Tectonic Framework and Geologic History (North Atlan!C and Iceland)

Tectonic Framework and Geologic History (North Atlanc and Iceland) Sarah Slotznick Enrichment Trip 2014 Ingibjörg Kaldal Iceland Geosurvey Present Day North Atlanc 1, | 5 5a*L~~~~~~I | ____I____-__ I____ I_____ I____I____I_ WIN 58II I 30.W 25 W Fig. 2 (left). The location of Reykjanes Ridge, southwest of Iceland, and the area of Fig. 3. The 1000-fathom submarine contour is shown, together with the 500-fathom contours for Rockall Bank. Fig. 3 (right). Summary diagram of the magnetic anomalies observed over Reykjanes Ridge (see Fig. 2). Straight lines indicate the axis of the ridge and the central positive anomaly (17). only additional parameter being the (21) in reconstructing the fit of the Ridge (half width, 600 km) is ap- rate of spreading; the scale (Fig. 4) continents around the Atlantic. In this proximately 1 centimeter per annum has recently received striking independ- instance the deep to the southeast of -that is, the rate of "drifting" is ap- ent confirmation from the work of Rockall may represent an initial abor- proximately 2 centimeters per annum. on January 8, 2012 Opdyke et al. (20) on deep-sea sedi- tive split; the oceanic area to the mentary cores. northwest, centered on Reykjanes Ridge, a subsequent and more persistent site Other Ridges of spreading of the ocean floor. There Reykjanes Ridge is every indication from the existing The model proposed by Vine and bathymetry (17) that the ridge crest Matthews (6) and developed by Vine Observed anomaly profiles obtained is linear and not interrupted or off- and Wilson (8) has been applied to during four crossings of the crest of set transverse four areas on by fractures. widely separated the www.sciencemag.org Reykjanes Ridge are compared (Fig. 5) This area therefore, 1200 kilometers midoceanic ridge system (Figs. 6-9) 1, | 5 5a*L~~~~~~I | ____I____-__ I____ I_____ I____I____I_ WIN 58II I 30.W 25 W with simulations obtained by assump- in width, may well record a compara- by assumption of the reversal time Fig. 2 (left). The location of Reykjanes Ridge, southwest of Iceland,tionandof thereversalarea oftimeFig.scalesVine 1966 3. Thefor1000-fathomthe last submarinetively simplecontourand straightforwardVogt 1986 ex- scale shown in Fig. 4 and a rate is shown, together with the 500-fathom contours for Rockall Bank.4 millionFig.years3 (right).and a rateSummaryof spreadingdiagram ampleof theofmagneticdrifting and spreading. The of spreading compatible with the width anomalies observed over Reykjanes Ridge (see Fig. 2). Straight lines indicate the axis of the ridge and the central positive anomaly (17). of 1 centimeter per annum for each oldest rocks in the Thulean or Brito- of the central anomaly. An observed limb of the ridge. The model assumed is Arctic Tertiary Igneous province oc- profile across Juan de Fuca Ridge, analogous to the one I have described cur in northwestern Scotland and east- southwest of Vancouver Island (Fig. only additional parameter being the (21) in reconstructing(8,themodelfit of2),thebut Ridgethe depths(half width,have 600ern Greenland.km) is ap-Preliminary potassium- 1) (8, 19), is compared (Fig. 6) with Downloaded from rate of spreading; the scale (Fig. 4) continents around thebeenAtlantic.madeIncompatiblethis proximatelywith the 1depthcentimeterargon perdatesannumfrom Arran, Mull, and a simulated profile based on a rate has recently received striking independ- instance the deep to tothethesoutheastridge crestof in -thatthis areais, andthe withrate of other"drifting"centersis ap-in the British Isles sug- of spreading of 2.9 centimeters per ent confirmation from the work of Rockall may representthean altitudeinitial abor-at whichproximatelythe survey2 centimeterswas gest anperageannum.of approximately on January 8, 2012 60 inil- annum per limb of the spreading sys- Opdyke et al. (20) on deep-sea sedi- tive split; the oceanicflown.areaIn toperformancethe of the survey, lion years (perhaps slightly greater) tem. A profile across the East Pacific mentary cores. northwest, centered on 58Reykjanesparallel Ridge,courses were flown normal (22). If it is assumed that this igneous Rise, just north of the Eltanin Frac- a subsequent and moreto thepersistentridge axis,site butOtherthe crestRidgeswas not activity indicates the initiation of drift ture Zone (23), is compared (Fig. 7) of spreading of the oceantraversedfloor. byTherethe first four and last in this area, then the implied average with a computed profile based on a Reykjanes Ridge is every indication fromfive courses:the existingthus crossingsThe model15, 25, proposed35, ratebyofVinespreadingand from Reykjanes rate of spreading of 4.4 centimeters bathymetry (17) that andthe45ridgeare showncrest asMatthewsbeing representa-(6) and developed by Vine Observed anomaly profiles obtained is linear and not interruptedtive. The orcorrelationoff- andbetweenWilsonthe(8)ob-has been applied to Jaramillo Olduval during four crossings of the crest of set transverse four areas on by fractures. widely separated the www.sciencemag.org served and computed anomalies is very BRUNHES I 4- GAUSS Reykjanes Ridge are compared (Fig. 5) area system This therefore,encouraging1200 kilometersand suggestsmidoceanica rateridgeof NORMAL(Figs.FIELD6-9) ZZA W] VA rz //////-- ---- -r i i A A with simulations obtained by assump- in width, may well recordspreadinga compara-of rather bylessassumptionthan 1 centi-of the reversal time N I r//, 1 ----. tion of reversal time scales for the last tively simple and straightforwardmeter per annum.ex- scale shown in Fig. 4 K-Arand AGEa rate0 1I0 2-0 3-0 4.0 M. YRS 4 million years and a rate of spreading ample of drifting and spreading.When oneTheappliesof spreadingthe conceptcompatibleof with the width REVERSED FIELD | / of 1 centimeter per annum for each oldest rocks in the Thuleancontinentalor Brito-drift to thisof theregion,centralit seemsanomaly. An observed ////r A /Z - limb of the ridge. The model assumed is Arctic Tertiary Igneousreasonableprovincetooc-assumeprofilethatacrossRockallJuan de Fuca Ridge, < MATUYAMA > | eGILBERT analogous to the one I have described cur in northwestern ScotlandBank, southeastand east- of southwestthe ridge (Fig.of Vancouver2), Island (Fig. Mammoth (8, model 2), but the depths have ern Greenland. Preliminaryis a continentalpotassium- fragment,1) (8, 19),as wasis comparedas- Fig.(Fig.4. Geomagnetic-polarity6) with epochsDownloaded from deduced from paleomagnetic results and potassium- been made compatible with the depth argon dates from Arran,sumed Mull,by Bullard,and Everett,a simulatedand Smithprofile basedargon ondating.a rate[Based on Cox, Doell, and Dalrymple, and Doell and Dalrymple (7)1 to the ridge crest in this area and with other centers in the British16 DECEMBERIsles sug-1966 of spreading of 2.9 centimeters per 1407 the altitude at which the survey was gest an age of approximately 60 inil- annum per limb of the spreading sys- flown. In performance of the survey, lion years (perhaps slightly greater) tem. A profile across the East Pacific 58 parallel courses were flown normal (22). If it is assumed that this igneous Rise, just north of the Eltanin Frac- to the ridge axis, but the crest was not activity indicates the initiation of drift ture Zone (23), is compared (Fig. 7) traversed by the first four and last in this area, then the implied average with a computed profile based on a five courses: thus crossings 15, 25, 35, rate of spreading from Reykjanes rate of spreading of 4.4 centimeters and 45 are shown as being representa- tive. The correlation between the ob- Jaramillo Olduval served and computed anomalies is very BRUNHES I 4- GAUSS encouraging and suggests a rate of NORMAL FIELD ZZA W] VA rz //////-- ---- -r i i A A spreading of rather less than 1 centi- N I r//, 1 ----. meter per annum. K-Ar AGE 0 1I0 2-0 3-0 4.0 M. YRS When one applies the concept of REVERSED FIELD | continental drift to this region, it seems ////r A //Z - reasonable to assume that Rockall < MATUYAMA > | eGILBERT Bank, southeast of the ridge (Fig. 2), Mammoth is a continental fragment, as was as- Fig. 4. Geomagnetic-polarity epochs deduced from paleomagnetic results and potassium- sumed by Bullard, Everett, and Smith argon dating. [Based on Cox, Doell, and Dalrymple, and Doell and Dalrymple (7)1 16 DECEMBER 1966 1407 T.H. Torsvik et al. / Earth and Planetary Science Letters 291 (2010) 106–112 107 we describe a single important example, namely the calculation of net Geochem. Geophys. Geosyst.). In Section 3 we explore NR, not only for lithosphere rotation (NR). If mantle convection is the principal driving present times but for the past 150 Ma. mechanism for plate motions, NR should be zero unless individual lithospheric plates have different couplings to the underlying mantle 2. Global plate polygons ﬂow. A proper reference frame with appropriate NR is important for discussions of poloidal/toroidal partitioning of plate motions (Lith- Building global plate polygons through Earth history (Fig. 1; gow-Bertelloni et al. 1993). Most plate models predict westward drift Supplementary data) requires knowledge of relative plate motions of the lithosphere with respect to the deep mantle, which has been from both continental and oceanic areas. The uncertainty in ascribed to lateral viscosity variations (Ricard et al. 1991; O'Connell constraining these motions increases for older times, due to the et al., 1991). Westward drift estimates vary considerably (1.5–9 cm/ destruction (through subduction) or distortion (such as collision) of year) and are usually larger than those calculated from geodynamic relative motion. For example, more than half of the oceanic crust models (Becker 2006). However, comparison of westward drift created since the Jurassic has been consumed by subduction, therefore estimates with geodynamic models is problematic, since all geody- past plate boundary conﬁguration has to be restored by making namic models are based on simplifying assumptions. Recently, seismic assumptions based on limited geological constraints (like the age of anisotropy has emerged as a further tool to estimate NR for recent preserved ophiolites or slab-window related volcanism) and the rules times (Becker 2008;Plate Reconstrucons Kreemer 2009; Conrad and Behn, submitted of plate tectonics.

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