A purely structural restoration of the NFP20‐East cross section and potential tectonic overpressure in the Adula nappe (central Alps)

Abstract

Common extrusion-type models for the high-to ultrahigh-pressure Adula nappe require a major normal fault along the top of this unit which is not conveyed in the structural record. This implies that such a normal fault existed but was completely erased during later deformational stages. However, there is evidence that decompression occurred during top-to-the-foreland thrusting. We performed a new, purely structural kinematic restoration of the central part of the NFP20-East cross section in order to estimate the burial depths of individual units without converting petrological pressure data into depth under the assumption that pressures were lithostatic. The results show that pressures within most of the units were close to but somewhat higher than lithostatic for several stages of the tectono-metamorphic history. Only for the maximum burial stage of the Adula nappe, we estimate local tectonic overpressures of 40 to 80% of the lithostatic pressures. Accepting such an amount of overpressure, which is moderate compared to values theoretically possible, the Adula nappe was probably not subducted to subcrustal depth. We propose that the structural record of the Penninic nappe stack is quite complete and suggest that the decay of tectonic overpressure is a feasible explanation for decompression from eclogite-to amphibolite-facies conditions during thrusting. Consequently, exhumation and convergence rates of the Eocene to Oligocene Alps may be smaller than previously assumed.

Thermochronometry Across the Austroalpine‐Pennine Boundary, Central Alps, Switzerland: Orogen‐Perpendicular Normal Fault Slip on a Major “Overthrust” and Its Implications for Orogenesis

“…For a nominal inclination of the channel of 45° (a modest angle compared to modern Mediterranean Benioff zones), the lower end of the Penninic mass (southern part of the Adula nappe) would have lain at a paleodepth of at least 10 + 80/(cos 45) = 74 km, corresponding to a pressure of about 2.2 GPa, which is within the observed range of 2.0–3.0 GPa estimates for (U)HP metamorphism at circa 32–34 Ma in the Monte Duria area (Hermann et al, ; Herwartz et al, ; Meyre et al, ). Assuming this geometric analysis is basically correct, it provides independent confirmation that pressures recorded by (U)HP metamophic assemblages reflect paleodepth, rather than zones of localized “tectonic overpressure” (e.g., Mancktelow, , ; Pleuger & Podladchikov, ; Schenker et al, ). In further support of this analysis, in the most southern, deepest exposed levels of the Pennine zone, near exposures of UHP eclogites at Cima Lunga and Alpe Arami, detailed zircon geochronology and geochemistry of Alpine migmatites indicate that syntectonic partial melting and crystallization of metasedimentary and metaigneous rocks occurred over a protracted period, under isothermal conditions of 620 to 700°C, from 31 Ma to 22 Ma (Rubatto et al, ).…”

Comparison of the earth's crust geothermal field in Northern Tien Shan region with seismic characteristics

“…The ratio of the total pressure (with allowance for thermobaric) and lithostatic pressure in the region of anomalous temperatures is 1.3÷1.4. The calculations are in the range of the experimental data of mineral equilibrium, showing that the tectonic pressure can reach up to 100% of the lithostatic and the values of the first gigopascals [11]. o C in the area of anomalous heating and the isobaric surface of thermoelastic stresses of 0.35 GPa can be attributed to the criteria that controls the distribution of earthquake hypocenters in the earth's crust (Fig.…”

Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides

“…Already now, however, the data set allows some inferences to be drawn regarding the ongoing discussion on tectonic v . lithostatic pressure in collisional orogens (Mancktelow, ; Li et al ., ; Pleuger & Podladchikov, ; Gerya, ; Schenker et al ., ).…”

“…In view of the large areal extent of these gneisses in the SNC, more UHP rocks are likely to be found in the future. Already now, however, the data set allows some inferences to be drawn regarding the ongoing discussion on tectonic v. lithostatic pressure in collisional orogens (Mancktelow, 2008;Li et al, 2010;Pleuger & Podladchikov, 2014;Gerya, 2015;Schenker et al, 2015).…”

“…Smith, 1980;Dong et al, 2002), or the result of local overpressure (e.g. Vrijmoed et al, 2009;Pleuger & Podladchikov, 2014). The differences could also result from the response of a specific protolith to the deep subduction process, that is its chemistry and/or tendency to dehydrate during the progression of metamorphism, eventually leading to H 2 O-depleted conditions at peak pressure, slowing down or preventing critical mineral reactions (e.g.…”