Heat and Groundwater Flow in the NE Sector of the Morocco Hot Line
M. Verdoya1, A. Rimi2, A.E. Barkaoui3, Y. Zarhloule3, P. Chiozzi1
1. DISTAV - Università di Genova, Viale Benedetto XV, 5, 16132 Genova, Italy
2. Institut Scientifique, Université Mohammed V, Avenue Ibn Battouta, B.P. 703 Rabat, 10106 Morocco 3. Laboratory of Hydrogeology-Environment, Faculty of Sciences, University Mohamed 1st, Oujda, Morocco
Abstract
The Moroccan portion of the Atlas chain is characterised by widespread volcanic activity that occurred 14.6-0.3 Ma ago within a SW-NE trending belt, called Morocco Hot Line (MHL). The volcanic belt extends from the Atlantic coast to the Mediterranean Sea. Geophysical modelling relying on the combination of elevation, geoid anomaly and heat flow data indicated that the lithosphere is strikingly thinned beneath the Atlas chain and thus a hot, buoyant lithosphere is required to explain the relief. A mantle plume, extending from the Canary Islands to central Europe, was invoked to account for such a lithosphere thinning. The geochemical features of volcanism are generally consistent with this hypothesis, being mostly intraplate and alkaline, with the exception the NE termination of MHL, where there is evidence of different sublithospheric mantle sources suggesting an origin partly related to subduction processes. This paper focuses on the thermal features of the NE portion of MHL, where a number of geothermal data have been collected over the past two decades. This zone shows high geothermal gradient and heat flow values, which might be related to deep-seated processes. However, it is hydrogeologically characterised by the occurrence of Liassic limestone forming an extensive, often artesian, deep reservoir hosting moderately hot groundwater. The occurrence of a thermal aquifer may boost the temperature gradient in the overlying, impermeable, marly cover, thus locally yielding enhanced heat flow. We therefore carefully revise the available dataset to investigate the contribution of advection. Temperature data available from water and oil wells were reprocessed and analysed in combination with thermal conductivity measurements on a wide set of lithotypes.
Keywords: lithosphere thinning, heat flow, groundwater,
In its eastward extension to the Mediterranean coast of Morocco and Algeria, MHO includes volcanic products dated 6.2-0.8 Ma.
A small mantle plume related to form part of a hot and deep mantle reservoir system extending from the Canary Islands to central Europe was invoked to account for lithospheric thinning. The lack of correlation between the age and the geographic position of the Moroccan alkaline volcanoes is indeed typical of hot lines and contrasts with the regular trends observed for Hawaiian-type hot spots.
Temperature depth profiles from boreholes characterized by upward flow of hot water fed by the carbonatic formation are available to 350-500 m depth. Temperature logs were analyzed by
matching thermal data with models of vertical temperature distribution which incorporate both heat and mass transfer. Thermo-hydraulic parameters were calculated from the coefficients of the advective models obtained by means of the least-square fitting method. The inferred temperature gradient above the advectively perturbed carbonatic formation exceeds 50 mK/m, thus locally boosting the heat-flow density to values larger than 100 mW/m2. Analytical modeling of heat and
water transfer involved in the deep circulation was attempted along selected hydro-geological cross-sections. Several hypotheses of basal heat-flow density, porosity, aquifer thickness and water velocity were tested in order to fit the borehole thermal records. The results show that temperatures of the deep aquifer are compatible with a topographically driven flow in the carbonatic formations down to 600-1500 m depth, under conditions of enhanced geothermal gradient.
The geothermal fluid is characterised by a complex deep circulation and it ascends through complex fault systems. The Liasic reservoir of Eastern Morocco belongs to the Atlas domain (Fig. 1) which is characterised by a succession of NE-SW horst and graben structures of pre-Miocene age and by Plio-Quaternary basaltic volcanics, produced by crustal thinning.
These ranges form the main recharge zones of deep water reservoirs. In the Middle Atlas there are several hydrogeological basins with minor shallow aquifers in the Plio-Quaternary terrains. A marly substratum separates the shallow groundwater from the deeper artesian aquifer, occurring in the high-permeability Liassic carbonatic sequences and extending throughout the region. The top of the carbonatic aquifer ranges from 200 to 1300 m depth, and water temperatures as higher as 50 °C were recorded to about 500 m depth. Moreover, numerous springs with temperature larger than 40 °C and a flow which may reach 40 l/s occur in this area.
The aim of this paper is to contribute to the hydro-geothermal characterization of northwestern Morocco by presenting results of measurements of rock thermo-physical properties and analyzing temperature data available from water wells reaching the Mesozoic terrains. A number of thermal conductivity measurements were carried out on a set of samples representative of the stratigraphic sequence of the Middle Atlas. The carbonatic lithotypes forming the deep aquifer show a relatively high thermal conductivity. Values range from 2.0-3.1 W/(m K) in limestones to 4.6-5.0 W/(m K) in dolomites. Temperature depth profiles from boreholes characterized by upward flow of hot water fed by the carbonatic formation are available to 350-500 m depth. Temperature logs were analyzed by matching thermal data with models of vertical temperature distribution which incorporate both heat and mass transfer. Thermo-hydraulic parameters were calculated from the coefficients of the advective models obtained by means of the least-square fitting method. The inferred temperature gradient above the advectively perturbed carbonatic formation exceeds 50 mK/m, thus locally boosting the heat-flow density to values larger than 100 mW/m2. Analytical modeling of heat and
water transfer involved in the deep circulation was attempted along selected hydro-geological cross-sections. Several hypotheses of basal heat-flow density, porosity, aquifer thickness and water velocity were tested in order to fit the borehole thermal records. The results show that temperatures of the
deep aquifer are compatible with a topographically driven flow in the carbonatic formations down to 600-1500 m depth, under conditions of enhanced geothermal gradient.
Fig. 1. Structural sketch map of the Moroccan Atlas (after Boccaletti et al., 1985 and Michard et al., 2006). (1) Late to post-orogenic fills and foreland covers; (2) Tertiary-Quaternary magmatic rocks; African units deformed during the Apenninic-Maghrebid orogeny: (3) pelagic and platform sequences of the African margin and (4) parts of the Hauts Plateaux and African foreland; (5) main folded Atlas Zone and (6) Hauts Plateaux and eastern Meseta with deformations partially related to the Atlas and the external Tell. Full squares, boreholes with available temperature-depth logs. Sampling areas for thermal conductivity measurements are denoted with rectangles A-G. a-a’ and b-b’ location of geological cross-sections used for thermal modeling.
35° 20' N 35° 00' N 34° 40' N 34° 20' N 3° 00' W 2° 40' W 2° 20' W 2° 00' W 1° 40' W Mediterranean Sea Melilla Oujda Mou louya Ch aref 1 2 3 4 5 6 Kebdana
