18 Tectonic Processes in the Circum-Atlantic Orogens II: A Celebration of the Career of Rob Strachan

Saturday, 29 March 2025
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NEW PALEOLATITUDE CONSTRAINT FOR AVALONIA AND LAURENTIA AT 415 MA FROM THE KNOYDART FORMATION RED BEDS, ANTIGONISH HIGHLANDS, NOVA SCOTIA

Paper number: 18-2
Program: Theme Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

Paleomagnetic results to constrain global paleogeography are sparse for the Devonian Period, which is proposed to have had significant apparent polar wander. To help constrain the paleoposition of Laurentia, we report preliminary paleomagnetic results from the Early Devonian Knoydart Formation in the Avalonian terrane Antigonish highlands, Nova Scotia, Canada.

Twelve paleomagnetic sites were block sampled from the lower stratigraphic part of the Knoydart Formation at its iconic locality along the stream bed of McAras Brook. Sampled lithology consists of gently southwest-dipping lavender, red and green siltstone, mudstone in sub-decimetre to massive metre-thick sandstone beds, likely representing a marine-influenced deltaic depositional setting. The Knoydart Formation bears bony fish index fossils including Pteraspis crouchi, which constrain the section age to Lochkovian (419.0 - 412.4 Ma).

Stepwise thermal demagnetization of specimens from the 76 block samples reveals hematite as the main remanence carrier, with minor magnetite in some beds. Hematite shows some remanence loss between 610-670° C, and sharp unblocking at 670-680° C, consistent with the behaviour of detrital hematite grains. Most sites show stable, ESE and up-directed characteristic remanence magnetizations (ChRMs) upon simple bedding tilt-correction, which we interpret as normal polarity. Samples from the stratigraphically uppermost two sites carry reversed polarity, WNW, down ChRMs. The presence of a polarity reversal in the sampled section, along with a positive conglomerate test from Knoydart clasts in the succeeding McAras Brook Formation suggest that the ChRM in the Knoydart Formation is primary, of Early Devonian (~415 Ma) age, placing Avalonia at ~27° S and constraining Laurentia to have straddled the equator at this time.

A low southern paleolatitude is consistent with existing Early Devonian paleomagnetic results from Laurentia, offering independent confirmation that Avalonia (Antigonish highlands) had arrived at Laurentia within paleomagnetic resolution by 415 Ma. The Knoydart ChRMs have a high declination anomaly that implies 90° CW vertical-axis rotation compared with the Laurentia reference frame, most likely due to Devonian local km-scale block rotation along the west edge of the Antigonish highlands.

Geological Society of America Abstracts with Programs. Vol. 57, No. 3

doi: 10.1130/abs/2025NE-408655

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