrust events, in 1942 (Mw = 7.8), 1958 (Mw = 7.7), and 1979 (Mw = 8.2), whose rupture zones abut one another. Multichannel seismic reflection and bathymetric data acquired during the SISTEUR cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones. The Paleogene‐Neogene Jama Quininde and Esmeraldas crustal faults define a ∼200‐km‐long margin crustal block that coincides with the 1942 earthquake rupture zone. Subduction of the buoyant Carnegie Ridge is inferred to partially lock the plate interface along central Ecuador. However, coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge. We report on a newly identified Manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones. During the earthquake cycle the fault is associated with high‐stress concentration on the plate interface. An outer basement high, which bounds the margin seaward of the 1958 rupture zone, may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault. Coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami. We propose a model of weak transverse faults which reduce coupling between adjacent margin segments, together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake.

Microboring and taphonomy in Middle Oxfordian to lowermost Kimmeridgian (Upper Jurassic) from the Prebetic Zone (southern Iberia)

Microboring was investigated through thin section analysis of Oxfordian rocks from the Prebetic Zone (Betic Cordillera, southern Spain). Microboring description and distribution were analysed, as was their correlation with traits such as the type and size of bioclasts and microtaphonomic features (encrustation and fragmentation). Single and branching microboring are the two major morphological groups differentiated. Microborers were not identified but by their patterns of distribution in the bioclasts and in the environmental setting are assumed to be phototrophic. Microboring only affected bioclasts, with higher values of microboring index ( MB i) registered in thicker ammonoid shells and serpulids, and lower ones in ostracods, foraminifera and small bivalves, gastropods and echinoderms. A very close, direct relationship exists between bioclast size and microboring abundance, as well as between the microboring ( MB i) and encrustation indexes ( E i) used. In contrast, a clear relationship between MB i and the fragmentation index ( F i) has not been proved. Microboring activity has been interpreted to be related to both bioclast-size and time exposure of bioclasts on the seafloor, the latter resulting directly proportional to the rate of sedimentation in low energy environments. The studied features ( MB i, F i, E i and bioclast mean size) were analysed taking into account the stratigraphic intervals investigated (I=Transversarium Zone to lower Bifurcatus Zone, II=upper Bifurcatus Zone, III=Bimammatum Zone, and IV=Planula Zone p. p.), and the trends registered were interpreted as a final consequence of relative sea-level fluctuations. Stratigraphic intervals I and IV, characterised by higher values in MB i and E i, were related to lower sedimentary rates during increasing distality in a context of relative sea-level rise, which is consistent with the development of Transgressive System Tract (TST) conditions. Stratigraphic intervals II and III, showing lower values in MB i and E i, are related to higher sedimentation rates and decreasing distance from shore during relative sea-level falls, which are in accordance with developing Highstand System Tract (HST) and Shelf Margin Wedge (SMW) conditions. Data obtained and interpretations agree with the general ecostratigraphic context in the studied area previously proposed by the authors.

Microboring and taphonomy in Middle Oxfordian to lowermost Kimmeridgian (Upper Jurassic) from the Prebetic Zone (southern Iberia)

Abstract Microboring was investigated through thin section analysis of Oxfordian rocks from the Prebetic Zone (Betic Cordillera, southern Spain). Microboring description and distribution were analysed, as was their correlation with traits such as the type and size of bioclasts and microtaphonomic features (encrustation and fragmentation). Single and branching microboring are the two major morphological groups differentiated. Microborers were not identified but by their patterns of distribution in the bioclasts and in the environmental setting are assumed to be phototrophic. Microboring only affected bioclasts, with higher values of microboring index (MBi) registered in thicker ammonoid shells and serpulids, and lower ones in ostracods, foraminifera and small bivalves, gastropods and echinoderms. A very close, direct relationship exists between bioclast size and microboring abundance, as well as between the microboring (MBi) and encrustation indexes (Ei) used. In contrast, a clear relationship between MBi and the fragmentation index (Fi) has not been proved. Microboring activity has been interpreted to be related to both bioclast-size and time exposure of bioclasts on the seafloor, the latter resulting directly proportional to the rate of sedimentation in low energy environments. The studied features (MBi, Fi, Ei and bioclast mean size) were analysed taking into account the stratigraphic intervals investigated (I=Transversarium Zone to lower Bifurcatus Zone, II=upper Bifurcatus Zone, III=Bimammatum Zone, and IV=Planula Zone p.p.), and the trends registered were interpreted as a final consequence of relative sea-level fluctuations. Stratigraphic intervals I and IV, characterised by higher values in MBi and Ei, were related to lower sedimentary rates during increasing distality in a context of relative sea-level rise, which is consistent with the development of Transgressive System Tract (TST) conditions. Stratigraphic intervals II and III, showing lower values in MBi and Ei, are related to higher sedimentation rates and decreasing distance from shore during relative sea-level falls, which are in accordance with developing Highstand System Tract (HST) and Shelf Margin Wedge (SMW) conditions. Data obtained and interpretations agree with the general ecostratigraphic context in the studied area previously proposed by the authors.

Deep structures of the Ecuador convergent margin and the Carnegie Ridge, possible consequence on great earthquakes recurrence interval

The deep structure of the Ecuador subduction zone and adjacent Carnegie Ridge (CR) was investigated using on‐shore off‐shore wide‐angle seimics. A crustal model obtained by 2‐D inversion of traveltimes reveals the overthickened (14 km) oceanic crust of the CR that underthrusts the high velocity (>6 km/s) basement of the upper plate margin wedge, interpreted as part of the accreted oceanic terranes described on‐shore. The plate interface dips 4° to 10° east from the trench to a depth of 15 km. Shadow zones observed on the margin OBS records are interpreted as a low velocity zone consisting of a thin layer of underthrust sediments, and the 3‐km‐thick CR layer 2, whose average seismic velocity of 5.1 km/s is slower than that of the margin wedge. Increased interplate coupling related to the subduction of the thick, buoyant CR may account for an apparent local increased recurrence interval between great interplate earthquakes.

Comparison of three surgical procedures of differing complexity in the correction of trachomatous upper lid entropion: a prospective study

objective To compare the efficacy of three common surgical procedures of increasing complexity in the correction of trachomatous entropion. materials and methods In a prospective study, lids with moderate or severe (without lid gap) trachomatous entropion were randomly allocated to undergo either terminal tarsal rotation (I, n = 30), tarsal rotation with tarso-conjunctival advancement (II, n = 30), or anterior lamellar repositioning with lid margin split and wedge resection of tarsus (III, n = 30). The procedures were compared for improvement of symptoms, duration of surgery, cosmesis, rate and type of complications, anatomical correction, failure and recurrence. One-way and repeated-measure ANOVA, Chi-square and Fisher's exact tests were used. results The study included 90 eyes of 77 patients (age range: 30-85 years). Symptomatic improvement was comparable after each procedure (p > 0.05). Procedure I, the simplest in technique, took significantly less time (p < 0.001). The three procedures were comparable in achieving cosmesis (p = 1.0), anatomical correction (p = 0.35), and rate of complications (p = 0.43). Failure of surgery was seen in two lids (procedure II), and recurrence in one lid (procedure III). conclusion In developing countries, where manpower and other resources are limited and patient-load high, ophthalmic surgeons should choose a procedure that is simple, quick and effective. This study suggests that terminal tarsal rotation after transverse tarsotomy should be the procedure of choice in the correction of moderate or severe (without lid gap) trachomatous entropion.

The Speading Pattern of Ambilateral, Cogged Belted Plot Fertilizer Distributors

The mechanization of fertilizer distribution in the case of different types of fertilizers often poses problems. As a result, I have designed and constructed a brand new type of plot fertilizer distributor. This machine of moderate price can be controlled easily; it provides accurate distribution and versatility. The main part of this machine is an ambilateral cogged belt. I briefly present the operation and the advantages of cogged belts.I investigated the spreading patterns at the front and the end part of the experimental plot. I demonstrate a method never seen so far in the investigation of spreading patterns. My method also shows marginal divergences and flat wedge effects in spreading patterns.

Heat flow over the descending Nazca plate in central Chile, 32°S to 41°S: observations from ODP Leg 202 and the occurrence of natural gas hydrates

Bottom simulating reflectors (BSRs) were detected in multichannel seismic reflection data acquired in the vicinity of Isla Mocha across the southern Chile margin and near 33°S. Geothermal gradients were determined from the depth of the BSR that is interpreted to mark the thermally controlled base of a gas hydrate layer. Ground truth for the assessment and additional thermal constraints were provided by downhole measurements obtained during Ocean Drilling Program (ODP) Leg 202 in Site 1233 at 41°S and Sites 1234 and 1235 near 36°S. Both BSR-derived data and downhole temperatures were used to calculate heat flow anomalies and provide new constraints on the thermal regime of the continental slope and downgoing slab in Chile between 32°S and 41°S. Downhole chemical logs of Th, U, and K from Site 859 of ODP Leg 141 have been used to assess the radiogenic heat production in the margin wedge. Heat production is low (∼0.8 μW/m3). However, knowledge of this reduces the errors of estimating the contribution from frictional heating along the subduction thrust fault. With respect to the Eocene age of the incoming oceanic lithosphere, heat flow appears to decrease landward of the deformation front as expected due to the advective transport of heat into the subduction zone by the downgoing slab. Calculations of conductive fore-arc heat flow show that the modelled seafloor heat flow agrees with the measured heat flow only if there is negligible frictional heating. At 33°S, temperatures in the fault zone reach 100°C approximately 60 km landward of the deformation front and are coincident with the onset of earthquake activity and hence mark the up-dip limit of the seismogenic zone. The up-dip limit shifts seaward going to the south, reflecting the progressive southward decrease of lithospheric age of the subducting plate.

Mixed siliciclastic–cool-water carbonate deposits over a tide-dominated epeiric platform: the Faluns of l’Anjou formation (Miocene, W. France)

The Tortonian bioclastic sands of Anjou (W France) are weakly cemented mixed siliciclastic–carbonate deposits. The carbonate fraction can reach up to 90%, and is irregularly spread over the area of the original depositional platform. The temperate marine water character is demonstrated by the lack of ooids, green algae, and biohermal scleractinians, and is dominated by numerous species of bryozoans and bivalves, associated with red algae, barnacles, and echinoderms (bryomol facies; Heterozoan association). Skeletal grains are weakly cemented. The presence of large submarine dunes indicates a platform bounded by a tide-domination of a succession that developed rapidly under highstand and shelf margin wedge system tracts. The tide-dominated “Faluns de l’Anjou” provides a model different from many other examples of temperate carbonate settings, which are often wave-dominated.

Magnetic fabrics from the Costa Rica margin: sediment deformation during the initial dewatering and underplating process

Drilling off of the Costa Rica margin during Ocean Drilling Program Leg 170 penetrated the complete sediment section seaward of the Middle Americas Trench (MAT) at one location (Site 1039), and sections through the margin wedge, décollement, and subducting sediments at two locations (Sites 1040 and 1043) near the toe of the wedge. Analyses of these sediments and their structures indicated that the margin wedge is not constructed by offscraping of the present incoming sediments, but is an older (perhaps accreted) structure. Nearly the entire section of sediments entering the MAT is thrust beneath the margin wedge. Anisotropy of magnetic susceptibility (AMS) and paleomagnetic data provide insight into the initial stages of fabric development associated with formation of the active décollement fault. Within the décollement zone, fabric development is localized to the upper few meters of the décollement, as indicated by higher AMS P values and by fabric orientations. Below the décollement, the uppermost 180 m of the underthrust hemipelagic sediment section is also deformed, as indicated by comparison of fabrics and their orientations between the seaward reference site and the sites that penetrated the décollement. AMS foliations become progressively more anisotropic, and the orientation of these fabrics, following reorientation of the cores using paleomagnetism, dip landward within the underthrust sediment package. These data indicate that deformation involves thinning of the incoming sediment package by compaction produced by distributed shearing of the upper portion of the sediment package. The fabric data suggest that the uppermost 180 m of the underthrust sediments are undergoing incipient underplating beneath the margin wedge.