Fluid Circulation In The Active Emerged Asal Rift (East Africa, Djibouti) Inferred From Self-Potential And Telluric-Telluric Prospecting

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Journal Article: Fluid Circulation In The Active Emerged Asal Rift (East Africa, Djibouti) Inferred From Self-Potential And Telluric-Telluric Prospecting

Abstract
In 1996 Self-Potential (SP) and Telluric-Telluric (TT) surveys were performed in the Asal rift, in the Republic of Djibouti, in order to study water circulation through the rift, from Ghoubbet Strait to Asal lake located 11 km Northwest, and at the altitude of 155 m below sea level. SP surveys reveal two main positive anomalies, several kilometres wide, up to 150 mV in amplitude, parallel to the rift and separated by a relatively large (1 km) negative anomaly. The northern anomaly is well centred above the active rift. The central part of the anomaly is cut by a sharp and narrow negative anomaly (200 mV in amplitude, 150 m width) due to a major fault running from Ghoubbet Strait to Asal lake. The two positive anomalies are interpreted by ground seawater flowing toward Asal lake and warmed up during their advance by the strong geothermal flux existing beneath the rift. The negative anomalies are most probably due to large and rapid fluid transfer along faults and high permeable geologic beds bending toward Asal lake. The rapid flow would prevent any large heating of the ground water. The largest negative anomaly, which isolates the two positive ones, gives new valuable information on the tectonics. The extension of the northern positive anomaly to the Southwest is stopped by a fault located about 1 km to the North of a normal fault considered until now, as a major rift structure. The 16 Telluric-Telluric (TT) surveys point out three behaviours of the telluric field. Near Ghoubbet Strait and inland the telluric field is well polarised in a N90-100°E direction and the amplitude expresses a medium to a low resistivity beneath the recording sites. This pattern is the signature of the coast effect and the regional polarisation of the field. Along normal faults bordering the rift the SE-NW polarisation seems to be controlled by the rift tectonics. Inside the rift the telluric field becomes weaker, indicating a decay of the resistivity. In the central part of the rift the telluric field amplitude becomes 1/3 of the generally recorded amplitude and the polarisation cannot be estimated. It is corresponding to very low resistive areas in agreement with high positive SP values.

Authors 
M. Mlynarski and J. Zlotnicki








Published Journal 
Tectonophysics, 2001





DOI 
10.1016/S0040-1951(01)00127-5


 

Citation

M. Mlynarski,J. Zlotnicki. 2001. Fluid Circulation In The Active Emerged Asal Rift (East Africa, Djibouti) Inferred From Self-Potential And Telluric-Telluric Prospecting. Tectonophysics. (!) .