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




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


We draw the following conclusions from this study:


  1. New seismic data show the Amazon margin is underlain by >9at least 10 km of sediment, approximately a significant part of which 25% of which washas been deposited in the asince the mid-Late Miocene in a Amazon deep-sea fan systemn system.




  1. The sediments overlie is underlain by crust which that has a similar velocity structure to oceanic crust, but is unusually thin. The thin crust extends over distances of up 250 km and is attributed to either slow seafloor spreading or reduced magma supply following the initial break-up of South America and Africa in the Early Cretaceous.




  1. The sediment load of the Amazon fan3D process-oriented gravity and flexure modelling suggests the fan hashas flexed the underlying continental and oceanic basement lithosphere by > 2 km over a horizontal distance of > 500 km. Comparisons of the observed flexure the observed flexure to predictions of simple elastic plate models suggest that the fan and was emplaced on oceanic lithosphere that has increased its strength with age since rifting that is unusually strong.




  1. Discrepancies between seismic Moho and the flexed MohoComparisons of the gravity effect of the seismic structure to observed free-air gravity anomaly data suggest are suggestive of lateral changes in the sub-crustal mantle density with the upper fan overlying a denser mantle than the lower fan.




  1. The origin of these lateral mantle density differencesdensity changes in the mantle is not clear, but it they might be due to proximityrelated to the thermal structure of the Cearra Rise, an oceanic plateau that formed on or near a mid-ocean ridge during the Late Cretaceous/Early Tertiary.




  1. The emplacement superposition of the sediments of the Amazon fan on a strong, dense, and hence cold lithosphere creates horizontal stresses and flexures that extend into areas that flank the main region of extend for some hundreds of kmsubsidence. accounts for TheseThe stresses and flexures are consistent with with thethe stress field implied by borehole break-out data and with the evolution of the main landforms of the Amapá shelf coastal plain region.landscape evolution models……




  1. The seismic and gravity data suggest that despite its proximity to fracture’leaky’ transform faults zones (and the Ceara Rise) that the Amazon margin is a non-volcanic margin. The main differences with other non-volcanic margins (e.g. West Iberia, Newfoundland) is the thicker sediment and the fact that it has a relatively narrow ocean/continent transition zone and lacks any evidence of extreme extension and mantle serpentinisation.


Acknowledgements


We are grateful to the Master, Officers, and Ccrew of the RRS Discovery, the support staff and sea going technicians of NERC’s UKORS and David Close (Oxford), Tiago Cunha (Oxford) and Luis Drehmer (LAGEMAR, Niteroi) ffor their help at sea

. This work was supported by the NERC/LINK Ocean Margins programmeWe thank Anna Krabbenhoeft and Cord Papenberg (IFM-GEOMAR) and Tom Oliva (Seamap, UK) for their ocean bottom instrument support, BP, Petrobras and GETECH (UK) for providing unpublished data prior to the cruise, and Alberto Figueiredo (LAGEMAR, Niteroi) and Jesus Berrocal (University Sao Paulo) for logistical help. This work was supported by thethe NERC/LINK Ocean Margins programmeprogramme. and a tied graduate studentship.


Figure Captions


Figure 1. Perspective view (azimuth = 135 o, elevation = 30o) of the topography of the NE Brazil continental margin in the vicinity of the Amazon river, delta, and deep-sea fan. The topography has been constructed from a 1 1 minute GEBCO [GEBCOBODC, 20031975] grid. The dotted lines show (from west to east) the 35, 100 and 4000 m bathymetric contours. The thick dashed yellow line shows the approximate landward limit of the subsidence caused by the load of the Amazon fan, details of which are shown in Figure 16c.


Figure 2. Location map of the Amazon fan, offshore the NE Brazil continental margin. The contours show bathymetry at 500 m intervals, except on the shelf where 20 m contours are shown (dashed lines). The thick grey solid lines show the location of the mainthe RRS Discovery cruise D275 crustal transects, Lines A and B. The thick grey dashed line show the transect along which the data of Houtz et al. [1978] has been projected. Filled black triangles locate the oOcean bBottom iInstruments deployed during RRS Discovery cruisecruise D275 D275. Large triangle shows the location of OBS 406, arrivals at which are shown in Fig.Figure 6. Solid red lines show the location of seismic reflection profiles acquired during D275. Note that seismic reflection profile Line B ends at the southernmost OBS/OBH. Solid orange lines show the location of Petrobras 18 s reflection lines. The grey box seaward of the shelf break delineates the blocks where BP and partners have acquired 2D and 3D seismic and well data. The tectonic elements (grey lines) are based on [Carozzi [i, 1981]. Grey lines with short bars show Miocene growth faults on the upper slope. Grey lines with long bars show syn-rift (Early Cretaceous) growth faults on the shelf. MG = Mexiana Graben. LAB = Lower Amazon Basin. MAB = Middle Amazon Basin. FZ = Fracture Zone. NBR = North Brazilian Ridge. Filled blue circles show the location of selected Petrobras wells. Filled white circles with an interior black circle show scientific ocean drilling sites (DSDP Leg 39; ODP Legs 154, 155, 205). Large filled black square shows the location of “Point A” of [Silva and Maciel [, 1998].


Figure 3. Subsidence analysis at Point A (Fig.Figure 2). a) Geohistory analysis showing paleobathymetry and sediment accumulation through time at Point A based on [Silva and Maciel [, 1998]. b) Tectonic subsidence obtained by backstripping the paleobathymetry and stratigraphy data in a). Solid line shows the sediment accumulation through time. Dashed line shows the accumulation corrected for compaction using the assumed Porosity Vs. Depth curve shown in the inset. The curve is generally based on the lithology models of Carozzi [1981]. Grey shade shows the tectonic subsidence. Thick solid line shows the predicted subsidence of a 115 Ma mid-oceanic ridge based on a cooling plate model. Thick dashed lines show the predicted subsidence of continental lithosphere that has been stretched for 25 Myr during 115 to 90 Ma by factors of 1.5, 3.0 and 4.5.


Figure 4. Seismic reflection profile of the Amazon margin, offshore NE Brazil. The profile has been constructed from a merge of Line 2 3 of BP and Line B of [Rodger, et al. [, 2006]. The inset map shows the location of the merged profile. The coloured lines on Line 2 3 show prominent seismic reflectors that have been dated by BP on the basis of well-ties. Black lines show structural interpretations.


Figure 5. Seismic reflection profile of a portion of Line B (Fig.Figure 4) showing reflector terminations (black horizontal arrows) in the region of unconformity U1 (red dashed line)..


Figure 6. Seismic reflection profile Line E (Figs. 2, 4) over the upper and middle Amazon fan. The data has been processed using standard techniques. The figure shows a final time-migrated stack displayed as a weighted “super-gather” of every 5 Common Mid-Points (CMPs). a) Final stack. b) Grey-shaded final stack with interpretation. Top of oceanic crust is recognised as a rough, high-amplitude, reflector at ~8.5 s Two-Way Travel Time (TWTT). The overlying sediments have been sub-divided into 4 main units on the basis of their reflector character and geometry. The unit ages are tentative and are based on correlations with nearby BP seismic and well data. U1 = Middle/Late Miocene unconformity. Thick horizontal arrows show reflector terminations. 1, 2 show individual channel-levee systems identified by [Damuth, et al. [, 1983]. The systems form part of the Western Levee Complex is composed of several individual, coalescing systems. Short thick vertical lines show ODP sites that are within 5 km of Line E.


Figure 7. Seismic data recorded on the hydrophone component of OBS 406 on the middle Amazon fan (Fig.Figure 2). a) Original data, band-passed to improve clarity, showing water wave, sediment, crust, and mantle arrivals. b) Comparison of picked arrivals (thick lines) to the modelled arrivals (thin lines). The modelled arrivals have been computed using the velocity model shown in c) and RAYINVR [Zelt and Smith, 1992]. Note the limited range (~5 km) over which the Pg event appears as a first arrival. This is due to a combination of relatively high velocities in the lower part of the sediment ‘wedge’ and the unusually thin oceanic crust that underlies it. S = Sediment. OC = Ocean Crust. c) Ray trace of the best fitbest-fit velocity model.


Figure 8. The best fitbest-fit velocity model, obtained by combining arrivals from all 8 OBSs and OBHs along Line F (Fig.Figure 2). Thick lines show model layer boundaries that are generally marked by small increments in velocity. Thin lines show velocity contours at 0.25 km s-1 intervals. The model shows the seafloor and an up to 12 km thick sedimentary layer that overlies a ~4 km thick crust with oceanic crust-type velocities.


Figure 9. Seismic reflection profile Line F over the upper and middle Amazon fan with selected Velocity Vs. TWTT profiles superimposed. The reflection profile has been processed using standard techniques. The figure shows a final time-migrated stack displayed as a weighted “super-gather” of every 5 Common Mid-Points (CMPs). The thick lines show a plot of Velocity Vs. TWTT at OBS402, OBS404, OBS406 and OBS408. Note the decrease in velocities in the sediment layers in a seaward direction and the abrupt increase in velocity at the top of the oceanic crust and at the ‘layer2/layer3’ boundary. There is a hint of a Moho reflection at ~10 s TWTT at the western end of the profile. Thick horizontal arrows show reflector terminations that delineate unconformity U1. The profile crosses the eastern levee complex of [Damuth, et al. [, 1983] and the slump debris flow that flanks it to the east.


Figure 10. Sediment thickness maps of the NE Brazil margin in the region of the Amazon fan. The maps have been constructed from the previous compilations by [Kumar [, 1978], and [Braga [, 1991], and BP (P. Bentham, pers. comm..) as well as the seismic data presented in this paper. a) Middle/Late Miocene to Rrecent. b) Early Cretaceous – Late/Middle Miocene. c) Total. Numbers in the insets show sediment volumes in km3.


Figure 11. Comparison of observed and calculated gravity anomalies along Line B (Fig.Figure 2). a) Observed and calculated gravity anomalies. The observed gravity (filled circles and squares) is based on D275 and other shipboard data (offshore) and GETECH land data (onshore) and has had a long wavelength field to degree and order 20 (i.e. wavelengths > 2000 km) subtracted removed from it. The calculated gravity is based on the density and elastic parameters listed in Table 1 and either a constant Te of 10, 30 and 50 km (thick dashed lines) or one that varies with age since rifting (thick solid lines) where Te (lower Late Cretaceous- Middle-Late Miocene layer) = 13 km and Te (upper Late Miocene-Recent layer) = 35 km. Shaded region shows the effect on the 2 layer model of varying the average sediment density by ± 100 kg m-3. b) Crustal model corresponding to the 2 layer model in a). 2/3 = the step in velocity that defines the transition from the upper to the lower oceanic crust in Fig. 4. OCB = Ocean/Continent Boundary. Inset shows the RMS difference between observed and calculated gravity anomalies. White filled star = best fitbest-fit two2 layer model with Te = 13 km for the lower layer and a Te = 35 km for the upper layer. White filled circles = single layer constant Te of 10, 30 and 50 km model.


Figure 12. Plot of Te Vs. age since the initiation of rifting for selected rifted margins subject to large sediment loads during their evolution. The plot includes the Amazon fan (this paper) and East India [Krishna, et al., 2000] margins that were loaded by large deep-sea fan systems of the Amazon River and Bay of Bengal respectively and the South China Sea [Lin and Watts, 2002] and Arabian plate [Ali and Watts, 2009] margins which were loaded by orogenic loads in Taiwan and United Arab Emirates-Oman respectively. Also shown are data from the Wilkes Land, [Close, 2005] and the Western platform, New Zealand [Holt and Stern, 1991] margins. Thick solid lines show the calculated Te during and following rifting based on the numerical modelling of [Burov and Poliakov [, 2001]. Thin solid and dashed lines show the depth to the 300, 450 and 600 oC isotherms based on the cooling plate model of Parsons and Sclater [1977]


Figure 132. Comparison of observed and calculated gravity anomalies along seismic reflection profile Line B. a) Comparison of the gravity effect of the sediment/top oceanic basement interface computed assuming 2-dimensions (solid line) and 3-dimensions(dashed line). b) Comparison of the observed (thick dashed line with grey shade) and calculated (solid lines) gravity anomalies. Thin solid lines show the gravity effect of the water, and sediment, and the oceanic crust. Thick solid line shows the sum. The very thick solid line shows the combined gravity effect of the water, sediment, oceanic crust and variable density sub-crustal mantle. c) Density model derived from the velocity structure in [Rodger, et al. [, 2006]Rodger et al using the velocity Vs. density relationships of Nafe and -Drake [1963] (sediments) and Carlson & and Raskin [1984] (crust). Light grey = sediments. Dark grey = oceanic crust. d) Comparison of the ‘observed’ tectonic subsidence derived from backstripping to the calculated subsidence based on the age grid of Müller et al. (1997) and a cooling plate model [Parsons and Sclater, 1977].


Figure 143. Comparison of observed and calculated gravity anomalies along seismic reflection profile Line F. Line thickness, annotations, and symbols are as defined in Fig.Figure 11.


Figure 154. Flexural bending stress associated with the loading of the Amazon fan. a) Orientation and magnitude of the maximum horizontal stress in the uppermost part of the flexed plate. Red = tension. Blue = compression. Thick solid line shows the stress orientations based on the borehole break-out measurements of [Lima, et al. [, 1997]. b) Stress profile in the uppermost part of the flexed part. Note that the peak compressive stress occurs at the shelf break, in the region of the maximum sediment thickness while the peak tensile stress occurs in flanking regions over the maximum curvature of the flexed continental and oceanic basement.


Figure 165. Comparison of the observed topography/bathymetry and calculated flexure due to fan loading along Line B and its extension to the SW. a) Comparison of the observed topography/bathymetry to the calculated flexure profile based on a uniform model with Te = 10 and Te = 50 km and a 2 layer model with Te = 35 km (upper layer) and Te = 13 km (lower layer). Thick dashed line shows the sum flexure due to the fan and margin load based on a 2-layer model. Note that none of the calculated curves account for the topography in coastal regions that comprise a 25 m high Pleistocene (?) terrace. b) Comparison of the observed topography/bathymetry to the calculated flexure based on a 2-layer model with Te = 35 km (lower layer) and Te = 13 km (upper layer). The calculated curves have been adjusted vertically assuming that the terrace is of marine origin and that sea-level prior to the Pleistocene stood ~ 25 m higher that it does at the present day. Note that there is now a closer agreement between the topography and calculated flexure.

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