HR: 1340h
AN: U33A-0030 [Abstracts]
TI: The Position of Baltica During 700-450 Ma Ago: New Data from the Janisjarvi Impact Structure
AU: * Salminen, J
EM: johanna.m.salminen@helsinki.fi
AF: Laboratory for Solid Earth Geophysics, University of Helsinki, P.O.Box 64, Helsinki, 00014
Finland
AU: Masaitis, V L
EM: viktor-masaitis@vsegei.ru
AF: Karpinsky Geological Institute, Sredny prospect 74, St. Petersburg, 199106
Russian Federation
AU: Naumov, M
EM: mikhail-naumov@vsegei.ru
AF: Karpinsky Geological Institute, Sredny prospect 74, St. Petersburg, 199106
Russian Federation
AU: Deutsch, A
EM: deutsca@uni_muenster.de
AF: Institut fr Planetologie, University of Muenster, Wilhelm-Klemm-Str. 10, Mnster, 48149
Germany
AU: Pesonen, L J
EM: lauri.pesonen@helsinki.fi
AF: Laboratory for Solid Earth Geophysics, University of Helsinki, P.O.Box 64, Helsinki, 00014
Finland
AB:
Paleomagnetic reconstructions of continents are often hampered by the lack of dated poles for key intervals. Impact rocks can
occasionally provide a solution to the problem since they can be accurately dated and they often yield reliable
paleomagnetic data. We present new paleomagnetic and rock magnetic data for Baltica derived from the Janisjarvi impact
structure, Russian Karelia, the southeastern part of Fennoscandian Shield. Isotopic datings (40Ar-39Ar, 40K-40Ar) yield an
age $\sim$ 700 Ma for this impact event.
Some 50 oriented hand samples were collected from the Janisjarvi impact structure. The a.f. demagnetization is able to
isolate a hard and stable remanence component in the impactites (tagamites, suevites and breccias). The same component was
also identified in several Svecofennian ($\sim$ 1880 Ma) target rocks on and near the shoreline of Lake Janisjarvi. The
target rocks retained also a primary Svecofennian component thus providing a fully positive impact test. Demagnetization
data, coupled with rock magnetic results, suggest that the impact component, of dual polarity, is primary and either of
thermal or thermochemical origin. The remanent magnetization direction from 16 sites is D = 74\deg, I = 72\deg (alfa95 =
2.1\deg). This yields a pole position Plat. = 54\deg N, Plon. = 96\deg E (A95 = 3.5\deg) which places Baltica to 60S
paleolatitude at 700 Ma. When plotting the pole on the calibrated APWP of Baltica a magnetization age of 500-478 Ma is
obtained. Five possible reasons may explain the conflicting isotopic and paleomagnetic ages: 1. post-impact tilting of the
impactites, 2. remagnetization, 3. error in the isotopic ages, 4. error in the APWP, or 5. inaccurate pole. The Late
Precambrian to Lower Ordovician reconstructions of Baltica based on the new data from the Janisjarvi impact structure will be
discussed.
DE: 8155 Plate motions--general
DE: 1724 Ocean sciences
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1620 Climate dynamics (3309)
SC: Union [U]
MN: 2004 AGU Fall Meeting