Heat flow in the westernmost part of the Alpine Mediterranean system (the Rif, Morocco)

TECTONOPHYSICS ELSEVIER

Tectonophysics 285 (1998) 135-146

Heat flow in the westernmost part of the Alpine Mediterranean system (the Rif, Morocco) A b d e l k r i m R i m i a,*, A h m e d C h a l o u a n h, L a h c e n B a h i c a Scientific Institute, Department of Earth Physics, Chari~ lbn Battouta, B.I~ 703, Rabat-Agdal, Morocco ~ FaculO' of Sciences, Department of'Earth Sciences, Charigt lbn Battouta, B.P. 1014, Rabat-Agdal, Morocco c Mohammadia Engineering School, Charid lbn Sina, B.P. 765, Rabat-Agdal, Morocco Received 15 May 1997; accepted 23 June 1997

Abstract

Heat flow density (HFD) and thermal gradient calculated on the basis of oil well data in the Rif vary from 50 to 90 mW/m 2 and from 20 to 50°C/km, respectively. Short-term variations in the west can be attributed to water circulation and lateral heat conductivities contrast along the Pre-Rif thrusting front and at the limits between the main structural units (the Gharb basin, the South Rifian ridges and the Pre-Rif nappe). High HFD and thermal gradient values are determined in the southwest of the study area. This zone extends the central Moroccan massif which is well known by its high thermal manifestations. The External Rif shows a tendency of increasing HFD toward northeastern Morocco and the Alboran Sea. In agreement with the geophysical observations (electrical conductive structures, great negative Bouguer anomaly, shear velocity attenuation, high upper mantle P-velocity), the thermal structure of the External Rif would be a consequence of the presence at subcrustal depths of hot and low-density asthenospheric material overlying a high-velocity lithospheric body beneath the Rif-Betic belt. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: thermal structure; geophysical anomalies; asthenosphere overlying lithosphere; Rif, Morocco

1. I n t r o d u c t i o n

The study area has a long history of hydrocarbon production. Saqualli (1970) has catalogued the thermal springs of the Gharb and the Pre-Rif, most of them being located along the Pre-Rifian thrust front (Fig. 1). Using measurements of temperature and thermal conductivity in shallow boreholes and geochemical sampling of thermal springs, Bahi et al. (1983) described an N E - S W geothermal lineament from Nador to Kenitra running through Taza. Re*Corresponding author. Tel.: +212-7-774543; tax: +212-7774540.

cently, Benabidate (1994) studied the thermal gradients of the region, by using geothermometers, shallow temperature determinations and some bottom hole temperatures (BHT). However, the BHT correction used a rough law which underestimated thermal gradients and the shallow well measurements were affected by the fluid circulation. On the basis of the available oil exploration data (temperatures, geophysical and lithological loggings), the specific aim of this study is to define the thermal regime in light of the tectonic context and other geophysical observations. On the other hand, such an investigation provides important data for a better understanding of the hydrocarbon maturation and migration.

0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 9 5 1 ( 9 7 ) 0 0 1 8 5 - 6

136

A. Rimi et aL I Teclonophysics 285 (1998) 135 146 i

8°w

~°

4"

J-';

IBERIA

~,

1

*

5

-"

2

~

6

c ? +.,, 3.

2 7

C5 -1+ "36

~

........ *~ Alboran

"~kX'~"

,

.,~,,~*',~" ~ -~

+,,o o°+++

MOROCCO

. . . . .

+

+

Fig. 1. Geophysical observations superimposed on the structural sketch of northwestern Morocco: 1 -- negative Bouguer gravity (-150 < A < -50 reGal): 2 = electrical conductivity anomaly defined structure; 3 = thermal spring (t > 30°C): 4 = Neogene volcanic rocks; 5 = volcanic centers; 6 = ultramatic rocks; 7 = main thrust front: 8 = major fault. Main structural domains: (a) Internal Rift (b) Flysch zone; (el northern External Rif; (d) Pre-Rif; (e) South Rifian Ridges: (J) Paleozoic massif.

2. G e o l o g i c a l

context

The major tectonic processes in the region have been the Triassic-Jurassic rifting in the Atlantic and subsequent separation of the continental plates, resulting in the definition of the Atlantic margin. This was followed by the Tertiary collision between Africa and Eurasia resulting in development of the R i f - B e t i c orogenic belt. This arcade belt trends northward to the Betic fold chain across southern Spain. The Rif can be divided into three main zones (Fig. 1): an Internal zone which belongs to the Alboran domain, an Intermediate Flysch zone and an External zone which is a foreland told-and-thrust belt formed from the Mesozoic and Cenozoic sedimentary cover of the African margin. The south of the External Rif includes the following units: - The Pre-Rif is a post-tectonic basin where Upper Miocene marls and Jurassic to Middle Miocene chaotic blocks accumulated. - The Gharb basin is a subsiding depression filled with Jurassic to Miocene sediments and subse-

quently overlain by a series of thrust sheets. These thrusts are, in turn, overlain unconformably by Late Miocene sediments. - By Late Miocene to Pliocene, the marl deposition passes into molasse towards the foreland. A major northeast-southwest Sidi Fill fault trend (SF) separates this molasse into an eastern area above the Middle Jurassic carbonate platform, and a western area where the Miocene series overly the Hercynian Meseta domain. Within an Alpine compressional regime, salt tectonic and left-lateral movements along this Hercynian fault gave rise to the imbricate thrusts of the South-Rifian ridges. These folded and heavily faulted structures are considered as the Atlasic parautochthon and belong to the postHercynian basins which bound the African, American and west European margins (Faugeres, 1981). The Internal Rif and the eastern Elysch zone are separated by a major tectonic feature referred to as the N70°E-striking left-lateral Jebha-Chrafate (JC) fault zone. To the east, the External Rit' units are crossed by a N E - S W - t r e n d i n g lineament (the Nekor fault, NK), also a left-lateral fault marked by intensive seismic activity (Cherkaoui, 1991 ). Various hypotheses have been proposed to understand the geodynamics of the Alboran basin and its surrounding mountain chains, in conjunction with the convergence of the Eurasian and African lithospheric plates. In their simplest form, these models can be brought together into major groups including, indentation tectonics (Andrieux et al., 1971; Le Blanc and Olivier, 1984), some kind of subduction processes between the African and European plates (Morley, 1992; Blanco and Spakman, 1993; Bufforn et al., 1995; Pasquale et al., 1996), gravity and driven tectonics off a high located in the Alboran Sea (Loomis, 1975; Weijermars, 1985; D o N a s and Oyarzun, 1989; Platt and Vissers, 1989; Seber et al., 1996b). 3. T h e r m a l

data

3.1. T e m p e r a t u r e s

In petroleum exploration wells, temperatures are measured at great depths and may be therefore quite representative of the deep thermal regime. However, the correction of the bottom hole temperatures is still debated. In some cases where multiple BHT

A. Rimi et al./Tectonophysics 285 (1998) 135 146

are measured at the same depth at successive times, and if the duration of mud circulation and the time elapsed after the end of circulation (shut-in time) are found, it is possible to extrapolate the equilibrium temperature using the Homer plot correction (Fertl and Wichmann, 1977). Unfortunately, the required specific drilling parameters are generally unknown and in most cases only one BHT is available at the same depth. Even then, this technique still underestimates the correction by 5 to 10%, because of the assumption that the borehole is as a constant axial line heat source, the error increasing with decreased shut-in time (Drury, 1984; Beck and Balling, 1988). During the drill steam tests (DST), reservoir temperatures are recorded in conjunction with bottom hole pressure surveys. The maximum value among the recorded temperatures of each reservoir can be regarded as the actual formation temperatures, provided that measurements are recorded in formation fluid (Sekiguchi, 1984; Perrier and Raiga-C16menceau, 1984; Ben Dhia, 1988). Owing to the high cost of the tests, only a few temperatures are available. The reliability of these temperatures depends on both the quality and the sampling time; tests producing just mud during a few minutes are turned down. Since the BHT remain numerous, evenly distributed and exist at a wide range of depths, they may be, once corrected, representative of a deep thermal state. A correction law of BHT as a function of depth can be established comparing BHT to DST. First, 19 DST temperatures measured in fresh water are selected and fitted versus depth (Fig. 2A) as: TDST (°C)

=

24.2 + 30.3D (km)

(1)

137

represented and linearly fitted versus depth (Fig. 2B). Some temperatures wandering more than 10°C from this law are thought influenced by water circulation and are not taken into account. The differences 6T (°C) = TDST ( ° C ) - - TBHT ( ° C ) , between each bhT and its corresponding 'DST temperature', calculated at the same depth by Eq. 1, are plotted versus depth and fitted by an average polynomial law as: ~T(°C) = - 1 . 9 + 5 . 7 D + 0 . 4 5 D 2, D being the depth in km. The obtained curve (Fig. 2C) is used as a correction factor to adjust all the BHT values. The

200

(A) 150

1 O0

50

Reservoir temperatures T = 3 0 . 3 " D + 24.2 N = 19; R = 0.92; rms = 5.9°C

....=Jl=~"===~ ~~

~-~ 0

0

200

0 V

(B) 150

100 L 50

i

B o t t o m hole temperatures T = 24.5 *D + 26.4 N = 461 ; R = 0.91 ; rms = 8.3°C

- •

~U

0

L

40

(C)

,. 4

20

O.

E

_. 0

" "P

; ~ l _ - ' .

i

" .......... .

__, i

I

~

•

!

Temperature correction 6T= -1.9 + 5.7D + 0.45D ~ N = 384

-20

D being the depth (km). The uncorrected BHT are -40 200

Fig. 2. (A) Drill steam test temperatures represent the actual formation temperatures" only those recorded in fresh water were plotted and fitted versus depth. (B) Uncorrected temperatures for all the study area versus depth; the mean thermal gradient is lower than that obtained by corrected temperatures. (C) The correction of the bottom hole temperatures is attempted by second-order polynomial fitting the difference between the DST geotherm and BHT versus depth: c~T = - 1.9 + 5.7D 4- 0.045 D z. 3T in °C and D in km. (D) Corrected temperatures versus depth for all the Rif. The average geotherm is linearly approximated by TD(°C) = 30.9D (kin) 4- 24. l.

(D)

•

150 •

a m

100

•.

.ml.lr

• .

mR

50

Corrected temperatures T = 31.7*D + 23.0 N = 461; R = 0.94; rms = 8.2 °C

r/i'-"= 0

0.5

1.5 E:)

e

2.5 Io

t

h

3.5 (krT1)

4.5

138

A. Rim± et al. / Tectonophysics 285 (I 998) 135 146

negative correction between the surface and 325 m depth confirms that the circulation fluid is hotter at these depths than the enclosing beds. In spite of its restricted validity over the area for which it is calibrated (Deming, 1989), similar statistical procedures have yielded successful estimates of the deep thermal state (Takhersit and Lesquer, 1988; Lucazeau and Ben Dhia, 1989; Rim±, 1993). As a precautionary measure, an a-priori uncertainty of 10°C will be attributed to these corrected temperatures during their processing.

3.2. Thermal conductivities The thermal conductivity can be determined by the relation:

where )~, )~m and )~,~. are the thermal conductivity of the sediment, the matrix and the formation fluid, respectively. Since lithological samples were not available for measurement of their thermal conductivities, the evaluation is based on the geometric model described by Brigaud et al. (1990): )~m = HL~'i ;~ is the conductivity of the ith main component and p~ its proportion in the formation. The lithological profile of each well is finely divided into simple facies, which are in turn decomposed in terms of basic constituents. The content of each constituent is estimated using a table elaborated by Brigaud (1986). Table 1 shows the thermal conductivities of the basic elements (Brigaud et al., 1990). The influence of the temperature variation on the thermal Table 1 Thermal conductivity (in W/m°C) of basic elements of sedimentary rocks after Brigaud et al. (1990)

conductivity (Roy et al., 1981) is adjusted by using the relations given by Chapman et al. (1984): )Vm.r = )Vmm)"[293/(273 -- T)] where ),m.7 is the matrix conductivity at temperature T (°C) and ,kin,:0 is the matrix conductivity under laboratory conditions at 20°C, T represents the corrected temperature as a function of depth D (km) (Fig. 2D). Kappelmeyer and Haenel (1974) had estimated the variation in the water thermal conductivity by the following relations: 5Lw.T = 0 . 5 6 + 0 . 0 0 3 T

;%.T = 0 . 4 4 2 + 1 n T ,

0's27,

0 < T _< 5 0 ° C

T >50°C

The geophysical logging is done just in the potential reservoir levels. Also the majority of wells in the study area do not dispose of these logging. A regional porosity-depth law (Fig. 3) is established on the basis of the following porosity determinations: - scattered porosity data obtained for the marlysandy post-nappe Miocene in the Gharb basin, from measured travel time (At), bulk density (Pb) and the hydrogen index (@m) (Areski, 1990), by plotting all the porosity values versus depth: q)t) = 0.36 e x p ( - 0 . 2 8 D ) , D being the depth in km. - well logging and laboratory porosity measurements in the Cretaceous and Jurassic carbonate facies were taken from the drilling reports. The data fit gives: 9)/) = 0 . 4 0 e x p ( - 0 . 9 5 D ) , D being the depth (km). - for the marly-argillaceous levels of depth less than 600 m where no data are available, the porositydepth relation is approximated by the fit of the DSDP porosity measurements in the Moroccan Atlantic margin (a few tens of kilometers southward of the study area) (Hinz el al., 1984): 9)D = 0.59 e x p ( - 0 . 9 6 D ) where D is the depth in km.

Thermal conductivity Quartz Calcite Clay Dolomite Anhydrite Halite Water

7.7 3.4 2.7 5.5 6.0 6.6 0.6

4- 1.2 4- 0.3 ± 0.7 4- (1.5 ± 0.4 ± 0.6

4. Estimation of the heat flow density The least squares fit of the corrected temperature versus depth (Fig. 2D) in the study area shows a high correlation coefficient (94%) and a small root mean square (spread) of 8.4°C. This distribution suggests that the lateral temperature variations are very small,

A. Rimi et al./Tectonophysics 285 (1998) 1 3 5 - 1 4 6

15

30

/ /.J A / " ., Z/- . f / C • " :~'SJ ~=' "' :" ,-;; .-J :.- :.:.. :

. /~..

/

/

: ."

-',,,I-~,.:,

~:..:-.::~.." !..: ;:~,: ,: : •.

]" / B "'::~. : ! . . .

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m

45

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Porosity 60

(%)

. •

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.1...

.

-

/ tt J ,r i L - J

I

Depth

(km)

139

A-priori input data include corrected temperatures (with an uncertainty of 10°C), estimated conductivities (with a relative error of 10 to 30%, depending on the evaluation of mineralogy) and the ground surface temperature (To = 19.5 ± 7.0°C), according to the observations reduced to the sea level, of Roux (1937). Each Ti leads to an individual estimation of q. The inversion algorithm treats simultaneously all the available Ti at different depths Di and returns the optimal values of q, Ti, Xj and their a-posteriori standard deviations. Hence the precision of the estimation increases with the number n of temperatures T/. Owing to the lack of information on the surface heat flow density, an a-priori value of 100 mW/m 2 is assumed with a very large error of 100 mW/m 2. The a-posteriori standard deviation on the heat flow density is 16 mW/m 2 on average. 5. Thermal structure features

Fig. 3. Porosity versus depth in the study area. A regional p o r o s i t y - d e p t h law is established on the basis of: A = well logging and laboratory porosity measurements in the Cretaceous and Jurassic carbonate facies: ~PD = 0 . 4 0 e x p ( - - 0 . 9 5 D ) ; B = scattered porosity data obtained for the m a r l y - s a n d y post-nappe Miocene in the Gharb basin (Areski, 1990): ~0D = 0 . 3 6 e x p ( - 0 . 2 8 D ) ; C = for the levels of depth less than 600 m, the DSDP porosity measurements in the m a r l y - a r g i l l a c e o u s series of the Moroccan Atlantic margin (a few tens of km southward of the study area) (Hinz et al., 1984) is applied: ~0D = 0.59 e x p ( - 0 . 9 6 D ) , D is the depth in kin.

and the heat transfer can be therefore considered essentially by vertical conduction. For such a heat transfer mode, Vasseur et al. (1985) had improved the thermal resistance method (Bullard, 1939) in one dimension, by a stochastic inversion procedure which allows the simultaneous processing of the corrected temperature data set and the conductivities of the lithological layers in each borehole. On the basis of the equation linking the heat flow density to the thermal conductivity L and the thermal gradient (3T/3D), the underground temperature TD at a depth D is obtained by integration from the surface (D = 0), to the depth D, of the relation q = X(3T/6D):

Ti = To + q ~

Hij)~i

ij=l

where n is the total number of layers and thickness of the jth layer.

Oij

is the

For each site, the inverted temperatures (including the surface temperature) obtained in each borehole or a set of boreholes, are fitted versus depth to give the mean thermal gradient (Table 2). The heat flow density and the thermal gradient are not uniform throughout the study area. Their respective values are 40-95 mW/m 2 and 20-60°C/km. Contour maps of the HFD and the thermal gradient (Fig. 4) show two abnormally hot domains in the east and the southwest, which are separated by a region characterized by short-term thermal variations.

5.1. Short-term thermal anomalies These rapid thermal variations are noticed at the boumdary separating the main structural units (Figs. 1 and 4), the Gharb basin, the Pre-Rifian thrust and the South Rifian ridges. These perturbations can be attributed to lateral heat conductivity heterogeneities (salt diapir, shallow granitic or quartzitic basement owing to fault movements) and/or to an important water circulation. A complex hydrogeological framework points out the existence of two opposite senses of water flow (Ostapenko, 1985), the percolation waters from the Palaeozoic massif and the South Rifian ridges towards the northeast and the north, while the expulsion waters go to the southwest. In the Gharb, the Miocene hydrodynamism is

A. Rimi et al. / Tectonophysics 285 (1998) 135-146

140

Table 2 T h e r m a l gradient and heat flow density results Site symbol

Latitude (")

Longitude (°)

alg ans I ba7 cgd drj hal 1 kba ksr laral m a 101 maml mare2 nrtl nrt2 nrt3 nrt4 nrt5 obd2 okt olin o m 101 onzl onz2 onz3 oyfl pkb sah 1 sd I se to twila akll01 b atm I bbl bfs I isal ka kdh I mb I b rm I smal til01 til02 nadorl grfl tafratal x rrl msdl akl-2-3 a16 am azhl bd

34-52.7N 34-86.0N 34-22.6N 34-42.0N 34-49.8N 34-65.5N 34-76.5N 34-52.0N 35-20.0N 34-44.6N 34-12.7N 33-99.6N 34-53.0N 34-52.7N 34-49.0N 34-58.0N 34-52.9N 34 7 1 . 0 N 34-50.6N 34-66.0N 34-04.0N 34-52.3N 34-50.3N 34-53.0N 34-58.2N 34-29.8N 34-00.0N 34-23.0N 34-32.6N 34 0 5 . 0 N 34-35.2N 34-46.3N 34-47.6N 34 3 1 . 0 N 34-32.2N 34-79.0N 34-30.0N 34-09.7N 34-22.0N 34-22.4N 34-32.7N 34-29.5N 34-23.4N 35-32.6N 34-27.9N 34-22.0N 33-02.1N 34 2 4 . 5 N 34-01.0N 33-87.6N 33-92.0N 34-13.0N 34-22.0N

06-06.0W 06-15.0W 05-68.9W 06-06.0W 06-09.4W 06-05.9W 05-95.0W 05-97.7W 06 54.0W 06-21.9W 06-52.9W 06-54.4W 05-91.8W 05-84.2W 05-85.8W 05 8 7 . 8 W 05 8 7 . 4 W 06-16.0W 06-08.3W 06-12.0W 06-02.3W 05-96.9W 05-96.7W 05-97.9W 06-06.7W 05-76.5W 06-00.0W 05-58.0W 06-49.8W 05-59.9W 06 4 5 . 8 W 03-96.2W 03-87.0W 05-36.0W 04-33.4W 05 4 5 . 5 W 05-34.0W 03-52.0W 05-34.0W 04-52.0W 05-37.0W 04-08.8W 04-43.1W 02 6 6 . 2 W 03-42.4W 03-10.6W 03-20.8W 03-47.0W 05-64.0W 05 7 3 . 7 W 05-73.0W 05-38.5W 05-68.0W

Number of temperatures 5 2 3 10 3 1 4 3 1 2 2 1 1 2 1 2 1 3 2 3 1 2 1 1 l 8 1 2 4 2 1 2 3 3 4 3 2 5 2 4 4 3 1 4 3 1 5 4 3 1 5 3 22

A v e r a g e thermal g r a d i e n t (°C/km) 29 28 35 28 36 42 39 27 29 34 46 61 29 28 31 53 27 32 33 33 46 28 26 48 26 22 39 27 34 35 37 29 38 25 34 21 34 37 23 33 23 46 37 37 36 35 21 37 33 36 40 22 35

Residual m e a n square (°C/kin) 5 1 4 5 3 ~ 1 1 ~ 2 2 ~' ~ 1 " I ~' 2 1 5 ~' 1 ~ ~ ~ 2 ~' 3 2 1 ~' 3 6 l 3 1 1 2 8 2 3 1 a 7 2 a 3 8 1 a 2 3 4

Surface heat flow density ( m W / m ) 55 48 69 52 61 65 63 49 46 66 81 89 53 51 50 92 47 59 54 63 73 47 42 88 58 81 67 56 63 61 73 61 81 52 72 48 63 79 96 65 56 81 68 86 92 76 47 90 53 58 67 41 66

Standard deviation (roW/m) 10 20 7 8 13 28 23 13 16 7 27 34 14 16 23 I1 23 15 16 13 27 18 20 12 19 30 14 11 12 25 15 I1 8 8 5 10 2I 6 14 6 6 19 19 13 16 11 9 7 27 25 25 9 20

A. Rimi et al./Tectonophysics 285 (1998) 135 146

141

Table 2 (continued) Site symbol

Latitude (°)

Longitude (o)

N u m b e r of temperatures

Average thermal gradient (°C/km)

Residual mean square (°C/km)

Surface heat flow density ( m W / m )

Standard deviation (mW/m)

bh bk brl bt I 01 dc 1 dfl01 dgr dka dm dogl dz gd2 gdal hr hrn3 hs kh km ks3 kz lil lr mhl nzb I b2 nzal a3 nzl-5 nz3-4 nz13-14 nz6 nza2 ob ool or3 or4-5-6-7 or4b or2-8 ot8 sal sf zr dj I kj mg 1 my I of ss I jebha

33-98.7N 34-13.0N 34-32.8N 34-07.7N 33-95.0N 34-11.5N 34-31 .ON 34-64.4N 34-14.0N 34-31.0N 34-12.0N 33-66.0N 34-19.7N 34-31.8N 34-36.5N 34-27.0N 34-15.0N 34-18.8N 34-10.8N 34-13.0N 34-13.6N 34-10.7N 34-08.8N 34-15.4N 34-13.6N 34-14.5N 34 13.2N 34-12.9N 34-10.8N 34-12.9N 34-09.6N 33-88.5N 34-16.8N 34-20.4N 34-02.4N 34-23.6N 34-16.8N 34-18.5N 34-13.4N 34-17.0N 34-01.8N 33-99.7N 34-04.7N 34-07.5N 33-93.9N 33-91 .ON 35-34.1N

05-76.3W 05-55.0W 05-61.4W 06-00.3W 05-41.6W 05-98.1W 05-56.0W 06-05.0W 05-89.0W 05-69.0W 05-94.0W 05-46.0W 05-05.0W 05-61.0W 05-62.0W 05-65.5W 05-87.0W 05-73.5W 05-88.6W 05-92.5W 05-89.0W 05-94.8W 05-87.3W 05-44.5W 05-86.0W 05-46.0W 05-46.1W 05-92.3W 05-41.4W 05-51.7W 05-96.9W 05-47.8W 05-82.4W 05-81.0W 05-63.1W 05-77.1W 05-76.8W 05-82.0W 05-90.5W 05-85.0W 05-92.6W 05-92.5W 05-21.0W 05-17.4W 05-71.0W 05-36.0W 04-27.1W

2 l1 1 1 1 1 6 1 19 4 6 5 2 15 3 3 47 4 1 5 1 3 1 3 3 6 5 2 2 1 48 1 10 12 1 6 3 5 15 18 1 1 2 1 7 2 8

49 32 23 43 28 46 28 47 35 32 38 30 28 28 28 25 36 29 29 36 33 41 27 28 33 28 29 37 39 35 32 31 30 31 32 30 44 36 35 39 38 31 30 40 28 26 33

1 5 a a a ~ 5 a 3 1 4 2 2 5 5 10 6 2 a 4 a 2 a 4 5 2 1 7 1 a 4 ~ 4 3 a 4 3 3 5 3 ~ ~ 2 ~ 6 1 3

79 61 40 69 50 76 66 84 64 55 64 65 51 59 57 51 62 52 58 62 56 71 55 52 51 53 52 64 60 55 63 52 67 64 55 71 77 77 65 69 59 51 54 74 67 44 71

22 17 15 24 18 18 10 24 15 9 16 7 10 18 8 10 13 19 18 15 17 18 29 15 20 13 20 15 33 26 18 18 6 9 24 7 21 8 15 12 28 23 10 19 12 17 9

A site may have one or several boreholes that are very close to each other. The inverted temperatures (including the surface temperature) obtained in each borehole or a set of boreholes, are fitted versus depth to give the mean thermal gradient and its residual mean square (RMS). The heat flow density of the site is the average of the values obtained in each well. a No R M S is given when only one temperature is used. b The heat flow density and thermal gradient values in the two boreholes A k l l 0 1 and M B I were not considered in calculating the isolines. Their holes are in karst and in a salt dome, respectively.

142

A. Rimi et al./Tectonophysics 285 (1998) 135 146

35°N

C

34°N D 6°W

5°W

',-~

/' " Tangiers

~,~i,~,~ #

I

,

,/ b

34°N

/

3°W

(B

"--.

Alboran sea

o 35°N

4°W

)

, I

,

)

/ ° "~" /

s)

~C/ "~-"

/x

,,'

~4~

/'

+

/'

+/

a

C

6°W

5°W

4°W

3°W

Fig. 4. (A) Surface heat flow density isolines in the Rif; the contour interval is 5 m W / m 2. (B) Geothermal gradient isolines in the Rif with a contour interval of 5°C/kin i . The two maps are drawn by the S U R F E R surface mapping program (version 6.01 ), using kriging as gridding method. Triangles and squares sizes are proportional to the HFD and the thermal gradient values, respectively: the + represents thermal spring hotter than 30°C.

143

A. Rimi et al./Tectonophysics 285 (1998) 135-146

essentially conditioned by an upward fluid migration from the underthrust series. Water springs hotter than 30°C are stretched along the Pre-Rifian thrust front and the boundaries between the major structural units. Their occurrences are localized preferentially between the 32 and 37°C/km thermal gradient isolines. These fractured zones favour their resurgences by an upward fluid migration.

100]

I •

• "="

"

"===

• ~

(A)

E ~-~

E

"

~

v

~9

25

"O

100

s • •

t u d ~ • •

y

area q0=2.B D+58 N = 238; r = 0.21

(B)

0 75

5.2. Long-term thermal anomalies -r

In the southwest of the study area, remarkable high HFD (75-85 mW/m 2) and thermal gradient (40-55°C/km) are observed (Fig. 4). This domain, where Hercynian granitic basement is attained at only 500 m depth, represents the northern part of the Moroccan central massif well known by its high thermal surface manifestations, linked to intensive magmatic intrusions and Quaternary basaltic volcanism (Rimi and Lucazeau, 1991). Fig. 4 displays, over the area extending the PreRifian thrust toward northeastern Morocco, a longterm hot geothermal trend. With HFD and thermal gradient ranges of 70-85 mW/m 2 and 35-45°C/km, respectively, the thermal regime of this area appears overprinted by recent heating processes.

6. Origin of the long-term surface thermal anomaly Radioactive heat generated within the sediments could not have a great influence unless for depths exceeding 5 km (Rybach, 1986). As no published results on heat generation in the Rif are available, an estimation is attempted by using an empirical A[IxW/m 3] = 0.0158 (GR[API] to 0.8) (Bticker and Rybach, 1996). The data are taken from the gammaray log in the deepest wells of the zone. The estimated radiogenic heat production is comprised between 0.35 and 0.99 # W / m 3. Drill-hole data show a total Pre-Rifian sedimentary column of 8 km (Seber et al., 1996b) while the aeromagnetic survey interpretation gives 10 km (Demnati, 1972). Anyhow, if the heat production range estimated is applied, the contribution of the sediments in the surface heat flow would not be greater than 10 mW/m 2 at the most. Fig. 5A shows a slight increase of the HFD with depth over the study area as a whole, while a more

50

•

Preriflan

thrusts

q0 = 4.3"D + 53 N = 13;r=0.35

25 0

I

I

I

I

I

I

1

2

3

4

5

6

Depth

(kin)

Fig. 5. (A) Heat flow density in terms of the deepest BHT measured in all the study area. (B) Heat flow density in terms of the deepest BHT measured in the Pre-Rifian thrusts.

sensitive downward component particularly appears in the Pre-Rifian nappe (Fig. 5B). The rapid Neogene subsidence in the Pre-Rifian basin (Morel, 1988) and a thick low heat conductivity marly blanket could favour an absorption of the surface heat flow (Lucazeau and Le Douaran, 1985). The deepest levels are thus expected to be hotter. It appears that the surface thermal anomaly could be rather the result of a deep process linked with the lower crustal and upper mantle structures.

7. Discussion Physical proprieties within the lithosphere (density, electrical conductivity, magnetic and seismic properties) are sensitive to the temperature distribution. In the light of this interdependence and within the context of the Betic-Rif belt formation, the thermal regime of the Rif will be considered. Both gravity and refraction data suggest that the crust is relatively thin beneath the Rif (30 km) and decreases toward the Alboran basin (20 kin) (Demnati, 1972; Tadili et al., 1986). Two electrical conductivity anomalies (Fig. 1) were found using deep geomagnetic soundings in the Rif. They were correlated with abnormally hot and deep-rooted structures parallel to the Jebha-Chrafate fault in the west and the Nekor fault in the east (LEGSP and DMGM,

144

A. Rimi et al./Tectonophysics 285 (1998) 135-146

1977; Menvielle and Rossignol, 1982; Menvielle and le Mouel, 1985). These two N E - S W sinistral faults have played a fundamental role in the western Mediterranean formation (Andrieux et al., 1971; Le Blanc and Olivier, 1984; Michard et al., 1992). The high surface heat flow and the upheaval of the isotherms in these areas mean that the depth of the conductive structures do not exceed a few tens of kilometres (Adfim, 1980). A low EW-oriented Bouguer gravity, with a minimum of - 1 5 0 mGal (Van Den Bosh, 1981), is located between the meridians 4°W and 5°W and the parallels 34°N and 34.5°N (Fig. 1). This density anomaly cannot be explained neither by topographic compensation (average elevation is only about 600 m) or by a deep basin filled with low-density sediments. Recent three-dimensional gravity modelling (Seber et al., 1996b) suggests that the 8 km of sediments thickened by imbricated thrusting and foreland basins can only account for one half of the observed density anomaly in the Rif region. The rest of the gravity anomaly requires the presence of low-density material at subcrustal depths. Previously, an analysis of teleseismic P-wave travel times and high shear wave attenuation (Seber et al., 1996a), pointed out a low-velocity asthenospheric material overlying a high-velocity lithospheric body beneath the Rif-Betic area. Volcanism is intensive since the Oligocene (Girod and Girod, 1977) and scavenged xenoliths by Pliocene-Quaternary alkaline basalts in the Rif and southeast of Spain reflect a lower-crust and upper-mantle thermal process during extensional tectonics (Vielzeuf, 1983; Dautria and Girod, 1986). On the other hand, a study of the vertical distribution of heat generation along seismic profiles showed that the mantle heat flow contribution in the Rif and Alboran Sea offshore northern Morocco is comprised between 47 and 57 mW/m 2 (Rimi, 1997). Finally, the reported surface heat flow results, 55-90 mW/m 2 in the Alboran Sea (Foucher et al., 1976) and 6 0 100 mW/m 2 in southeastern Spain (Albert Beltran, 1979), confirm that the high thermal anomaly in the Rif-Betic could result from a regional increase of the mantle heat flow. Among the current tectonic models put forward to explain the geometry and kinematics evolution of the Betic-Rif orogenic belt, the one proposed by Platt and Vissers (1989) incorporates the observed

thermal and geophysical features in the Rif and the Alboran Sea. A significant phase of extensional detachment tectonics is associated with a convective removal of a thickened lithospheric root (delamination) and subsequent radial thrusting off an uplifted collisional ridge in the Alboran Sea. The delaminated lithospheric body in the upper mantle would extend over about 200 km beneath the continental regions of both African and Iberian plates (Seber et al., 1996a). However from the available results, we cannot constrain the spatial variations of the lithospheric structure beneath the Rif. Further studies involving thermal lithospheric modelling by integration of thermal, density and seismic data are in progress to test this promising hypothesis of the Rif-Betic southern branch formation.

8. Conclusion The HFD and thermal gradient values in the Rif (Morocco) range from 50 to 90 mW/m 2 and from 20 to 50°C/kin, respectively. Short-term variations in the west are related to fluid movement and thermal conductivity heterogeneities meeting salt diapirs or shallow granitic basement. Water convection along the Pre-Rif thrusting front and the structural unit boundaries favour resurgence of hot springs mostly located within a 32-37°C/km thermal gradient range. The southwestern thermal anomaly of the study area extends that of the central Moroccan massif marked by large granitic intrusions, Quaternary volcanism and hot springs. The External Rif shows a tendency of increasing HFD toward northeastern Morocco and the Alboran Sea. This high long-term thermal anomaly is consistent with the observed geophysical properties (deep rooted zones of high electrical conductivity, large negative Bouguer anomaly, shear velocity attenuation within a hot and low-density asthenospheric material overlying an upper mantle body with higher seismic velocity).

Acknowledgements Dr. Ahmed Benyaich, geologist of the Rif, encouraged this work. He tragically passed away during the preparation of the article, God rest his soul. The data were kindly provided by the Moroccan O.N.A.R.E.P.

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