Fast and partitioned postglacial rebound of southwestern Iceland Guillaume Biessy, Olivier Dauteuil, Brigitte van Vliet-Lanoë, A. Wayolle To cite this version: Guillaume Biessy, Olivier Dauteuil, Brigitte van Vliet-Lanoë, A. Wayolle. Fast and partitioned post- glacial rebound of southwestern Iceland. Tectonics, American Geophysical Union (AGU), 2008, 27 (3), pp.TC3002. 10.1029/2007TC002177. insu-00286999 HAL Id: insu-00286999 https://hal-insu.archives-ouvertes.fr/insu-00286999 Submitted on 29 Jun 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. TECTONICS, VOL. 27, TC3002, doi:10.1029/2007TC002177, 2008 Fast and partitioned postglacial rebound of southwestern Iceland G. Biessy,1 O. Dauteuil,1 B. Van Vliet-Lanoe¨,2 and A. Wayolle2 Received 24 June 2007; revised 11 January 2008; accepted 15 February 2008; published 9 May 2008. [1] Located both on the Mid-Atlantic Ridge and postglacial rebound of southwestern Iceland, Tectonics, 27, above a mantle plume, Iceland is subject to horizontal TC3002, doi:10.1029/2007TC002177. and vertical motions. Many studies described these deformations in terms of rifting episodes that have combined both extensional tectonics and magmatism. 1. Introduction However, few studies have described the glacio- [2] In Iceland, the conjunction of the Mid-Atlantic Ridge isostatic response induced by the retreat of the and a mantle plume creates intense tectonic and magmatic Weichselian ice cap. The melting of this ice cap activities. These combined processes lead to deformation induced a postglacial rebound for the whole of Iceland with large horizontal and vertical components. Horizontal that may be controlled by the geodynamic setting and motions are mainly induced by the divergence between the the rheological layering of the lithosphere. This study North American and European plates, trending at N110°E with a half spreading rate of 0.9 cm/a [DeMets et al., 1994]. is devoted to (1) understanding the Holocene rebound However, single rifting events generate higher strain rates on the southwestern coast and (2) estimating the that may reach tens of centimeters to meters per year asthenosphere viscosity and depth beneath Iceland. [Gudmundsson et al., 1999; Zobin, 1999; Dauteuil et al., Two stages of holocene evolution were determined by 2001; Bellou et al., 2005]. Whereas the origin of the means of GPS profiles, morphological observations, horizontal motions is well established, vertical motions and data compilation. The first stage corresponds to a result from multiple processes [Dauteuil et al., 2005], such vertical uplift of 67.5 to 157.5 m. It started at 10,000 as glacio-isostasy, regional tilting, transform zones, hot spot years BP and ended at 8500 years BP implying uplift doming, deep intrusion or volcanism. Each process has its rates between 4.5 and 10.5 cm/a. It was a quick own characteristic wavelength (meters to hundreds of kilo- isostatic response to the fast ice retreat. The second meters), rate (mm/a to m/a) and displacement amount (cm to stage had vertical motion of tens of meters with a hundreds of meters). The present analysis on the postglacial rebound, which occurred after the retreat of the last Weich- probable tectonic origin and started at 8500 years BP. selian ice cap on Iceland, was the major aim of this study. The uplift rate is 1 to 2 orders of magnitude slower This rebound was poorly studied because it was assumed to than the one during the first stage. Uplift partitioning have a small vertical component compared to the tectonic during the first stage was controlled by the thermal and magmatic processes. state of the lithosphere, the highest geothermal flux [3] The Icelandic plateau underwent several glaciations inducing the maximum uplift rates. The relaxation during the Quaternary. According to Einarsson and Albertsson time for uplift provides a viscosity estimate of 5.4– [1988], 15–23 glaciations affected Iceland during the past 5.8 Â 1019 Pa s for the asthenosphere. This value is three million years, including the last one, the Weichselian similar to those determined for glacial areas in glaciation ended at 9000 years BP. During a glacial stage, different continental contexts. However, the flexural the ice sheet increases the vertical loading of the lithosphere wavelength indicates a shallower asthenosphere than and leads to a subsidence of the bedrock surface, controlled by both the lithosphere elasticity and the asthenosphere that occurring in continental domains. Therefore this viscosity [Stewart et al., 2000]. A rough estimate shows that study highlights a coupling between the thermal the island subsides 300 m under an ice sheet 1000 m thick. structure of the Icelandic asthenosphere and the During the deglaciation, the retreat of the ice cap unloads glacial rebound. Citation: Biessy, G., O. Dauteuil, B. Van the basement and induces an uplift of Iceland with a Vliet-Lanoe¨, and A. Wayolle (2008), Fast and partitioned complex pattern: the quick decrease of the vertical stresses generates an elastic rebound with basement uplift, faulting and seismicity. In such a context, the horizontal stresses are gradually relaxed by the viscoelastic return flow of the mantle material [Stewart et al., 2000]. Furthermore, the high geothermal gradient due to the geodynamic pattern of Ice- 1Ge´osciences Rennes, UMR 6118 CNRS, Universite´ de Rennes 1, land induces a thin lithosphere [Bourgeois, 2000] and a Rennes, France. 2 mantle viscosity at shallow depths that is smaller than in a Processus et Bilan des Domaines Se´dimentaires, UMR 8110 CNRS, Universite´ de Lille 1, Villeneuve d’Ascq, France. normal continental context. This raises a question: does the specific Icelandic context emphasize the glacial rebound? Copyright 2008 by the American Geophysical Union. While horizontal deformation has been widely studied in 0278-7407/08/2007TC002177 TC3002 1of18 TC3002 BIESSY ET AL.: POST-GLACIAL REBOUND IN ICELAND TC3002 Figure 1. Geological setting of Iceland [Johannesson and Saemundsson, 1998]. Iceland lies at the junction between the Reykjanes Ridge in the southwest, the Kolbeinsey Ridge in the north, and a hot spot, whose apex is located under the Vatnajo¨kull ice cap. Current tectono-volcanic activity occurs in the Neovolcanic Zone, composed of three main segments, the Northern (NVZ), Western (WVZ), and Eastern (EVZ) Volcanic Zones. The Snaefellsjo¨kull peninsula (SnVFZ), Ho¨fsjo¨kull (HFVZ) and O¨ raefajo¨kull- Snaefell (OSnVFZ) flanck zones are also active. SISZ, South Iceland Seismic Zone; TFZ, Tjo¨rnes Fracture Zone. Iceland, the vertical motions were not so intensively stud- due to the plume [Kaban et al., 2002]. In Iceland, there is a ied. The purpose of this paper is to determine uplift rates on consensus that crustal thickness varies from about 40 km the southwestern coast of Iceland and its spatial variations. under Vatnajo¨kull to less than 20 km under the northern part To determine the rebound pattern, we focused on (1) field of the North Volcanic Zone and the Reykjanes Peninsula work based on a detailed mapping of coastal surfaces with [Menke et al., 1998; Darbyshire et al., 2000; Kaban et al., high-resolution GPS, (2) analysis of geological and geo- 2002; Foulger et al., 2003]. The extensional processes are morphologic data, and (3) estimate of vertical motions controlled by rift jumps that are largely emphasized by including eustatic variations since the Last Glacial Maxi- magma supply [Helgason, 1984, 1985; Garcia et al., 2003] mum. These uplift rates allowed us to determine a mean and a succession of volcanic roll-overs [Bourgeois et al., viscosity for the Icelandic asthenosphere. 2005]. [5] The build-up of the Icelandic plateau began 25 Ma ago. The present direction of divergence between the North 2. Geological Setting American and Eurasian plates is N110°E with a half 2.1. Geodynamic Framework spreading rate of 0.9 cm/a [DeMets et al., 1994]. Nowadays the crustal accretion connecting the ridges of Reykjanes and [4] The Iceland Plateau and the Greenland-Faeroe Ridge Kolbeinsey crosses Iceland from the southwest to the north are conspicuous bathymetric features in the NE Atlantic inside the Neovolcanic Zone. This area covered by inter- Ocean (Figure 1). These shallow areas have an anomalously glacial to subglacial volcanic formations younger than thick oceanic crust resulting from a high magmatic supply 2of18 TC3002 BIESSY ET AL.: POST-GLACIAL REBOUND IN ICELAND TC3002 Figure 2. Models of processes generating vertical motions in Iceland (modified from Dauteuil et al. [2005]). See text for further explanations. 800 ka is subdivided into three rift systems: the Western result from subglacial volcanic eruptions or ruptured ice- Volcanic Zone (WVZ) that extends from the Reykjanes dammed lakes (joku¨lhlaup). Thus, they are located down- peninsula to the Langjo¨kull glacier, the Eastern Volcanic stream of ice caps, often in the vicinity of an active volcanic Zone (EVZ) from the Vestmann Islands to the Vatnajo¨kull zone. A strandflat is a coastal platform common to mid to glacier, and the Northern Volcanic Zone (NVZ) from the high latitudes and of variable extent [Van Vliet-Lanoe¨, Vatnajo¨kull glacier to the northern coast of Iceland (Figure 1). 2005]. These surfaces are quite extensive along the northern The external zone located on both sides of the Neovolcanic Atlantic [Guilcher et al., 1994]. The processes responsible Zone is made up of basalt lava flows emplaced from 16 Ma for their formation are still debated: marine abrasion, glacial to 800 ka [Saemundsson, 1978, 1979].