Crustal Structure in the Western Part of Romania from Local Seismic Tomography

Multi‐kilometre long pathway of geofluids migration: Clues concerning an ophiolite serpentinization setting possibly responsible for the inferred abiotic provenance of methane in thermal water outflows of the South‐West Carpathians (Romania)

“…The latter likely enables a slightly concentrated, Na–Cl–(SO 4 )‐type groundwater—similar to the CH 4 ‐conveying thermal water of Mehadica—to dilute, in various degrees, a more concentrated Na–Ca–Cl‐type groundwater encountered at Băile Herculane. The WNW–ESE orientation of the corresponding flowpath is consistent with the direction of a minimum P‐wave velocity anomaly that Zaharia et al (2017) had detected by means of seismic tomography above ~15 km depth (Figure 7). A consequently interpreted permeable fractured zone could be the result of sub‐horizontal, SSW–NNE‐directed spreading—like the one indicated by the normal‐fault mechanism (Figures 1b and 7) determined (International Seismological Centre, 2019) for the 18 July 1991, MS = 5.6 earthquake.…”

“…Fence diagram illustrating—for the horizontal domain enclosed in the black rectangle of Figure 1a—the seismo‐structural setting and the correspondingly conjectured geofluids flow patterns. Figure 1a indicates also the positions of transects A–A′ (geological cross‐section from Ştefănescu et al., 1988, simplified) and B–B′ (vertical section of anomalies (%) of P‐wave velocity, according to Zaharia et al, 2017, simplified). No vertical exaggeration.…”

“…A possible additional signature of that present‐day extensional tectonic activity could be a low P‐wave velocity structure that Zaharia et al (2017) had detected, above ~15 km depth, by means of seismic tomography. Worth noticing is this low velocity lineament WNW–ESE orientation (which is consistent also with the earthquake epicentres distribution—Figure 1b): that direction strongly deviates from the SSW–NNE structural trend (delineated by the boundaries between the nappes and by the major Cerna‐Jiu fault), which the region had inherited from previous epochs of tectonic deformation.…”

“…(b) Map of the domain (red dashed‐dotted rectangle in Figure 1a) that incorporates the inferred geofluids migration pathway between Băile Herculane and Mehadica. There are additionally indicated the positions of the outflows sampled in the framework of the present study, the P‐wave velocity anomalies (according to Zaharia et al, 2017), computed—for the depth of 5 km—as per cent with respect to the optimized 1D velocity model, as well as the epicentres of local earthquakes (data retrieved from International Seismological Centre, 2019, and from Shebalin et al., 1998), larger symbols corresponding to the events of 11 October 1910 (MS = 4.3) and 18 July 1991 (MS = 5.6, with displayed focal mechanism provided by the National Earthquake Information Center– https://earthquake.usgs.gov/earthquakes/eventpage/usp0004uaj/focal‐mechanism?source=us&code=choy19910718115630). The surface geology background is compiled, in a simplified form, from Berza, Balintoni, Iancu, Seghedi, and Hann (1994), Iancu et al.…”

“…We therefore primarily used, within the present study, ionic composition analyses which we had recently conducted on thermal water discharges that conveyed the inferred abiotic CH 4 . Those water composition data were corroborated with previously published chemical and isotopic gas analyses (Cosma, Italiano, Baciu, Ristoiu, and Etiope, 2003; Cosma & Ristoiu, 1999; Filipescu & Humă, 1979; Ionescu et al., 2017; Mastan, Cosma, & Znamirovschi, 1982; Mastan, Znamirovschi, and Cosma, 1982; Mastan, Znamirovschi, Cosma, & Pop, 1981), with geological‐structural information (Iancu, Berza, Seghedi, & Marunţiu, 2005; Iancu & Seghedi, 2017; Iancu, Seghedi, Mărunţiu, & Strusievicz, 1990; Năstăseanu, Iancu, & Russo‐Săndulescu, 1988; Plissart, Diot, Monnier, & Mărunţiu, 2018; Plissart et al., 2017), as well as with recent seismic tomography results (Zaharia, Grecu, Popa, Oros, & Radulian, 2017), and with data on the local seismic activity (International Seismological Centre, 2019; Shebalin et al., 1998).…”