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The cyclic interplay of seismic and aseismic deformation in the lower continental crust

Applicant Dr. Sascha Zertani
Subject Area Mineralogy, Petrology and Geochemistry
Geology
Term from 2021 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 461241592
 
Final Report Year 2024

Final Report Abstract

Most earthquakes occur in the upper ~15 km of the crust. Below, classical models of crustal strength predict the continental crust to flow, and not host earthquakes. However, geophysical, petrological, and structural observations show that earthquakes occur also in the deep portion of the crust, if it is dry. This project was aimed at understanding how brittle (seismic) and ductile (aseismic) deformation interact in such a lower-crustal setting. For this, the former lower continental crust – now exposed at the surface – in Lofoten, northern Norway, was chosen as a natural laboratory. This lower-crustal block has a somewhat enigmatic history. It is well-known for the occurrences of pseudotachylytes, solidified frictional melt indicative of former earthquakes, and ductile shear zones, both of which have been interpreted to have formed at amphibolite-facies conditions. Based on mutual cross-cutting relationships, previous studies have demonstrated that brittle and ductile deformation occurred cyclically. However, most of the rock volume is devoid of any related structures and preserves the Paleoproterozoic magmatic rocks. The pseudotachylytes and shear zones have been assumed to have formed during the Caledonian collision (430-400 Ma). However, no clear geochronological data was available so far. In this project we were able to determine the age of the pseudotachylytes to be 426 ± 11 Ma. This was achieved by in-situ apatite U-Pb dating coupled with detailed microstructural analysis. This age is not only important to understand the geological history of these pseudotachylytes, which are used in many studies to investigate processes of deep crustal seismicity, but is also the first time that the age of lower-crustal pseudotachylytes was determined directly. Another enigmatic aspect of the tectonometamorphic history of this lower-crustal block is scattered occurrences of eclogites, that indicate deep burial of the rocks to high-pressure conditions. When in the geological history this metamorphic event occurred is important to understand the tectonic context in which the lower-crustal seismicity occurred. Similarly, the sparse occurrence of eclogite is puzzling, as it raises the question why the bulk volume of the Lofoten block remained unaltered at high-pressure conditions, and eclogitization occurred only locally. Prior age dating of this event has provided an age of 399 Ma. We were able to shed light on the sequence of events with regard to eclogitization in the area. Structural analysis combined with electron microscope and geochronological analyses show that eclogitization occurred before amphibolite-facies deformation, which had seized at 413 Ma. Eclogitization thus occurred early in the collisional history or before. Further, our analyses show that eclogitization was spatially restricted as Paleoproterozoic fabrics served as channels for deformation, fluid-flow, and metamorphism, leaving the remainder of the rock volume unaffected.

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