Seismic triggering of landslides and turbidity currents offshore Portugal

Article Volume 12, Number 12 14 December 2011 Q12011, doi:10.1029/2011GC003839 ISSN: 1525-2027

Seismic triggering of landslides and turbidity currents offshore Portugal D. G. Masson National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, UK ([email protected])

R. G. Arzola National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, UK Atlantic Petroleum, 26/28 Hammersmith Rd., London W6 7BA, UK

R. B. Wynn, J. E. Hunt, and P. P. E. Weaver National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, UK [1] Sediments in deep water basins often include turbidites that record sediment input from adjacent conti-

nental margins. In seismically active areas, where turbidity currents are triggered by earthquakes, the basinal turbidite sequence may thus contain a record of palaeoseismicity, which can be used to infer the frequency of earthquakes affecting the margins of the basin. This is particularly useful where large earthquakes have a recurrence interval than is greater than the historical record. However, turbidity currents can be triggered by several processes, and it is often difficult to trace individual turbidites to their precise source areas and to assign a definite trigger to a particular turbidite. Here, we demonstrate that turbidites emplaced at 6600 and 8300 Cal yr BP in the Tagus Abyssal Plain, off Portugal, correlate with erosional hiatuses in two submarine canyons on the continental margin. The turbidites are sourced from simultaneous landsliding in both canyons, requiring regional triggers interpreted as earthquakes. An earthquake recurrence interval for the continental margin of 4000 years is estimated by extrapolation to deeper turbidites in the basin sequence. However, the example of the 1755 earthquake, which caused widespread devastation in southwest Iberia, shows that palaeoseismic interpretations must be made with caution. The 1755 earthquake had a magnitude >8.5 and yet the associated turbidite in the abyssal plain is typically 5 cm thick, while older turbidites can be >1 m thick. Given the large 1755 earthquake magnitude, the difference in turbidite thickness is unlikely to be related to the relative size of triggering earthquakes. Instead, we suggest that the offshore location of the 1755 earthquake, coupled with low sedimentation rates during the Holocene, may have limited the size of the associated turbidite. Components: 11,100 words, 9 figures, 1 table. Keywords: palaeoseismicity; submarine canyon; turbidites. Index Terms: 3002 Marine Geology and Geophysics: Continental shelf and slope processes (4219); 3022 Marine Geology and Geophysics: Marine sediments: processes and transport; 3045 Marine Geology and Geophysics: Seafloor morphology, geology, and geophysics. Received 17 August 2011; Revised 25 October 2011; Accepted 25 October 2011; Published 14 December 2011. Masson, D. G., R. G. Arzola, R. B. Wynn, J. E. Hunt, and P. P. E. Weaver (2011), Seismic triggering of landslides and turbidity currents offshore Portugal, Geochem. Geophys. Geosyst., 12, Q12011, doi:10.1029/2011GC003839. Copyright 2011 by the American Geophysical Union

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MASSON ET AL.: SEISMIC TRIGGERING OF TURBIDITY CURRENTS

1. Introduction [2] Deep-water sedimentary basins contain records of sediment input from adjacent continental margins. Large-scale changes in sediment input to a margin, for example due to climate and associated sea level change, and catastrophic events such as continental slope landslides, are among the events likely to be recorded in the adjacent basin. A major aim of studies of such basins is to decipher what these records mean in terms of margin evolution. The value of basin sedimentary records has been proven in areas such as the Moroccan turbidite system where a dateable record of volcaniclastic turbidites related to Canary Island flank collapses, which are difficult to date in proximal settings near the islands, is preserved in the Agadir and Madeira Abyssal Plain basins [Weaver and Kuijpers, 1983; Weaver et al., 1992; Masson et al., 2002; Wynn et al., 2002].

1.1. Turbidite Palaeoseismology [3] The concept of turbidite palaeoseismology was originally developed for the Cascadia continental margin off the western United States [Adams, 1990; Goldfinger et al., 2003]. The basic principle of this concept is that turbidity currents that are generated simultaneously in different areas along the margin, and flow through separate channels to amalgamate on the basin floor, cannot be generated by purely sedimentological processes, but must have an external trigger. In seismically active areas, this trigger is likely to be a large regional earthquake, as demonstrated elsewhere by the sequential downslope failure of submarine cables impacted by turbidity currents generated by historical earthquakes [e.g., Heezen and Ewing, 1952; Hsu et al., 2008]. [4] The field of submarine palaeoseismicity has expanded rapidly over the last 20 years, with published studies from a wide variety of locations around the world (see Goldfinger [2011] for a complete list of locations). The major challenge facing this discipline, especially for prehistoric turbidites where there is no independent record of an earthquake, is finding criteria that can be used to separate turbidites generated by earthquakes from those generated by other triggers that include slope loading by sedimentation, flank collapse of island volcanoes, storm waves, tilting due to margin subsidence or salt movement, changes in methane hydrate stability or hyperpycnal flows [Masson et al., 2010a; Goldfinger, 2011]. Thus most studies have been undertaken on recent sedimentary sequences, where turbidites can be positively

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correlated with their triggering earthquakes. For example, multiple coarse fraction pulses, with or without contrasts in mineralogy, have been identified in turbidites that were generated by historical earthquakes [Nakajima and Kanai, 2000; Goldfinger et al., 2008]. This has been interpreted as a signal of multiple turbidity currents from different sources triggered simultaneously by a large earthquake affecting a relatively broad area. However, turbidites with multiple coarse bases are also associated with volcanic island flank collapse [Wynn and Masson, 2003]. Although the mineralogy of the multiple volcanic bases is very similar, in some cases they can be distinguished using geochemical techniques, because the individual flows, although originating from the same source, eroded geochemically distinct areas of the volcanic edifice [Hunt et al., 2011]. This could give a false impression of multiple sources. The converse situation, where multiple earthquake-triggered failures give rise to a single fining-upward turbidite in a distal basin is also possible, especially where the initial failures occur over a relatively limited area (e.g., 1929 Grand Banks turbidite [Piper et al., 1999]). In addition, where at turbidite is generated from a single landslide, there is no obvious reason why the triggering mechanism would have any influence on the resulting deposit, typically a single finingupward Bouma sequence turbidite. A good knowledge of the geology of the source area is thus a prerequisite for concluding that the turbidite had a seismic trigger. [5] Gorsline et al. [2000] showed that earthquake

generated turbidites offshore southern California could be separated from storm generated tubidites on the basis of their greater volume and areal extent. However, it is not clear to what extent these criteria are dependent on the local sedimentary environment (e.g., sediment input rates and processes, slope stability factors) from which the turbidites originated. It seems unlikely that these distinguishing criteria can be extrapolated with confidence to all ancient basins, as suggested on the basis of the California study [Gorsline et al., 2000]. Complex crossbedding and an increase in terrigenous sediment content relative to other turbidites in the same basin have been attributed to the earthquake triggering of a turbidite associated with the January 2010 Haiti earthquake [McHugh et al., 2011]. However, as with the California example, it is difficult to understand which of the characteristics of this turbidite are a direct consequence of seismic triggering, and which are related to its depositional environment, in this case in a small enclosed basin. Complex cross 2 of 19


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bedding is common in turbidites generated by a variety of processes in many different sedimentological environments; not all can be related to seismic triggering (e.g., cross-bedding in volcaniclastic turbidites in the Madeira Abyssal Plain [Rothwell et al., 1992].

The widespread extent of this erosion strongly suggests that the erosional events had external triggers of regional extent, most likely significant earthquakes in the vicinity of the Portuguese margin near 38°N.

[6] Goldfinger [2011] argues that “virtually all”

1.3. Study Area

palaeoseismic studies invoke the “synchronous triggering” test, even when additional sedimentological arguments for seismic triggering are proposed [e.g., Nakajima and Kanai, 2000; Goldfinger et al., 2008]. This appears to be a reasonable appraisal of submarine palaeoseismic studies at the present time. Studies based on sedimentological criteria [e.g., Gorsline et al., 2000; McHugh et al., 2011] may be applicable in a local, wellunderstood context, but are difficult to extrapolate for use in global recognition of seismically generated turbidites.

1.2. Turbidites of the Tagus Abyssal Plain and Iberian Continental Margin [7] The most recent turbidite in the distal Tagus Abyssal Plain (TAP), off Portugal, has been correlated with the 1755 earthquake that devastated much of SW Iberia [Thomson and Weaver, 1994]. This suggests that the sequence of thick turbidites in the TAP [Lebreiro, 1995] might contain a prehistoric record of seismicity on the Portuguese margin [Vizcaino et al., 2006; Gràcia et al., 2010]. Furthermore, Gràcia et al. [2010] have shown that all the major earthquakes affecting SW Iberia in the last 2000 years left their mark in the marine record, albeit with variable expression in terms of magnitude and extent. A better understanding of the way in which turbidites in the marine record reflect the event that triggered them would thus be a major step forward in understanding the geohazard threat posed by earthquakes along that margin. However, interpreting the turbidite record has been difficult because to date it has only rarely been possible to correlate individual turbidites in the basin with their source areas along the margin [e.g., Goldfinger et al., 2008; McHugh et al., 2011], and without knowledge of the source it is hard to distinguish between different possible turbidity current triggers. For example, turbidity currents could be derived from landslides resulting from high sediment input to the margin, with or without a seismic trigger. In this paper, we demonstrate that turbidites deposited in the TAP at 6600 Cal yr BP and 8300 Cal yr BP have the same age as erosional events in the Setubal and Cascais Canyons offshore Portugal.

[8] The area discussed in this paper lies immediately to the north of the Africa-Eurasia plate boundary southwest of Iberia (Figure 1). The exact location and structure of the plate boundary in this area is the subject of ongoing discussion [e.g., Zitellini et al., 2009, and references therein]. It appears to be represented by a broad and complex deformation zone dominated by NE trending thrust faults and NW trending strike-slip faults; an accretionary wedge has also been identified under the Gulf of Cadiz (Figure 1) [Gutscher, 2004; Zitellini et al., 2009]. The epicenter of the 1755 earthquake is located within this complex deformation zone and modeling of the arrival times of the associated tsunami at various places around the N. Atlantic gives a best estimate of 200 km off Cape St Vicente [Baptista et al., 1998b]. The fault that generated the tsunami has not been positively identified, although the Horseshoe Fault has been suggested as the most likely candidate [Stich et al., 2007]. [9] Setubal and Cascais Canyons cut the Portuguese continental margin between 38° and 39°N (Figure 1). Cascais Canyon cuts the steepest part of the margin. Consequently it is relatively short and has steep axial gradients that typically range from 5 to 20° [Lastras et al., 2009]. It has a v-shaped cross section for most of its length but broadens and becomes flat floored beyond 4500 m before merging with the TAP between 4700 and 4800 m. [10] Setubal Canyon, and its tributary Lisbon

Canyon, deeply incise the continental shelf in the vicinity of the Sado and Tagus Rivers respectively. Setubal Canyon is twice the length of Cascais Canyon but crosses the same depth range, hence axial gradients are much lower, rarely exceeding 5° [Lastras et al., 2009]. Although some sections of the upper and middle canyon (as defined by Lastras et al. [2009]) are v-shaped, much of the canyon has a distinct flat-floored axial channel ranging from a few hundred to a thousand meters in width. Below 4200 m depth, the canyon widens abruptly into a broad flat-floored channel that gradually increases in width from 2 to 5 km over a distance of about 25 km, where the canyon merges with the TAP between 4800 and 4900 m.

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Figure 1. Location of Setubal and Cascais Canyons and the Tagus Abyssal Plain, offshore southern Portugal. Regional bathymetry (pale colors and 100 m contour interval) from GEBCO. More detailed bathymetry of canyons (dark colors) based on EM12 multibeam data. Labeled cores are those described in detail in this paper, except for MD03 2701 that is described by Gràcia et al. [2010] and cores 252–34 and 218–36, described by de Stigter et al. [2011]. Other coring sites used to delimit the sandy canyon mouth lobe are also shown; note the failed coring sites (red symbols) that prevented the acquisition of a complete core transect between the canyons and the abyssal plain. Inset top left shows location of study area, inset bottom right summarizes the tectonic setting; in both cases the rectangle shows the area of the main figure. Locations of major thrusts (blue), strike-slip faults (red) and the limits of the accretionary prism in the Gulf of Cadiz (green) are from Zitellini et al. [2009]. Locations of historical earthquakes with magnitudes >6.0 (black dots with dates) are from Gutscher [2004]. The likely location of the 1755 earthquake is after Baptista et al. [1998b]. [11] The Tagus Abyssal Plain (sensu lato) is defined

as the broad basin that extends west from the base of the Portuguese slope between 37° and 38.5°N (Figure 1). The northeastern part of this area slopes gently toward the southwest with an average gradient of 0.1°; this area is underlain by a sandy depositional fan related to the two canyons [Lebreiro, 1995] (Figure 1). The deepest and flattest part of the TAP lies to the southwest, with an area of 18,000 km2 enclosed by the 5100 m contour.

2. Materials and Methods 2.1. Sediment Cores [12] Piston cores used in this study were collected

during RRS Discovery Cruise 187, RRS Charles Darwin Cruises 157 and 179 and RRS James Cook Cruise 27 (Figure 1 and Table 1). In the canyons, a large number of cores were collected in order to gain

an understanding of the heterogeneous sedimentary environment. Cores were collected mainly from terraces adjacent to but elevated above the canyon axis. These terraces typically preserve a record of predominantly muddy sediments that is relatively easy to core, more or less continuous, and contains dateable material (planktonic foraminifera). In contrast, the canyon axis is typified by sand, gravel and landslide deposits that are difficult to sample and contain little in situ dateable material [Arzola et al., 2008]. Core sites in the canyons were chosen on the basis of TOBI (Towed Ocean Bottom Instrument) 30 kHz deep-towed side-scan sonar data, which cover all of the canyon floor and most of its walls (Figure 2) [Arzola et al., 2008]. Only relatively wide and flat terraces were chosen for coring, because of possible errors of up to 200 m in absolute positioning of the side-scan data coupled with possible offset of the corer on the seabed

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Table 1. Core Locationsa

Core

Location

Latitude

Longitude

Water Depth (m)

CD56820 CD56837 CD56822 CD56825 CD56843 CD56414 D11931 D11951

Cascais Canyon Cascais Canyon Setubal Canyon Setubal Canyon Setubal Canyon Setubal Canyon Tagus AP Tagus AP

38° 17.97′ N 38° 22.47′ N 38° 08.97′ N 38° 04.01′ N 38° 07.32′ N 38° 08.54′ N 37° 34.2′ N 37° 20.8′ N

9° 46.78′ W 9° 53.47′ W 9° 37.01′ W 9° 44.26′ W 9° 59.38′ W 10° 21.29′ W 12° 15.3′ W 11° 51.6′ W

3250 4260 3155 3810 4330 4785 5065 5080

Canyon Axis Depth (m)

Height Above Thalweg (m)

3400 4430 3300 3815 4450 4810

150 170 145