Electrical conductivity anomaly of the northern Carpathians and the deep structure of the orogen

Abstract

Anomalia przewodności elektrycznej Karpat Północnych a wgłębna budowa orogenu The Carpathian conductivity anomaly, also called geoelectrical anomaly, is constrained by the presence of high-conductivity rock series at depths of 10 to 25 km, in different segments of the orogen. This paper aims at locating the source of the anomaly in the framework of crustal blocks and at its geological interpretation. The faulted and locally depressed to 20 km surface of the high-resistivity Precambrian basement of the Polish Carpathians is presented basing on the results of magnetotelluric sounding (MTS). The major faults, of Neogene age, are oriented SW-NE in the west and SE-NW in the east, running symmetrically to the Kraśnik-Rzeszów- Rymanów-Debrecen line (ca. 22° E). The high-conductivity layer overlying the crystalline basement is probably composed of metamorphosed Palaeozoic strata, including coal-bearing Carboniferous rocks. The sole presence and extent of these rocks, however, are not sufficient to generate the regional conductivity anomaly whose outline is clearly related to that of the Tertiary Carpathian orogen. The results of MTS sounding have been compared with those of seismic refraction studies in the Ukrainian Carpathians. Two belts of depressions of variable depth have been distinguished in the basement. The southern belt includes a collision suture, i.e. the contact zone between the lowered Central European block and the blocks of the Central West and East Carpathians. Between Bardejov-Wysowa and the Marmarosh Massif the source of the geoelectrical anomaly is situated close to the northern margin of the southern basement depression. Farther to the south, up to 46° N near Sf. Gheorghe, Romanian geologists place this source on a deep-seated fault that borders the Central East Carpathian block from E. To the east of the Slovak-Ukrainian border, the southern depression is overlain by folded flysch complexes of the Rakhov, Porkulets, “Black Flysch”, and Ceahlau units, interpreted as the infill of the Outer Dacides rift, as well as by the Dukla unit flysch. The rift was situated in the marginal part of the European continent. Its western continuation, presently covered by the Dukla and Magura Nappes, is documented by relics of dark Doggerian and Neocomian flysch strata, as well as by basalts exposed within the Grajcarek Unit, north of the Pieniny Klippen Belt west of Krynica. The farther continuation of this zone can be found in the Penninicum (Piemontais?) of the Alps. The source of the Carpathian geoelectrical anomaly is situated near the collision suture; however, the structure of the Ukrainian Carpathians basement and a shift o f the anomaly source towards the Carpathian Foredeep in the Focçani region, Romania, may indicate a relationship between the anomaly and the lowered, marginal part of the continental crust. It is suggested that an important factor generating the Carpathian geoelectrical anomaly is graphite originated due to post-Oligocene migration and graphitization of organic substance within deeply buried strata of the Jurassic-Cretaceous rift and within fault zones in the crystalline basement. The near-surface manifestation of this process is the presence of veins bearing hydrothermal mineral association, including authigenic quartz (“Marmarosh diamonds”). This quartz contains inclusions of anthraxolite, a hard bitumen showing traces of incipient graphitization. Such veins are ubiquitous in the Carpathians, close to the axis of the anomaly.