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




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Gravity and seismic modelling


An additionalAOne test of thethe flexure model of the model is to compare the calculated crustal structuredepth to Moho that is implied by combined backstripping and the gravity and flexure modelling to the depth of the observed seismic structureMoho. The depth to the two Moho’s should agree – if the only processes to have affected the margin are rifting, crustal thinning and sediment loading.


Fig.Figure 1321cb, for example, compares, for example, the depth to the seismic Moho to the flexed Moho. Moho (see [Watts and Marr, 1995] for definition). The seismic Moho is based on Fig.Figure 3 of in [Rodger, et al. [, 2006] and the8. The flexed Moho has been calculated from the best fitbest-fit Te model by adding the flexure due to sediment loading to the backstrip Moho (i.e. the Moho that was calculated from the sediment backstrip assuming Airy isostasy). While the two Moho curves are in general agreement, there are differences. The flexed Moho is shallower than the seismic Moho landward of the point ‘X’ and deeper in a seaward direction. This could mean indicate that our assumptions that the water-filled subsidence (i.e. the backstrip) is compensated for only by thickness variations of a uniform density crust that overlies a uniform density mantle are incorrect. Alternatively, our the assumptions are correct and processes other than rifting and sediment loading have occurred at the margin. Such processes could include magmatic underplating, which thickens the crust,, and or tectonic erosionextreme extension and mantle serpentinisation that would thin it.


Gravity and seismic modelling


In order to better understand the cause of the differences between the observed seismic and calculated modelled flexed Moho Fig.21b, we converted the seismic velocitiesy along Lines B and F to density. ,We then calculated the gravity anomaly expected for the seismic structure and, and then compared it to the observed gravity anomaly.


Figs. 132bc compares the observed and calculated gravity anomalies along seismic Line B. The seismic velocity structure was simplified into a number of layers, each of which was converted to density using the relationships of [Nafe and Drake [e, 1963] for sediments and [Carlson and Raskin [, 1984] for oceanic crust. The gravity effect of the layerseach layer was thenwas calculated using a 2D line-integral method [(e.g. [Talwani, 1965]). The calculated anomaly (thin black line) comprises a negative that increases in a seaward direction due to the increase in water depth and the lateral decrease in sediment density and a positive that increases in a seaward direction due to a shallowing of the crust. The sum of these two contributions (medium black line, Fig.Figure 132b) shows small values (0-25 mGal) along the southwest end of the profile and large values (~70 mGal) along the northeast end.


The transition along Line B from small to high calculated gravitylarge anomalies occurs at ~140 km which, interestingly, also marks the transition from a region to the southwest where the flexed Moho is shallower than the seismic Moho and a region to the northeast where the flexed Moho is deeper (Fig.Figure 132b,c). Since the gravity effect of the water, sediments and crust is included in the calculated sum profile, then these observations suggest thatthen the mantle beneath the northeast end of the profile must be lower in density than the mantle to the southwest. This observation conclusionconclusion is consistent with the tectonic subsidence determined from backstrippingbackstrip profile (Fig.Figure 132d) which shows that the tectonic tectonic subsidence and uplift decreases from ~6.75 km in the southwest to ~45.9 km to the northeast, a difference of ~1.5 8 km. Unfortunately, we cannot uniquely determine the lateral or vertical extent of the low-density mantle that is required to fit the gravity data. However, a mantle that decreases in density from 3330 kg m-3 beneath the southwest end of the profileLine B to 3300 kg m-3 beneath the northeast end and extends to a depth of 125 km (i.e. the base of the thermal lithosphere) is in accord with ourthe gravity observations (thick black line - Fig.Figure 132b). It is also in general agreementin accord with tectonic subsidenceisostatic considerations which suggests that a 6.5 km deep, 4.2 km thick , oceanic crust that overlies a 3330 kg m-3 mantle should isostaticallywould be in isostatic balanceequilibrium with a 4.2 km deep, 8.2 km thick oceanic crust that overlies a 3300 kg m-3 mantle. This The difference in depth implied of 2.3 km compares with the ‘observed’ difference in the average tectonic subsidence between the southwest and northeast ends of Line B of ~1.8 km (Fig.Figure 132d).


The gravity calculations in Figure 13b were carried out inassume have been carried out in 2D rather than 3D. Unfortunately, it is difficult to quantify the effects of dimensionality on the calculations as the velocity and, hence, density data were acquiredhave been acquired along only two transects of the fan. We have, however, theHowever, the bathymetry sediment thickness, and hence top of the crust, is available in grid form and so we are able to compare a 2D and 3D calculation of the gravity effect of the sediment/water interfacecrust interface. Fig.Figure 132a shows that apart from a vertical shift, which results from the ‘slab’ assumption in the 2D calculations, there is little difference between the 2D and 3D gravity calculations. Because iso-velocity contours are likely to be facies controlled and generally follow the bathymetrybasement, we conclude believe that a full calculation in 3D would not significantly alter our results.


Fig.Figure 143 shows the comparison along seismic Line F where the velocity and density structure and calculated gravity contribution of the water, sediment and oceanic crust were computed in the same way as for seismic Line B. The figure shows similar results for the density structure and calculated gravity anomalies as for seismicfor seismic Line F as for Line B. The main difference is that the thin oceanic crust extends over a much greater distance along Line F (~250 km) than Line B (~100 km), which results in a shallower Moho and, hence, a larger contribution to the positive gravity anomaly due toof the water, sediment and crust. Isostatic balancingAn isostatic balance of a 125 km column of water, crust and mantle predicts that a 6.5 km deep, 4.2 km thick oceanic crust that overlies a 3330 kg m-3 mantle should be 5.7 km deep if it overlies a 3300 kg m-3 mantle. The difference in predicted depth of 0.8 km compares well with the observed difference in tectonic subsidence of ~1.0 km along the profile.


We conclude from this combined seismic and gravity modelling that the Amazon fan is underlain by a mantle that is characterised by lateral changes in density. The origin of these changes is not clear, but the mantle density decreases in a NE direction which suggest that they may be related to the Cearra Rise. It has been suggested, for example, that the Cearra Rise is an oceanic plateau that formed on or near a mid-ocean ridge crest during the Paleocene.Late Cretaceous (~80 Ma) [Kumar and Embley, 1977]. The lateral changes in density may therefore reflect, at least in part, temperature anomalies associated with the thermal structure of the risethe flank of this feature..


Discussion


Thin oceanic crust


Probably theOne of the most important significant results to have emerged from our seismic datastudy is the observationthat the thick sediment of the Amazon fan is underlain by a uniformly thin oceanic crust underlies the Amazon fan. The thickness of oceanic crust along Line B, southwest of 140 km model offset is 4.30±0.43 km, for example, whilst the thickness along Line F is 4.21±0.31 km. This is significantly thinner than ‘normal’ oceanic crust, which in the Atlantic is typically in the range 6.5-8.5 km [(e.g. [White 1992]). Comparisons with velocity Vs. depth profiles, however, suggest that the velocity of layers 2 and 3 along Lines B and F are generally similar to ‘normal’ crust. This suggests that the thin crust represents an attenuated version of ‘normal’ oceanic crust that was formed by seafloor spreading at a mid-ocean ridge, rather than by transtension, for example, at a transform fault or fracture zone.


Modelling studies cooling [Reid and Jackson, 1981; ] [Bown and White, 1994] suggest that thin oceanic crust is generated when slow spreading rates inhibit decompression melting and, hence, magmna supply due to conductive cooling. Such a model isSuch models are in accord with gravity anomaly [Coakley and Cochran, 1998] and magnetic anomaly [Brozena, et al., 2003] datastudies overn the Gakkel Ridge (GR), . They are also consistent with the results of dredging geochemical [Dick, et al., 2003] and seismic refraction [Minshull, et al., 2006; ] [Jokat and Schmidt-Aursch, 2007] studies on the on the South-West Indian Ridge (SWIR)) and GR. [Dick, et al., 2003] and, more recently, seismic refraction [Minshull, et al., 2006] [Jokat and Schmidt-Aursch, 2007] data on the SWIR. The SWIR, which is spreading at a full rate of 11-18 mm a-1, has a 2.2-5.4 km thick oceanic crust. The mean thickness of the oceanic crust isof 4.2 km, is the same which as similar to the thickness of oceanicthe crust that underliesbeneath the Amazon fan. The velocity structure is also similar, the main difference being a lower velocity at the top of layer 2 and bottom of layer 3 at the SWIR. Unfortunately, mantle velocities at the SWIR were constrained only from sparse Pn critical distancesarrivals and so we cannot compare these velocities. The GR, which is spreading at a full rate of 10-13 mm a-1, has magmatic segments of 3.5 km thick crust that abut 1.4-2.9 km thick crust in amagmatic segments. The main difference with the Amazon fan is the spatial variability and the almost complete absence at the Gakkel Ridge of a layer 3; a typical profile comprising a layer 2 that overlies low velocity mantle (~7.8 km s-1).


Thin oceanic crust has previously been documented in non-volcanicat non-volcanic rifted margins settings, most notably at the West Iberia [Whitmarsh, et al., 1996] and Newfoundland [Hopper, et al., 2004] [Hopper, et al.,, 2006] margins}. The velocity structure and spatial extent and spatial extent of the Amazon margin thin oceanic crust is most similar to the crust that abuts the Peridotite ridges of the Iberia Abyssal Plain and highly stretched continental crust seaward of Flemish Cap. We therefore follow Rodger et al. [2006] in suggesting that the thin crust was generated at a magma-starved either a slow spreading spreading centre or one with a reduced magmatic supply. Thin oceanic crust is usually formed at ultra-slow full spreading rates below ~20 mm a-1 [e.g. Dick et al. 2003]. Unfortunately, the absence of well-identified magnetic anomalies preclude means that it is difficult to estimate the spreading rate the determination of the spreading rate of the thin oceanic crust that underlies the Amazon fan. However, plate tectonic reconstructions [Müller et al., 1997] suggest full rates that could be as low as ~26 mm a-1.


Flexural stress, borehole break-outs, and seismicity


Flexure due to sediment loading generates a localized lithosphere stressstress in the lithosphere that might be manifest in borehole break-out data, faulting, and seismicity patterns. Immediately beneath a load, for example, the upper part of the plate that behaves elastically on long time-scales (>105 a) would experience compression whilst flanking regions would be subject to tension. These flexural stresses would be superimposed on the regional stress field and, depending on their magnitude and orientation, either compete with or enhance these stresses. It is interesting therefore to compute the flexural flexural bending stresses due to fan loading and compare them to other proxies for the stress regime.


Fig.Figure 154 shows a prediction of the magnitude and orientation of the maximum horizontal flexural bending flexural bending stress induced by Amazon fan loading. The stresses have been computed assuming an elastic plate model withwith the best-fit value of Te = 35 km (Fig.Figure 11) and the parameters listed in Table 1. The figure shows that the fan load induces stresses in the uppermost upper half part of the flexed flexed plate that reach magnitudes of up to 200 MPa beneath the shelf break where they are dominantly compressional. Stresses are of smaller magnitude in flanking regions (< 100 MPa) where they are mainly tensile.


The black thick solid bars in Fig.Figure 154a show the orientation of the horizontal stress field, SHmax, of the Amazon margin according tobased on the borehole break-out data of [Lima, et al. [, 1997]. These data suggest how a a regional horizontal stress field, SHmax, that trends generallydominant WNW-ESE trend thattrend , sub-parallel to the Equatorial Atlanticlocal fracture zone trend. This suggests they that the borehole break-out data reflects maybe associated with a ‘push’ force from the a mid-ocean ridge ‘push’ force. Indeed, Indeed, [Coblentz and Richardson [, 1996] have used finite element modelling of absolute plate motions subject to various plate boundary forces, including a ridge push force from the South Atlantic mid-oceanic Ridgeridge, to predict a WNW-ESE regional stress orientation in Brazil.


In the models of [Coblentz and Richardson, 1996]. They also predict that if the, the ridge force is applied across the entire 100 km thickness of thethick ‘short-term’ elastic seismic lithosphere thickness of the lithosphere, then the equivalent which is equivalent to a SHmax is in the range 5-25 MPa. If, however, the load force is supported applied across the thinner ‘long-term’ elastic elastic thickness then the stress field would increaseincreasesincreases to 15-75 MPa. Irrespective, the models suggest that the ridge push stress appearstherefore appears to be is smaller than the flexural bending stresses generated by fan loading, which locally as Fig. 14 suggests, exceed 200 MP (Fig.Figure 154a)a. We believe that this is reflected in the borehole break-out data of [Lima, et al. [, 1997] which shows two wells sites on the outer r Amapa shelf which where show with athe stress orientation (10-30o) that deviates significantly (by >840o) from the regional stress field. stress orientatio n. The deviation (>40o) from the general WNW-ESE trending regional stress field is substantial. Therefore,Therefore, flexural bending stresses may well have modifiedgenerated by sediment loading have significantly modified the regional stress field in the Amazon fan region.

the regional stress field, by rotating stress directions by some 60-80o.


At volcanic loads such as Hawai’i, a convincing case has been made for a linka link has been established between flexural bending stresses, faulting, and seismicity [Pritchard, et al., 2007]. An interesting question therefore is whether the fan loading is has been sufficient to generateis associated with seismicity. Fig.Figure 1b in AssumpçãoAssumpcao [1998] shows that while while there have been a few earthquakes there have been a few earthquakes in the region of the Amazon river estuarymargin region, the, the majority of historical seismicity (Mb < 6) appears to beis focussed to the east, along the Brazilian coast between Fortaleza and Natal. Here, focalFocal mechanism solutions [Assumpcao, 1998] suggest suggest a strike-slip regime with a maximum horizontal compression sub-parallel to the coastline, consistent with a ridge ‘push’ force. . The absence of seismicity in at the Amazon margin might therefore be due to the fact that rifting has resulted in faults that do not have a favourable orientation to be re-activated by a ridge ‘push’ force. In addition, [Bezerra and Vita-Finzi [i, 2000], however, suggest that that there is a set of NE-SW faults several km in the Portugar basin region of NE Brazilinland of Forteleza and and Natal that were seismically active during the Neogene. One possibility is thatThey attributed the the fault-induced seismicity se earthquakes are a response to flexural bending caused by sediment loading offshore. If this is theWe would therefore expect a similar pattern of pattern of faults in the crystalline basement rocks of the Amazon margin. Unfortunately, flexure has been significant enoughresulted in the that basement isbeing deeply buried and, hence, obscured in much of the Amazon estuary region. case, then it is puzzling why the Amazon margin, which has the largest Neogene sediment loads, is not more seismically active. The lack of seismicity in the Amazon river mouth region is therefore puzzling given the break out data. Clearly, we need more monitoring of the coastal regions of the Amazon fan……….


Flexure and landscape evolution


Seismic and gravity data suggest that the crystalline basement that underlies the Amazon fan is characterised by a long-term n elastic thickness (35 km) that greatly exceeds the crustal thickness (~4.2 km). This observation implies The flexural loading model that generally accounts for the available gravity and seismic data suggests that all the crust and a significant part of the mantle and mantle ismust be involved in the support of the Amazon fan load. Although we haveThis is in accord with the strong seismic data that showsevidence for flexure of the top and base of the oceanic crust. We have less , we have less evidenceevidence, however, for mantle involvement. The only indication from our seismic refraction data, for example, is the westerly dip of the iso-velocity contours > 8.0 km s-1 along Line F that generally follow the dip of the top and base of the oceanic crust.


A One consequence of flexural loadingflexural loading is that regions of subsidence offshore will bearewill be flanked by uplifts that in the case of the Amazon margin would have modified both onshore and offshorein onshore regions regions. In Rio Grande do Norte, for example, GEBCO 1 1 minute digital elevation data [BODC, 2003] reveal a number of topographic topographic surfaces, the highest of which is 750-1199 m above present day sea-level [Peulvast, et al., 2008]. In addition, there is evidence in this region of exhumed granites that project through Cenomanian-Turonian cover sediments [Peulvast, et al., 2008]. The timing of the exhumation and consequential uplift and erosion is difficult to constrain. The youngest topographic surface, however, is believed to coincide with the deposition of the Barreiras Formation: a coarse fluvial sediment which was deposited on an eroded coastal plain [Peulvast, et al., 2008].


The age of the Barreiras Formation is problematic. [Bezerra, et al. [2001, , 2006] consider it as to be Pliocene-Pleistocene in age. However, [Peulvast, et al. [, 2008] consider it the onshore equivalent of the Miocene siliciclastic shelf sediments offshore. The formation may indeedmay indeed reflect an increase in clastic supply and a coupled uplift in the hinterland and subsidence in the coastal plain. Oxygen-isotope data [Miller, et al., 2005] indicate that global sea-level was continuing to fall during the late Miocene and so by late Pliocene, the rate may have been large enough to exceed the rate of tectonic subsidence , and expose the Barreiras Formation to incision. Despite the incision, the Barreiras Formation outcrops out along much of the Brazilian coastline between Rio de Janeiro and Amapáa. In Rio Grande do Norte, south of Natal, the formation forms a 1 km long 20 m high cliff of braided stream deposits [de _Araujo, et al., 2006]. In Amapáa, the formation makes up the older coastal plain sediments to the west of the Holocene outcrop [Nittrouer, et al., 1996] where it forms a distinct terrace about 20-~25 m above sea-level. [Peulvast, et al. [, 2008] suggest that the Neogene uplift is related to a flexural response to sediment loads on the margin.


To test this possibility, we compare in Fig.Figure 15 16 an observed topography profile of the Amapáa coastal region to the calculated flexure due to fan loading. The topography B and has been constructed from a GEBCO 1 1 minute grid [BODC, 2003] while the flexure has been calculated assuming Te = 10 km and Te = 50 km (thin solid line) and our best fitbest-fit model with Te = 35 km (thick solid line). The maximum uplift for the best fitbest-fit flexure model in regions flanking the subsidence is 26 m, which is similar to the height of the Amapáa terrace (~24 m). However, the node (i.e. the point of no uplift or subsidence) is at -456 km rather than at the base of the terrace which is ~ 30 km further seaward.


One possible explanation of this discrepancy comes is that the Barreiras Formation is younger than Amazon fan loading and its flexural compensation. In this case, the fluvial sediments of the Barreiras Formation would have been deposited on an already flexed surface. If this is correct, then it implies that the coast would have been at the top of the present day terrace, some 80 km further inland, and therefore that global sea-level was higher at the Amazon margin in the Miocene-Pliocene than at the present day, by about 25 m.


To test thise possibility, we have shifted the flexure based on our best fitbest-fit flexure model vertically by 25 m and compared it to the observed topography. Fig.Figure 163b shows that the shifted curve accounts well for the position of from which sediments could subsequently prograde seaward, thereby forming the terracethe top of the terrace well. Moreover, when the flexural uplift due to fan and pre-fan loading are summed (thick dashed line) then they explain both the position, height, and both the landward and oceanward dip of the crest of the Gurupée arch.


Comparison of the crustal structure at the Amazon marginfan with other other non-volcanic rifted margins


The flexural loading model used in this paper attemptsWe have used the simple models of flexure discussed earlier to takequantifys into accountthe contribution of both rifting and sediment loading to the crustal structure, gravity anomaly, and landscape of the Amazon fan margin. It is basedThe models are based on the assumptions that a state of isostatic equilibrium prevails before and after stretchingextension, the thickness of zero elevation, unstretched, pre-rift d, unloaded, continental crust is 31.2 km, the average density of continental and oceanic crust is the samesimilar, and that there is little or no flexural strength during rifting. Any of these assumptions may be incorrect. Despite this, model predictionsflexure models, particularly of the crustal structure, provide a useful reference that can be used to compare to the obuseful reference that can be used for comparative margin studiesserved seismic structure.


Fig.Figure 175 shows a comparison ofcompares the predicted crustal structure based on the best fit Te fit flexure mmodel to the observed seismic structure alongfor 3 transects of the Amazon fan; Line B which crosses the centre of the upper, middle and lower fan and Lines A and CC, two “reference” lines that cross the NE Brazil margin to the north and south of the main fan load respectively. The transects have been stacked on the ‘hinge zone’ which we interpret marks the boundary between unstretched and highly stretched continental crust. The figure shows that the Amazon margin is characterised by a relatively abrupt change from unstretched to stretched continental crust, a narrow zone of crust that is transitional between oceanic and continental crust, and thin oceanic crust. Comparisons of the observed seismic Moho to the flexed Moho derived from gravity and flexure modelling is indicative of the processes that may have modified the margin, other than by rifting and sediment loading. The figure shows thereFig. XX shows that there is general agreement between the observed seismic and modelled flexed Moho in the region of the oceanic crust along Line B and in the transition zone and oceanic crust along Line A, north of the fan. The main discrepancies are beneath the Demerara Pplateauare beneath the landward end of Line A where the seismic Moho is deeper than the flexed Moho by some 8 km and beneath the oceanward end of Line C C where the seismic Moho appears is shallower than the flexed Moho by some 8 kma similar amount. These discrepancies


Discrepancies between the observed and calculated crustal structure are of interest are of interest because they may indicatereflect processes, other than rifting and sediment loading, that may have modified the margin during or during or after rifting. The deeper seismic Moho beneath the Demerara Plateaushelf, for example, may be due to magmatic underplating that has locally thickened the crust and caused uplift. Indeed, XXit has already been suggested [Campos, et al., 1974] that the NE Brazil margin was associated with magmatism due to its association with fracture zones, one of which, St Paul’s, extends into the chain of small seamounts that make up the North Brazilian Ridge (Figure 2). We see no evidence, however, in our our data of either a seaward- dipping reflector sequence or its associated high P-wave velocity (> 7.2 km s-1) lower crustal body [Greenroyd et al., 2007, ][Greenroyd, et al., 2008]. We therefore believe that despite the possibility of underplating beneath the Demerara Plateau, the NE Brazil margin inin the landward end of Line A, the Amazon fan region is a non-volcanic margin. The shallow seismic Moho beneath the oceanic crust is indicative ofmay be due to thin oceanic crust that is underlain by a relatively low density mantle.


Some insight into the processes presentthat are occcurring at the Amazon fan margin comes from comparing its structure based on seismic and gravity and flexure modelling to other non-volcanic rifted margins. Fig. 15 showsWe compare in Fig.Figure 175, for example, compares the 3the 3three margin transects Amazon fan trto two transects of transects of the West Iberia and conjugate Newfoundland non-volcanic margins. The figure shows that while there is general somegeneral agreement between the observed seismic and modelled flexed Moho in the region of the stretched continental crust at each these margins, there are, unlike the Amazon margin, large discrepancies, particularly in the in the region of the transition zone. For example, thethe seismic Moho is significantly shallower than the flexed Moho in the transition zone of the West Iberia and Newfoundland margins than it is at the Amazon marginseismic Moho is significantly shallower than the flexed Moho by ~ 10 km in the transition zone at both these margins. This is indicative that crustal material has been removed at the West Iberia and Newfoundland margins. We attribute thisththee removal to extreme extension at these margins whichthat has led to the replacement of crust by mantle, which in turn has experienced various degrees of serpentinissiation.


These comparisonsThe comparisons in Figure 17 suggest both similarities and differences between suggest the Amazon fan margin, however, has similarities and differences in a number of respects from otheandr non-volcanic margins like Iberia and Newfoundland non-volcanic margins. The main similarities are the relatively thin oceanic crust and the relatively abrupt change from unstretched to stretched continental crust. The main differences between the Amazon fan margin and the West Iberia and Newfoundland margins is are a a narrow transition, the a general coincidence of the observed seismic and modelled flexed Moho, and the lack ofof any evidence of extreme extension and mantle serpentinisation in the transition.

.


a comparison of the seismic and flexed Moho at two other non-volcanic rifted margins. The figure shows that unlike Line A north of the fan, the seismic Moho is much shallower than the flexed Moho. The reason for this is that the crust has been replaced by sepentinised mantle.

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