Thermal segmentation along the N. Ecuador–S. Colombia margin (1–4°N): Prominent influence of sedimentation rate in the trench

Earth and Planetary Science Letters 272 (2008) 296–308

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Thermal segmentation along the N. Ecuador–S. Colombia margin (1–4°N): Prominent influence of sedimentation rate in the trench Boris Marcaillou a,⁎, George Spence a, Kelin Wang b, Jean-Yves Collot c, Alessandra Ribodetti c a b c

School of Earth and Ocean Science, University of Victoria, Victoria, B.C., Canada Geological Survey of Canada, Pacific Geoscience Center, Sidney, B.C., Canada Geosciences Azur, Université de Nice Sophia-Antipolis, IRD, Université Pierre et Marie Curie, CNRS, Observatoire de la côte d’Azur, Villefranche sur mer, France

A R T I C L E

I N F O

Article history: Received 24 September 2007 Received in revised form 25 April 2008 Accepted 29 April 2008 Available online 16 May 2008 Editor: C.P. Jaupart Keywords: seismogenic zone seismotectonic convergent margin thermal modelling seismic reflection Ecuador Colombia

A B S T R A C T Along the deformation front of the North Ecuador–South Colombia (NESC) margin, both surface heat flow and trench sediment thickness show prominent along-strike variations, indicating significant spatial variations in sedimentation rate. Investigating these variations helps us address the important question of how trench sedimentation influences the temperature distribution along the interplate contact and the extent of the megathrust seismogenic zone. We examine this issue by analysing 1/ a new dense reflection data set, 2/ pre-stack depth migration of selected multichannel seismic reflection lines, 3/ numerous newlyidentified bottom-simulating reflectors and 4/ the first heat probe measurements in the region. We develop thermal models that include sediment deposition and compaction on the cooling oceanic plate as well as viscous corner flow in the mantle wedge. We estimate that the temperature from 60–150 °C to 350–450 °C, commonly associated with the updip and downdip limits of the seismogenic zone, extends along the plate interface over a downdip distance of 160 to 190 ± 20 km. We conclude that the updip limit of the seismogenic zone for the great megathrust earthquake of 1979 is associated with low-temperature (60–70 °C) processes. Our models also suggest that 60–70% of the two-fold decrease in measured heat flow from 3°N to 2.8°N is related to an abrupt southward increase in sedimentation rate in the trench. Such a change may potentially induce a landward shift of the 60–150 °C isotherms, and thus the updip limit of the seismogenic zone, by 10 to 20 km. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Subduction thrust faults are predominantly aseismic at depths shallower than a few kilometres and deeper than 40 to 50 km (Tichelaar and Ruff, 1993). In between these depths, the seismogenic zone is defined as the limited region of interplate contact that can generate thrust earthquakes (Hyndman et al., 1997). It is commonly acknowledged that the stick-slip behaviour responsible for earthquakes along the thrust fault, and thus the location of the updip and downdip limits of this seismogenic zone, is at least partially temperature-dependent (Scholz, 1990). The onset of stable-sliding at the downdip limit coincides with temperatures of 350–450 °C (Scholz, 1990; Ruff and Tichelaar, 1996). Thermal models of the Alaska (Hyndman et al., 1997; Oleskevich et al., 1999), Cascadia (Hyndman and Wang, 1993, 1995), Japan (Wang et al., 1995a), Mexico (Currie et al., 2002) and Nicaragua (Newman et al., 2002) subduction zones show that the updip limit of seismicity occurs around 100–150 °C. In these subduction zones, most of the authors ⁎ Corresponding author. E-mail address: [email protected] (B. Marcaillou). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.04.049

invoke a change of frictional properties related to the smectite to illite transformation in the underthrusted sediments in this temperature range (Vrolijk, 1990). Moreover, thermally-controlled diagenetic and low-grade metamorphic reactions may lead to the onset of stick-slip behaviour along the thrust fault at temperatures as low as ∼ 60 °C (Moore and Saffer, 2001; Saffer and Marone, 2003). The respective influences of incoming plate age (Newman et al., 2002), interplate contact dip (Gutscher and Peacock, 2003), hydrothermal cooling (Harris and Wang, 2002; Fisher et al., 2003), thermal parameters and convergence rates (Dumitru, 1991) on the seafloor heat flow and temperature distribution along the interplate contact have been investigated. The influence of the incoming plate sedimentation rate has been pointed out (Hyndman and Wang, 1993) but has never been quantified. The sedimentation rate is rarely specified and often neglected in thermal studies. An initial analysis of the thermal regime along the North Ecuador–South Colombia (NESC) margin was based on thermal modelling of heat flow derived from Bottom-Simulating Reflectors (BSRs) along seven margin-perpendicular multichannel seismic lines (Marcaillou et al., 2006) recorded during the SISTEUR cruise in 2000 (Collot et al., 2002). This study resulted in two main conclusions for

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the NESC margin. First, every margin segment is characterized by a relatively uniform surface heat-flow value that is clearly different from neighbouring segments. Second, the updip limit of the seismogenic zone may primarily be controlled by low-temperature (∼ 50–60 °C) processes, except in the area of the Mw = 7.7 1958 earthquakes where this limit is likely related to structural features in the upper plate. The 2005 AMADEUS cruise (Collot et al., 2005) provided new seismic reflection lines showing clear BSRs, bathymetric data and the first in-situ marine heat-flow measurements on the NESC margin, over both the overriding and incoming plates. Thus, in the present study, we have expanded our previous study by (1) determining BSR-derived heat flow along a much denser set of seismic lines to ensure that the heat-flow pattern within each segment is well-sampled, (2) measuring heat flow to compare with and calibrate the values derived from BSRs, (3) assessing the Nazca plate's thermal regime before subduction, with sediment heat-flow measurements, (4) carrying out pre-stack depth migration of selected lines to improve the images of the margin structure. Furthermore, we have used a more sophisticated thermal modelling approach than Marcaillou et al., (2006). Our models include the influence of sedimentation and a continental mantle wedge flow. The models yield estimates of the temperature distribution along the subduction interface and the width and location of the thermally-defined seismogenic zone. Moreover, geotherm estimates for the cooling of the oceanic lithosphere allow us to investigate the influence of along-strike variations in sedimentation rate on heatflow values and on the temperature distribution along the interplate contact.

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2. Regional setting 2.1. Structure The Nazca plate underthrusts the North Andean margin at a rate of 5.4 cm/yr with a N96° azimuth (Fig. 1) (Trenkamp et al., 2002). Magnetic lineations sub-perpendicular to the trench indicate a southward aging of the Nazca plate from ∼12 to 20 Ma off the NESC margin (Hardy, 1991). In the trench, the basement of the Nazca plate is covered by a thick layer of sediment mainly consisting of turbidites (Collot et al., 2005). These terrigeneous sediments are transported to the trench of the Tumaco and Manglares segments through the Mira, Patia and Esmeraldas Canyons (M. Ca., P.Ca. and E.Ca. in Fig. 1a). The last of these is associated with a channel (E.Ch.) meandering in the trench and constructing a deep turbidite system (Fig.1a) (Collot et al., 2005). In contrast, the Sanquianga Canyon (S.Ca.) and the Mira and Patia Canyons diverge from the Patia Promontory, transporting sediments toward the trench, to the north and the south of the Patia segment. The basement of the North Andean margin consists of mafic to ultramafic oceanic terranes (Goosens and Rose, 1973; Juteau et al., 1977) accreted to the paleo-South American margin during Cretaceous to Paleogene times (Gansser, 1973; Toussaint and Restrepo, 1994) and overlain by a Cretaceous volcano-sedimentary sequence and a thick Cenozoic forearc sedimentary basin (Deniaud, 2000; Jaillard et al., 1995). The Patia Promontory and the Manglares Fault divide the NESC margin into morphotectonic segments with different tectonic and structural features, particularly at the deformation front (Collot et al., 2004; Marcaillou and Collot, in press). In the Patia segment, the margin is fronted by a ∼35-km-wide accretionary wedge, while in the Tumaco segment the wedge is reduced to less than 10 km in width. This wedge

Fig. 1. a- Bathymetry of the North Ecuador–South Colombia margin and the incoming Nazca plate acquired during the AMADEUS cruise (Collot et al., 2005). The shaded areas with dashed outlines show the wedges: the Outer Basement High (OBH) and the accretionary prism (Acc. Prism). E.Ca.: Esmeraldas Canyon; E.Ch.: Esmeraldas Channel (Collot et al., 2005); M.Ca.: Mira Canyon, P.Ca.: Patia Canyon and S.Ca.: Sanquianga Canyon. b- Location of seismic and heat-flow data: the red and blue lines are multichannel seismic lines recorded during the AMADEUS and SISTEUR cruises (Collot et al., 2002), respectively. The sections overlain with black and yellow lines are multichannel seismic sections showing a clear bottom-simulating reflector. Solid and open circles represent the locations of heat-flow measurements by “Lister-type” probe and outrigger respectively. On both maps, the single arrow is the Nazca — South America plate convergence vector derived from GPS studies (Trenkamp et al., 2002) and double arrows show the margin segments.

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Kelleher, 1972), 1942 (Mw = 7.9) (Swenson and Beck, 1996), 1958, (Mw = 7.8) and 1979 (Mw = 8.2) (Beck and Ruff, 1984; Herd et al., 1981; Kanamori and McNally, 1982) (Fig. 2). The coseismic rupture defined by inversion of seismic data for the 1979 event appears to have extended very close to the trench in the Tumaco segment (Beck and Ruff, 1984; Herd et al., 1981; Kanamori and McNally, 1982), whereas in 1958 it stopped ∼30 km landward of the trench in the Manglares segment (Swenson and Beck, 1996) (Fig. 2). A recent model by Collot et al. (Collot et al., 2004) suggested that in this latter segment, the coseismic rupture propagated upwards towards the seafloor along a mega-thrust splay fault associated with the landward side of the OBH. The aftershocks that occurred within three months after the main shocks beneath the overriding plate (Mendoza and Dewey, 1984) are distributed in an area extending seaward to near the deformation front for the 1942 and 1979 earthquakes, but restricted to 40 km landward for the 1958 event (Fig. 2) (Swenson and Beck, 1996; Beck and Ruff, 1984; Mendoza and Dewey, 1984). These authors reported the aftershocks as thrust-faulting events although the fault plane solutions have not been published. The distribution of thrust events in the Harvard University Centroid Moment Tensor (CMT) archive shows similarly varying patterns along the NESC margin (Fig. 2). The consistent seaward extents of rupture zones, three-month thrust aftershock distribution and CMT archive events suggest that the seismogenic zone extends to near the trench beneath the Patia and Tumaco segments but is restricted farther landward beneath the Manglares segment (Marcaillou et al., 2006). 3. Data and processing Fig. 2. Location of the epicentres (stars) with focal mechanisms and rupture zones (dashed ellipses) of the 20th century great subduction earthquakes that occurred beneath the North Ecuador–South Colombia margin. The solid and open circles are the 1958 and 1979 three-month aftershocks, located beneath the margin and interpreted as thrust events (Swenson and Beck, 1996; Mendoza and Dewey, 1984). The “beach-balls” indicate the location of the Harvard University archive of Centroid-Moment Tensor (CMT). OBH: Outer Basement High.

separates the trench from a compressional margin basement and forearc basin. The Manglares segment is fronted by a 30-km-wide Outer Basement High (OBH) that bounds an up to 5-km-thick, 60-km-wide and currently non-deforming forearc basin. Vp velocities (4.5 km/s) and reflectivity analysis suggest that the OBH has an oceanic origin and has undergone intense fluid alteration and likely basal erosion (Agudelo, 2005).

3.1. Seismic reflection data During the 2005 AMADEUS cruise along the NESC margin, we recorded 55,000 km2 of continuous swath bathymetry and 85 new sixchannel seismic reflection lines that supplemented the 360-channel seismic lines recorded during the 2000 SISTEUR cruise (Fig. 1b). The lines were processed using Seismic Unix and Geovecteur® (Fig. 3). To obtain depth-migrated seismic images and quantitative 2-D velocity models, we applied pre-stack depth migration (PSDM) to selected

2.2. Thermal regime On land, 16 heat-flow measurements made by Ingeominas (Colombian Geological Survey) across the coastal area, the western cordillera and the eastern cordillera, range from 43 to 65 mW m−2 (Alfaro et al., 2000). On the NESC margin, a previous thermal study, based on seven SISTEUR cruise seismic lines (Fig. 1b), yielded BSR-derived heat flow of 100–110 mW m−2 for the Patia segment (2 lines), 55–60 mW m−2 for the Tumaco segment (3 lines), and 70-80 mW m−2 for the Manglares segment (2 lines) (Marcaillou et al., 2006). 25 km landward from the trench, BSR-derived heat flow was approximately constant at 40– 50 mW m− 2 along every line. Marcaillou et al., (2006) concluded that each morphotectonic segment is characterized by a homogeneous heat flow value that is different from neighbouring segments. They proposed several possible causes for these variations: the southward aging of the Nazca plate, variations in subduction angle, hydrothermal cooling and/ or the structure of the overriding plate. 2.3. Seismicity and seismogenic zone Along the North Andean margin, four great subduction earthquakes occurred in 1906 (Mw = 8.8) (Kanamori and Given, 1981;

Fig. 3. Seismic processing applied to the multichannel seismic reflection data sets using Seismic Unix and Geovecteur®. A preserved-amplitude pre-stack depth migration (Thierry et al., 1999a; Jin et al., 1992) applied to selected SISTEUR lines resulted in depth seismic images (Fig. 6).

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estimated thermal conductivity of 1.15 W/m/K at 500 m depth. The product of thermal conductivity and thermal gradient between the BSR and the seabed provides a heat-flow estimate. During the AMADEUS cruise, we made the first marine heat-flow measurements along North Ecuador and South Colombia. We collected 76 measurements distributed among twelve different locations from the trench to the upper slope (Fig. 1b). We used both a “Lister-type” probe with thermal conductivities measured in-situ (measurements labelled P1 to P79), and a combination of outrigger temperature measurements on a piston corer with laboratory measurements of thermal conductivity from the sediment cores (measurements labelled K5, 7, 8 and 9). Measurements collected along sections of seismic lines SIS-40 and 47, where heat flow was also estimated from BSRs (Fig. 1b), indicate that the difference between the heat-flow measurements and the BSR-derived values is less than 10% (Fig. 4), which probably represents the uncertainties in both types of measurements. 3.3. Thermal modelling

Fig. 4. Sediment heat-flow probe measurements (stars) and BSR-derived heat flow (solid squares) along lines (a) SIS-40, and (b) SIS-47. The error bars show the uncertainty in the BSR-derived heat-flow calculation estimated after (Marcaillou et al., 2006).

SISTEUR lines, using a preserved-amplitude approach (“Ray + Born diffraction tomography”) (Thierry et al., 1999a; Jin et al., 1992). Several applications to 2-D and 3-D data sets have been presented for the acoustic case (Lambaré et al., 1992; Thierry et al., 1999b; Operto et al., 2003; Ribodetti et al., 2000). This method is very sensitive to the background velocity macro-model. To obtain a reliable migrated image (correct geometry and correct velocity perturbations of seismic reflectors) and to enable quantitative estimation of velocities and their associated errors, we use the classical approach known as “migration-velocity-analysis” (Al-Yahya, 1987). Local correction functions for the velocity macro-model can be estimated from semblance panels and Iso-X panels (Al-Yahya, 1987). The latter represent the traces sorted by common-angle gathers and extracted from each partially migrated image at a given x-coordinate. An iterative correction of this macro-model is performed to obtain flat reflectors in Iso-X panels and 1-centered coherency index “gamma” in semblance panels (Al-Yahya, 1987). When these mandatory conditions are satisfied, the Iso-X panels are stacked to produce a reliable final migrated image.

We develop thermal models for the three morphotectonically distinct segments, along lines SIS-40, 37 and 42, to investigate the temperature distribution along the subduction interface at the depths of interplate seismicity. The models are constrained by the measured heat flow and BSR-derived values. The 2-D steady-state thermal modelling method applied to the NESC margin as well as its uncertainties were described in detail by Marcaillou et al., (2006). Here, we address the main aspects of the method as well as the substantial improvements brought about by a more sophisticated approach. On the oceanic plate, the surface heat flow and the geotherm were calculated using a 1-D model of a cooling lithosphere with the effect of sediment deposition and compaction (Hutchison, 1985; Wang and Davis, 1992). In this approach, two layers are considered (Fig. 5). The upper layer consists of sediment grains and pore water distributed according to the porosity function θ(z) for normally-consolidated sediment.  z θðzÞ ¼ θ0 exp − λ

ð1Þ

where θ0 is the seafloor porosity and λ a constant (Hutchison, 1985; Rubey and Hubbert, 1960). The lower layer represents the igneous basement. The 1-D coordinate system is fixed to the seafloor, Z0, with

3.2. Heat flow The BSR is recognized to represent the base of the methane-hydrate stability field (Shipley et al., 1979). The NESC seismic data set provided us with more than 800 km of BSRs divided among 83 sections that cover a region extending ∼ 50 km landward from the trench in all three morphotectonic segments (Fig. 1b). The depth of BSR beneath the seafloor can be calculated from the vertical travel-time between the seafloor and the BSR reflectors. A thermal gradient between these reflectors is assumed (Yamano et al., 1982; Townend, 1997). From the trench to the upper slope, temperatures at the seafloor were measured using a Conductivity Temperature Density (CTD) profiler (Flueh et al., 2001) and temperature outriggers mounted on piston core barrels. The pressure at the BSR depth is calculated assuming hydrostatic pressure within the sediment column. The corresponding temperatures are obtained using the methane hydrate temperature-pressure phase diagram in a pure methane-seawater system (Dickens and QuinbyHunt, 1994). The thermal conductivities of sediments retrieved in piston cores are 0.8 ± 0.5 W/m/K (Collot et al., 2005). At greater depth, we use an empirical expression calculated from Leg 112 data on the Peruvian margin (Suess and von Huene, 1988) that results in an

Fig. 5. Simplified two-layer sediment (S)–basement (B) model used for 1D heat-flow calculations on the oceanic plate (Hutchison, 1985). As sediment is added at Z0 at a rate V0, the sediment–basement interface ZB moves downward.

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the z axis pointing downward. Sediment input to the system at Z0 at a rate V0 induces a downward movement of the sediment–basement interface in this reference frame. The temperature at depth z in the sediment layer is given by: @ @T @T K −ðρw cw Vw θ þ ρs cs Vs ð1−θÞÞ þA @z @z @z @T ¼ ðρw cw θ þ ρs cs ð1−θÞÞ @t

ð2Þ

Table 1 Thermal parameters used in thermal modelling of the NESC margin Layer Rock type

Thermal conductivity λ (W m− 1 K− 1)

Heat generation A (μW m− 3)

Thermal capacity ρc (MJ m− 3 K− 1)

1 2

2.0 ± 0.5 3.0 ± 0.5

1.0 ± 0.5 0.05 ± 0.04

3.3 3.3

3.1 3.1

0.01 0.01

3.3 3.3

3.5 2.9

1.0 ± 0.5 0.02

3.3 3.3

3 4

and at depth z' in the basement by: @2T @T @T ¼ ρB cB KB 2 −ρB cB VB @z @t @z

ð3Þ

where A is internal heat production and T(z,t) is the temperature at depth z and time t. ρw, cw and Vw(z,t) are the pore fluid density, specific heat and velocity, respectively. ρs, cs and Vs(z,t) are the sediment particle density, specific heat and velocity, respectively. K(z,t) is the composite thermal conductivity at depth z. KB, ρB, cB and VB are the basement conductivity, density, specific heat (all assumed constant) and velocity, respectively. The calculation of Vs and Vw is based on conservation of the mass flux of the solid fraction and the incompressible fluid. We used the finite-element code developed by Wang and Davis (Wang and Davis, 1992) for this calculation. On the margin, we used 2-D steady-state finite element models (Wang et al., 1995b; Peacock and Wang, 1999). For every model, the margin structure is represented by a finite element mesh based on the seismic structural image of the margin. Depth sections from reflection seismic lines and OBS lines (Agudelo, 2005; Gailler et al., 2007; Meissner et al.,1976; Agudelo et al., 2002) image the crustal structure over a model length of 300 km to a maximum depth of 20 km. For depths shallower than 5 km, Iso-X panels derived from the pre-stack depth migrations indicate that the estimated error in reflector depth is ∼5%. Intermediatedepth earthquake locations provide an estimate of the geometry of the downgoing slab to ∼50 km depths (Engdahl et al., 1998; Guillier et al., 2001). The Nazca plate age is estimated from magnetic lineations (Hardy, 1991) to be ∼12, ∼14 and ∼19 ± 2 Myr at the trench along lines SIS-40, 37 and 42, respectively. The models include corner flow in the mantle wedge simulated using the analytical solution (Batchelor, 1967) for isoviscuous corner flow driven by the incoming plate. The thermal parameters and uncertainties (Table 1) are estimated as described by Marcaillou et al. (Marcaillou et al., 2006). The landward boundary of the model is placed sufficiently far from the seismogenic zone to avoid edge effects. Temperatures at the bottom boundaries are set at 1450 °C and, at the top boundary, are constrained by measurements from the CTD (Flueh et al., 2001) and outriggers mounted on the piston core barrels. The main control on the thermal regime is provided by the seaward boundary condition, i.e. the thermal structure of the incoming oceanic plate. This thermal structure is based on the geotherms derived from the 1-D model described above. 4. Results The pre-stack depth migration of selected lines results in greater confidence and higher precision for the subduction zone depth structure than on the time-migrated sections and is important to construct accurate 2-D grids for thermal models. It also allows us to estimate the sedimentation rate over the oceanic plate that affects the geotherm at the thermal model oceanic boundary.

5 6

Compacted sediments Upper plate crust (oceanic origin) Fixed continental mantle Flowing continental mantle Underthrusted sediments Oceanic lithosphere

In these models, the thermal conductivity and heat generation of layers 1, 2 and 5 were tested for three different values.

4.1. Margin structure We specifically emphasize here the description of structural features that may control the heat-flow variations by focussing on the structure of the trench and the lower slope where the variation in heat flow is greatest. At these depths, the Iso-X panels indicate a 5% error in estimating depths and thicknesses (Fig. 6). 4.1.1. Trench fill High-amplitude, well-bedded, sub-parallel and continuous reflectors overlying a highly diffracting seismic facies image the sedimentary trench fill over the oceanic basement (Fig. 6). On line SIS-37 (Fig. 6b), reflectors marking the top of the oceanic crust and deep sediments regularly step down landwards (for instance by 500 m at −12 and −5 km and by 600 m at −8 and −1 km) along landward-dipping planes inferred to be normal faults. We infer the presence of similar normal faults beneath the trench along all seismic lines in the Tumaco segment. Beneath the deformation front, the top of the oceanic basement is deeper on line SIS-37 (8 km) than on lines SIS-40 (5.5 km) and 42 (6.0 km). The sediment layer thickens considerably towards the trench axis (Table 2), particularly off the Tumaco segment where the sediments are ∼280% thicker than off the Patia segment (Fig. 6). This considerable thickening off the Tumaco segment appears to be related to normal faulting and subsequent deepening of the top of the oceanic basement. 4.1.2. Downgoing plate Beneath the lower slope, seismic reflection lines show highamplitude reflectors interpreted as the top of the oceanic crust (To) and the décollement level (De) (Marcaillou et al., 2006; Collot et al., 2004). These reflectors delineate an interplate layer, the so-called subduction channel (Sc), which consists of underthrusted sediments and/or materials derived from the erosion of the upper plate (Shreve and Cloos, 1986). The pre-stack depth migration provides an accurate estimate of the variations in subduction channel thickness and décollement dip between these lines. Landward of the deformation front, at 5 km, the subduction channel thickness is 0.7 km in line SIS40, 2.6 km in line SIS-37 and 2.2 km in line SIS-42 (Fig. 6). Thus the subduction channel is more than 350% thicker at 5 km and remains substantially thicker farther landward in line SIS-37 than in line SIS40. Over 30 km distance landward from the deformation front, the décollement dips landward at 5.1° in line SIS-40, 7.3° in line SIS-37 and 6.9° in line SIS-42. Thus the décollement is ∼ 40% steeper beneath the margin's lower slope in line SIS-37 than in line SIS-40.

Fig. 6. Along-strike variations in morphotectonic features of the lower slope at the North Ecuador–South Colombia margin. The seismic images are pre-stack depth migrations of segments of lines (a) SIS-40, (b) SIS-37 and (c) SIS-42. Dots mark the main reflectors: the top of the oceanic crust (To), the décollement (De), the backstop (bs) and the base of the forearc basin (B). These reflectors bound the subduction channel (Sc), the margin basement and the Cenozoic sediment layer (Cs). Arrows indicate the location of the bottomsimulating reflectors. Equally-spaced semblance and Iso-X panels indicate that the uncertainty in depth for these reflectors is ∼5% until 5-km-deep beneath the seafloor. Gamma in semblance panels is a coherency index.

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Table 2 Sediment thickness (z) and sedimentation rate in m/yr at locations 30, 15 and 0 km west of the subduction line, over the Nazca plate off lines SIS-40, 37 and 42 −30 km

SIS-40 SIS-37 SIS-42

−15 km

the period of time for the oceanic plate to travel from -30 km to the trench. 4.3. Along-strike variations in surface heat flow

0 km

z (km)

Sed rate (m/yr)

z (km)

Sed rate (m/yr)

Zz(km)

Sed rate (m/yr)

0.6 0.55 0.6

6.6 10− 5 5.3 10− 5 4.1 10− 5

1.3 2.6 0.9

3.5 10− 3 1.1 10− 2 1.5 10− 3

1.5 4.2 2.7

0.9 10− 3 7.4 10− 3 7.4 10− 3

A less than ± 15% error is considered in the sedimentation rate calculation.

4.1.3. Upper plate structure On line SIS-40 (Fig. 6a), the 35-km-wide accretionary wedge is 3.1km-thick at 10 km and up to 4-km-thick landward. Landward from the backstop (bs), a 0.8- to 1-km-thick layer of roughly-bedded reflectors, interpreted as Cenozoic sediments (Cs) (Marcaillou and Collot, in press) lies over the margin basement. On line SIS-37 (Fig. 6b), landward of the 10-km-wide and up to 4.7-km-thick accretionary wedge, a 1.3- to 1.8-km-thick layer of well-bedded reflectors images the Cenozoic sediment layer underlain by a high-amplitude reflector, labelled B and interpreted as the top of the margin basement (Collot et al., 2004; Marcaillou and Collot, in press). On line SIS-42, the margin is fronted by the OBH, which extends from 0 to 25 km (Fig. 6c). The lower slope is covered by a 0.6- to 1.0-km-thick Cenozoic sediment layer. Thus, beneath the margin's lower slope, the structure and thickness of the upper plate vary considerably between morphotectonic segments. In particular, at a similar distance from the trench, the margin wedge is thicker in line SIS-37 than in line SIS-40. 4.2. Sedimentation rate over the Nazca plate

The BSR-derived heat-flow values, comparable to in-situ heat-flow measurement values, are summarized in a map (Fig. 8). In the Patia segment, two heat-flow probe measurements in the trench, 95 and 121 mW m− 2, are consistent with the BSR-derived heat flow at the deformation front (Fig. 8). Similarly, in the Tumaco segment, the probe measurements in the trench, 64-65 mW m− 2 seaward of line SIS-37 and 54-56 mW m− 2 seaward of line SIS-47, are similar to the BSRderived heat flow near the deformation front (Fig. 8). These results reinforce the conclusion of our previous study, based on seven sparse seismic lines (Marcaillou et al., 2006), that every morphotectonic segment is characterized at the deformation front by a homogeneous heat-flow value that differs markedly from the neighbouring segments (Fig. 8). A 300-km-long finite element mesh was used for thermal modelling along lines SIS-40, 37 and 42 (Fig. 9), which represent the Patia, Tumaco and Manglares segments respectively. The modelling yields heat-flow estimates at the seafloor of the oceanic plate, the continental slope, and the continental shelf (Fig. 10). On the Nazca plate, the 1-D models provided a calculated heat flow for every reference location at which sedimentation rate was calculated (Table 3 and Fig. 10). At the first location, −30 km, these results are closely consistent with theoretical oceanic heat-flow values of ∼130, ∼120 and ∼100 mW m− 2 for 12,14 and 19 Myr-old oceanic plates respectively (Stein and Stein, 1992). At the trench axis beneath lines SIS-40 and 37, the calculated seafloor heat flows (Table 3 and Fig. 10) compare well with the heat-flow measurements in

The marked increase in sediment thickness over the downgoing plate towards the trench axis as well as thickness differences along the trench suggest appreciable variations in sedimentation rate over the Nazca plate. We have used the velocity structure obtained from the pre-stack depth migration of five multichannel seismic lines (SIS-42, 45, 47, 37 and 40) to perform a depth conversion of time-migrated lines recorded on the Nazca plate (Ama-06, 07, 55, 56, 57 and SIS-44, 33, 35, 39). These depth sections are used to construct a map of the sediment thickness over the Nazca plate from the Yaquina Graben to the trench axis (Fig. 7). For each of the three segments, we measured sediment thicknesses z at three reference locations, namely 30, 15 and 0 km west of the trench axis (Table 2). Fig. 7 shows that along line Ama-55, ∼ 30 km from the trench, the sedimentary layer over the oceanic basement is relatively thin (0.5 to 1 km) along the margin. Approaching the trench, at −15 km and 0 km, the sediment layer thickens considerably, in particular off the Tumaco segment (Table 2). The thickness of sediment before compaction Z0 is estimated from the current thickness by a decompaction equation: Z0 ¼ ½Zi −Zi−1 þ λðθi −θi−1 Þ=ð1−θ0 Þ

ð4Þ

where Zi, Zi- 1, θi and θi- 1 are sediment thicknesses and porosities at two successive reference locations. With a 5.4 cm/yr relative plate velocity, a N96° convergence direction (Trenkamp et al., 2002) and a N35°-trending trench, the trench-normal convergence rate is 4.4 cm/ yr. The Nazca plate has thus travelled the distance between the reference locations (from −30 to −15 km and from −15 to 0 km) in T ≈ 350,000 years. Sedimentation rates between these reference locations can therefore be estimated by Z0/T, with an uncertainty of about 15% (Table 2). For the sedimentation rate at −30 km, we assume that the accumulated sediments have been deposited uniformly since the ocean crust formed. Thus, the accumulation time is given by the plate age in the trench minus 2T, which approximately corresponds to

Fig. 7. Variations of the oceanic sediment thickness (in km) from the Yaquina Graben to the trench. White lines indicate the multichannel seismic lines recorded over the oceanic plate. Red labels indicate those that were processed with a pre-stack depth migration. Black labels indicate those that were depth-converted using velocity models derived from the pre-stack depth migrations.

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4.4. Temperatures along the interplate contact A primary objective of the thermal modelling is to provide estimates of temperature along the interplate contact, since the seismogenic zone is thought to be located between 60–150 °C and 350–450 °C (Hyndman and Wang, 1993). The predicted depths and horizontal distances from the deformation front associated with the boundaries of this thermally-defined seismogenic zone (Fig. 11) are given in Table 4. 5. Discussion 5.1. Seismogenic zone length The seismicity data (Fig. 2) suggest that, in the Patia and Tumaco segments, the updip limit of the seismogenic zone is located close to the trench, where temperatures are as low as 60–70 °C. This highlights the influence of low-temperature processes on the onset of stick-slip seismogenic sliding on the interplate contact. In the Manglares segment, the updip limit is shifted landward of the splay fault (Fig. 2). The difficulty in constraining the deep margin structure results in a ∼20 km uncertainty in distance and ∼18 km uncertainty in depth for the 350° and 450 °C contours (Table 4). We cannot deduce any variations in the downdip extent of the seismogenic zone along the NESC margin, because of uncertainties in the 350° and 450 °C contours location and in the continental Moho location (Gailler et al., 2007; Meissner et al., 1976), whose intersection with the interplate contact may possibly be shallower than the 350 °C isotherm. However, the seismogenic zone

Fig. 8. Heat-flow variations along the North Ecuador–South Colombian margin measured with a “Lister-type” heat-flow probe (solid dots) or outriggers on piston corers (open circles) and derived from bottom-simulating reflectors (white lines) along multichannel seismic lines. The black line indicates the 200 m isobath. Some measurements were collected in the trench off the Patia and Tumaco segments (dots and values in mW m− 2). This map demonstrates that the margin is thermally segmented: at the deformation front and in the trench, each segment is characterized by a homogeneous heat-flow value that is clearly different from those of neighbouring segments.

the trench for the Patia and Tumaco segments (Fig. 8 and Fig. 10). No measurement was recorded off the Manglares segment. Thus, when approaching the trench from the ocean basin, the calculated and measured heat flows decrease substantially, particularly in the central Tumaco segment where the calculated heat flow in the trench is half the value at 30 km westward. The estimated errors (Table 3), related to ±2 Myr uncertainties in the Nazca plate age and ±15% in the estimated sedimentation rate, indicate that the variations in calculated heat flow from one location to another are significant. Landward of the deformation front, the 2-D models result in calculated heat flow that is closely consistent with the heat-flow measurements and BSR-derived heat flow, for lines Sis-40 and 42, from the deformation front to the upper slope (Figs. 10a and c). For line SIS-37, the calculated heat flow is ∼ 10 mW m− 2 higher than the BSRderived heat flow within 10 km of the deformation front, but calculated and measured heat flows are consistent beyond this distance (Fig. 10b). The local discrepancy near the deformation front is possibly related to fluid circulation between the thick underthrusted sediments and the upper crust. From the deformation front to the upper slope, the uncertainties are estimated to be less than 10%. However, landward of the coastline, the geometry of the interplate contact is not constrained by seismic data but by interplate seismicity (Engdahl et al., 1998; Guillier et al., 2001), and errors increase to ±20%.

Fig. 9. Finite-element mesh used for 2-D thermal modelling along the margin. (a) Entire mesh for the model along line SIS-37 and detailed meshes for models along lines (b) SIS40, (c) SIS-37 and (d) SIS-42. Layers 1 to 6 represent the upper sediments, the margin oceanic basement, the fixed and flowing parts of the upper plate mantle, the underthrusted sediments and the downgoing lithosphere, respectively. The BSRs are located at the margin's lower slope (black frames).

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Fig. 10. Calculated and measured heat flow along lines (a) SIS-40, (b) SIS-37, and (c) SIS-42. Greater detail over the distance range 0–60 km is given in (d), (e) and (f). The inset in (d) shows an expanded horizontal scale over a region where detailed heat-flow probe measurements (green open circles) were made. The BSR-derived heat flow is indicated by red crosses. The calculated heat flow seaward of the deformation front (blue cross) has an error of 1–5%. Landward of the deformation front (blue line) errors vary from 9% at 0 km to 20% at 60 km (shaded area). The calculated heat flow matches the measurements and BSR-derived values indicating that these models are representative of the thermal regime in each morphotectonic segment.

may extend 160 to 190 km± 20 km from the deformation front (Table 4) and thus 50 to 60 km landward of the coastline, a region with a large human population. 5.2. Influence of the sedimentation rate on the seismogenic zone location On the Nazca plate, the calculated heat flow for model SIS-37 is lower than that for SIS-40 by ∼47 mW m− 2 in the trench and by ∼ 15 mW m− 2 at a location 30 km seaward of the trench (Table 3). On the lower continental slope, at 5 km, the calculated heat flow for model SIS-37 is 31 mW m− 2 lower than for model SIS-40 (Fig. 10). Moreover, temperatures along the interplate contact are colder for model SIS-37 than for model SIS-40 (Fig. 11). However, despite these heat-flow variations, the 60 °C isotherm is located at the same distance from the deformation front (3 ± 3 km from the trench; Table 4) in both cases. We carried out four tests to investigate what aspects of the subduction zone control the southward decrease in heat flow from the Patia to the Tumaco segment (Fig. 8). These tests are also aimed at highlighting the influence of these aspects on the temperature distribution along the interplate contact. In the structural model for line SIS-40, we changed a given test parameter to the corresponding value taken from model SIS-37, with all other parameters remaining unchanged. As described in the following sections, the test parameters are the Nazca plate age, the upper plate structure, the décollement dip and the sedimentation rate. 5.2.1. Nazca plate age In model SIS-40, the Nazca plate was aged from 12 Myr to the SIS-37 value of 14 Myr. This change reduced the calculated heat flow on the Nazca plate by 13.5 mW m− 2 at −30 km and by 11 mW m− 2 in the trench (Fig.12f). Over the margin, from 0 to 60 km landward of the deformation front, the older downgoing plate resulted in a similarly trending but lower heat-flow distribution. The dashed line (Fig. 12g) shows that an older plate age also resulted in a colder interplate contact: the 60 and 150 °C isotherms were shifted landward by 3 and 11 km respectively.

5.2.2. Upper plate structure In model SIS-40, the topography and the geometry of layers 1 and 2 were modified to reflect the lower slope structure of line SIS-37. The upper plate is thicker with a shorter accretionary wedge and a relatively thick upper plate basement (Fig. 6b and Fig. 12a). This change had a small influence on the calculated heat flow between 0 and 20 km, except around 22 km, where the heat flow was increased by 5 mW m− 2. This local increase is related to the thickening of layer 2 in the model landward of 10 km that represents the eastern edge of the 10-km-wide accretionary prism. This test also resulted in a warmer interplate contact (thin dotted line in Fig. 12g): the temperature beneath the deformation front increased from 60 to 75 °C, and the 150 °C isotherm was shifted trenchward by 5 km. This warming is mainly related to a deeper interplate contact below the seafloor due to a thicker upper plate for line SIS-37 than for line SIS-40. 5.2.3. Décollement dip The décollement dip beneath the lower slope was increased from the SIS-40 value of ∼ 5.1 ° to the SIS-37 value of ∼7.3° (Fig. 12b). Landward of 20 km, this dip varied only by a small amount from the SIS-40 value. From 0 to 20 km, the dip increase resulted in lower heat flow, reduced by 7 mW m−2 at 5 km for instance. This test had only a small influence on the temperature distribution along the interplate contact (Fig. 12g) because of the low variation in dip value, restricted to 20 km from the deformation front.

Table 3 Heat flow and uncertainties in mW m− 2 calculated with a 1D model (Hutchison, 1985; Wang and Davis, 1992) at the surface of the Nazca plate, along lines SIS-40, 37 and 42 at −30, −15 and 0 km westward from the trench axis

SIS-40 SIS-37 SIS-42

−30 km

−15 km

0 km

135.5 ± 6.9 120.3 ± 4.7 102 ± 3.0

105.5 ± 6.4 52.8 ± 4.6 96.3 ± 3.0

108.3 ± 6.2 64 ± 4.6 74.4 ± 3.2

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decrease in the trench is related to a higher sedimentation rate in the Tumaco segment than in the Patia segment. This high sedimentation rate is also responsible of 65 % for the heat-flow decrease between the Patia and Tumaco segments at 5 km, on the margin's lower slope (Fig.12). In comparison, the Nazca plate age and the décollement dip only account for 19% and 22%, respectively, of the heat-flow variation at 5 km. Similar calculations at 60 km from the deformation front confirm that 60 to 70% of the heat-flow decrease between the Patia and the Tumaco segments is related to the southward increase in sedimentation rate over the Nazca plate. Moreover, the variation in sedimentation rate induced 85% of the landward shift of the 150 °C isotherm on the plate interface, between the Patia and Tumaco segments. That is, such an increase in sedimentation rate may induce a 10 to 20 km landward shift of the temperature range commonly associated with the updip limit of the seismogenic zone. However, the upper plate structure for model SIS-37 has an opposite effect on temperature by shifting the isotherms trenchward (Fig. 12g). Within 20 km from the deformation front, the warming effect of the upper plate structure on the interplate contact counteracts the cooling effect due to high sedimentation rates. This balance explains the seaward convergence of temperature curves and the similar ∼60 °C interface temperatures at the deformation front for both models. Nevertheless, neglecting the sedimentation rates in thermal modelling may lead to serious errors in estimating the distribution of the temperatures commonly associated with the updip limit of the seismogenic zone, even when the calculated heat flow at the surface fits measurements.

Fig. 11. Isotherm distribution across the margin from the deformation front to 300 km landward for the (a) Patia segment (line SIS-40), (b) Tumaco segment (line SIS-37) and the (c) Manglares segment (line SIS-42). Every 100 °C isotherm from 50° to 1450 °C is shown. The 60° to 150 °C contours and the 350° to 450 °C contours are commonly thought to correspond to the limits of the seismogenic zone (Hyndman and Wang, 1993; Moore and Saffer, 2001).

5.2.4. Sedimentation rate The sedimentation rate over the Nazca plate in thermal model SIS40 was modified to incorporate the oceanic sedimentation rates calculated for model SIS-37, with a 4-km-thick trench fill and a 3-kmthick subduction channel (Fig. 12c). On the Nazca plate this change reduced the calculated heat flow by 1.5 mW m− 2 at −30 km and by 36 mW m− 2 in the trench (Fig. 12f). On the continental slope, the higher sedimentation rate resulted in a much lower heat flow, reduced by 20 mW m− 2 at 5 km, for instance. At 60 km, the heat flow was reduced by 7 mW m− 2 only, showing that this parameter's impact decreases landward. This test resulted in cooling the interplate contact: the 60 and 150 °C isotherms are shifted landward by 9 and 19 km respectively (Fig. 12g). The plain black line (Fig. 12g) shows that between 25 and 170 km, the variation in the sedimentation rate produces most of the change in the interface temperature between lines SIS-40 and SIS-37. This influence is significantly greater than that of the incoming plate's age as shown in § 5.2.1. Beyond 180 km, all the curves converge, indicating that the influence of the sedimentation rate progressively disappears. In our models, the interplate contact, which is more than 70 km deep beyond this distance, intersects the viscous corner flow, which is likely the primary control on the thermal structure of the downgoing slab at greater depths. These tests indicate that, on the Nazca plate, 30 km westward from the trench, 90% of the heat-flow decrease from the Patia to the Tumaco segments is related to southward plate aging. In contrast, 85% of the

5.2.5. Origin of along-strike variation in sedimentation rate In the Tumaco and Manglares segments, several normal faults downthrow the top of the oceanic crust in the trench with up to 1 km vertical displacement and result in the top of the oceanic crust reaching depths as great as 8 km beneath the deformation front in the Tumaco segment (Fig. 6). The deeper ocean crust allows more sediment to accumulate at a higher sedimentation rate, with an abundant supply of sediments transported to the trench through the Esmeraldas, Mira-Patia and Sanquianga Canyons on the continental shelf and slope (Fig. 1) (Collot et al., 2005). The reasons for this normal faulting remain unclear. A number of authors have noted that bending-related normal faulting occurs at the outer trench wall across an area 50- to 100-km-wide by reactivation of the oceanic spreading fabric of the incoming plate (Kobayashi et al., 1998; Masson, 1991; Ranero et al., 2003). However, seismic data from various convergent margins show that normal faults may be absent or much less developed such as in southern Chile (Polonia et al., 2007), Java (Kopp et al., 2006) and portions of Nankai through (Tsuru et al., 2002). Moreover, in the Patia segment and off the Ecuador margin as far south as the Guayaquil area, the top of the oceanic crust shows no normal faulting or sparse faults with less than 200 m-high throws (Calahorrano, 2005; Sage et al., 2006; Marcaillou, 2003). Thus, off Ecuador–Colombia, only the Manglares and Tumaco segments exhibit such major crustal faults, suggesting the presence of specific geodynamic features on this restricted portion of the Nazca plate. Moreover, the distribution of hypocentres from the CMT Harvard catalogue indicates that intra-oceanic plate earthquake, with normal focal mechanism, are located off this margin section only (Fig. 2).

Table 4 Depths and distances from the deformation front of the intersection point between the interplate contact and the 60°, 150°, 350° and 450 °C isotherms SIS-40

60 °C 150 °C 350 °C 450 °C

SIS-37

SIS-42

Depth (km)

Distance (km)

Depth (km)

Distance (km)

Depth (km)

Distance (km)

5±1 9±2 48 ± 18 61 ± 18

3±3 34 ± 4 159 ± 20 183 ± 20

6±1 12 ± 2 58 ± 18 65 ± 18

3±3 54 ± 4 176 ± 20 188 ± 20

6±1 17 ± 2 60 ± 18 62 ± 18

18 ± 3 78 ± 4 182 ± 20 190 ± 20

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Fig. 12. Impact of the Nazca plate age, (a) the margin's lower slope structure, (b) the décollement dip and (c) the sedimentation rate over the incoming plate on (f) the margin heat flow and (g) the temperature distribution along the interplate contact. (d) and (e) show the SIS-40 and 37 meshes. The black line in graph (f) show that 60 to 70% of the heat-flow decrease between the Patia and the Tumaco segments is related to the southward increase in sedimentation rate in the trench. The black line in graph (g) indicates that such an increase can generate a ∼15 km landward shift of the 60 to 150 °C isotherms commonly associated with the updip limit of the seismogenic zone.

Based on crustal thickness estimates along the Cocos, Carnegie and Malpelo ridges, Sallares and Charvis (Sallares and Charvis, 2003) estimated the intensity of the Galapagos Hotspot melt anomaly and the migration of the Cocos-Nazca Spreading Centre with respect to the hotspot during the last 20 Myr. This study resulted in a reconstruction of the relative position between the hotspot, the spreading centre and the transform faults zones through time. This model of the tectonic evolution suggests that a fossil transform fault zone with a SW–NE trend is currently located in the trench beneath the NESC margin where it is partially subducted (Sallares and Charvis, 2003). It is noteworthy that the crustal normal faults are located near this inferred fossil transform fault zone. Thus, one possible explanation for the normal faulting in the NESC trench is that plate bending near the trench has reactivated a fossil transform fault with normal throws. This interpretation implies that the tectonic history of the downgoing plate may have an indirect influence on the seismogenic zone, through its control on trench sediment distribution and subsequent cooling of the margin front. 6. Conclusion Based on dense multichannel seismic lines, pre-stack depth migrations of selected lines, numerous BSR-derived heat flow and

heat-flow measurements on both the incoming and overriding plates, we determined the heat-flow pattern and performed steady-state thermal models for the subduction zone in the three morphotectonic segments of the NESC margin. We significantly improved the results of a first study of the thermal control on the seismogenic zone and megathrust earthquakes in this region. In particular, the new detailed heat-flow pattern, confirmed by the first heat-flow measurements in the area, reinforces the conclusion that the margin is thermally segmented and that the heat flow significantly decreases from the Patia segment to the Tumaco segment. Moreover we specifically investigated the influence of the along-strike variations in trench sedimentation rate on the thermal regime of the margin and the temperature distribution along the interplate contact. The conclusions of this study can be summarized as follows: 1/ The temperature ranges from 60–150 °C to 350–450 °C, commonly associated with the updip and downdip limits of the seismogenic zone along the plate interface, extend over a distance of 160 to 190 ± 20 km. These new thermal models confirm that the updip limit of the seismogenic zone on the NESC margin is generally controlled by low-temperature (60–70 °C) processes, except for the 1958 event, where the updip limit of the seismogenic zone is more likely related

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to prominent structural features in the upper plate landward of the deformation front. 2/ 60 to 70 % of the heat-flow decrease between the Patia and the Tumaco segments is related to the southward increase in trench sedimentation rate. Such sedimentation rate variations may induce an up to ∼20 km landward shift of the 150 °C isotherm. The prominent cooling effect of the increased trench sedimentation rate persists as far as 180 km landward of the trench and progressively disappears beyond. However, at the trench itself, the heating effect associated with a different upper plate structure compensates for the cooling effect of fast sedimentation rates so that the 60 °C isotherm is located at the same 3 ± 3 km distance from the trench in both the Patia and Tumaco segments. Thus, in general, neglecting sedimentation rate in thermal modelling may lead to serious errors in estimating temperature along the interplate contact. 3/ The substantially higher sedimentation rate in the Tumaco segment is associated with normal faulting of the top of the oceanic crust, which locally deepens the crust, increase the volumetric capacity of the trench and allows the rapid accumulation of thick sediments transported through prominent canyons on the continental shelf and slope. This faulting is likely related to inherited structures in the oceanic plate, reactivated by the plate bending, which is consistent with a transform fault zone located nearby the trench as proposed by Sallares and Charvis (Sallares and Charvis, 2003)'s geodynamical model. Acknowledgements This publication is a contribution of UMR Geosciences Azur 6526 and GDR-Marges and this research was supported by a grant from the Natural Sciences and Engineering Research Council, which provided funds for the heat flow survey and for post-doctoral support to BM. This is also Geological Survey of Canada publication 20070627. We are grateful to IFREMETR, the crews and technical and scientific staffs of the SISTEUR and AMADEUS experiments. We also thank Ikuko Wada and Jiangheng He for discussion and technical help with the modelling procedure and we are especially grateful to Trevor Lewis for his expertise in running the heat-flow probe. References Agudelo, W., 2005. Imagerie sismique quantitative de la marge convergente d'EquateurColombie, Phd thesis, Université de Pierre et Marie Curie, Paris. Agudelo, W., Charvis, P., Collot, J.Y., Marcaillou, B., Michaud, F., 2002. Structure of the Southwestern Colombia convergent margin from the Sisteur seismic reflectionrefraction experiment, EGS XXVII, Nice, France. Alfaro, C., Bernal, N., Ramirez, G., Escovar, R., 2000. Colombia, country update, Proceedings World Geothermal Congress, Kyushu - Tohoku, Japan. Al-Yahya, K.M., Velocity analysis by iterative profile migration, Phd thesis, Stanford University, 1987. Batchelor, G.K., 1967. An introduction to fluid dynamics. Cambridge University Press, New York, p. 635. Beck, S.L., Ruff, L.J., 1984. The rupture process of the great 1979 Colombia earthquake: evidence for the asperity model. J. Geophys. Res. 89, 9281–9291. Calahorrano, A., Structure de la marge du Golfe de Guayaquil (Equateur) et propriétés physiques du chenal de subduction, à partir de données de sismique marine reflexion et refraction, PhD thesis, Université Pierre et Marie Curie, Paris, 2005, 312 pp. Collot, J.Y., Charvis, P., Gutscher, M.A., Operto, S., 2002. Exploring the Ecuador-Colombia active margin and interplate seismogenic zone, EOS. Trans. Am. Geophys. Union 83, 189–190. Collot, J.Y., Marcaillou, B., Sage, F., Michaud, F., Agudelo, W., Charvis, P., Graindorge, D., Gutscher, M.A., Spence, G.D., 2004. Are rupture zone limits of great subduction earthquakes controlled by upper plate structures? Evidence from multichannel seismic reflection data acquired across the N-Ecuador–SW Colombia margin. J. Geophys. Res. 109, B11103. doi:10.1029/2004JB003060. Collot, J.Y., Migeon, S., Spence, G.D., Legonidec, Y., Marcaillou, B., Schneider, J.L., Michaud, F., Alvarado, A., Lebrun, J.F., Sosson, M., 2005. Mapping the seafloor of the Ecuador-SW Colombia margin helps understand great subduction earthquakes, EOS. Trans. Am. Geophys. Union 86, 463–465.

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