Late Quaternary geomorphologic evolution of submarine canyons as a ...

Marine Geology 315–318 (2012) 77–97

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Late Quaternary geomorphologic evolution of submarine canyons as a marker of active deformation on convergent margins: The example of the South Colombian margin Gueorgui Ratzov a, d,⁎, Marc Sosson b, Jean-Yves Collot a, Sébastien Migeon c a

Géoazur, Université de Nice Sophia-Antipolis, Institut de Recherche pour le Développement (UR 082), Observatoire de la Côte d'Azur, La Darse B.P. 48 06235 Villefranche-sur-Mer Cedex, France Géoazur, Université de Nice Sophia-Antipolis, Centre National de la Recherche Scientifique (UMR 7329), Observatoire de la Côte d'Azur, 250 av Einstein 06560 Valbonne, France c Géoazur, Université de Nice Sophia-Antipolis, Université Pierre et Marie Curie, Observatoire de la Côte d'Azur, La Darse B.P. 48 06235 Villefranche-sur-Mer Cedex, France d Université de Brest, UMR 6538 Domaines Océaniques, 29280 Plouzané, France b

a r t i c l e

i n f o

Article history: Received 9 November 2011 Received in revised form 21 May 2012 Accepted 22 May 2012 Available online 13 June 2012 Communicated by D.J.W. Piper Keywords: submarine canyon convergent margins active tectonics

a b s t r a c t The morphology of Patia and Mira canyons on the South Colombian convergent margin reflects an interplay between tectonic deformation, sea-level variation and canyon evolution, and provides new insight into the age and location of margin deformation over the last ~150 ka. Multibeam bathymetry, seismic, and sedimentary data reveal that tectonically active structural highs control canyon incision and the canyon's course. The canyons developed across the margin in five major stages. First, the upper slope was incised by headward and downward erosion during the Pleistocene, infilling a structurally bounded slope basin. The basin periodically spilled over and breached the accretionary prism at ~150 ka, leading to the development of isolated sediment lobes in the trench. The prism was efficiently breached leading to a well-developed trench channel–levee system at 53–67 ka. Today, the system shows limited activity. Antecedent streams, convex-up axial incision profiles, and increasing height/width ratio indicate an active uplift of the structural highs since at least ~ 150 ka and support localized shortening through the margin accommodated by out-of-sequence structures thrusts and folds. An 80 m-high scarp where the canyon crosses a fault on the middle slope further supports active uplift related to a major thrust. Previous seismostratigraphic studies of the margin have demonstrated that active uplift occurred during the Early Pliocene; here we demonstrate that uplift continued throughout the Late Pleistocene. Comparisons with canyons on other convergent margins reveal that features relating to margin deformation and canyon age (tortuous path, convex-up profiles, abandoned canyon paths, overincision, abrupt canyon turns) are generally restricted to accretionary margins. Because of their complex morphology, accretionary margins appear more favorable for the occurrence of such features than margins undergoing tectonic erosion with more simple morphology. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Submarine canyons are common features of continental margins where they act as major conduits for sediment transport from the shelf to deep ocean basins (Normark and Carlson, 2003). Canyon initiation occurs by upslope retrogressive erosion (Twichell and Roberts, 1982; Farre et al., 1983; Pratson and Coakley, 1996) or downslope incision by gravity-flows (Daly, 1936; Pratson et al., 1994; Pratson and Coakley, 1996; Mulder et al., 1997), the relative importance of these two processes being hotly debated. Although they act on both passive and active margins, tectonic deformation in the case of the latter exerts strong control on canyon geometry and evolution (Hagen et al., 1996; McHugh et al., 1998; Kukowski et al., 2001; Greene et al., 2002; Laursen and Normark, 2002; Soh and Tokuyama, 2002; Huyghe et al., ⁎ Corresponding author. Tel.: + 33 298498757. E-mail address: [email protected] (G. Ratzov). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2012.05.005

2004; Mountjoy et al., 2009). Hence, detailed study of canyon shape, sedimentary fill, and timing of incision can provide important constraints on recent tectonic evolution of active margins. Previous studies of the south Colombian convergent margin have documented active deformation and a complex history of tectonic evolution over the last ~30 Ma at the front of the margin. Compression of the offshore forearc basin was accommodated during several tectonic phases, the last one being more intense since the Late Miocene (Marcaillou and Collot, 2008; Lopez, 2009). Chronostratigraphic studies based on seismic-reflection data (Marcaillou and Collot, 2008; Lopez, 2009) however do not constrain late Quaternary seafloor tectonic deformation. Such information is crucial for understanding local and transient versus regional and long-term tectonic processes and their various causes. The southern Colombian margin is crossed by the Mira and Patia submarine canyon systems, first mapped in 2005 during the Amadeus Cruise (R/V L'Atalante) and described by Collot et al. (2005). In map view, these canyons exhibit a Z-shape morphology, suggesting

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G. Ratzov et al. / Marine Geology 315–318 (2012) 77–97

Fig. 1. Structural map of the North Ecuador–South Colombia convergent margin. The black arrow represents the convergence rate and direction inferred from GPS study (Trenkamp et al., 2002) of the Nazca plate underthrusting the South America plate. Modified from Collot et al., 2009.

that their paths are structurally controlled. Specific markers of recent seafloor deformation, such as canyon shape and degree of incision, thus represent important but largely unexplored tools for constraining recent tectonic deformation. In this study, we employ high-resolution multibeam bathymetry data, together with seismic-reflection profiles and short cores, to 1) define the main structural elements of the margin, 2) constrain margin uplift and subsidence since the canyon's inception, and 3) derive a chronology of the major stages of canyon incision and therefore of recent phases of the margin deformation. Our approach enables new insight into relationships between locations and ages of seafloor vertical motions and margin deformation processes. We then compare our results with canyons on other convergent margins in order to determine key features that could be used to constrain recent deformation in subduction zones. 2. Geological setting and present-day sediment input The SW Colombia convergent margin is located at the boundary of the South America Plate, where the Nazca Plate is subducting eastward with a convergence rate of 58 mm yr− 1 (Trenkamp et al., 2002) (Fig. 1).

The front of the margin consists of an accretionary prism (Mountney and Westbrook, 1997) widening northward from Latitude l° to 5° N (Marcaillou and Collot, 2008; Marcaillou et al., 2008). NNE trending, parallel-to-the-margin folds and structural highs, such as the Tumaco High (Figs. 2 and 3), the Tumaco Antiform (Collot et al., 2004), the Patia promontory, and the Guaiquer ridge (Figs. 2 and 3), represent the seaward boundary of the Manglares forearc Basin (Marcaillou and Collot, 2008). Onshore, sediments derived from the Andes were trapped within the Borbon-Tumaco forearc basin, bounded landward by the Western Cordillera, and southward along strike by the Remolino High (Lopez, 2009). The margin basement, commonly inferred to be Cretaceous accreted terranes of oceanic plateau origin (Goosens and Rose, 1973; Juteau et al., 1977; Feininger and Bristow, 1980; Kerr et al., 1996) is overlain by the Borbon – Tumaco basin onshore, and the Manglares Basin offshore (Fig. 1). The main sedimentary units onshore, defined by unconformities (Evans and Wittaker, 1982; Benítez, 1995; Jaillard et al., 1997), can be correlated offshore with four major seismic units within the Manglares basin (Marcaillou and Collot, 2008; Lopez, 2009): U1) Middle Eocene to Lower Oligocene (40–31 Ma); U2) Oligocene to Lower Miocene (30– 21 Ma); U5) Upper Miocene (7.5–5 Ma); and U8) Pleistocene (Table 1).

Fig. 2. Bathymetric map of the studied area. White dashed lines represent the limits between the defined morphological domains (upper, middle, and lower slope, and the trench). Bold isobaths are every 500 m, while thin isobaths are every 100 m. The data was collected using a Simrad EM12D multibeam system (13 kHz, 162 beams).


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Fig. 3. Seismic line SIS 33 (modified from Marcaillou, 2003) outlining the present state of margin deformation along canyons. In-sequence deformation is only visible on the margin front within the accretionary prism, whereas the Guaiquer ridge and Tumaco High demonstrate out-of-sequence deformation.

Table 1 Chronostratigraphic relationships between the seismic units encountered in the trench, middle, and upper slopes. Concerning ages estimations, please refer to Tables 2, 3, and 4. Trench

Middle slope

Upper slope

Seismic Unit unit properties

Interpretation Seismic Unit unit properties

Ta

Ua Steady channel-levée system

Overbank deposits Well bedded continuous reflectors; low amplitude

Episodic channels

Ub

Well bedded continuous reflectors; low to high amplitude

Slope basin infill

Uc

Well bedded discontinuous to continuous reflectors; high amplitude Well bedded continuous reflectors; medium to low amplitude

Deep water hemipelagites and low energy turbidite environment (Lopez, 2009) Deep water and low energy turbidite, alternating with mass transport deposits (Lopez, 2009) Deep water hemipelagites and low energy turbidite environment (Lopez, 2009) Deep to shallow marine flood basalts mixed with sediments and volcanic sills (Lopez, 2009)

Tb

Tc

Poorly continuous to chaotic reflectors, low amplitude Poorly Continuous reflectors; medium amplitude Well bedded continuous reflectors; medium to high amplitude

Interpretation

Equivalent to unit of other publications

Seismic unit

Properties

U8 (Lopez, 2009); A (Marcaillou and Collot 2009)

D1 D2 Well bedded continuous reflectors; medium amplitude

Tentative age Interpretation

Patia and Mira rivers submarine deltas

53–67 ka

155 ka Pleistocene

Turbidites/ hemipelagites trench fill

Ud

Ue

Uf

Discontinuous to continuous reflectors; high amplitude Chaotic (acoustic basement)

U5 (Lopez, 2009); B (Marcaillou and Collot, 2008)

Upper Miocene (7.5–5 Ma) (Marcaillou et al., 2008; Lopez, 2009)

U2 (Lopez, 2009); C (Maracaillou and Collot 2008)

Oligocene to Lower Miocene (30-21 Ma) (Lopez, 2009)

U1 (Lopez, 2009); D (Marcaillou and Collot, 2008)

Middle Eocene to Lower Oligocene (40–31 Ma) (Lopez, 2009)

HR (Lopez, 2009)

Late Paleocene to Early Eocene (Lopez, 2009)


4. Results

Table 2 Properties of the multichannel seismic datasets used in this study. Dataset

Amadeus (AMA)

Agenzia Nacionale de Hidrocarburos (ANH)

Sisteur (SIS)

Year Streamer length Number of traces Total volume of Gis air guns Frequency of the source Theoretical vertical resolution at the seafloor

2005 300 m 6 300ci 17–85 Hz ~ 10 m

1973 2300 m 24 1200ci 8–60 Hz ~ 14 m

2000 4500 m 360 2896ci 13–36 Hz ~ 20 m

1982 2375 m 96 3050ci 14–65 Hz ~ 12 m

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The continental slope is divided into three main morphological domains: the upper, middle and lower slopes. The upper and middle slopes are separated by the Tumaco High and Antiform, while the Guaiquer ridge separates the middle and lower slopes (Figs. 2 and 3). The Patia and Mira canyons incise the upper slope, and merge on the middle slope (Fig. 2) to form a single canyon, which crosses the lower slope and reaches the subduction trench. 4.1. The upper continental slope

According to Marcaillou and Collot (2008) and Lopez (2009), episodes of uplift and subsidence deformed the seafloor after the accretion of the oceanic terranes. A major regional tectonic phase of uplift (Jaillard et al., 1997), initiated during Middle Eocene to Early Oligocene, created the Tumaco High and the Patia Promontory, and peaked from Late Oligocene to Early Miocene (~30 to ~20 Ma). Following a period of subsidence and sedimentation during the Early-Middle Miocene (~ 20 to ~ 12 Ma), intense Middle Miocene shortening led to widespread uplift from ~10 to 7.5 Ma (Lopez, 2009) and probably an emergence of structural highs (Marcaillou and Collot, 2008). Uncertainty exists as to whether uplift of the margin has increased since the Early Pliocene (Marcaillou and Collot, 2008), or since the Early Pleistocene (~1.8 Ma) (Lopez, 2009). At present, the study area is seismically very active, with the occurrence of four major subduction earthquakes (7.7b Mwb 8.8) since 1906 (Beck and Ruff, 1984; Mendoza and Dewey, 1984; Swenson and Beck, 1996). Today, sediment is supplied to the margin by the Mira and Patia rivers (Fig. 2). Both drainage systems are located within a humid tropical climatic zone, characterized by relatively high temperatures, rainfall rates, and humidity (Restrepo and Lopez, 2008). The 272 km-long Mira river drains a watershed of 9530 km 2, with an average precipitation rate of 4703 mm yr − 1 between 1970 and 2001, and a total sediment yield of 1025 t km − 2 y − 1. Although longer (415 km) and draining a larger basin (23,700 km 2), the Patia river has a smaller sediment yield (972 t km − 2 y − 1), probably due to a lower average rate of precipitation (3296 mm yr − 1).

3. Geophysical and sedimentology data The Amadeus survey (2005) provided bathymetry data (150-m resolution) (Fig. 2) that we reprocessed using the software CARAIBES (Ifremer) for water depths shallower than 2000 m to obtain a grid resolution of 60 m. We generated bathymetric profiles and estimated slope angle values; we also define an incision ratio, I = (H/W) * 100, where, in cross section, H is the canyon wall height and W is the canyon width at the top of canyon walls. This ratio indicates the importance of canyon incision relative to width, and thus serves as a general indicator of the incision intensity. Our dataset of Multi-Channel Seismic lines (Table 2) consists of 6 channel AMA lines collected during the Amadeus survey, 24 and 96 channel ANH lines provided by the Colombian Agencia Nacional de Hidrocarburos and 360 channel SIS lines collected during the Sisteur cruise (Fig. 4). A single, 3.47 m-long Kullenberg-type core (Kama 08) was also collected during the Amadeus survey. It was split into 1-m long sections, described onboard, and X-radiographed using the SCOPIX system (University of Bordeaux 1; Migeon et al., 1999). Grain size was measured using a COULTER Laser granulometer. Finally, AMS 14C dating was performed on foraminifer samples, and radiocarbon dates were corrected for marine reservoir age (400 yr) and calibrated to calendar years (see Table 3).

The upper slope extends from the shelf break (~200 mbsl) seaward to the N30°-trending Tumaco High and Tumaco Antiform (~1200 mbsl). The Tumaco High consists of a series of ~20–30 km long, b200 m-high, and 1 to 1.5 km-large elongate antiforms, while the Tumaco Antiform corresponds to a major ~20 km-long, 3 km-large, and ~500 m-high antiform. This domain is bounded to the north by the Patia Promontory. At the shelf break, the slope angle is up to 5°, and the continental slope is incised by numerous ~150 to ~600 m-wide and up to 50 m-deep gullies (Fig. 2). Recent deposits consist of well-layered, medium-amplitude, and high-frequency continuous seismic reflectors (Fig. 5A, and B, and Table 1). Along-strike, seismic data (Fig. 5A) indicate slope deposits mainly composed of two sedimentary units, D1 and D2. Unit D2 was incised by the Mira canyon, while unit D1, for which progradation is indicated by southward downlapping reflectors (Lopez, 2009), was incised by tributaries of the Patia canyon. Both units correspond to deltaic deposits (Martinez et al., 1995; Lopez, 2009) interpreted as fed by sediment transported from the Mira and Patia rivers. The Mira canyon incises the shelf break in a westward direction (Fig. 6) at a latitude of 1°45′ N, then, at ~ 900 m water depth, it bends northeastwards, parallel to the Tumaco High (Fig. 6A). In the bend the canyon exhibits a secondary curved bed with a steep western flank that might be interpreted as an abandoned path (Segment A in Fig. 6A). Downstream (Segment B in Fig. 6A), the canyon evolves from a broad, >3 km-wide and b100 m-deep U-shape (Fig. 6C) (incision ratio I = ~2), to a narrow, ~2 km-wide and up to 300 m-deep V-shape (I = ~10)(Fig. 6D). Upstream from this transition, the canyon axis has a slope angle value lower than 2° at the shelf break (Fig. 6B), evolving to an almost horizontal slope (0.3–0.4°) downstream. The path of the canyon segment is almost linear where it incises along the Tumaco High (Segment B in Fig. 6A). The canyon stream profile in its upper part is concave-up (Fig. 6B), suggesting an equilibrium state (Langbein and Leopold, 1964; Pirmez et al., 2000). Further downstream (Segment C in Fig. 6A), the Mira canyon crosses a threshold between the Tumaco High and the Tumaco Antiform. There, the canyon exhibits a series of four bends, probably caused by seafloor irregularities related to the lateral terminations of the Tumaco High and the Tumaco Antiform structures. The slopes of the canyon walls within the threshold are over ~30° (Fig. 6D), considered to be over incised in regard to their slope angle compared to the rest of the canyon, and with a maximum I value of ~35. The along stream profile is convex-up along the threshold, and outlined by a stronger slope angle (1.5–2°) (Fig. 6B). The Patia canyon consists of three main tributaries, named A, B, and C, from north to south (Fig. 7A), incising the upper slope in a westward direction. Tributary A is characterized by the deepest incision (~500 m) within the shelf break with a V-shape cross section, while tributaries B and C are only 200 m deep, with a smoother U-shape profile (Fig. 7B). 700 to 1500 m-large slope failures are abundant at the head of tributary A, located on a steep slope (>20°). The stream profile of tributary A (Fig. 7C) shows a concave-up morphology across most of the upper slope, with I = ~5. Then, it changes to a convex-up morphology across the Tumaco Antiform, its slope increasing from b1° to 2° (Fig. 7C), and I increasing to ~12. Tributaries B and C breach the upper continental

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Fig. 4. Position of the seismic datasets used in this study.

G. Ratzov et al. / Marine Geology 315–318 (2012) 77–97 Table 3 Radiocarbon ages of core Kama 08. Without further data on the marine reservoir age, raw ages were calibrated to calendar years using a standard 400 yr marine age reservoir difference (Reimer et al., 2009) using CALIB Rev 6.0/Marine09 dataset (Stuiver and Reimer, 1993). Final results are shown as the age median probability in column 6. Core

Sample depth (cm)

Material

14 C uncorrected age

2-sigma cal yr BP age ranges

Kama 08

239

3 530 ± 60

3 339 to 3513

3 418

Kama 08

338

Neogloboquadrina dutertrei Neogloboquadrina pachyderma Neogloboquadrina dutertrei Neogloboquadrina pachyderma

9 340 ± 70

10 117 to 10 249

10 188

Cal yr BP age median probability

slope and merge at a water depth of ~750 m. Tributary C extends westwards, and at the junction with tributary A (~1100 m water depth), its direction changes abruptly, thus going around the Tumaco Antiform, with I = ~ 12. The junction between tributaries C and A is outlined by an 80-m-high scarp (Fig. 7D) indicating that erosional processes within tributaries B–C increased recently as a result of southward stream migration. 4.2. The middle continental slope 4.2.1. General morphology and subsurface processes The morphology of the middle slope (1200 m to 2000 m water depth) is locally characterized by a ~ 20 to ~ 25 km-wide slope basin (Fig. 8A), bounded northeastward by the Patia Promontory, eastward and southeastward by the Tumaco Antiform and Tumaco High respectively, and westward by the Guaiquer Ridge (Fig. 2). The Mira canyon incises the slope basin along a NS direction. In that area, the canyon is less than 1000 m-wide, ~ 150 m deep, and I = ~ 10. It merges northward with the ~ 4 km-wide and 200 m-deep Patia canyon. Near the canyon's junction, the Patia canyon turns slightly in a westward direction, and its floor is nearly flat at a water depth of 2100 m (Fig. 8A), with a low I value (~3). There are two 8 to 12 km-large and 100 to 500 m-high scarps (S1 and S3 in Fig. 8A), located on the southern wall of Patia canyon, interpreted as slope failures related to tectonic activity and canyon incision (Ratzov et al., 2007). The scarp S1 is located immediately south of the bend of the Patia canyon, where overhanging hectometric blocks infill the canyon and form a dam (S1 in Fig. 8B), trapping up to 0.3 stwt (~ 270 m) of sediment upstream and resulting in a flat canyon floor over a distance of ~12 km (Fig. 8A). As the shallowest point of the dam lies ~30 m above the sedimentary infill level, the dam continues to prevent massive overspilling. 4.2.2. Seismic architecture Seismic data acquired across the slope basin reveals the presence of six main sedimentary units (Ua to Uf in Fig. 8B and C; Table 1) correlating with regional seismic units established by Marcaillou and Collot (2008) and Lopez (2009): – Deepest unit Uf is characterized by a chaotic seismic facies that represents the acoustic basement. It was interpreted by Lopez (2009) as resulting from submarine volcanic activity that initially filled the basin. – Unit Ue: the top of the acoustic basement is overlain by highamplitude seismic reflectors whose visibility depends on the type of seismic data and profile location (Fig. 8C). This unit is inferred to correlate with Unit U1 of Lopez (2009), consisting of Middle Eocene to Lower Oligocene hemipelagites and distal turbidites. – Unit Ud: this seismic facies consists of high-amplitude, well-bedded discontinuous to continuous reflectors. On dip-oriented seismic line

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ANH-2800 (Fig. 8D), its thickness varies from 0.3 stwt to 0.7 stwt. On along-strike profile AMA014 (Fig. 8B), this unit mimics a basin-like morphology between the Tumaco High and the Patia promontory. Unit Ud likely represents unit U2 of Lopez (2009) and could therefore be interpreted as Oligocene to Lower Miocene (30 to 21 Ma) deepwater basin deposits and distal turbidites. – Unit Uc slightly onlaps the underlying unit Ud (Fig. 8C), and is characterized by medium- to high-amplitude, well-bedded and continuous seismic reflectors. On dip-oriented seismic line ANH2800 (Fig. 8D), the unit pinches out on unit Ud along the Guaiquer ridge, indicating uplift of the ridge before deposition of Uc. This is further supported by a change in thickness of Uc (up to 0.25 stwt) beneath the Patia canyon. Unit Uc maximum thickness reaches 0.3 stwt southward on the slope of the Tumaco High (Fig. 6B). Unit Uc is correlated with Unit U5 of Lopez (2009) and therefore interpreted as Upper Miocene (7.5 to 5 Ma) deep-water hemipelagic deposits and distal turbidites. – Unit Ub pinches out on unit Uc along the Guaiquer ridge on line ANH-2800, and thickens eastward up to 0.35 stwt at the Tumaco Antiform (Fig. 8D–E). On strike-oriented seismic line AMA014, Ub deposits onlap unit Uc, and are horizontal and weakly deformed, except along the Tumaco High where they dip southward (Fig. 8B). The Patia canyon incises Ub; however, the incision does not reach the base of unit Ub (Fig. 8), suggesting that the canyon incision postdates or is coeval with unit Ub. – Ua is the youngest unit, with a seismic facies comprised of wellbedded and continuous low-amplitude reflectors. It pinches out on unit Ud along the Guaiquer ridge in a similar way to Ub (Fig. 8D), reaching a maximum thickness of ~0.15 stwt. Ua is mainly conformable on unit Ub, except along dip-oriented line ANH-2800 (Fig. 8D), where it slightly onlaps the underlying deformed unit Ub. Northward the unit slightly drapes the underlying deposits of Uc. Units Ub and Ua correspond to the uppermost unit U8 identified by Lopez (2009) as a Pleistocene sedimentary fan. 4.2.3. Lithology of units Ua and Ub On seismic profiles, the alternating high- and low-amplitude reflectors (Fig. 8E) of unit Ub likely correspond to alternating coarseand fine-grained deposits. The unit shows sharp onlap on the surrounding highs and is interpreted to consist of turbidite-prone deposits, whereas units Uc and Ud show few onlaps on the surrounding highs, and are probably the result of decantation (fine-grained deposits or sheet-like turbidites). A 3.44 m-long piston core (Kama08) collected in unit Ua on the southern bank of the Patia canyon (1334 mbsl) permits verification of inferences from seismic data (Fig. 9A). It contains homogeneous fine-grained olive-green clay sediment with a median grain size (Q50) of ~6–7 μm (Fig. 7B). The sediment is strongly bioturbated and contains monosulfides, suggesting a low sediment supply. X-ray images reveal seven fine-grained turbidites (Q50 ~ 12–25 μm; Fig. 7C) showing erosive bases with rare cross-laminae. Beneath ~150 cm, the sediment is dominated by fine-grained laminae visible only on X-ray images (Fig. 9D) that can be interpreted as lower-flow regime laminae within turbidite beds. All these deposits are therefore associated with lowenergy tractive currents. The presence of foraminifera implies that the sediment also contains a hemipelagic fraction. The whole core is thus interpreted to represent an alternation of fine-grained turbiditic and hemipelagic intervals stemming from overbank processes (Piper and Deptuck, 1997) associated with the canyon activity. This interpretation is also supported in seismic profiles by the onlaps of unit Ua on the basin flanks. 4.3. The lower continental slope The lower continental slope is structurally outlined by a ~20 km‐wide accretionary prism (Marcaillou, 2003) characterized by three main

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Fig. 5. Seismic lines highlighting sedimentation of the upper slope. (A) Along-strike seismic line ANH-1750 showing the two sedimentary fans 1 and 2 incised by the canyons. Fan 1 shows southward dipping downlaps implying a southward migration of the system. (B) Along-dip seismic line ANH-1973-87 demonstrating westward progradation of Fan 1. (C) Location of the seismic lines.

morphological ridges ranging from 1500 to 3000 mbsl (Fig. 10A). In map view, the canyon presents a Z-shape with an upper segment oriented along dip, a middle segment trending northeastward after a sharp 90° right turn and running parallel to the tectonic structures over ~8 km. This middle segment correlates with the presence of an 80 m-high scarp. After a 90° left turn, the lower segment trends northwestward until reaching the deformation front. The walls are up to 1200 m high and 35° steep, and the canyon bed widens from ~3 km at the middle slope to over 7 km across the deformation front. The transition between the middle slope and the lower slope is marked by a general convex-up stream profile (Fig. 7B), with slopes locally as steep as 6.5° and a maximum I value of ~19 (Fig. 7B). 4.4. The trench The transition between the canyon mouth and the trench consists of a ~30 km-wide and ~150 m-high deep-sea fan, located at ~3600 mbsl (Fig. 10A). Three main seismic units Ta to Tc appear on seismic data (Fig. 10B): – Seismic unit Tc, the deepest one, consists of well-stratified, subcontinuous reflectors interpreted as alternating hemipelagic and turbidite layers (Fig. 10C) characteristic of trench fill deposits (Thornburg and Kulm, 1987; Blumberg et al., 2008). The apparent lack of channelized systems or fan deposits in seismic data suggest relatively low turbidite activity and sheet-like deposits likely related to local, small-scale slope failures. The canyon is thus thought to be inactive during the deposition of this unit. – Seismic unit Tb located between ~ 0.7 and ~0.4 stwt below the seafloor is less reflective and more chaotic than Tc. It contains ~0.15 s-thick isolated symmetrical bodies that thin laterally with downlapping reflectors. They are interpreted as low-relief channel/ levees or lobes (Fig. 10C).

– Unit Ta is for the most part poorly reflective, and passes laterally to a well-stratified seismic facies that resembles Tc. The 0.4 s thick poorly reflective segment (Fig. 10C) is incised by a ~ 0.25 s-deep infilled channel axis. The lateral transition from poorly-reflective facies with discontinuous reflections to well-stratified seismic facies suggests axial channel deposits passing laterally to hemipelagic/ turbiditic trench fill. The sequence of deposits presented in the trench supports 1) a transition from hemipelagic/turbiditic deposits to small channels, suggesting that sediment supply to the trench was likely episodic; and 2) the recent construction of a turbidite channel axis at the terminus of the Patia canyon that could be the result of either an increasing activity of sediment transfer within the canyon or the development of the canyon itself toward the trench.

4.5. Onshore fluvial system connection Infilling of the channel axis in the trench at present suggests that sediment transport within the canyon has decreased. This may be either result of the dam (Fig. 8) on the middle slope blocking gravity flow or a change in the volume of sediment delivered to the canyons by the rivers. Investigating the inland connections of the canyons is therefore necessary to constrain the canyon's activity. The Patia river flows northwards along the Remolino High (Fig. 11A), crosses the high westwards, and flows toward the Pacific Ocean in a meandering way, forming a dendritic network. The widest (i.e. the most active) tributaries of the Patia river (130–180 m wide) (1 to 4 in Fig. 11A), which should channelize the highest volumes of particles, are currently located on the southern part of the delta and do not face the heads of the Patia canyon system. Contrastingly, only a narrow and likely less active river tributary (~80 m wide) (5 in


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Fig. 6. (A) Detailed bathymetric map (60 m grid resolution) of the Mira canyon incising the upper slope. The along stream canyon axis and flank profiles are represented in (B) along with the local slope gradient of the canyon axis. Cross-sections AA′ and BB′ are represented in (C) and (D).

Fig. 11A) faces the most incised canyon branch (A in Fig. 11), the biggest rivers (1 to 4 in Fig. 11A) being offset ~18 to ~40 km southward. The Mira river presents a single main tributary, wider (250–300 m) than those of the Patia river. The river reaches the Pacific Ocean ~13 km southwestward of the canyon head and the canyon head thus appears disconnected from the river, similar to the Patia canyon/river system. In summary, the present-day disconnection between the mouths of the active river tributaries and the main canyon heads indicates a southward migration of the respective sources of the canyons. The total drainage system and associated delta deposits of the Patia river appear to have shifted south-westward during the Quaternary (Gomez, 1986; Correa, 1996).

5. Discussion The most parsimonious explanation for the observations presented above is that the Patia-Mira canyon systems and their associated rivers were affected by tectonic evolution of the margin and its influence on sediment supply, probably combined with glacio-eustatic control. 5.1. Canyon incision versus margin deformation 5.1.1. Estimation of margin paleo-morphology prior to canyon incision Topographic features are obstacles to the main path of gravity flows, and thus may deflect the course of a canyon during its initial

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Fig. 7. (A) Detailed bathymetric map (60 m grid resolution) of the Patia Canyon incising the whole margin until the trench. The along stream canyon axis and flanks profiles are represented in (B) along with the local slope gradient of the canyon axis. The measurements were made from tributary A of the canyon. Red lines represent thrust faults crossed by the canyon, inferred from slope gradient variation and seismic profiles. Cross sections AA′ and BB′ are represented in (C) and (E). (D) outlines morphological scarps at the junction of canyon tributaries, implying a southward stream migration.


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Fig. 8. (A) Detailed bathymetric map (60 m grid resolution) of the middle slope. The white dashed line shows the limits of the slope basin, while the white bold line shows the limits of the canyon sedimentary infill. Thin isobaths are every 100 m, while bold isobaths are every 500 m. Parallel-to-the-margin seismic lines Ama 014 (B) and (C) and perpendicular-to-themargin line Anh-2800 (D) and (E) show the seismic units Ua to Uf and slope failure deposits S1 and S2. Location of the seismic lines is shown in (A).

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Fig. 9. (A) Bathymetric map showing the position of the sedimentary core Kama08. (B) Stratigraphic log and median grain size of core Kama08. Green layers represent turbidites. Red dots are 14C dating samples. (C) and (D) represent X-ray images of the core outlining respectively a turbidite and fine-grained laminae. (E) represents seismic line Anh 1300 along which the core was been collected. (F) shows the age vs depth chart obtained from 14C dating samples along with inferred sedimentation rates.

incision (Huyghe et al., 2004; Mountjoy et al., 2009). Therefore, the general course of canyons may provide information on the seafloor morphology prior to canyon inception: the Tumaco High (TH) and Tumaco Antiform (TA), the Patia promontory, and the Gauiquer ridge all appear to be main structures that guided the canyon's incipient course, and are therefore tentatively used here to estimate margin morphology prior to canyon incision. The abrupt 90° change of the Mira canyon course, from alongstrike to along-dip where the canyon encounters the TH (Fig. 5A), together with the absence of paleo-canyon evidence across the TH, support that topography predated the canyon system. Prior to merging with the Patia canyon, the Mira canyon meanders at the periclinal termination of the TH folds where the canyon passes through a saddle

between the TH and TA, suggesting that the saddle pre-dates the canyon. Moreover, the Patia canyon rounds the TA on its northern termination, indicating a second saddle on the upper continental slope (Fig. 7). Consequently, the Tumaco High and the Tumaco Antiform necessarily predate canyon excavation. As indicated by onlaps of units Ua, Ub, and Uc on the southern flank of the Patia promontory (Fig. 8B), this flank was tilted prior to the deposition of these units. Incision of Ua and Ub by the Patia canyon indicates that the canyon started incising after uplift of the promontory. Because the Patia canyon exhibits a Z-shape where crossing the Guaiquer ridge and the accretionary prism (Fig. 10A), ridges were probably present on the lower continental slope prior to the canyon breaching it and rounding the ridges to reach the trench. In addition,


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Fig. 10. (A) Bathymetric map showing the lower slope and trench. The white dashed line delineates the limits of the deep sea fan located at the mouth of Patia Canyon. The inset (B) shows the Z-shape in map view where the canyon breaches the Guaiquer Ridge, as well as the oversteepened walls and an 80-m high ridge crossing the canyon floor associated with active uplift. (C) Seismic line Ama 07 crossing the trench and the Patia deep sea fan. Unit Tc is comprised of turbiditic/hemipelagic trench fill, unit Tb contains F isolated lobes and small channel–levee systems, and unit Ta represents a large channel–levee system incised by a well-developed and presently buried channel.

unit Ub, interpreted as predating or coeval with canyon activity, onlaps the tilted units Uc and Ud along the Guaiquer ridge (Fig. 8D, E) and thus attests to the presence of the ridge before canyon incision.

5.1.2. Margin tectonic deformation during canyon incision Analysis of the tilt of the canyon floor in cross section, the shape of the bathymetric profile along the axis of the canyons, and the geometry of deposits associated with canyons activity as well as their I ratio help

identify spatial variations in relative seafloor deformation since the canyon was initiated. 5.1.2.1. The Tumaco High (TH) and Tumaco Antiform (TA). Cross section AA′ in the Mira canyon reveals that the canyon floor is dipping about 1° landward (Fig. 6C). This dip is opposite to that expected from the erosion of a canyon floor, which is usually stronger along the external side of a bend. This reverse dip may imply that the seafloor was tilted landward by the uplift of the TH. This is also supported by migration

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Fig. 11. Landsat 7 images and sketches illustrating the connections between Patia (A) and Mira (B) canyon heads and their respective inland drainage systems.

of the canyon axis to the northeast that left an abandoned section at a water depth slightly shallower than the present-day canyon location (Fig. 6A). Moreover, we interpret the steep (> 30°) canyon walls across the saddle between the TH and the TA (cross section BB’ in Fig. 6D) and the high observed I value (~ 35) as the result of an antecedent stream and implying syn-incision seafloor uplift. As compared to rivers, antecedent streams have established a course prior to the growth of a structure and subsequently maintain their course across a developing zone of active differential uplift through stronger incision, rather than being deflected around it (Burbank et al., 1996).

The concave-up profile in the upper part of the Mira canyon (0–60 km in Fig. 6B) implies relative tectonic stability, allowing the canyon to approach an equilibrium profile, while its convex-up lower part (~ 70–85 km in Fig. 6B) suggests that local uplift dominates seafloor erosion across the saddle. Similarly, tributaries A to C of the Patia Canyon show a concave-up profile across the upper slope (Figs. 6B and 7C), attesting to a stable state, while their segments that cut across the TA show a convex-up morphology. Accordingly, both the Mira and Patia canyons are interpreted to cross uplifting tectonic features, as documented for river profiles in mountain belts such as the Himalayas (Seeber and Gornitz, 1983) or by submarine channels and canyons


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Fig. 12. Evolution model proposed for the study area. (A) Canyons are incised on the upper slope probably during a sea-level lowstand in the Pleistocene. They contour the Tumaco High and antiform, and the Patia promontory. Debris flows and turbidites transported within the canyons are trapped within the slope basin bounded by the already uplifted Guaiquer ridge, Patia promontory, and Tumaco High and Antiform. (B) The slope basin is progressively infilled (unit Ub), and part of the incoming sediment overspilled the Guaiquer ridge along a bathymetric threshold and start breaching the ridge and the accretionary prism at 150 ka. Communication with the trench is not completely efficient and results in the deposition of isolated lobes and migrating channel–levee systems. (C) The Guaiquer ridge and accretionary prism have been breached, resulting in a welldeveloped channel–levee system in the trench, and the deposition of fine grained overbank deposits in the slope basin (unit Ua) since 53–67 ka. Uplift is active along the Guaiquer ridge and the Tumaco High and Antiform, as evidenced by over-incised canyon walls and the abandonment of a meander in Mira. Moreover, the Patia drainage system is progressively migrating southwestward. (D) The present-day morphology is characterized by disconnection of canyon head with their sources and slope failure obstructing the canyon bed. The canyon floor in the slope basin and the channel in the trench are infilled.

Fig. 13. Along-dip seismic line SIS 35 showing the geometry of the trench fill southwestward from the Patia deep sea fan. Oc and the red dashed line represent the Nazca plate oceanic crust and its roof. Vs and the black dashed line represent the volcano-sedimentary and pelagic cover of the Nazca plate. Tf represents the trench fill. The green dashed line outlines the uppermost reflector onlapping on the Nazca plate, used for estimating the thickness and sedimentation rate of the trench fill. Assuming that the trench fill was deposited uniformly, the point A was at a similar position as point B at the time of its deposition. As the downgoing plate converged, the point A was buried by ~ 2500 m (decompacted depth) over ~ 615 ka.

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Table 4 Measurements made to estimate the mean sedimentation rate of the trench fill along seismic line SIS 35 (Fig. 13). The two lines represent data concerning the top and the bottom reflectors used for the calculations. The depth was measured on the seismic line in twt, then converted in meters assuming a 1800 m s− 1 seismic waves velocity as demonstrated by Calahorrano et al. (2008) offshore southern Ecuador. Porosity and decompaction of sediment were estimated using a decompaction law (Hutchinson, 1985; Marcaillou et al., 2008) and assuming 60% porosity at the seafloor. We then measured the distance between the top and bottom onlaps (dx), and using the apparent convergence rate along the seismic line, calculated the time between the two onlaps in order to estimate a mean sedimentation rate (see Mountney (1997) for details).

Top Bottom

Depth (stwt)

Depth (mbsf)

Porosity (%)

Decompacted depth (m)

dx (km)

Apparent convergence rate (mm.yr− 1)

dt (kyr)

Sedimentation rate (mm.yr− 1)

4.92 6.57

247 1735

50.9 19

275 2796

0 21.3

34.7 34.7

0 615

4.1

(Yeats et al., 1998; Huyghe et al., 2004; Bourget et al., 2011). Consequently, uplift of the TA and TH is considered active. The uplift is also supported by I values increasing from ~2 to ~35 and from ~5 to ~12 on the Mira and Patia canyons along the TH and TA, implying stronger incision across these structures. Moreover, the thrust observed in front of the TA (Fig. 8D) affects unit Ua, interpreted as overbank deposits of the canyon. Consequently, part of the thrusting and uplift occurred since incision of the canyon. 5.1.2.2. Guaiquer ridge and the accretionary prism. As indicated by the overall convex-up morphology of the stream profile of the Patia canyon (70–100 km in Fig. 7B), the Guaiquer ridge is interpreted as an uplifting feature. The unequivocal V cross-section, the steep walls (30°) (Fig. 7E), the maximum incision (up to ~1200 m), and the increase in I from ~3 to ~19 suggest an overincised antecedent stream and uplift of the Guaiquer ridge since canyon incision. Finally, the 80 m-high scarp that developed across the canyon floor (Fig. 8B) testifies to recent tectonic activity overriding canyon erosion. Moreover, the stream profile outlines the occurrence of folds across the accretionary prism in front of the Guaiquer ridge. Every slope variation within the lower continental slope occurs where the canyon crosses a fold, suggesting that the ridges were at least partly active recently (Fig. 7B). The frontal ridge, which partly dams the canyon mouth, implies active deformation at the subduction front that was not balanced by the erosive power of gravity flows bypassing the canyon. In summary, all major structural highs were already prominent seafloor reliefs prior to canyon inception. This preexisting morphology controlled the initial course of the canyons. The canyon stream profiles reveal that the Tumaco High and Antiform, the Guaiquer ridge, and the accretionary prism continued to experience uplift during the latest phases of canyon development, possibly with a gradual decrease in sediment transport.

5.2.1. Pleistocene Canyon incision on the upper continental slope The Patia and Mira canyons were likely initiated on the upper slope and connected to their sources, the Patia and Mira rivers (Fig. 12A). Indeed, the close connection between the rivers and the canyon heads permitted the delivery of enough material to enable canyon formation by downward incision and retrogressive erosion as proposed by Pratson and Coakley (1996) on a theoretical margin. Downward erosion is supported by the tortuous path of canyons contouring bathymetric highs, as opposed to retrogressive erosion, which evolves upslope on a straight path. The proximity of continental sources plays a major role in downward erosion (Pratson and Coakley, 1996) because of hyperpycnal currents entering canyons during major floods (Mulder and Alexander, 2001) and eroding the seafloor. This process can be particularly efficient during sealevel lowstands as direct connection between rivers and canyon heads are possible, thus directly providing particles (Aloisi et al., 1975; Bourget et al., 2010) that bypass and scour the canyon floor. The headward retrogressive erosion in the upper part of the Patia canyon is supported by the presence of numerous small-scaled scarps at the head of the tributary A (Fig. 7A). Accordingly, both downward cutting and retrogressive erosion incised the Mira and Patia Canyons. We propose an age of canyon incision on the upper slope by estimating the age of deltaic deposits incised by the canyons. Unit D1 (Fig. 5A and B) located on the upper slope corresponds to southwestward-spreading deltaic deposits (Lopez, 2009) and outlines the migration of the Patia river drainage system. This unit is inferred on land as Pleistocene without further age precisions (Gomez, 1986; Correa, 1996); as canyons incise and postdate this unit, their inception should be Pleistocene in age.

5.2. Main stages of canyon and margin evolution Based on the available data, we interpret five stages that characterize the evolution of canyon activity at this active margin.

Table 5 Estimation of the age of the transition from hemipelagic trench fill to episodic channel– levee system (see dashed line in Fig. 9B). The time slice was converted to depth assuming a 1800 m s− 1 seismic waves velocity as demonstrated by Calahorrano et al. (2008) offshore southern Ecuador, and decompacted using a decompaction law (Hutchinson, 1985; Marcaillou et al., 2008) and assuming 60% porosity at the seafloor. The thickness was finally converted to age using the sedimentation rate of the trench fill calculated along seismic line SIS 35. Depth below Depth below Porosity Decompacted Sedimentation rate Age (mm.yr− 1) (kyr) (%) depth the seafloor the seafloor (m) (m) (stwt) 0.57

515

42.6

633

4.1

~ 150

Fig. 14. Graph showing relations between the directivity ratio D and the mean continental slope angle α. D = CL/MIW; CL represents the canyon length and MIW represents the margin width incised by the canyon orthogonally to the margin direction.


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Table 6 Compilation of morphometric parameters and morphological features encountered along canyons on accretionary and erosive margins. The mean continental slope angle is calculated between the margin front and the shelf break. D is the ratio between the canyon length and the margin width measured at the canyon mouth. Canyon

Margin type

Mean slope D ratio (°)

Convex-up Abandoned Slope Deep sea Reference profiles path basin fan

Patia (Colombia) Barbados Kaoping (Taïwan) Makran San Antonio (Chile) Tenryu (Japan) Cook Strait (New Zealand) La Aguja (Colombia) Chiclayo (Peru) Costa Rica Esmeraldas (Ecuador) Guayaquil (Ecuador) Santa Elena (Ecuador)

Accretionary Accretionary Accretionary Accretionary Accretionary Accretionary Accretionary /strike slip Accretionary /strike slip Erosive Erosive Erosive Erosive Erosive

2.5 1 1.9 2 to 3 3.5 2.9 2.6 2.4 5.5 ~6 ~ 3.7 5.9 6.8

3 4 4 7 1 Unclear N/A 3 1 N/A

1.8 1.7 2.8 1.34 to 2 1.44 1.23 1.46 1.53 1.1 1.11 1.34 1.21 1.17

1 2

5.2.2. Infill of the slope basin surrounded by structural highs All the structural highs existed prior to the incision of the Patia canyon, as supported by the tilting and growth strata of units Uc and Ud. Hence, without a bathymetric low allowing sediment bypass, these highs, and especially the Guaiquer ridge, formed a dam for sediment input coming from canyons freshly initiated on the upper slope (Fig. 10A). Debris flows and turbidity currents that usually bypass canyons (Arzola et al., 2008; Bourget et al., 2010) were not able to overflow the surrounding topographic highs, resulting in rapid development of slope basin infill (Sinclair and Tomasso, 2002). Unit Ub most likely corresponds to this infill as it sharply onlaps the underlying units, and exhibits a reflective echofacies, probably related to the alternation between debrites/turbidites and hemipelagic deposits. 5.2.3. Overspilling of the slope basin at ~150 ka The next stage involves the beginning of overspill of the slope basin (Fig. 12B). It is suggested by the sedimentary record in the trench where a transition occurs from hemipelagic/turbiditic trench fill to the construction of episodic lobes and channel–levee systems. This transition indicates that sediment transited from the slope basin toward the trench episodically. The deposits Ub were likely confined within the slope basin until reaching a bathymetric low on the Guaiquer ridge. Probably only the finest-grained fraction of incoming turbidity currents transited over the threshold and reached the trench, rounding the accretionary prism ridges as indicated by the Z-shape in map view of the Patia canyon. Sinclair and Tomasso (2002) observed a similar evolution in the Gulf of Mexico (gravitationally driven deformation) and in the Southwestern French Alps (shortening tectonics) based on seismic and sedimentological data. Without direct dating of the trench deposits constituting this stage, we calculate a mean sedimentation rate of turbiditic/hemipelagic trench fill away from the deep sea fan (Fig. 13) based on the geometry of trench fill and the plate convergence rate as proposed by Mountney (1997). Assuming that the trench fill was deposited uniformly, and that the convergence rate remained unchanged during the late Quaternary, the age of sediment onlapping the subducting plate and becoming progressively buried as it approaches the trench can be estimated using the convergence rate. After decompaction, the overall trench sedimentation rate is estimated to be 4.1 m ka− 1 (Table 4). Using this sedimentation rate on the isochron between units Tb and Tc in the trench (Fig. 10C), we estimate the age of the transition to episodic fan deposits at ~150 ka (Table 5). 5.2.4. Canyon activity across the entire margin since ~53–67 ka The fourth stage of canyon evolution reflects an efficient connection between the shelf break and the trench. It is supported by the

Yes No Yes ? Yes Yes Yes Yes No Unlike Yes No No

Yes Yes Yes Yes Yes No No No No No No No No

Yes Yes Yes Yes Yes Yes Yes Yes No No Yes No No

This study Huyghe et al. (2004) Chiang and Yu (2006); Yu et al. (2009) Bourget et al. (2010) Hagen et al. (1996); Laursen and Normark (2002) Soh and Tokuyama (2002) Mountjoy et al. (2009) Restrepo-Correa and Ojeda (2010) Sosson et al. (1994) von Huene et al. (2000) Collot et al. (2009) Coronel (2002); Collot et al. (2009) Coronel (2002); Collot et al. (2009)

construction of a well-developed channel–levee system within the trench (Ta in Fig. 10C) that indicates a direct canyon-derived sediment input. Once the basin infilled to the threshold of the Guaiquer, incision of the Patia canyon axis resulted, through the accretionary prism downstream and through unit Ub in its upstream part, thus allowing a connection between the shelf and the trench. In the middle slope, this stage is highlighted by a change in seismic facies between units Ub and Ua. The transition from high amplitude facies of unit Ub, interpreted as contrast between muddy debrites and sand turbidites, to low-amplitude Ua facies, consisting of fine-grained overbank deposits (Fig. 9), outlines a major change in sediment supply and deposition processes. Once the connection with the trench was established, the majority of sediment bypassed the slope while only fine-grained deposits overbanked and built unit Ua. The age of Ua deposits in the slope basin is deduced from sedimentation rates calculated in core Kama08. Two 14C dates from pelagic foraminifera give ages of 3418±60 yr BP and 10,188 ±70 yr BP at 1.46 m and 3.37 m below the seafloor (Fig. 9B and Table 3), providing a mean sedimentation rate of 0.35 mm yr− 1. This sedimentation rate corresponds only to the Holocene and is representative of highstand sea-level; lowstand rates are generally lower. For ages greater than 10 ka, we used mean highstand/lowstand rate ratios calculated by Covault and Graham (2010) for 22 fans worldwide. Depending on whether systems are lowstand- or transgression-dominant, the ratio ranges between ~3 and 2.2 for ages >10 ka. Applying these sedimentation rates from the base of the core (~10 ka) to the base of unit Ua and accounting for compaction, the age of overbank deposits is estimated between ~53 and ~67 ka. We propose that most of canyon incision took place during this stage as the canyon was active from the upper continental slope to the trench. The canyon strongly incises the Guaiquer ridge and the accretionary prism as indicated by the increasing incision in these locations (up to 1200 m) (Fig. 7E). Moreover, the canyons experienced recent active uplift suggested by the general convex-up morphology and by antecedent processes along the canyon. 5.2.5. Holocene decline of canyons activity? Presently, canyon activity is decreased, as evidenced by channel infill in the trench. Because the infill is not draping the whole channel–levee system and because Holocene overbank deposits are observed on the slope basin, canyon activity appears to have decreased but not ceased. 5.3. Controls on canyon activity It is widely assumed that sediment supply is governed by relative sea level variations and glacioeustatic control, with turbidite systems predominantly growing during sea-level fall and lowstand (Vail et al.,

94


1977; Posamentier et al., 1991). These periods are characterized by subaerial exposure and erosion of the continental shelf, enhanced incision of rivers tending to return to an equilibrium profile, possible direct sediment input to canyon heads, and a decrease in accommodation space. As a result, canyon activity and sediment transport to the deep-sea environment are usually greater during sea-level fall and lowstand. Glacioeustatic sealevel control is particularly efficient on passive margins where no or little vertical seafloor motion occurs, but on tectonically active margins, where continental shelves may be narrow, and uplifts and subsidence might be significant, sediment input can also be sustained during periods of relative highstand (Bourget et al., 2010; Covault and Graham, 2010). Both estimated ages of canyon activity, corresponding 1) to the over-spilling of the slope basin and deposition of episodic fan lobes in the trench (~150 ka) and 2) to canyon activity across the whole margin with construction of the channel–levee system in the trench and overbank deposits in the slope basin (~ 53–67 ka), correlate with relative lowstands or a general fall in sea level (Waelbroeck et al., 2002). The overspill of the slope basin may have been facilitated by a significant amount of sediment delivered during the Maximum Glacial at ~ 150 ka, but a likely decrease in sedimentation during a highstand at ~120 ka probably prevented sustained activity downslope. During sea level fall, the amount of sediment likely increased enough to allow canyon activity from the shelf to the trench and the construction of the channel–levee system in the trench at 53–67 ka. The present-day decreasing activity may be associated with a landslide located in the slope basin that dammed the canyon. Moreover, highstand conditions are often associated with trapping of sediment on the shelf, and this could be particularly efficient offshore the tributaries of the Patia river where the continental shelf is up to ~ 10 km wide. The present-day disconnection of both Mira and Patia canyons from their major feeding rivers may also play a significant role in the recent decline of canyon activity. However, although decreased, the activity has not ceased as suggested by the Holocene overbank deposits. There is almost no continental shelf landward from the Mira canyon, suggesting that delivered particles cannot be completely trapped there. Moreover, although disconnected from their major sources, smaller rivers still feed the Patia canyon tributaries, and sediment brought by northward littoral drift cannot be totally excluded, as both canyon systems are located northward from their respective rivers. Finally, the margin undergoes strong subduction earthquakes that trigger submarine landslides and turbidity currents (Ratzov et al., 2010) and thus may feed the canyons and contribute to their Holocene activity. 5.4. Margin deformation and out-of-sequence structures Previous studies suggested several phases of uplift and subsidence across the margin (Marcaillou and Collot, 2008; Lopez, 2009). The polyphasic activity of both proximal and distal structures therefore implies out-of-sequence deformations. The Guaiquer ridge involves the basement of the margin (Fig. 3 and Lopez, 2009), which appears bound by a major crustal-scale reverse fault (Marcaillou, 2003). Moreover, Collot et al. (2004) (SIS-35, their Fig. 10) identified shortening distributed all along the margin wedge, the forearc basin and the Tumaco High. This compressional event deformed the youngest unconformity. Previous studies based on seismic stratigraphy and supporting outof-sequence deformation do not indicate whether the last deformation stage, peaking during the early Pliocene (Marcaillou and Collot, 2008) or Early Pleistocene (~1.8 Ma) (Lopez, 2009), was also active during the Quaternary. Such information is crucial for constraining whether uplift activity occurred as the result of short-term events or long-term processes, and ultimately as the result of different tectonic causes. Indeed, short-term (up to several ka) and discontinuous uplift imply local interplate coupling variations that might be caused by subducting

asperities and seamounts (Lallemand, 1999), or alternately by sediment patches subducting beneath the upper plate (Sage et al., 2006), whereas continuous uplift implies long term processes (102 to 10 3 ka) such as changes in slab dip or in convergence rates, among other parameters (Lallemand et al., 2005). Our analysis shows that out-of sequence tectonic deformation has been active during the Quaternary, and has remained active at least since ~150 ka, when canyons were incepted, suggesting a rather long-term deformation process. The presence of an 80-m high scarp across the canyon axis where it crosses the Guaiquer Ridge (Fig. 10B) supports the presence of an active splay fault, as suggested by Marcaillou (2003) (Fig. 3). This scarp may have originated because the canyon locally incises a substratum with a more rigid lithology, such as compacted sediment or oceanic basement, uplifted at the hanging wall of the fault. Alternatively, the ridge may be the result of a high uplift rate that cannot be compensated by the erosion of the canyon. In the case of the latter, a high uplift rate would imply that a significant part of the margin convergence could be accommodated along the fault. 5.5. Comparison with geomorphic features on other convergent margins Accretionary margins are characterized by frontal accretion of trench sediment with a complex morphology of ridges and piggy-back basins oriented along strike. Conversely, erosive margins undergo basal erosion along the plate interface, resulting in a narrow and steep subsiding continental slope, usually with simple and regular morphology that might be affected by normal faults. To outline whether features used in this work are particular to our case study, or rather are generic on convergent margins and could help constrain recent tectonic activities elsewhere, we compare our observations with those of canyons on other active margins 5.5.1. Features indicating uplift prior to canyon inception To constrain the location of structural highs at the time of canyon inception, we used the occurrence of sharp canyon turns. To quantify the deflection of the path induced by the highs, we use a “deflection ratio” D that we define as D = CL/MIW, where CL represents the canyon length and MIW the margin width incised by the canyon along dip. This parameter is slightly different from traditional sinuosity, as it represents a ratio between the length of the canyon and the shortest path to the trench instead of the shortest path to reach the canyon mouth. D appears greater on accretionary margins (~1.23–2.8) (D ~1.8 for Patia canyon) than on erosive margins (~1.1–1.34) (Fig. 14), supporting a more indirect path to the trench. Moreover, D appears to decrease on steeper margins (Fig. 14). Although greater mean slope angles observed on erosive margins could at least partly explain this trend, slope irregularities likely play a role in determining D. Indeed, as observed in this study and for the canyons San Antonio offshore Chile (Laursen and Normark, 2002), Cook Strait offshore New Zealand (Mountjoy et al., 2009), Makran (Bourget et al., 2011), Barbados (Huyghe et al., 2004), Kaoping offshore Taïwan (Chiang and Yu, 2006) and Tenryu offshore Japan (Soh and Tokuyama, 2002), abrupt changes of direction occur along structural ridges and thus tend to increase D. By contrast, on erosive margins where the physiography is usually simple with few or no major ridges inducing counter-slopes, canyons appear to be quite linear and reach the trench on an almost direct path (1.1 b D b 1.34). However, the D value may also increase on erosive margins if the canyon is rooted on major faults oblique to the margin, as exemplified by the Esmeraldas fault and canyon (D = 1.34) on the Ecuador margin (Collot et al., 2004). 5.5.2. Features indicating active uplift Convex-up stream profiles, antecedent structures, and abandoned canyon paths all suggest active uplift. Convex-up stream profiles occur where canyons cross active structures such as folds or thrusts; there, uplift tends to deform the seafloor. As such structures are common on


accretionary prisms, convex-up profiles may be numerous (3 in our study; 4 in the Barbados, up to 7 in the Makran) (Table 6). Across these structures, where uplift rates are significant and cannot be balanced by erosion from turbidity currents, structural highs form a topographic dam resulting in canyon infill or abandonment, as observed in our study (Fig. 6A) and the Cook Strait, Tenryu, Kaoping, and San Antonio canyon systems (Table 6). On erosive margins, although limited frontal prisms may occur, the overall simple morphology leads to less numerous convex-up profiles. Canyon profiles are the result of competition between erosion by gravity flows and uplift of the seafloor. When the uplift rate is higher than the erosion rate, the profile tends to be convex-up. The erosion rate however depends on parameters such as the nature of the seafloor and its resistance to erosion, the type of gravity flows responsible for transport and erosion, the nature of transported material (sand, mud, etc), or the number and frequency of gravity flows (Pirmez et al., 2000; Piper and Normark, 2009; Covault et al., 2011). As erosive margins are usually more sediment poor and therefore usually show a thinner sedimentary cover (Clift and Vannucchi, 2004), the margin basement can outcrop (Sosson et al., 1994), resulting in a seafloor with different physical properties from the sedimentary cover. Variations in along-stream profiles could therefore be interpreted as caused by different resistance to downcutting erosion (Hagen et al., 1996) rather than by tectonic uplift. 5.5.3. Features giving insights into the age of canyons and deformation To decipher the stages of activity of canyons and constrain the age of uplift, we mainly employed the sedimentary units of the trench/ deep-sea-fan and of the slope basin. Without original bathymetric data and seismic lines, we cannot discuss occurrence of such features on other convergent margins. Rather, we simply verify the presence of places where such features may be recorded. All investigated canyons located on accretionary margins exhibit a deep sea fan at their canyon mouth (Table 6). In contrast, on erosive margins and with the exception of the Esmeraldas canyon, none of the canyons feed a major deep-sea fan. This difference is likely related to the fact that erosive margins are fed by a reduced amount of continental sediment (Clift and Vannucchi, 2004). The Esmeraldas canyon is a particular case where the presence of a coastal cordillera on land deflects the major Andean drainage toward the Esmeraldas river, and thus a great sediment charge is captured by the canyon leading to a well-developed deep sea-fan. Moreover, six out of nine canyons on accretionary margins also exhibit slope basins on their path, whereas no slope basins appear on canyons located on erosive margins. Likewise, this observation is likely the result of the overall simpler morphology of erosive margins. 5.5.4. Implications for canyons as strain markers of deformation The observations we describe above are specific to accretionary margins. They usually exhibit a complex topography affected by diverse ridges and thrusts that are prominent and numerous enough to control canyon paths during their inception. Where the structures are active, the resulting uplifts appear sufficient to force canyon morphology. In contrast, margins dominated by basal erosion tend to undergo subsidence, possess a steep slope gradient, and demonstrate relatively simple morphology. Although normal faults can cross the entire margin, their vertical movements are likely insufficient to generate counter-slopes able to deflect canyon paths, resulting in rather linear canyons. Moreover, the usual absence of well-developed deep sea fans, likely because of the reduced sediment supply, also reduces the number of potentially accessible dating markers (i.e. sedimentary unit boundaries). Because of the fundamentally different deformation styles and sediment supply between accretionary and erosive margins, the features we describe should not be generalized to all convergent margins, but should be applicable at least in the case of accretionary margins with a complex morphology.

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6. Conclusions The joint analysis of bathymetric, seismic, and sedimentary data collected on the South Colombian convergent margin provides constraints of the origin and evolution of the Mira and Patia submarine canyon system and evidence of Late Quaternary tectonic activity on the margin. Our main results are the following: – The path and shape of the submarine Patia and Mira canyons were subject to structural control, enabling estimation of uplifted highs prior to canyon incision. – The history of canyon incision and evolution support the following main stages of morpho-tectonic development of the margin and sediment transfer from the upper slope to the trench: 1) canyon incisions (likely on the upper slope) caused by retrogressive erosion and downward cutting associated with the Patia and Mira rivers systems during the Pleistocene, 2) infill of a slope basin surrounded by uplifting structural highs, 3) overspill of the slope basin and breaching of the Guaiquer ridge and accretionary prism at ~150 ka, leading to the construction of isolated sediment lobes in the trench, and then to 4) a well developed trench channel–levee system at 53–67 ka. 5) Presently, canyon activity is decreased because of highstand conditions, daming by a slope failure, and river migration, but the canyon remains lightly active probably trapping some river discharge and channelizing earthquake-triggered landslides. – The Tumaco High, Antiform, and the Guaiquer ridge underwent uplift over the last ~150 ka as suggested by convex-up stream profiles, steep walls (~30°), and an increase in I values (incision height/ width), all supporting the occurrence of an antecedent stream. – Recent uplift is located close to the trench on the outer accretionary prism, where in-sequence deformation prevails, and at the middle and upper margin slopes where out-of-sequence tectonic structures develop. – The analysis of submarine canyon morphologies and deposits allow refinement of previous Ma-scale chronologies of the area based on seismic stratigraphy, and provides new insights into the tectonic history of the margin, supporting continuous out-of-sequence deformation during the Quaternary. – Comparison with canyons on other convergent margins reveals that the features related to margin deformation and canyon age (tortuous path, convex-up profiles, abandoned canyon paths, overincision, abrupt canyon turns) are mainly specific on accretionary margins. Because of their usually complex morphology, these margins appear more favorable for the occurrence of such features than erosive margins typified by simple morphology. 7. Acknowledgements This work is part of the Ph.D research (Université de Nice Sophia Antipolis) of G. Ratzov, supported by a grant of the French Ministère de l'Education Nationale, de la Recherche et de la Technologie. The AMADEUS project was funded by the Institut de Recherche pour le Développement and the Institut National des Sciences de l'Univers. We are grateful to IFREMER for providing ship time, and GENAVIR for geophysical equipments and technicians. We thank INOCAR in Guayaquil, INGEOMINAS in Bogotá as well as the French Embassies in Quito and Bogotá for help in obtaining research permits and for others logistics. We thank A. Lebot and J.-F. Lebrun for processing multibeam bathymetry and seismic reflection data, J.I. Martinez for having helped picking foraminifera used for radiocarbon dating, the INSU national facility with the LMC14 sma Artemis, for providing the radiocarbon dates. We are grateful to J.-N. Proust, A. Cattaneo, P. Barnes, H. Pouderoux, and D. Graindorge for motivating discussions on early versions of this publication. Finally we thank L. McNeill,

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