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2 Marine and Petroleum Geology 26 (09) Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: Heat flow anomalies in the Gulf of Cadiz and off Cape San Vincente, Portugal Ingo Grevemeyer a, *, Norbert Kaul b, Achim Kopf b,c a Leibniz Institute of Marine Sciences, IFM-GEOMAR, Wischhofstrasse 1-3, Kiel, Germany b Department of Earth Sciences, University of Bremen, Klagenfurter Strasse, Bremen, Germany c RCOM (Research Centre Ocean Margins), University of Bremen, Leobener Strasse, Bremen, Germany article info abstract Article history: Received 22 October 07 Received in revised form 18 July 08 Accepted 6 August 08 Available online 26 August 08 Keywords: Gulf of Cadiz Great Lisbon earthquake Heat flow Thrust faulting Shear stress Heat flow anomalies provide critical information in active tectonic environments. The Gulf of Cadiz and adjacent areas are affected by the plate convergence between Africa and Europe, causing widespread deformation and faulting. Active thrust faults cause lateral movement and advection of heat that produces systematic variations in surface heat flow. In December 03 new heat flow data were collected during the research vessel Sonne cruise SO175 in the Gulf of Cadiz over two sites of recent focused research activity: (i) the Gulf of Cadiz sedimentary prism and (ii) the Marques de Pombal escarpment. Both features have also been discussed as potential source areas of the Great Lisbon earthquake and tsunami of Background heat flow at the eastern terminus of the Horseshoe abyssal plain is about mw/m 2. Over the Gulf of Cadiz prism, heat flow decreases from w57 mw/m 2 to unusually low values of 45 mw/m 2 roughly 1 km eastward. Such low values and the heat flow trend are typical for active thrusting, supporting the idea of an east-dipping thrust fault. Slip rates are 10 5 mm per year, assuming that the fault dips at 2. A fault dipping at 5, however, would result into slip rates of mm per year, suggesting that subduction has largely ceased. Based on seismic data, the Marques de Pombal fault is interpreted as part of an active fault system located w100 km westward of Cape San Vincente. Heat flow over the fault is affected by refraction of heat caused by the 1 km high escarpment. Thermal models suggest that the slip rate along the fault must either be small or shear stresses acting on the fault are rather high. With respect to other fault zones, however, it is reasonable to assume that the fault s slip rate is small. Ó 08 Elsevier Ltd. All rights reserved. 1. Introduction The Atlantic Mediterranean transition zone to the west of the Gibraltar arc hosts the plate boundary between Europe and Africa. Plate convergence of 4 mm/yr (Argus et al., 1989) is accommodated over a wide and diffuse deformation zone (Sartori et al., 1994) characterized by significant and widespread seismic activity (e.g., Stich et al., 05; Buforn et al., 04). The region is also characterized by large earthquakes and tsunamis, such as the 1969 Mw ¼ 7.9 Horseshoe Abyssal Plain earthquake (Fukao, 1973) and the November 1, 1755 Great Lisbon earthquake with an estimated magnitude of Mw w 8.5 (Martinez-Solares et al., 1979; Johnston, 1996). Two features in the Gulf of Cadiz have been sites of intensive geological and geophysical surveys: the 1 km high Marques de Pombal escarpment to the west of Cape San Vincente (Zitellini et al., 01, 04; Gracia et al., 03) and the sedimentary wedge to west * Corresponding author. address: igrevemeyer@ifm-geomar.de (I. Grevemeyer). of the Gibraltar Arc (Maldonado et al., 1999; Torelli et al., 1997; Gutscher et al., 02; Thiebot and Gutscher, 06). The Marques de Pombal escarpment has generally been interpreted to be the surface expression of a thrust fault (Gracia et al., 03; Zitellini et al., 01, 04). The interpretation of the sedimentary wedge in the Gulf of Cadiz has been more controversial. This wedge is formed by large allochthonous masses with seismically chaotic reflections at the forefront of the Gibraltar Arc (e.g., Purdy, 1975; Torelli et al., 1997; Maldonado et al., 1999). The chaotic sedimentary melange is often interpreted as an olistostrome and shows signs of intense deformation and westward transport, attributed primarily to wrench faulting between Africa and Eurasia as well as gravity sliding (Torelli et al., 1997; Maldonado et al., 1999; Medialdea et al., 04). Other researchers, however, interpreted the sedimentary prism as accretionary complex of an east-dipping subduction zone (Gutscher et al., 02; Thiebot and Gutscher, 06). Both sites have also been proposed to be potential source areas of the Great Lisbon earthquake of During cruise SO175 of the German research vessel Sonne in December 03 heat flow determinations were carried out across the Marques de Pombal fault and in the Gulf of Cadiz (Fig. 1). While /$ see front matter Ó 08 Elsevier Ltd. All rights reserved. doi: /j.marpetgeo

3 796 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) W 12 W 11 W 10 W 9 W 8 W 7 W 6 W 5 W 39 N Portugal 38 N Tagus abyssal plain Marques de Pombal Spain 37 N Gorringe Bank 36 N Horseshoe abyssal plain Gulf of Cadiz prism 35 N 34 N Morocco 33 N Fig. 1. Location map of heat flow data obtained during research vessel Sonne cruise SO175 in December 03. Black dots are measurements obtained with a violin-bow design heat probe, white symbols mark station where MTL (minature temperature loggers) were attached to a gravity corer. seismic data have the power to image the sub-surface structure, they may tell us little about on-going tectonic movements. In contrast, heat flow varies little over inactive structures, but is inherently affected by advection of heat caused by relative motion between blocks, as for example caused by thrusting (e.g., Molnar and England, 1990). Heat flow surveying is therefore a powerful tool to study active tectonic environments. Here, we will use heat flow anomalies to gain a better understanding of fault motion at the Marques de Pombal escarpment and dynamics of the allochthonous masses at the forefront of the Gibraltar Arc. 2. Data description Crustal heat flow was obtained from measurements with a violin-bow design Lister probe (Hyndman et al., 1979; Lister, 1979). This probe obtains the geothermal gradient from 11 thermistors mounted in a lance that penetrates 3 m into a sedimented seabed. After penetration the frictional heating decays, while the probe remains motionless in the seafloor for 7 min. Equilibrium temperatures are calculated by extrapolating the decay of the frictional heating pulse (Hartmann and Villinger, 02). At every other station, in situ conductivity measurements were made by applying a s pulse of electric current along heater wires within the lance. The thermal decay of this calibrated heat pulse allows to estimate the conductivity at the location of in situ temperature measurements. Data from the individual thermistors were monitored in real time using a coaxial cable connecting the probe with the ship. In addition to measurements with the Lister probe, thermal gradients were measured by Miniature Temperature Loggers (MTL) mounted on outriggers (Pfender and Villinger, 02) attached to the gravity corer. All individual temperature and conductivity measurements were inverted to obtain surface heat flow. The complete processing sequence to obtain surface heat flow is described elsewhere (Hartmann and Villinger, 02). Results of all 52 heat flow measurements are summarized in Table Gulf of Cadiz sedimentary prism In the Gulf of Cadiz, two heat flow transects were obtained. The first transect is a 90 km long corridor located to the north of Coral Patch Ridge along latitude N. The second 1 km long SW NE trending profile is located further to the south and follows multi-channel seismic (MCS) line SISMA16 (Thiebot and Gutscher, 06) from the Seine abyssal plain across the prism. Each transect provided 16 successful heat flow determinations. The transect at N has four heat flow stations (GeoB9046, GeoB9047, GeoB9048 and GeoB9049). Heat flow values seaward of the toe of the sedimentary prism are highest (55 59 mw/m 2 ), systematically decreasing further towards the Gibraltar Arc to values of w mw/m 2 roughly 85 km eastward from the toe of the prism (Fig. 2). Heat flow values along profile SISMA16 vary significantly along the profile; values range between 172 and 42 mw/m 2 (Fig. 3). The highest determinations are from MTL outrigger measurements (stations GeoB9085 and GeoB9086) on a dome-like seafloor feature interpreted by Thiebot and Gutscher (06) as salt diapir. Salt is characterized by a thermal

4 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) Table 1 Geothermal data Station Date Lat-deg Lat-min Lon-deg Lon-min Depth [m] Inst-type N k q GeoB , , m-HP ,4 GeoB , , m-HP ,6 GeoB , , m-HP ,8 GeoB , , m-HP ,4 GeoB , , m-HP ,6 GeoB , , m-HP ,9 GeoB , , m-HP ,6 GeoB ,1 10 3, m-HP ,6 GeoB , , m-HP ,0 GeoB , , m-HP ,2 GeoB , , m-HP ,5 GeoB , , MTL GeoB , , MTL GeoB , , m-HP GeoB , , m-HP GeoB ,2 9, m-HP GeoB ,410 9, m-HP GeoB ,4 9 19, m-HP GeoB ,5 9 18, m-HP GeoB , , m-HP GeoB ,5 9 39, m-HP GeoB ,1 9 38, m-HP GeoB , , m-HP GeoB ,591 9, m-HP GeoB , , m-HP GeoB , , m-HP GeoB ,2 9 22, m-HP GeoB ,2 9 1, m-HP GeoB , , m-HP GeoB ,0 9 0, m-HP GeoB , , m-HP GeoB ,0 8 41, m-HP GeoB ,0 8 41, m-HP GeoB ,587 8, m-HP GeoB , , m-HP GeoB ,1 7 57, m-HP GeoB , , m-HP GeoB , , m-HP GeoB ,0 8 24, m-HP GeoB , , m-HP GeoB , , m-HP GeoB , ,4 92 3m-HP GeoB , , m-HP GeoB , , m-HP GeoB ,0 8, m-HP GeoB , , m-HP GeoB ,1 8 51, m-HB GeoB ,0 8 51, MTL GeoB , , MTL GeoB , , m-HP GeoB , , m-HP GeoB , , m-HP Inst-type (3m-HP: 3 m long heat probe, MTL outrigger on gravity corer), N number of temperature sensors, k thermal conductivity in W/m K, q heat flow in mw/m 2. conductivity k of w5.5 W/mK, suggesting a 5-fold increase of k with respect to normal sedimentary rocks. The 5-fold increase of k and refraction of heat may suggest that the high heat flow values are indeed related to salt diapirism and hence of local origin. A high degree of variability of heat flow values (station GeoB9082) roughly km landward of the toe of the wedge is located at a thrust fault identified in MCS data (Thiebot and Gutscher, 06). The effect of thrusting, fluid flow and topography on the heat flow anomaly will be discussed in the next chapter. However, averages for stations GeoB9087, GeoB9082 and GeoB9075 with values of 47, 49, and 45 mw/m 2, respectively, show little lateral variability in heat flow across the prism. Additional data were obtained further to the north at the eastern terminus of the Horseshoe abyssal plain across the toe of the Gulf of Cadiz sedimentary wedge (station GeoB9009). Heat flow values are between 52 and 59 mw/m 2, supporting observations that heat flow is high over the Horseshoe abyssal plain and decreases eastward Marques de Pombal escarpment off Cape San Vincente Geothermal measurements were obtained across the fault where seismic and side-scan sonar data revealed surface rupture (Gracia et al., 03). The heat flow transect comprises 11 heat probe penetrations (station GeoB9004) and two additional (offline) measurements (stations GeoB9006 and GeoB9008) with outriggers on gravity cores. One penetration on the steep fault scarp failed, resulting into 12 successful heat flow deployments. Measurements on the footwall show values of mw/m 2 (Fig. 4), while values on the hanging wall are generally below 45 mw/m 2, yielding profound differences between measurements on the footwall and hanging wall.

5 798 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) GeoB9046 GeoB9047 GeoB9048 GeoB Heat flow transect at Fig. 2. Heat flow transect across the Gulf of Cadiz sedimentary prism along latitude N to the north of Coral Patch Ridge. 3. Correction of heat flow for topography and under-thrusting A number of features may affect the measured heat flow anomaly. Channelled migration of warm fluids along faults or other local zones of high permeability is one explanation for localized heat flow anomalies (Fig. 5a). A thrust fault perturbs the surface heat flow by bringing colder near-surface sediments to greater depth. Assuming that such motion was sufficiently recent so that the heat has not been dissipated away, there is a possible relationship between escarpments and a heat flow low (Fig. 5b). Of course, thrust faults may govern fluid migration and hence both thrusting and fluid flow may affect measured heat flow (Fig. 5c). However, topography itself may affect the observed heat flow pattern by focusing and defocusing caused by the static seafloor topography (Lachenbruch, 1968; Ganguly et al., 00; Grevemeyer et al., 04). In troughs or regions of concave-upward topography, refraction of upward directed heat flow results in focusing of heat flow, while the opposite occurs over ridges or concave downward topography (Fig. 5d). Using the analytical solutions of Lachenbruch (1968) these effects can be quantified and data may be corrected for them. In this method the irregular seafloor morphology is replaced by a series of plane reference surfaces. For a plane slope of height H and angle b (between 0 and p/2), the surface heat flow q(x) at a distance x from the brink of the slope is qðxþ ¼Gð1 þ DqðxÞÞ ¼ G 1 þ 1 p tan b ln x x þ H cot b where G is the regional heat flow anomaly. To some degree all heat flow measurements are affected by these processes. However, in all cases where lateral heat flow variations are small, we did not apply any corrections. Three stations, however, have been corrected. Two stations were located in the vicinity of active thrusts on the Gulf of Cadiz sedimentary prism and one station surveyed the Marques de Pombal escarpment. Station GeoB9049 was obtained along a profile to the north of Coral Patch Ridge, roughly km to the east of the toe of the wedge. With respect to measurements obtained eastward or westward of the station, it shows significantly reduced heat flow values. Heat flow determinations indicated a trough like feature with the first and last measurement providing the highest values (Fig. 6a). We believe that these features are caused by underthrusting. Corrected heat flow would yield a background value of w42 mw/m 2. Station GeoB9082 shows the highest degree of scattering. It crossed an active thrust fault imaged in seismic reflection data (Thiebot and Gutscher, 06). However, the two westernmost and the easternmost measurements provide nearly constant values of 48 mw/m 2. In between, values increase systematically to a maximum of mw/m 2 before they decrease to a minimum of w mw/m 2. Measurements were obtained across a stairway-like topography. We calculate topographic corrections for two different approximations of the seafloor relief; a low angle slope and a steeper slope (Fig. 6b). The calculated approximation of the heat flow anomaly caused by the seafloor topography roughly mimics the observed variations in heat flow. We therefore consider a background heat flow of 48 mw/m 2 to be representative for GeoB9082. The Marques de Pombal escarpment has a steep slope and a relief of w1 km. We calculated the effect caused by topography and hence refraction of heat, surveying potential effects on station GeoB9004. Heat flow values to the west of the fault matches the prediction calculated from simplified sloping topography (Fig. 7). Over the hanging wall, heat flow was roughly 10 mw/m 2 lower than over the footwall. However, this feature seems to be caused by refraction of heat, as the model matches the values over the hanging wall quite nicely. Corrected values scatter by w5 mw/m 2 around a mean value of 59 mw/m 2. Two outliers over the slope might be caused by slope failure, as evidenced in side-scan sonar imaging (Gracia et al., 03), leaving the debris or slump masses in a state of thermal re-equilibration.

6 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) GeoB GeoB9086 GeoB9087 GeoB9082 GeoB Heat flow transect along SISMA Fig. 3. Heat flow transect along profile SISMA16 (Thiebot and Gutscher, 06) crossing the Gulf of Cadiz prism SW NE. Note that the scale for the heat flow changes above 100 mw/m 2. The highest values occur over a dome-like feature at profile km-28, interpreted as salt diapir. Salt has a thermal conductivity of w5.5 K/mW and would therefore increase heat flux above a salt dome. 4. Discussion 4.1. Gulf of Cadiz Heat flow data in the Gulf of Cadiz decrease systematically from 57 mw/m 2 obtained at the toe of the sedimentary wedge to values of w45 mw/m 2 roughly 1 km eastward (Fig. 8). Assuming that the sedimentary wedge is in thermal equilibrium and basal heat flux is constant, we would expect a constant heat flow over the entire wedge. However, the sedimentary wedge contains sediments from a continental source; radiogenic heating may therefore contribute to the obtained surface heat flow. As the thickness of the wedge increases eastward, radiogenic heat production would result into a heat flow anomaly that increases towards the Gibraltar Arc, too. Thus, both models are inconsistent with the observed data Depth [m] Fig. 4. Heat flow measurements with the violin-bow design heat probe across the Marques de Pombal escarpment (grey shading is the depth profile). 4.5

7 0 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) a Fluid flow b Under-thrusting fluids cold sediments c Fluid-flow + Under-thrusting d Topography Basal heat flow: mw m -2 fluids cold sediments Fig. 5. Effects of (a) fluids flow, (b) under-thrusting, (c) fluids flow plus thrusting, and (d) topography alone on heat flow. Heat flow anomalies shown in (a) to (c) ignore the impact of topography and hence refraction of heat. a Effect of under-thrusting on GeoB9049? expected heat flow Basal heat flow: 42 mw m active thrust? b Topographic effect on GeoB9082 Basal heat flow: 48 mw m Fig. 6. (a) A trough like low in heat flow of station GeoB9049 is suggested to be caused by under-thrusting of cold sediments. The background heat flow is in the order of w42 mw/ m 2. (b) Effect of refraction of heat caused by static topography on station GeoB9082. Two different approximations of topography are used and fit the observed trend reasonably well. In addition, fluid migration or under-thrusting may contribute to the observed heat flow anomaly. Values of w48 mw/m 2 obtained at some distance from the sloping topography support the idea that the observed anomaly is of local origin.

8 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) Basal heat flow: 59 mw m Fig. 7. Heat flow anomaly over the Marques de Pombal structure and the effect of heat refraction caused by the 1 km high escarpment. (bottom) Topography and approximation of topography used to model effects of heat focusing and defocusing, (middle) observed and modelled heat flow, (top) corrected heat flow. GeoB9046 GeoB9087 GeoB9047 GeoB9048 GeoB9082 GeoB9049 GeoB9075 dip = 5 q 0 = 57 mw m Distance from deformation front [km] Fig. 8. Heat flow anomalies over the Gulf of Cadiz prism and prediction from thermal models of thrusting, assuming that the fault dips at 5. Labels indicate slip rate in mm per year. Small black dots are actual measurements, large grey circles are either averages of closely spaces measurements or have been corrected for effects of local thrusting or topography (stations GeoB9049 and 9082, see Fig. 6).

9 2 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) High sedimentation rates may affect surface heat flow, lowering heat flow by 10 % (Hutchinson, 1985). To explain heat flow decreasing systematically eastward by rapid sediment blanketing would require sedimentation rates increasing towards the east. However, seismic reflection data did not support such a scenario (Thiebot and Gutscher, 06; Medialdea et al., 04), suggesting that observed heat flow pattern characterize crustal heat flow and originate at greater depth. To relate the surface heat flow anomaly in the Gulf of Cadiz to the thermal properties of a thrust fault, heat flow was modelled using a two dimensional analytical approximation of heat flow anomalies caused by thrusting (Molnar and England, 1990). The heat flow anomaly over the hanging wall is given by q ¼ q 0 þ sn S where q 0 is the heat flow through the footwall, n is the slip rate, s is the shear stress, and S is a devisor which accounts for advection of heat caused by the lateral movement between hanging wall and footwall. It can be expressed as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi S ¼ 1 þ b z f n sin d=k where z f is the depth to the fault plain, d is the dip angle of the thrust, k ¼ m 2 /s the thermal diffusivity of the crust, and b ¼ 1 for all forms of heating where the temperature along the fault zone is proportional to z f (constant shear stress acting on the fault zone). In cases where s is related to P (constant coefficient of friction), the temperature is proportional to z 2 f and b ¼ 1.3 (Molnar and England, 1990). In our preferred model, the shear stress s is proportional to the lithostatic pressure s ¼ ap. Here, P ¼ grz f (r: density of rock, g: gravity acceleration) while a ¼ m b (1 l * ) is a Under-thrusting with shear stress as a function of lithostatic pressure GeoB9046 GeoB9087 GeoB9047 GeoB9048 GeoB9082 GeoB9049 GeoB Sh = a P A 0 = Wm -3 v = 2 cm yr -1 dip = 2 q 0 = 57 mw m b Under-thrusting with constant shear stress Sh = const A 0 = W m -3 v = 2 cm yr -1 dip = 2 q 0 = 57 mw m Distance from deformation front [km] 10 0 Fig. 9. Heat flow anomalies over the Gulf of Cadiz prism are explained by thermal effects caused by thrusting (for explanation of symbols see Fig. 8). (a) Shear stress is a function of lithostatic pressure. Dashed line is a model without shear heating and radiogenic heat production. Solid lines are models with radiogenic heat production and different values of friction, (b) models with constant shear stresses.

10 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) a function of the coefficient of basal friction m b of the fault gouge and of the pore pressure ratio l *. l * is the ratio between the local hydrostatic, lithostatic, and pore pressure (e.g., Screaton et al., 02). Consequently, a defines the effective strength of a fault. For the calculation of analytical approximation of thrusting, we assumed that the heat flux in the eastern Horseshoe abyssal plain of 57 mw/m 2 characterizes the basal heat flow through the footwall. Heat flow values decrease towards the east, suggesting lateral movement between both blocks. Roughly 1 km from the deformation front heat flow is 45 mw/m 2. Based on seismic imaging we used an angle of 2 for the dip of the fault (Thiebot and Gutscher, 06). First, we consider different convergent rates, assuming that shear stresses acting on the fault are low, a feature observed elsewhere (e.g., Lachenbruch and Sass, 1992; Hyndman and Wang, 1993; Wang et al., 1995; Grevemeyer et al., 03). Considering the error limits, slip rates of 5 mm per year may fit the data. Thiebot and Gutscher (06) used a numerical model to calculate the thermal structure of an east-dipping subduction zone. Their model would fit the observed data using convergent rates of 10 mm. Lower rates of fault movement did not produce the observed trend of eastward decreasing heat flow values. To reproduce their results, we introduced radiogenic heat generation in the prism, using a radiogenic heat production of Wm 3 (e.g., Grevemeyer et al., 03). We have to note, however, that radiogenic heat production has only a minor effect on the observed heat flow trend (Fig. 9). To match the observed heat flow data with the modelled trends, the fault has to be weak. Thus a ¼ m b (1 l * ) ranges from 0.0 to w0.1 (Fig. 9). Assuming that constant shear stresses act on the fault, s would be in the order of 5 10 MPa. The idea of active subduction in the Gulf of Cadiz is still under debate. However, plate tectonic reconstructions of the Mediterranean Sea suggests that the Oligocene/Miocene evolution was driven by subduction and slab roll-back (e.g., Faccenna et al., 01). Remaining slabs may have been imaged under the Calabrian Arc (Lucente et al., 06) and under the Alboran Sea (Lonergan and White, 1997). Geochemical studies of magmatic rocks from the Alboran Sea suggest that subduction related volcanism occurred here until at least Miocene times (Duggen et al., 03, 04). The existence of a Miocene magmatic arc in the Alboran Sea is further supported by seismic reflection imaging (Booth-Rea et al., 07). Recent geophysical data may indicate that the subduction system in the Gulf of Cadiz is still active. Seismic reflection and wide-angle profiles in the Gulf of Cadiz are interpreted in terms of an accretionary wedge, with active thrust faults and an east-dipping decollement overlying an eastward dipping basement (Gutscher et al., 02; Thiebot and Gutscher, 06). Other researchers, however, suggest that accretion has stopped a few million years ago (Iribarren et al., 07). Thiebot and Gutscher (06) used a fault dip of 2 for their subduction zone model. However, the assessment of fault dip from seismic data is difficult and a trade off between seismic velocity and depth determination may cause large uncertainties. Thus, it is interesting to point out that a steeper fault dipping at 5 would reduce slip rates to only mm per year (Fig. 8). Such small slip rates may suggest that motion along the former subduction megathrust has largely ceased, supporting a scenario with an inactive accretionary complex in the Gulf of Cadiz (Iribarren et al., 07) Marques de Pombal fault Multi-channel seismic reflection imaging of the Marques de Pombal escarpment and thrust fault indicates that the fault breaches the seabed only at the centre of the structure and becomes a blind fault both at its northern and southern tips. In total the thrust fault appears to be at least 100 km in length, dipping at 24 in the first 11 km (Zitellini et al., 01, 04). Heat flow data were obtained across a portion of the fault where it breaches the seafloor. As discussed in the previous chapter, heat flow at the toe of the escarpment and over the hanging wall is affected by refraction of heat cause by the steep slope of the w1 km high escarpment. Correcting heat flow for those effects causes a nearly constant heat flow of 59 mw/m 2 (radiogenic heat production caused by rocks forming the escarpment would change surface heat flow by <1 mw/m 2 and is therefore neglected). Thus, heat flow over the Marques de Pombal fault provides little evidence for an active thrust fault. In Fig. 10 we compare the observed heat flow anomaly with the prediction from thermal models. First, we consider different slip rates. Considering that the fault is weak (see discussion above), only very small rates of fault movements of <1 mm per year would fit the observed heat flow trend, assuming that the fault dips at (Fig. 10a). If we consider, however, that the fault is strong, higher slip rates can be introduced. A fault that dips at 24 (Zitellini et al., 01, 04) and moves with mm per year, requires shear stresses of 105 MPa (assuming that shear stress is constant along the thrust) to fit the observed heat flow trend. Decreasing the dip of the fault to would reduce shear stresses to MPa (Fig. 10b). In all cases of slip rates >5 mm per year, the fault is either supporting only low fluid pressures (small l * ) or its intrinsic shear strength is high. However, most studies of shear heating and thermal properties of fault zones indicate that shear stresses are generally roughly 10 times lower, suggesting that the effective strength of crustal a Effect of slip rate on heat flow prediction 0 10 b Effect of shear stress on heat flow prediction Sh = 0 MPa dip = q 0 = 59 mw m Sh = const. v = 2 cm yr -1 dip = q0 = 59 mw m -2 0 Fig. 10. Corrected heat flow over Marques de Pombal escarpment (see Fig. 7 for observed data): heat flow (black dots measurement with Lister probe; grey dots offline measurements with outriggers) is compared to thermal effects caused by trusting. (a) Effect of slip rate (in mm per year), (b) models with a constant shear strength of MPa and a slip rate of mm per year could also fit the data. In this case, the fault would be rather strong (see text for discussion).

11 4 I. Grevemeyer et al. / Marine and Petroleum Geology 26 (09) faults is generally low (e.g., Lachenbruch and Sass, 1992; Hyndman and Wang, 1993; Wang et al., 1995; Grevemeyer et al., 03). We therefore suggest that motion along the Marques de Pombal fault is rather small. Another explanation might be that the fault is geologically young. In this case either the fault could be very strong or the hanging wall might not be in thermal equilibrium. 5. Conclusions Data from 52 marine heat flow determinations were obtained in the Gulf of Cadiz and over the Marques de Pombal escarpment off Cape San Vincente. Over the Gulf of Cadiz prism, heat flow decreases systematically towards the east. This pattern could be interpreted by active thrusting. Inferred slip rates along a thrust dipping at w2 are between 5 and mm per year. The new heat flow data are consistent with the prediction of heat flow derived from a finiteelement thermal model of an east-dipping subduction zone (Thiebot and Gutscher, 06). However, a fault dipping at 5 instead of 2 would suggest that motion along the thrust occurs only at rates of mm per year, favouring an interpretation that subduction has largely stopped. Heat flow anomalies over the Marques de Pombal fault have been affected by refraction of heat caused by the topography of the 1 km high escarpment. Corrected heat flow values show little lateral variation across the feature. Thermal models suggest that slip movement along the fault must either be small or shear heating is rather important to generate the observed heat flow pattern. We favour the interpretation that the fault slip is small. However, the observed pattern could also be explained, assuming that the fault is geologically young. Acknowledgements We are grateful to Bernd Heesemann, Marc-Andre Gutscher, and Emmanuelle Thiebot for assistance during data collection at sea. Research vessel Sonne cruise SO175 was funded by the German Ministry of Education, Science and Research (BMBF grant 03G0175A) and the Deutsche Forschungsgemeinschaft (via RCOM, Bremen). Critical reviews helped to facilitate the discussion of observations and improved the manuscript. We are particularly thankful to Joao Fonseca. Figures have been made using GMT software (Wessel and Smith, 1998). References Argus, D.F., Gordon, R.G., Demets, C., Stein, S., Closure of the Africa Eurasia North America plate motion circuit and tectonics of the Gloria fault. J. Geophys. Res. 94, Booth-Rea, G., Ranero, C.R., Martínez-Martínez, J.M., Grevemeyer, I., 07. Crustal types and Tertiary tectonic evolution of the Alborán Sea, western Mediterranean. Geochem. Geophys. Geosyst. 8, Q10005, doi: /07/gc Buforn, E., Bezzeghoud, M., Udías, A., Pro, C., 04. 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