Gagnagrunur fyrir íslenskar bergsegulmælingar (ICEPMAG v1.0)

Iceland's lavas preserve a unique record of Earth's magnetic field for the past sixteen million years, and were used by early pioneers of palaeomagnetism to test several concepts which became crucial to modern geoscience. Iceland represents one of very few high latitude (>60°) locations where long sequences of lavas suitable for palaeomagnetic research are accessible. Since the early 1950s, research in Iceland has produced a large collection of palaeomagnetic data which has not previously been collected into a comprehensive database. ICEPMAG (http://icepmag.org/) compiles palaeomagnetic data published in journal articles, academic theses and other databases from over 9,200 sampling sites in Iceland - one of the world's largest collections of palaeomagnetic data from a single location. ICEPMAG was constructed utilising the principles and structure of GEOMAGIA50, and maintains the vocabulary of MagIC to allow easy transfer to the global palaeomagnetic database. The ICEPMAG database can be searched through a publicly available website which provides a range of customisable constraints, including rock and sample/specimen types, age constraints, dating methods, palaeointensity methods, geographic constraints (by region, location and between specified coordinates), authors and years of publication, as well as statistical constraints such as directional polarity, alpha95 and precision parameter, k. Query results are presented in a results table, with options to produce interactive maps of site locations, VGP plots and downloadable spreadsheets. All the entries in ICEPMAG contain palaeointensity or palaeodirectional data: 8649 contain direction only, 218 intensity only, and 337 both direction and intensity. ICEPMAG will contribute an additional 50 studies and 6,570 sites to the global MagIC database.Íslensk hraunlög varðveita einstaka sögu segulsviðs jarðar síðustu sextán milljón ár, sem var notuð af frumkvöðlum fornsegulsviðsfræða til prófunar á tilgátum sem urðu mikilvægar fyrir nútíma jarðvísindi. Ísland er einn af fáum stöðum á háum breiddargráðum (>60° N) þar sem opnur eru í langar hraunlagasyrpur, hentugar til rannsókna á fornsegulsviði. Síðan snemma á sjötta áratug síðustu aldar hafa rannsóknir á Íslandi skilað miklu magni af segulsviðsgögnum sem hefur hingað til ekki verið safnað saman í gagnasafn. Í gagnasafninu ICEPMAG hafa verið tekin saman gögn varðandi fornsegulsviðið úr útgefnum ritum, prófritgerðum við háskóla og öðrum gagnasöfnum frá yfir 9200 sýnatökustöðum á Íslandi – eitt af stærstu söfnum fornsegulsviðs-gagna frá einum stað. ICEPMAG var búinn til með aðferðum og uppbyggingu GEOMAGIA50 og notar íðorðaforða MagIC til þess að auðvelda flutning yfir í alþjóðagagnasafn fornsegulsviðsmælinga. Hægt er að leita í ICEPMAG í gegnum opna vefsíðu sem býður upp á fjölbreytta sérhæfða leitarmöguleika, meðal annars berg og sýnagerðir, aldur, aldursgreiningaraðferðir, aðferðir við mælingu á styrk fornsegulsviðsins, landfræðilega legu (eftir svæðum, staðsetningu, og milli tiltekinna hnita), höfunda og útgáfuár, sem og tölfræðilegra þátta eins og segulstefnu, alpha95, og nákvæmnistuðul, k. Umbeðnar niðurstöður birtast í töflu, sem býður upp á möguleikann á gagnvirku korti af sýnatökustað, VGP gröf og töflu sem hægt er að hala niður. Fyrir alla sýnatökustaðina í ICEPMAG gagnasafninu hefur verið ákvörðuð annað hvort stefna eða styrkur segulmögnunarinnar: Fyrir 8649 sýnatökustaði hefur einungis segulstefna verið ákvörðuð, 218 einungis styrkur og fyrir 337 sýnatökustaði hvort tveggja. ICEPMAG mun bæta við 50 rannsóknum og 6570 mælistöðum við alþjóðlega MagIC gagnasafnið

Middle Neoproterozoic (Tonian) Polar Wander of South China: Paleomagnetism and ID‐TIMS U‐Pb Geochronology of the Laoshanya Formation

Abstract Paleomagnetic records of middle Neoproterozoic (820 to 780 Ma) rocks display high amplitude directional variations that lead to large discrepancies in paleogeographic reconstructions. Hypotheses to explain these data include rapid true polar wander (TPW), a geomagnetic field geometry that deviates from a predominantly axial dipole field, a hyper‐reversing field (&gt;10 reversals/Ma), and/or undiagnosed remagnetization. To test these hypotheses, we collected 1,057 oriented cores over a 85 m stratigraphic succession in the Laoshanya Formation (Yangjiaping, Hunan, China). High precision U‐Pb dating of two intercalated tuff layers constrain the age of the sediments between 809 and 804 Ma. Thermal demagnetization isolates three magnetization components residing in hematite which are not time‐progressive but conflated throughout the section. All samples possess a north and downward directed component in geographic coordinates at temperatures up to 660°C that is ascribed to a Cretaceous overprint. Two components isolated above 660°C reveal distinct directional clusters: one is interpreted as a depositional remanence, while the other appears to be the result of a mid‐Paleozoic (460 to 420 Ma) remagnetization, which is likely widespread throughout South China. The high‐temperature directions are subtly dependent on lithology; microscopic and rock magnetic analyses identify multiple generations of hematite that vary in concentration and distinguish the magnetization components. A comparison with other middle Neoproterozoic paleomagnetic studies in the region indicates that the sudden changes in paleomagnetic directions, used elsewhere to support the rapid TPW hypothesis (ca. 805 Ma), are better explained by mixtures of primary and remagnetized components, and/or vertical axis rotations.</p