High transient stress in the lower crust: Evidence from dry pseudotachylytes in granulites, Lofoten Archipelago, northern Norway

Kristina G Dunkel,Xin Zhong,Bjørn Jamtveit,Lars Vesterager Valen,Paal Ferdinand Arnestad

doi:10.1130/g48002.1

Abstract

AbstractSeismic activity below the standard seismogenic zone is difficult to investigate because the geological records of such earthquakes, pseudotachylytes, are typically reacted and/or deformed. Here, we describe unusually pristine pseudotachylytes in lower-crustal granulites from the Lofoten Archipelago, northern Norway. The pseudotachylytes have essentially the same mineralogical composition as their host (mainly plagioclase, alkali feldspar, orthopyroxene) and contain microstructures indicative of rapid cooling, i.e., feldspar microlites and spherulites and “cauliflower” garnets. Mylonites are absent, both in the wall rocks and among the pseudotachylyte clasts. The absence of features recording precursory ductile deformation rules out several commonly invoked mechanisms for triggering earthquakes in the lower crust, including thermal runaway, plastic instabilities, and downward propagation of seismic slip from the brittle to the ductile part of a fault. The anhydrous mineralogy of host and pseudotachylytes excludes dehydration-induced embrittlement. In the absence of such weakening mechanisms, stress levels in the lower crust must have been transiently high.

Highlights

The brittle-ductile transition, deformation is commonly assumed to be accommodated by crystal-plastic mechanisms, and earthquakes should not be possible
Seismic activity below the standard seismogenic zone is difficult to investigate because the geological records of such earthquakes, pseudotachylytes, are typically reacted and/or deformed
∼15 km depth for the continental crust), deformation is commonly assumed to be accommodated by crystal-plastic mechanisms, and earthquakes should not be possible

Summary

INTRODUCTION

∼15 km depth for the continental crust), deformation is commonly assumed to be accommodated by crystal-plastic mechanisms, and earthquakes should not be possible. Their preserved microstructures provide the first glimpse into the incipient stages of Caledonian crustal transformation processes in an area with a high density of lower-crustal earthquakes (e.g., Steltenpohl et al, 2006; Moecher and Steltenpohl, 2011; Menegon et al, 2017; Jamtveit et al, 2019) and advance our understanding of earthquakes under high confining pressure. Pseudotachylytes are abundant and have been reported from numerous localities in the Lofoten-Vesterålen area (Fig. 1A) They are often coeval with eclogite- or amphibolitefacies shear zones (Steltenpohl et al, 2006; Moecher and Steltenpohl, 2011; Menegon et al, 2017), indicating formation under lower-crustal conditions

PSEUDOTACHYLYTES ON MOSKENESØYA

MICROSTRUCTURES OF HOST AND PSEUDOTACHYLYTE

AMBIENT CONDITIONS DURING PSEUDOTACHYLYTE FORMATION

GENERATION OF EARTHQUAKES IN DRY LOWER CRUST