Further paleomagnetic evidence for oroclinal rotation in the central folded Appalachians from the Bloomsburg and the Mauch Chunk Formations
Renewed paleomagnetic investigations of red beds of the Upper Silurian Bloomsburg and the Lower Carboniferous Mauch Chunk Formations were undertaken with the objective of obtaining evidence regarding the possibility of oroclinal bending as contributing to the arcuate structural trend of the Pennsylvania salient. These formations crop out on both limbs of the salient and earlier, but less definitive paleomagnetic studies on these units indicate that early acquired magnetizations can be recovered. Oriented samples were obtained from nine sites on the southern limb of the salient and eight sites from the northern limb in the Bloomsburg. The natural remanent magnetizations are multivectorial, dominated by a component (B) with a distributed spectrum of unblocking temperatures ranging up to 670°C, and a component (C) with a higher and very discrete distribution of unblocking temperatures. The B component is uniformly of reverse polarity, shows a statistically significant synfolding character, and represents a Late Paleozoic remagnetization. The C component passes fold tests with normal and reverse polarity site means. The C component directions from the southern limb (345.1°/-31.6°) and the northern limb (359.3°/-29.7°) are significantly different in declination (14.2°±10.4°) but not in inclination (1.9°±9°). Samples were also analyzed from seven additional sites in the Mauch Chunk on the southern limb of the salient. Inclusion of these new data gives a revised estimate of the difference between southern and northern limb mean directions of prefolding magnetizations in the Mauch Chunk of 23.3°±12.5 in declination and 4.8°±11° in inclination. Paleomagnetic data from the Bloomsburg, Mauch Chunk, and revised results recently reported for the Upper Devonian Catskill Formation together indicate 22.8°±11.9° of relative rotation, accounting for approximately half the present change in structural trend around the Pennsylvania salient. The oroclinal rotation can be regarded as a tightening of a less arcuate depositional package that developed across a basement reentrant, to achieve a curvature closer to that of the earlier zigzag continental margin outline.
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