Dynamics of volcanic systems in Iceland: example of tectonism and volcanism at juxtaposed hot spot and mid-ocean ridge systems

Dynamics of volcanic systems in Iceland: example of tectonism and volcanism at juxtaposed hot spot and mid-ocean ridge systems
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  See discussions, stats, and author profiles for this publication at: Dynamics of Volcanic Systems in Iceland:Example of Tectonism and Volcanism at Juxtaposed Hot Spot and Mid-Ocean...  Article   in  Annual Review of Earth and Planetary Sciences · August 2000 DOI: 10.1146/ CITATIONS 154 READS 193 1 author:Some of the authors of this publication are also working on these related projects: Rock Fractures in Geological Processes   View projectNew methods for forecasting the permeability of fractured reservoirs   View projectAgust GudmundssonRoyal Holloway, University of London 339   PUBLICATIONS   7,047   CITATIONS   SEE PROFILE All content following this page was uploaded by Agust Gudmundsson on 12 January 2017. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  Annu. Rev. Earth Planet. Sci. 2000. 28:107–40Copyright  2000 by Annual Reviews. All rights reserved 0084–6597/00/0515–0107$14.00  107 D YNAMICS OF  V OLCANIC  S YSTEMS IN I CELAND :  Example of Tectonism and Volcanismat Juxtaposed Hot Spot and Mid-OceanRidge Systems Agust Gudmundsson Geological Institute, University of Bergen, Allegaten 41, N-5007 Bergen, Norway,e-mail: Key Words  central volcano, magma chamber, mantle plume, crustal structure,seismicity Abstract  Volcanicsystemsareswarmsoftectonicfracturesandbasaltvolcanoesformed as a result of plate-pull (as the plates are pulled apart) associated with themid-ocean ridges and the magma dynamics of the Iceland Mantle Plume. Most sys-tems are 40–150 km long, 5–20 km wide, and develop a central volcano. They supplymagma to all eruptions in Iceland. Data obtained in the last few years have greatlyimproved our knowledge of their volcanotectonic environment; as a result, the geom-etry of the plume is better constrained, and the crust, previously considered thin (  10km), is now modeled as thick (  20–40 km). Depending on the location of the vol-canic systems, their activity either decreases or increases faulting in the two mainseismic zones. From this, we can infer that emplacement of the feeder-dike to thelargest historical eruption in Iceland (that of Laki in 1783) increased shear stress inthe South Iceland Seismic Zone and almost certainly triggered the largest (M  7.1 in1784) historical earthquake in Iceland. INTRODUCTION Iceland owes its existence to the mantle plume that supplies magmatoitsvolcanicsystems (Figure 1). Most of these systems are giant swarms of cracksthatdevelopwithin the volcanic zones in response to crustal spreading by plate pull. Theirdynamics are controlled partly by the plate dynamics of the North American andEurasian plates, and partly by the fluid dynamics of the Iceland Mantle Plume.Thus, they offer a fascinating opportunity to study the evolution of volcanotec-tonic systems that develop at a juxtaposed mantle plume and mid-ocean ridgesystem.Understanding of the volcanic systems of Iceland has improved greatly in thepast 20 years since the concept was introduced (Jakobsson 1979a, Saemundsson * An addendum was added to this review on 20 April 2010. Read the addendum online at *Addendum    A  n  n  u .   R  e  v .   E  a  r   t   h   P   l  a  n  e   t .   S  c   i .   2   0   0   0 .   2   8  :   1   0   7  -   1   4   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  a  n  n  u  a   l  r  e  v   i  e  w  s .  o  r  g   b  y   2   1   7 .   4   2 .   2   0   1 .   1   5  o  n   0   5   /   0   9   /   1   3 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .  108  GUDMUNDSSON Figure 1  Volcanic zones and systems in Iceland. The systems are: Tr  Theystareykir,Kr  Krafla, Fr  Fremri-Namur, As  Askja, Kv  Kverkfjoll, Th  Thordarhyrna,Gr  Grimsvotn, Ha  Hamarinn, Ba  Bardarbunga, Tu  Tungnafellsjokull, Hg  Hagongur, Ka   Katla, Ey   Eyjafjallajokull, Ve  Vestmannaeyjar, Ti  Tindfjalla- jokull, Va  Vatnafjoll, He  Hekla, Hj  Hofsjokull, Ke  Kerlingarfjoll, La1&2  Langjokull, He   Hengill. The other systems on the Reykjanes Peninsula (from east towest) are Brennisteinsfjoll, Trolladyngja, and Reykjanes. Off-coast are Eldey, Geirfuglas-ker, and Eldeyjarbodi. In the Snaefellsnes Volcanic Zone are the systems Sn  Snaefells- jokull, Ly    Lysuskard, and Lj    Ljosufjoll. Outside the East Volcanic Zone are thesystems Or  Oraefajokull, Es  Esjufjoll, and Sn  Snaefell. The rift zone (its axis isindicated in Figure 3) comprises the North Volcanic Zone, the West Volcanic Zone, andthe East Volcanic Zone to the south tips of the volcanic systems of Bardarbunga andGrimsvotn. Also indicated are the main ocean-ridge discontinuities, the Husavik-FlateyFault of the Tjornes Fracture Zone, and the South Iceland Seismic Zone (SISZ), locatedbetween the overlapping West and East Volcanic Zones. Data are from Jakobsson (1979a),Saemundsson (1979), AT Gudmundsson & Kjartansson (1996), and Johannesson & Sae-mundsson (1998). 1979). Not only have we obtained a wealth of new data on their structure, geo-chemistry, and dynamics, but the volcanotectonic environment in which theydevelop is now much better understood. During these two decades, extensivestudies have been done involving active and inactive volcanic systems, accuratemeasurements of local and regional plate movements, and geochemical and geo-physical exploration of the Iceland Mantle Plume.Most references in this review define the current state of knowledge of thedynamics of volcanic systems in Iceland and their environment. A few references    A  n  n  u .   R  e  v .   E  a  r   t   h   P   l  a  n  e   t .   S  c   i .   2   0   0   0 .   2   8  :   1   0   7  -   1   4   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  a  n  n  u  a   l  r  e  v   i  e  w  s .  o  r  g   b  y   2   1   7 .   4   2 .   2   0   1 .   1   5  o  n   0   5   /   0   9   /   1   3 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .  VOLCANIC SYSTEMS  109 guide the reader to topics that, although important and connected to the dynamicsof the volcanic systems, are not of central concern in this paper. Many citationsrefer to papers published in the last few years, but I have also included referencesto several older papers of historic importance. Although this is primarily a reviewpaper, I decided to present a number of suggestions, some of which are new, soas to provide a consistent overall picture of the volcanic systems in Iceland andtheir interaction with the mantle plume and the ocean-ridge discontinuities.Manyof these suggestions will doubtless have to be refined and improved in the future.I begin with a review of the volcanotectonic environment in Iceland and howthe volcanic systems interact with it. This part summarizes relevant knowledgeof the plate boundaries and plate movements in Iceland and its mantle plume, aswell as models on locking and unlocking of the main seismic zones by activityin the volcanic systems. Next, I summarize the systems’ infrastructure, focusingon the rift-zone systems. These summaries form a basis for understanding thedynamic evolution of the volcanic systems, the subject of the third main part of the paper. In that part, I focus on the general evolution of the systems—frombirth, through maturity, to death—and the effects that the formation of a shallowmagma chamber has on their subsequent development. PLATE BOUNDARIES AND MOVEMENTS Iceland is located at the junction between the Kolbeinsey Ridge in the north andthe Reykjanes Ridge in the south (Figures 1 and 2). The Kolbeinsey Ridge trendsroughly N10  E, subperpendicular to the spreading vector. Its seismic structure,essentially uniform, indicates a crustal thickness exceeding that of a normal mid-ocean crust by 1–1.5 km (Kodaira et al 1998). In contrast totheKolbeinseyRidge,the Reykjanes Ridge, trending roughly N36  E, is an oblique-spreading ridge. Itsneovolcanic zone consists of axial volcanic ridges, each of which trends subper-pendicular to the spreading vector, with an average spacing of 14 km and anoverlap of roughly one third of their lengths in an overall en echelon arrangement(Searle et al 1998).The surface expression of the mid-ocean ridge in Iceland is the zone of activevolcanism: the neovolcanic zone. This zone, covered with rocks of the Bruhnesmagnetic epoch (  0.8 Ma), corresponds to the magnetic plate boundaries of themid-ocean ridges, not to their neovolcanic zones which, by definition(Macdonald1982), are of Holocene age. The neovolcanic zone has three main segments: theNorth Volcanic Zone, the West Volcanic Zone and the East Volcanic Zone (Figure1). The Snaefellsnes Volcanic Zone, an old Tertiary volcanic zone reactivated at2 Ma, is propagating to the southeast, whereas the southern part of the EastVolcanic Zone is propagating to the southwest.In addition to rift zones and propagating rifts, the plate boundaries in Icelandinclude a transform fault and an overlapping spreading center (Figure 1). InNorthIceland the Tjornes Fracture Zone, which is partly exposed on land, is a transform    A  n  n  u .   R  e  v .   E  a  r   t   h   P   l  a  n  e   t .   S  c   i .   2   0   0   0 .   2   8  :   1   0   7  -   1   4   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  a  n  n  u  a   l  r  e  v   i  e  w  s .  o  r  g   b  y   2   1   7 .   4   2 .   2   0   1 .   1   5  o  n   0   5   /   0   9   /   1   3 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .  110  GUDMUNDSSON Figure 2  Dike injections in the volcanic systems can temporarily lock (suppress earth-quakes) or unlock (trigger earthquakes) in the main seismic zones in Iceland, the Husavik-Flatey Fault, and the South Iceland Seismic Zone. Static magma overpressure in a regionaldike can reach 10–50 MPa, resulting in a high horizontal compressive stress (cf Figures3 and 4). Dike injection in the parts of the volcanic zones marked by crosses would tendto suppress, whereas dike injection in the parts marked by parallel ticks would tend totrigger, earthquakes in the seismic zones fault connecting the plate boundary of the Kolbeinsey Ridge with that of theNorth Volcanic Zone. The Fracture Zone’s main section is the Husavik-FlateyFault, a major dextral strike-slip fault that correspondstothetransform-tectonizedzone of a typical oceanic transform fault. The South Iceland Seismic Zone (SIZS)is a zone of complex faulting located between the overlapping parts of the WestVolcanic Zone and the East Volcanic Zone (Figure 1).The magnitude and direction of plate movements in the Iceland area are nowwell known (DeMetz et al 1990, 1994). The size and direction of the spreadingvector varies slightly from the north to the southern part of the country: At theTjornes Fracture Zone the total spreading (opening) rate is 1.79 cm/year in thedirection N106  E, but 1.85 cm/year in the direction N104  E at the South IcelandSeismic Zone (DeMets et al 1990, 1994; Sigmundsson et al 1995; Jonsson et al 1997; Vadon&Sigmundsson1997).For Icelandasawhole,theaveragespreadingrate is 1.8 cm/year in the direction N105  E.    A  n  n  u .   R  e  v .   E  a  r   t   h   P   l  a  n  e   t .   S  c   i .   2   0   0   0 .   2   8  :   1   0   7  -   1   4   0 .   D  o  w  n   l  o  a   d  e   d   f  r  o  m   w  w  w .  a  n  n  u  a   l  r  e  v   i  e  w  s .  o  r  g   b  y   2   1   7 .   4   2 .   2   0   1 .   1   5  o  n   0   5   /   0   9   /   1   3 .   F  o  r  p  e  r  s  o  n  a   l  u  s  e  o  n   l  y .
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