Multicomponent magnetizations from the Mississippian Mauch Chunk
Formation of the central Appalachians and their tectonic implications
Previous paleomagnetic study of Mississippian red beds of the Mauch Chunk Formation from the central Appalachians reported shallow inclination directions. A positive fold test was obtained after thermal demagnetization, typically to only 550°C. The near-equatorial position for North America indicated by these results has been used to support the idea of a circa 15° latitudinal tectonic offset of the Acadia displaced terrain in the northern Appalachians. The present study of 153 samples from 29 sites occupied in the Mauch Chunk Formation of eastern and southern Pennsylvania reveals multicomponent magnetizations. A characteristic component (Declination (D) = 161.0°, Inclination (I)=27.8°, alpha95 (a95)=7.9°) with unblocking temperatures usually concentrated above 650°C and with dual polarity (10 sites normal, 13 sites reversed) is isolated in 23 sites and passes a fold test at the 99% confidence level. It is interpreted as an early acquired hematitic (detrital or chemical) magnetization of pre-Alleghanian orogeny and pre-Kiaman Reverse Polarity Interval age; a southern hemisphere paleolatitude of 15° for the sampling area is indicated. Another component has generally lower unblocking temperatures which, however, can extend up to 650°C or more and therefore is also likely to be carried by hematite. This secondary magnetization (D=170.6°, I=-5.3°, a95=5.0°) isolated in 25 sites is of uniform reverse polarity, and application of fold tests suggests that it is of synfolding origin; a near-equatorial paleolatitude (3°N) at the time of acquisition is obtained, consistent with Permo-Carboniferous Kiaman directions from North America. Incomplete removal of the secondary component can explain the previous results from the Mauch Chunk. Comparison of the present paleomagnetic results with those from similar age rocks in the Canadian Maritimes does not support a latitudinal offset of Acadia with respect to interior North America in the Early Carboniferous. In fact, according to these and other paleomagnetic data thought to be representative of this age, the Atlantic-bordering continents already were near to a Pangea configuration. The Alleghanian and particularly the Hercynian orogenies therefore may not be associated with the closure of a large (latitudinal) ocean later in the Carboniferous and Permian. Alleghanian deformation, however may have resulted in the partial bending of the Pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations.
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Also Published In
- Journal of Geophysical Research