Morphology and tectonics of Icelandic rifts western branch

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Abstract

Iceland is a unique example of a place, where rift zone of Mid-Atlantic Ridge appears onshore. Its morphological and tectonic features considerably differ from typical mid-oceanic ridge rift zones. The morphology and geodynamics of Icelandic rift western branch are formed by Iceland plume thermal influence that generated the North Atlantic Large Igneous Province. Icelandic rift western branch is characterized by ceasing tectonic and magmatic activity. Overlapping with the Eastern Rift Zone it forms rotating block of Hreppar Microplate that leads to tectono-magmatic activity decline northwards. Based on morphometric analysis of normal faults, the relative activity degree of individual parts of volcanic systems was revealed. For some parts, the activity changes in late Quaternary were traced. Obtained inferences demonstrate explicit differences in contemporary tectonic structure and dynamics of the rift zones and volcanic systems within them. For instance, transtensive Reykjanes Rift Zone, the southernmost one, has decreasing eastwards tectono-magmatic activity, which is connected with influence decrease of Reykjanes Ridge adjoining from the south-west. Its gradual southward shifting is observed that is explained by similar southward propagation of the most active Eastern Rift Zone and by the formation of new transtensive zone aggregating contemporary Reykjanes Rift Zone and South-Iceland Seismic Zone. In contrast, the Western Rift Zone develops independently from Reykjanes Rift Zone. It has the largest extension center in the area of Thingvallavatn Lake. In its northern part as within the Central Rift Zone, Holocene tectono-magmatic activity is very faint and is linked to glacioisostatic reactivation of more ancient structures. Revealed structural heterogeneities are traced in rift zone morphology as well. For example, within Western and Central Rift Zones, well-developed shield volcanoes are common. They consist of hyaloclasts predominantly. Within fissure swarms, individual lava shields are observed. In contrast, Reykjanes Rift Zone is characterized by absence of topographically expressed central volcanoes, and within fissure swarms, the chains of volcanic cones are present.

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V. A. Bogoliubskii

Lomonosov Moscow State University

Author for correspondence.
Email: bogolubskiyv@yandex.ru

The Earth Science Museum; Faculty of Geology

Russian Federation, Leninskiye Gory, 1, Moscow, 119991

E. P. Dubinin

Lomonosov Moscow State University

Email: edubinin08@rambler.ru

The Earth Science Museum; Faculty of Geology; Faculty of Geography

Russian Federation, Leninskiye Gory, 1, Moscow, 119991

A. A. Lukashov

Lomonosov Moscow State University

Email: smoluk@yandex.ru

Faculty of Geography

Russian Federation, Leninskiye Gory, 1, Moscow, 119991

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2. Fig. 1. Location of Icelandic rift zones and adjacent spreading ridges. 1 – volcanic spreading segments, 2 – amagmatic structures of extension and shear. The study area is highlighted in red. Abbreviations: RZ – rift zone, TZ – transform zone, SIZ – South Iceland Seismic Zone, MP – microplate.

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3. Fig. 2. Relief of the rift zones of southwestern Iceland (photo by A. Lukashov). a – the rift tract of Thingvellir on the Exarau River, disturbed by a series of longitudinal grabens (Heingill ES, Western RES); b – a lava flow with a roof partially intact from collapse, 1 km to the east of the Thrihnukagigur mountain (Brennysteinsfjell ES, Reykjanes RES); c – the “Valley of Smokes” of Reykjadalur, abundant in thermal springs, 3 km to the north of the Hveragerdi mountain (“Geyser Garden”), at the southeastern foot of the Heingill volcano.

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4. Fig. 3. Example of fault scarp parameter extraction for the Reykjanes RZ section based on the ArcticDEM DEM [Porter et al., 2018]. a – fragment of the shaded relief raster (azimuth – 100°, altitude – 45°): examples of length (D) and Euclidean distance (distance between fault scarps) (Ep) parameters are shown, the blue rectangle shows the profiles in Fig. 3b; 1 – scarps with dipping eastern rhumbs, 2 – with dipping western rhumbs; b – variations in the parameters of horizontal (Ga), vertical (Ba) amplitude, longitudinal curvature (K) and steepness. The red lines and dots indicate the boundaries of the fault scarp on the profile, determined by the automated method. The green dot indicates the extracted values ​​of the Pk parameter.

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5. Fig. 4. Location of profiles through volcanic systems of the western branch of the Icelandic rifts. 1 – east-dipping normal fault scarps, 2 – west-dipping normal fault scarps, 3 – profile positions, 4 – volcanic system boundaries, 5 – strike-slip zone, 6 – glaciers. DEM and volcanic system boundary data [Special…, 2019].

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6. Fig. 5. Changes in the parameters of normal fault scarps of the western branch of the Icelandic rifts. a – total horizontal amplitude, b – total vertical amplitude, c – arithmetic mean of the length, d – arithmetic mean of the distance between fault scarps, d – arithmetic mean of the maximum modulus of longitudinal curvature. Normal fault scarps: Reykjanes RB: 1 – Reykjanes, 2 – Krysuvik, 3 – Brennisteinsfjell, 4 – Heingill; Western RB: 5 – Prestahnukur, 6 – Langjökull; Central RB: 7 – Hofsjökull, 8 – Tungnafellsjökull (a – western-dipping scarps, b – eastern-dipping scarps).

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7. Fig. 5. Continuation.

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8. Fig. 6. Distribution of parameter values ​​by profiles. a – horizontal amplitude. Total amplitude values ​​(m): 1 – >500, 2 – 400‒500, 3 – 300‒400, 4 – 200‒300, 5 – 100‒200, 6 – 50‒100, 7 – < 50; 8 – boundaries of volcanic systems, 9 – shear zone, 10 – glaciers; b – maximum modulus of longitudinal curvature. Average values: 1 – <2, 2 – 2‒4, 3 – 4‒6, 4 – 6‒8, 5 – 8‒10, 6 – 10‒12, 7 – >12; 8-10 – similar to A. DEM and data on the boundaries of volcanic systems [Special…, 2019].

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9. Fig. 7. Tectonic map of the western branch of the Icelandic rifts. Structures: central volcanoes: 1 - stratovolcanoes, 2 - hyaloclastic edifices of shield volcanoes, 3 - rhyolite domes; volcanic edifices of fissure families: 4 - active large lava domes, 5 - inactive lava domes, 6 - active volcanic apparatuses (craters, cinder cones); lava flows: 7 - Holocene lava flows, 8 - pre-Holocene lavas, including those covered by sediments; hyaloclastic edifices: 9 - late Quaternary hyaloclastic edifices, 10 - late Quaternary hyaloclastic domes covered by lavas, 11 - early-middle Quaternary and pre-Quaternary hyaloclastic edifices; others: 12 - geothermal fields. Volcanic systems: central volcanoes: 13 – embryonic, 14 – highly active, 15 – low active, 16 – inactive; volcanic activity of fissure families: 17 – high, 18 – moderate, 19 – low, 20 – absent; tectonic activity of fissure families: 21 – high, 22 – moderate, 23 – low. Other designations: 25 – glaciers, 26 – rift zone boundaries. DEM and data on volcanic system boundaries [Special…, 2019].

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