Ductile Transition Depth Research Articles

The heat flow of Europa

The heat flow from Europa has profound implications for ice shell thickness and structure, as well as for the existence of an internal ocean, which is strongly suggested by magnetic data. The brittle–ductile transition depth and the effective elastic thickness of the lithosphere are here used to perform heat flow estimations for Europa. Results give preferred heat flow values (for a typical geological strain rate of 10 −15 s −1 ) of 70–110 mW m −2 for a brittle–ductile transition 2 km deep (the usually accepted upper limit for the brittle–ductile transition depth in the ice shell of Europa), 24–35 mW m −2 for an effective elastic thickness of 2.9 km supporting a plateau near the Cilix impact crater, and >130 mW m −2 for effective elastic thicknesses of ⩽0.4 km proposed for the lithosphere loaded by ridges and domes. These values are clearly higher than those produced by radiogenic heating, thus implying an important role for tidal heating. The ⩾19–25 km thick ice shell proposed from the analysis of size and depth of impact structures suggests a heat flow of ⩽30–45 mW m −2 reaching the ice shell base, which in turn would imply an important contribution to the heat flow from tidal heating within the ice shell. Tidally heated convection in the ice shell could be capable to supply ∼100 mW m −2 for superplastic flow, and, at the Cilix crater region, ∼35–50 mW m −2 for dislocation creep, which suggests local variations in the dominant flow mechanism for convection. The very high heat flows maybe related to ridges and domes could be originated by preferential heating at special settings.

High precision earthquake locations in Koyna‐Warna seismic zone reveal depth variation in brittle‐ductile transition zone

The Koyna‐Warna region located in the southwestern part of Deccan Volcanic Province is known for sustained seismicity during the last four decades. Catalog P and S phase data of local earthquakes recorded by the Koyna‐Warna digital network were used to relocate 609 earthquakes using double difference algorithm. The relocated seismicity patterns delineate the presence of three fault segments and variation in the brittle–ductile transition depths amongst the individual faults as the earthquake foci in the north are limited up to 11 km depth whilst in the south they are confined up to 6–8 km. The focal mechanism solutions also indicate distinct styles of faulting amongst the three inferred fault segments. The variation in brittle‐ductile transition zone is probably controlled due to the geometry of the fault system and the effective stress.

RETRACTED: Is the plastic flow uniformly distributed below the seismogenic region?

We compared the cutoff depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in intraplate Japan with the brittle–ductile transition depth of the widely accepted strength profile model of the crust. We successfully determined the temperature profile from borehole measurements, since almost the same geothermal gradients were observed at two boreholes located about 4 km apart from each other, and the thermal conductivity and heat production were also measured by taking numerous core samples. We found that the cutoff depth was much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 3×10−15 s−1. This small strain rate implies, however, that plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10−13 s−1, the cutoff depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region, even in the intraplate region.

Heat flux and seismicity in the Fennoscandian Shield

The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheological structure is analysed from the Kola Orogen to the Sorgenfrei–Tornquist zone. Moho temperatures and the heat flux from the mantle are typical of cratonic areas. The deep thermal field shows a cold root in the north-eastern sector. Larger lateral variations of mantle heat flux and Moho temperatures occur in the southern area and at the edges of the shield. The thickness of the thermal lithosphere is maximum (200–220 km), where the mantle heat flux is minimum (about 15–20 mW m −2). Three lithospheric cross-sections illustrate the expected lateral variation in viscosity and failure mode throughout the lithosphere as a consequence of the different geothermal conditions and the rheological stratification. In the upper crust, the depth of the brittle–ductile transition varies on average from 30, in the north-east, to 15 km, in the south-west. In the subcrustal mantle, the models predict a mainly ductile behaviour. At a depth of 60 km, the upper mantle viscosity is maximum (2.5×10 23 to 25.0×10 23 Pa s) beneath the Archean province and minimum (2.5×10 21 to 10.0×10 21 Pa s) below the Svecofennian and Sveconorwegian units. The comparison of the rheological calculations with the seismic activity shows a general agreement with the brittle–ductile transition depth expected in the Archaean and Proterozoic provinces. There is a difference both in number of earthquakes and distribution versus depth between areas of different age. The occurrence of larger magnitude seismic events near the base of the seismogenic zone, in the region of inferred peak of shear resistance, suggests some causal relationship. The Proterozoic areas show a more regular decrease of shocks with depth and, compared to the Archean provinces, a tendency to shallowing.