Continental collisions and the origin of subcrustal continental earthquakes

Abstract

The existence of subcrustal continental earthquakes beneath the Alpine-Himalayan Belt was recognised more than 60 years ago. There is general agreement that most of those beneath the western part of the belt in the Mediterranean result from the subduction of oceanic lithosphere. There is less agreement about the origin of those beneath Vrancea in Romania, the Hindu Kush, and the Pamir. Because there is little evidence for the former existence of oceanic lithosphere beneath these regions, many authors have argued that these seismic zones result from the separation of the mantle part of the continental lithosphere from the crust before it sinks into the mantle. However, this model has become steadily less satisfactory. Detailed studies of the depth of earthquakes beneath all stable regions of continents have shown that substantial subcrustal earthquakes, with magnitudes greater than 5.5, are rare. We show that this distribution is controlled by temperature, with material hotter than ϳ600°C being aseismic. This simple rule accounts for the distribution of almost all earthquakes in oceanic and continental lithosphere, including those in subduction zones. We argue that the subcrustal continental earthquakes must also result from the subduction of oceanic lithosphere. This proposal is not new but has generally been dismissed because of the lack of surface geological evidence that suitable pieces of oceanic lithosphere existed. However, the depth distribution of continental earthquakes makes it steadily harder to avoid.Résumé : L'existence de tremblements de terre continentaux subcrustaux sous la ceinture alpine-himalayenne est connue depuis plus de 60 ans. Il est généralement convenu que la plupart des séismes sous la partie occidentale de la ceinture dans la Méditerranée résultent de la subduction de lithosphère océanique. L'origine des séismes sous les régions de Vrancea, en Roumanie, de l'Hindou Kouch et du Pamir fait toutefois moins consensus. Parce qu'il y a peu de preuves de l'existence passée de lithosphère océanique sous ces régions, de nombreux auteurs ont argué que ces zones sismiques sont le résultat de la séparation de la partie mantellique de la lithosphère continentale et de la croûte avant que cette partie mantellique ne sombre dans le manteau. Ce modèle s'avère toutefois de moins en moins satisfaisant. Des études détaillées de la profondeur des tremblements de terre sous toutes les régions stables de continents ont démontré que les grands séismes subcrustaux, de magnitudes supérieures à 5,5, sont rares. Nous démontrons que cette répartition est contrôlée par la température, les matériaux de plus de ϳ600°C étant asismiques. Cette règle simple explique la répartition de presque tous les séismes dans la lithosphère océanique et continentale, y compris ceux qui se produisent dans des zones de subduction. Nous arguons que les séismes continentaux subcrustaux doivent aussi résulter de la subduction de lithosphère océanique. Si cette proposition n'est pas nouvelle, elle a généralement été rejetée en raison de ...

The Brittle‐Plastic Transition, Earthquakes, Temperatures, and Strain Rates

“…In proposing the~600°C cutoff depth of seismicity, McKenzie et al (2005McKenzie et al ( , 2019 exploited earthquakes in oceanic intraplate settings, for which estimates of strain rates are approximate. Sums of seismic moments from intraplate earthquakes in oceanic lithosphere yield average strain rates of~1.6 × 10 −18 s −1 (Wiens & Stein, 1983) or <3 × 10 −20 s −1 for lithosphere older than~40 Ma (Sasajima & Ito, 2016).…”

A discussion of “hidden subduction” in orogenic belts

“…We see on our planet various examples of such cases in various stages of their evolution from the not-yet choked South Caspian Basin to the entirely choked Songpan-Ganzi and the North Caspian areas. Does that save the McKenzie et al (2019) model? In principle it does, but whether it also applies to the Band-ı Turkestan/Hindu Kush/ Pamirs case can only be decided by a careful geological reconstruction of the evolution of that area.…”

“…The McKenzie et al (2019) model depends upon whether patches of oceanic lithosphere can remain in suture zones in areas of incomplete suturing, buried by thick sedimentary blankets, to be subducted later long after collision. What might trigger the subduction of these patches?…”

Ali Mehmet Celâl Şengör: A geologist who unravels the histories of continents and oceans

“…Instead, some researchers argued that detachment of the mantle part of the continental lithosphere from the crust generates earthquakes beneath these regions. McKenzie et al (2019) argue that the detachment model does not provide a satisfactory explanation for the origin of subcrustal continental earthquakes beneath Vrancea, the Hindu Kush, and the Pamir because the mantle part of the continental lithosphere in these regions is mostly aseismic. They demonstrate that substantial subcrustal earthquakes beneath all the stable regions of continents, with magnitudes greater than 5.5, are rare, their distribution being controlled by temperature, with material hotter than ϳ600 C being aseismic.…”

“…They demonstrate that substantial subcrustal earthquakes beneath all the stable regions of continents, with magnitudes greater than 5.5, are rare, their distribution being controlled by temperature, with material hotter than ϳ600 C being aseismic. On the basis of this observation, McKenzie et al (2019) propose that the majority of subcrustal continental earthquakes result from subduction of oceanic lithosphere.…”