The manifestations of modern degassing in the heat flow and deep structure (on the example of Lohvytsky block of Dnieper-Donetsk basin)

Abstract

The article contains heat flow calculations in the Lokhvytsky block and a scheme of the distribution of heat flow. We found the dependence of heat flow distribution with zones of activated deep faults of the crystal basement, sedimentary cover of Sribnenska depression and its surroundings. At the borders of Lokhvytsky block the heat flow in the area changes only a little, staying close to the average value — 40 mWt/m2. At all deposits the gradient increases by 20—30 °С/m, which happens at the intersection of tectonic, stratigraphic and lithological borders, which in many cases control the location of hydrocarbon deposits as well. Changes of temperature and composition of waters at the depth where the gradient jumps let us assume that their reason is modern hydrothermal activity. To find its manifestations, we incorporated geophysical research that determined the fault zones and zones of decompression, to which hydrocarbon deposits tend. The connection between thermal water discharge zones and faults is confirmed by their enrichment with hydrogen, helium, and the presence of native metal particles. Fault zones, which limit the Sribnenska depression and the Kherson—Smolensk transregional tectonic seam are shown at the crust structure, established by a two-dimensional speed model of seismic Р waves in the Еarth’s crust (GEORIFT 2013 profile). The results of seismotomographic research show the complex structure of the mantle, directly under the study area. Geophysical data demonstrate that faults manifested in the crystalline basement and sedimentary strata control the location of high-velocity blocks in the crust. The continuation of faults to the boundary of the crust and mantle is unquestionable. The inhomogeneities of the structure of the upper mantle and the transition layer, reflected in the seismotomographic model, can be traced directly under the Sribne depression to the lower mantle. This allows us to assume, as a working hypothesis, the existence of a chain of geodynamic processes that are manifested on the surface in hydrothermal activity.