Three types of structural?geological units, extending to the neighboring countries and farther onto the Eurasia continent, occur in Romania.
The first type includes platform regions which after the Assyntic, Caledonian or Hercinian orogeneses lost their mobility as geosynclines. This category contains the East-European Platform, Scythian Platform and the Moesian Platform. Being very old units, their crust is thick (45-50km) and rigid (without earthquakes) with a heat yield of 1.1 cal/cm3/s. In their case, main emissive segments for the radiogenic heat are concentrated linearly around transcrustal fractures, or on tectonic remobilized basement areas. In the first category, important geothermal resources are Mihai Bravu?Insuratei sector, on the Capidava-Ovidiu fault and North Bucharest-Otopeni-Snagov sector. The second category includes the Optasi-Bals?Slatina structural uplift, with segments affected by rift genesis and bimodal magmatism (acid and basic) between the Triassic and the Permian. On an almost 100 km2, the Upper Jurassic-Lower Cretaceous limestone aquifer, situated at 1600-2300 m depth, has the water resources with a surface temperature of 60-75°C.
The second type of structures is represented by geological regions active in the last two eras: the Carpathians and the North Dobrogea. The North Dobrogea orogene, with an intraplate rifting and two moments of orogenesis (Jurassic and Lower Cretaceous), lacks geothermal resources. In the Carpathians, the main geothermal areas are the result of: a) disintegration of radioactive elements inside the granite masses, possible of Tertiary age (Herculane Spa) ; b) some important tectonic events such as Southern Carpathian thrusting from the end of the Upper Cretaceous (Calimanesti-Caciulata); c) thermal remnants of the Neogene-Quaternary volcanism in the Southern Apuseni and western Eastern Carpathians. The third type of units is the great post-tectonic depressions. They contain big parts of the alpine orogene fell at depth of 2000-3000m, covered by an Upper Senonian-Holocene sedimentary pile. This category includes the post-orogene Transylvanian and Pannonian basins that have a distinct compressional tectonic regime. The Transylvanian Basin has a thin crust (30km), implicitly a thin granite layer (12 km), a low thermal flux (40mW/m2) and a heat yield of 1.72 cal/cm3/s.
The Pannonian Basin, of the same crust thickness, has a heat yield of 2.4cal/cm3/s, an average thermal flux around 95mW/m2, and temperature gradients of 50-70°C/km. So, between the towns of Satu Mare and Timisoara, geothermal fields are found in Pannonian siltstones, with water resources of 50-105°C surface temperature. The most important mineral water resources of this depression are the geothermal waters billeted in Triassic ? Cretaceous fissured limestones (flow rate 400 l/s at about 70-105°C).