Deep structure model and dynamics of mud volcanoes, southwest absheron peninsula (Azerbaijan)

Abstract

Measurements have been made of gravitational field, geodetic uplift, regional horizontal tectonic movement, thermal patterns, and radioactivity in the general area of the Lokbatan mud volcano after its explosion on October 25, 2001, as well as in the crater itself. In addition, geochemical measurements of vitrinite reflectance with depth have been done and isotopic variations of methane and ethane are also made available. This massive compendium of information represents the first time such a detailed investigation has been possible of the deep structural effects of a mud volcano and also of the sources of mud and gas at outflow time. The data are integrated into a combined picture that shows the roots of both the mud outflow and of the gas causing the flaming eruption are at several kilometres depth into the sedimentary pile. The overall behaviour is best served by a model in which a relatively thin jet of liquefied mud is extruded from depth due to action of the varying tectonic stresses in the region, as adduced from the global positioning system (GPS) tectonic movement data. The variation of Bouguer gravity across a profile including the Lokbatan mud volcano, and combined with the geodetic vertical motion immediately after and long after (10 months) the explosion, confirms this basic model. The focusing of heat flux around the volcano prior to the explosion, and the thermal measurements made with time after the explosion both in the crater and also in the immediate vicinity of the Lokbatan volcano, are in accord with a thin hot jet model in which liquefied mud, with entrained gas from deeper in the sediments, rises through a neck region and, due to the Rayleigh-Bernard convective instability, produces a high temperature region. The geochemical evidence, showing low vitrinite maturity (<0.6%) to a depth of around 6 km, also indicates production of oil and gas from greater depths, as do the isotopic carbon measurements of methane and ethane in the unburnt gases. In short, it would seem that tectonic "squeezing" of a low-strength plastic mud layer from depth through a narrow vent with entrained gas and mud is the primary driver for mud volcano explosions. In the general regional, approximately linearly arranged lines of mud volcanoes, with their apparent focus centred on Shemakha from where the lines fan out, is also a strong indication of the basic tectonic origin. The combination of a rapidly filled sedimentary region, with unconsolidated (or deconsolidated) muds occupying a domain at several kilometre depth, and bracketed above and below by more competent formations, together with the active horizontal stress variations as measured by the GPS network, together form the basis for the spectacular mud volcano effects in this part of Azerbaijan.