Seismic structure, gravity anomalies, and flexure of the Amazon continental margin, ne brazil




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Seismic structure, gravity anomalies, and flexure of the Amazon continental margin, NE Brazil


D R A F T - 2827/10/11/08A. B. Watts1, M. Rodger1, C. Peirce2, C. J. Greenroyd2, R. W. Hobbs2

1Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK

2Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK

A. B. Watts, M. Rodger, C. Peirce, C. Greenroyd, R. W. Hobbs


Abstract

We have used seismicSeismic and gravity data gave been used to determine the structure of the sediments, crust and upper mantle that underlie the Amazon continental margin, offshore NE Brazil. Seismic reflection profile data reveal a major unconformity at ~7 s Two Way Travel Time (TWTT) twhichhat we interpret as marking the onset of the transcontinental Amazon Rriver and the formation of the Amazonthe Amazon a deep-sea fan system in the mid-Late Miocene. Seismic refraction data show mean sediment velocities that decrease by > 1.5 km s-1 in a seaward direction. We attribute this decrease decrease to facies changes associated with the oceanward sediment progradation of the sediment andand the development of topset, foreset and bottom set beds. Seismic refraction data show that the sediments are underlain by oceanic crust which that has a similar velocity structure, but is unusually thin (~4.2 km) compared to elsewhere in the Atlantic Ocean. We attribute the thin oceanic crust to either slow seafloor spreading or a limited magma supply during the initial rifting of South America and Africa in the Early Cretaceous. The seismic data has been used to construct a new sediment thickness grid, which, that, together with gravity anomaly data, suggests the together with gravity anomaly data, shows that the sediments of the Amazon fanfan loaded a lithosphere with an unusually high flexural strength. Whilst a flexure model explains the gravity data well, therTheree areWhile a high-strength lithosphere explains the overall depth of the seismic Moho, there are discrepancies (of up to 2 km)) between beneath the upper fan, where the modelled flexed Moho is shallower than the seismic Moho, and beneath the middle fan where it is deeper. the depths of the calculated flexed Moho and the observed seismic Moho, however,. Gravity which gravityGravity and seismic modelling suggest these discrepancies these discrepancies are caused by lateral changes in the sub-crustal mantle mantle density such that the sub-crustal mantle that underliesmantle underlying the upper fan is denser than it is beneath the lower middle fan. We attribute these lateral density differences to proximity to the Ceara Rise, which is believed to have formed during the Late Cretaceous at a mid-ocean ridge. Fan loading of a relatively strong, dense and, hence cold, lithosphere predicts stress orientations that are consistent with borehole break-out data and the location and height of inlandionshore, presumed marine, (marine?) terracesthe GurupéGurupe Arch onshore. Despite its proximity to ‘“leaky’” transform faults and fracture zonesEquatorial Atlantic Fracture zones,, the margin that underlies the Amazon fan margin appears to be of non-volcanic origin. The main differences s with other non-volcanic margins, such as West Iberia and Newfoundland, is a thicker greater sediment accumulation, a narrower transitizone of transitional crustonal zone, and a lack of any any evidence for extreme extension and mantle serpentinisation.


Keywords: Continental margins. Seismic reflection and refractionStructure. Sediment thickness. Crust and mantleMarine geophysics. Crustal structure. Gravity Anomalies. Isostasy. Lithospheric Flexuresy and flexure of the lithosphere. Stress. Isostasy..Flexure. Mantle


Introduction


Rifted continental margins form by the break-up of continents and the development of new ocean basins. While the mechanics of break-up is still not fully understood, most workers invoke some form of heating and thinning of the lithosphere at the time of rifting [e.g. (e.g. [Buck et al., 1999]; – volcanic margins; [Lavier and Manatschal, 2006] non-volcanic margins). Initially, regional uplift may dominate, but as extension proceeds, the rifted crust thins which causes a localized subsidence and the accumulation of syn-rift sediments on one or more “hanging walls” which are separated from “footwalls” by growth on normal faults. Following the cessation of rifting, the heated and thinned sub-crustal mantle cools and post-rift sediments accumulate, usually over by overstepping much wider areas than the syn-rift. Syn-rift and post-rift sedimentary sequences are often separated by a basin-wide unconformity.


The thickness of syn-rift to post-rift sediment in a rift-type basin depends on the mode of extension as well as other factors such as the sediment flux. Simple shear TheThe “pure-shear” models model of extension [McKenzie [, 1978] predicts that predicts that the ratio of syn-rift to post-rift water-filled subsidence in a rift-type basin is similar, irrespective of the amount of heating and thinning. Therefore, if the sediment flux is uniform throughout margin evolution then there should be an equal proportion of syn-rift to post-rift sediment. Indeed, many margins have an approximately equal thickness of syn-rift and post-rift sediment. However, others have a significantly more post-rift than syn-rift sediment. This has been attributed at some margins (e.g. Labrador Sea) to depth-dependant extension [e.g Royden and Keen 1980] which can limit the thickness of the syn-rift compared to the post-rift. However, in others such as the northern Gulf of Mexico [Galloway, 1989], Niger [Doust and Omatsola, 1990], and Voring Plateau margins margin which rifted in the Triassic and was loaded by a prograding river delta in the Cenozoic [Galloway, 1989], Niger margin which rifted in the Cretaceous and was loaded }; in the Late Eocene [Doust and Omatsola, 1990], and the Voring Plateau margin which rifted in the Paleocene and was loaded in the Pliocene-Pleistocen}e [[; Kjeldstad, et al., 2003] this has been attributed to loading late in their evolution by unusually large prograding prograding delta systems.


The imposition of large, localized, loads, late in the history of a rifted margin, , on a rifted margin has implications for its thermal and mechanical evolution. This is particularly well seen at margins that have been subject to large orogenic loads. For example, thrust and fold loading of the Guiana transform margin has moved the entire post-rift sedimentary sequence from relatively shallow to deep depths and into the oil and gas window [Summa, et al., 2003] whilst in Taiwan, similar loads have re-activated the syn-rift faults of the underlying South China sea rifted margin and caused seismicity [Lin and Watts, 2002]. Margins that have been loaded by large, localized, sediment loads might therefore be not only be therefore be not only good targets for oil and gashydrocarbon eprospectivexploration, but be ‘tipping points’ for subduction initiation [Cloetingh, et al., 1982].


In 2003, during cruise D275 ofduring cruise D275 of RRS Discovery we carried out the first coincident seismic reflection and refraction study along 43 transects of the Amazon fan margin, offshore NE Brazil. Previous studies suggest that the fan is a large, localized, sediment load that was emplaced during the Late Miocene on an Early Cretaceous rifted margin following uplift in the Bolivian Andes and the development of the trans-continental Amazon River. The margin therefore makes an ideal focus site for ‘source to sink’ studies. [Rodger, et al. [, 2006] describe the preliminary seismic and gravity results along from Line B, a single coincident seismic reflection and refraction transect of the marginfan while Greenroyd et al. [2007] present the results from Line A, a coincident transect just to the north-west of the fan, arguing that the fan had loaded relatively thin oceanic crust and strong mantle. We present in this paper the seismic reflection and refraction datathe results from the the 2 other main transects (Lines E and F) acquired during D275 on the upper part of the fan. The combined data set has been used, together with the available industry data , to construct a new sediment thickness grid. We have used the this grid, together with 3D flexure and gravity modelling techniques, to quantify the role of lithospheric flexure in contributing to the stress state., t onshore landscape evolutionthe onshore topography, and crustal and mantle structure of the Amazon fan margin.


Geological and geophysical setting


For much of its 1000 km length, the NE Brazil margin displays the morphology of a typical Atlantic-type passive continental margin with a shelf, slope, and rise. However, in the region ofseaward of the modern Amazon delta the slope and rise locally widens (Fig.Figure 1) and individual bathymetric contours are deflected outwards seaward by up to a few hundred km (Fig.Figure 2).


The regional geology of the Amazon margin has been known since the work of [Edgar and Ewing [, 1968] and [Hayes and Ewing [, 1969, 1970]. These workers used 2 ship seismic refraction data to argue that the margin “contains up to 11 km of low- and high-velocity sedimentary and possibly volcanic rock that overlies a thin (3 km) oceanic layer.” [Campos, eet al. [, 1974] suggested proposed thatsuggested the tectonic configuration framework of the NE Brazil margin was established in the Early Cretaceous. According to them, rifting was followed by magmatism, possibly associated with the St Paul’s and Romanche Fracture Zones, which peaked in the Late Cretaceous to Early Tertiary. The magmatism was, in turn, followed d, in turn, by the accumulation of a thick wedge of marine sediments, which led to the development of growth faults and “huge” roll-over anticlines.


[Damuth [, 1975] used echo-sounder and bottom sampling data during the program tto argue that the deflection of the bathymetric contours at the Amazon margin was caused by a deep-sea fan system that was supplied with sediment by the transcontinental Amazon and Para river systems. [Kumar [, 1978] compiled all the available seismic data and used it to construct a the first sediment isopach map of the region. He used the result of recent scientific ocean drilling (DSDP Site 354) on the Cearra Rise to date a prominent seismic reflector (l, labelled by him as LM), as Llate Miocene (~6 Ma) and suggested from extrapolation that the fan initiated in the Eearly Miocene (~22 Ma).


Subsequent re-examination of the echo-sounder, bottom sampling and seismic data [Damuth and Embley, 1981], together with GLORIA side-scan sonar and high-resolution seismic reflection profile data, led [Damuth, et al. [, 1983] to the recognition of a succession of channels and associated levee structures, suggesting that sediment had been supplied to the fan at distinctspecific times in and along distinct pathways. They suggest that it would have taken~ 2-8 Myr to deposit all these levees sequentially. Presently, the fan is inactive. S and sediment is accumulation is occurringaccumulatingaccumulation is limited on to the inner and middle shelf in the Amazon delta [Nittrouer, et al., 1996]. The delta formed at a time of high present day sea-level above relict sands that date from the low stand associated with the Last Glacial Maxima (LGM). However, the presence of sub-marine terraces suggest that the delta continues to be influenced by sea-level, particularly during rate changeschanges in its rate, as occurred, for example, during the Younger Dryas [Hubscher, et al., 2002].


Plate tectonic reconstructions report [e.g. Asmus, 1981] suggest that the main tectonic elements of the margin werethe Amazon margin formed following the the initiation of rifting apart of South America and Africa during the Neocomian-Barremian (~130 Ma). Satellite-derived gravity) and that it that anomaly data confirm suggest that margin development was influenced by fracture zonestransform faults at the mid-oceanic ridge. The North Brazilian Ridge (NBR, Fig.Figure 2), for example, is a chain of small seamounts that mark the westward extension of the St. Paul’s Fracture Zone which intersects the margin at ~1o N. Seismic reflection profiles of the shelf [Pereira da Siva, 1989] show it to be underlain by half-grabens and fault-bounded tilted blocks which progressively down-step in a seaward direction. The half-grabens are infilled by syn-rift fluvial and lacustrine facies of Albian-Aptian and, possibly, Triassic age. The syn-rift is, in turn, overlain by a thick Cretaceous to Oligocene sedimentary post-rift sequence that is dominated by an extensivethean development during the Paleogene of an extensive Paleogene carbonate platform beneath the outer shelf [Carozzi, 1981].


The Amazon margin has long been of commercial interest and a number of deep wells and seismic reflection profiles have been acquireddrilled on the shelf and slope. While a large number of deep commercial wells have now been drilled in the Amazon margin Unfortunately, few details of their the lithology, paleoenvironments, andor biostratigraphy of these wellshic age have been published. [Silva and Maciel [, 1998], however, presented a geohistory analysis [Van Hinte, 1978] of a reconstructed stratigraphic section at a “Point A” near the shelf break. Fig. 3a shows that theaThe section shows a gentle increase in the sediment accumulation gently increased from 115 to 6.6 Ma and thenthat was followed by an abruptly increased due to the emplacement of the Amazon fan (Figure 3a). The backstripBackstripping [Steckler and Watts, 1978] . Fig. 3b shows that the backstrip curve has therevealsshows general form of ana relatively smooth exponential exponential decrease to the present daydecrease in the tectonic subsidence, indicating that the Late Miocene increase in sediment flux to the fan and associated decrease in water depth have been reasonably well accounted for. The backstrip curve curve is similar to what would be predicted if all the sediments that had accumulated at the sitePoint A had been deposited on 115 May oceanic crust. However,The same curve could, however, also the curve could also be explained, however, by a stretching stretching model in which the continental crust rifted from 115 to 90 Ma for 25 Myr and thinned by at least a factor of ~3.2. The rift durationTheis rifting history is generally consistent with the Early Cretaceous Caciporé formation [Brandao and Feijo, 1994] as being a syn-rift deposit and the thin crust is in accord with gravity anomaly data [Cochran, 1973; ], [Mello and Bender, 1988; 8], [Braga, 1991], although these datathe latter data are unable to distinguish whether the fan is underlain by oceanic or continental crust.


Recently, commercial exploration of the Amazon margin has been opened up to foreign companies and BP have was has been able to acquireacquired a closely spaced network of 2-D and 3-D seismic and well data in 2 two largetwo ‘’blocks’ on the outer shelf and upper fan (Thick grey line, Fig.Figure 2). Two regional reflection profiles (Profiles 2 and 5) were published by [Cobbold, et al. [., 2004], together with interpretations based on data from > 50 wells showing top Pliocene, top mid-Miocene, top Oligocene, top Cenomanian and the depth to the pre-rift basement. The data show that since ~10.5 Ma (i.e. Middle/Late Miocene), up to 10 km of clastic clastic sediment has accumulated in the Amazon fan region. They also confirm suggest that the outer shelf and upper fan is are characterised by “thin-skin” style extensional and compressional structures that root into a common basal detachment surface, some hundreds of km long.


The first seismic refraction profile data acquired over the NE Brazil margin using fixed ocean bottom instrument receivers were published by Rodger et al. [2006] and Greenroyd et al. [2007]. These data support the earlier suggestions of Hayes and Ewing [, 1969, 1970] and Houtz et al. [1978] that the Amazon fan is underlain by at least 10 km of sediment and unusually thin oceanic crust. Rodger et al. [2006] implied that Amazon fan had been emplaced on a rift-type margin, attributing the thin crust to ultra-slow spreading. Greenroyd et al. [2007], however, suggested that that itthe French Guiana margin some 200 km to the NW of the fan developed in an oblique setting and that the thin crust was most probably due to accretion at one or more closely spaced transform faults.

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