WEBVTT Kind: captions; language: nb-no 00:00:00,329 --> 00:00:06,032 [Automatic captions by Autotekst using OpenAI Whisper V3. May contain recognition errors.] Hi, I am Sascha Zertani, a postdoctoral researcher at the Njord Center of the University of Oslo. 00:00:06,492 --> 00:00:14,015 Today I want to show you the results of a paper we recently published together with colleagues from the Natural History Museum in Stockholm, Sweden. 00:00:14,695 --> 00:00:25,160 The study was published in the journal Earth and Planetary Science Letters and is important because it is the first time that the age of a fossil lower crustal earthquake was determined directly. 00:00:26,340 --> 00:00:33,223 Plate tectonics causes continents to collide and form mountain belts during which earthquakes occur within the Earth's crust. 00:00:34,143 --> 00:00:45,608 While most of the earthquakes on Earth occur at a depth of 10 to 15 kilometers during continental collision, some of them also occur in the lower part of the crust at depths greater than 20 kilometers. 00:00:46,485 --> 00:00:58,967 These unusually deep earthquakes represent a challenge for the scientific community because classic models of the mechanical behavior of the continental crust would predict the lower crust to flow rather than break. 00:01:00,071 --> 00:01:07,498 However, many studies have provided evidence that rocks can fracture at this depth if they are particularly dry. 00:01:08,338 --> 00:01:13,003 In the rock record, earthquakes may be preserved as so-called pseudotachylytes. 00:01:13,263 --> 00:01:25,133 These are typically very dark and fine-grained layers or networks in the rock that form when the rock fractures and the two blocks slide along each other at high velocities of around one meter per second. 00:01:25,971 --> 00:01:33,862 As they do so, energy is released in the form of heat, sort of like when you slide your hands along each other very quickly. 00:01:34,663 --> 00:01:40,050 This heat causes melting of the rock, which are the black layers that are preserved in the rock. 00:01:41,351 --> 00:01:52,796 It is widely accepted that pseudotachylytes are evidence of former earthquakes, but until now there has not been a successful attempt to determine the age of such a fossil earthquake in lower crustal rocks. 00:01:53,557 --> 00:02:07,063 However, determining the age will provide information on when during the collision of continents earthquakes in the lower crust occur, which is important to understand the mechanical response to plate motions in the deep portions of the continents. 00:02:09,281 --> 00:02:16,047 One example of a former lower crust and now exposed at the Earth's surface is found in Lofoten in northern Norway. 00:02:16,767 --> 00:02:29,597 There we find pseudotachylytes formed during earthquakes that occurred at 30 km depth, some of which contain a mineral called apatite, which can be dated using the decay of uranium to lead. 00:02:30,555 --> 00:02:39,318 From there, we collected several samples and determined if they contain apatite, and finally chose three samples to try to date the apatite grains. 00:02:40,378 --> 00:02:47,941 The apatite grains did not grow from the melt, but survived melting of the rock during the earthquake, so-called survivor clasts. 00:02:48,914 --> 00:02:56,636 First, we analyzed the apatite using electron microscopy techniques and found that not all apatites show the same features. 00:02:57,416 --> 00:03:05,779 To date the apatite grains, we went to the Natural History Museum in Stockholm to use a method called secondary ion mass spectrometry, or SIMS. 00:03:07,116 --> 00:03:12,141 With this method, we can measure the isotopes of uranium and lead and calculate the age. 00:03:12,862 --> 00:03:20,389 We found two age groups, one at 426 million years and one with scattered ages ranging from about 380 to 170 million years. 00:03:23,152 --> 00:03:40,305 Our prior analysis using scanning electron microscopy techniques shows that the apatite that gives the 426 million year age was plastically deformed, which requires relatively high temperatures, which occurred during the short thermal spike produced by the earthquake. 00:03:41,368 --> 00:03:49,815 This brief phase of deformation led to resetting of the clock that measures the age of the grain, the uranium-lead isotope system. 00:03:50,656 --> 00:03:55,941 The earthquakes in the Lofoten block therefore occurred at around 426 million years ago. 00:03:56,461 --> 00:04:04,808 During this phase, the continental collision that formed the Scandinavian mountain belt was at the beginning of the main collisional phase. 00:04:05,589 --> 00:04:15,609 Essentially, the two plates collided and the lower crustal rocks were incorporated into the collision zone where they fractured and produced earthquakes. 00:04:17,506 --> 00:04:37,230 The younger ages show us that there was interaction with fluids along the same structures, the pseudotachylytes, long after they formed, suggesting that these structures are the pathways for fluids to infiltrate the otherwise completely dry lower crust. 00:04:38,770 --> 00:04:46,752 The age of 426 million years is the first successful attempt to date a fossil lower crustal earthquake.