Jökull


Jökull - 01.01.2016, Page 89

Jökull - 01.01.2016, Page 89
The Kleifakot geomagnetic instability event in NW-Iceland profiles) it was not certain where in the time sequence they belong. The excursion zone is underlain by the normal-polarity zone DD 3–20 of Kristjánsson and Jóhannesson (1996). New sampling in Ísafjörður, 2014–2016 As a test of the persistence of Kleifakot-event se- quences in the lava pile, additional sampling for pa- leomagnetic studies was carried out in 2014-2016 in profiles on the coasts of Ísafjörður (Figure 2b). The main one was DW near the Álftagrófará stream (Fig- ure 3), with flow DW 0 at sea level. Also sampled were DQ along the old road across mt. Eyrarfjall, and DI at the Eyrargil gully, flows number 1 in all these profiles being just above the present road. Flow DI 2 could be followed northeast to the stream Laugará where it was sampled at road level as site DP 1. In this continuation, 6 to 8 core samples were initially drilled at each site and oriented in azimuth using sun shots, sightings on distant geographic ob- jects, and/or sightings on nearby objects with posi- tions known from GPS measurements. Additional samples were subsequently collected when deemed necessary, cf. the preceding section. The measure- ments were made in an Institut Dr. Förster static flux- gate magnetometer setup, as in previous projects. AF demagnetization was carried out in a Molspin tumbler demagnetizer at the peak-field steps listed above. Site-average results are shown in Table 1. Very similar remanence directions are sometimes found in some successive sites, such as DW 5 and 5A having visually different lithology. Sites DQ 0 and 1 which were sampled below and above the coastal road re- spectively, also have identical directions. Site DF 0 was cored at sea level 700 m northeast of DF 1, where sampled above the road in 1983. In the latter cases, these sites may each belong to one flow of over 20 m thickness. The directions obviously have excellent internal consistency, with all the α-angles except in DF 0 being smaller than 3.5◦. Values of the precision constant k mostly lie in the range 350–1100. All the samples collected are used in computing average re- manence intensities (after 10 mT AF treatment) which are generally low relative to the overall mean values of 3–4 Am−1 for lava collections in Iceland outside the volcanic zones (Kristjánsson, 2013, 2015). Table 1. Paleomagnetic results from Ísafjörður lava sites, 2014–2016. – Niðurstöður nýlegra bergsegul- mælinga á kjarnasýnum úr Ísafirði. SITE N n Dec Inc Lon Lat Alf J10 POL DW Álftagrófará – Ísafjörður East DW 0 15 11 156 +70 354 +31 2 0.97 NT DW 1 14 10 110 +18 46 +1 3 1.00 E DW 2 7 7 80 +14 74 +11 3 1.14 NT DW 3 Inaccessible DW 4 10 10 64 +41 82 +32 3 0.91 NT DW 5 13 6 204 -38 306 -43 3 0.62 R DW 5A 7 7 206 -40 304 -44 3 1.53 R DW 6 7 7 173 +31 344 -7 2 2.44 E DW 7 7 7 176 +42 341 +0 3 0.63 E DW 8 8 8 261 +62 276 +35 2 7.91 NT DW 9 6 6 32 +59 107 +59 3 1.19 N DQ Eyrarfjall road – Ísafjörður West DQ 0 7 7 69 -7 88 +5 3 1.33 E DQ 1 8 6 71 -3 86 +6 3 0.63 E DQ 2 7 5 233 -53 266 -45 3 1.07 R DQ 3 8 8 354 +49 166 +54 3 0.31 N DI Eyrargil – Ísafjörður West DI 1 7 7 73 -4 84 +5 2 1.07 E DI 2 6 6 239 -55 259 -44 3 0.52 R DI 3 7 7 4 +28 152 +39 2 0.75 NT DI 4 7 7 354 +63 169 +68 3 0.32 N DP Laugará – Ísafjörður West DP 1 8 7 226 -50 276 -45 2 1.05 R DF Kleifakotsmúli – Ísafjörður East DF 0 7 7 295 +44 236 +34 7 0.58 NT N = number of core samples collected. n = number of sam- ples used in averaging of remanence directions. Dec = dec- lination, inc = inclination, lon = VGP longitude, lat = VGP latitude, alf = 95% confidence angle (α95) for the average direction, all in degrees. J10 = mean remanence intensity (all samples) after demagnetization at 10 mT peak field, Am−1. Pol = polarity (N = normal, R = reverse, T = VGP latitude below 40◦, E = VGP latitude below 10◦). – n er fjöldi nothæfra sýna af þeim N sem safnað var. Dec, inc lýsa segulstefnunni, lon, lat og pol lýsa staðsetningu sýnd- ar-segulskauts, alf er 95% óvissuhorn segulstefnunnar, J10 er meðalstyrkur segulmögnunar sýnanna. JÖKULL No. 66, 2016 89
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