CONTRASTING SILURIAN-DEVONIAN RELATIVE PLATE MOTION IN THE CALEDONIDES: SINISTRAL TRANSPRESSION TO SINISTRAL TRANSTENSION

The late Silurian to mid or late Devonian interval in the Caledonides was a period dominated, sequentially, by sinistral transpression, strike-slip and transtension during the development of mainly non-marine "red-bed" basins following the Ludlow/Pridoli transition from marine to terrestrial sedimentation. The tectonic event that led to and generated the sinistral Devonian basins was the highly-oblique closure of the Iapetus Ocean between Laurentia and Baltica and between Laurentia and Avalonia. We examine the diachronous closure of Iapetus, the contrasting tectonic modes arising from that closure, and the nature and origin of subsequent Devonian deformation north and south of the Iapetus Suture in terms of progressively-changing, sinistrally-dominated relative plate motion between Laurentia and Avalonia/Baltica. We suggest that, from about 435 to 395 Ma, there was about 1200 km of sinistral strike-slip relative motion between Laurentia and Baltica. Our lower and upper estimates of Silurian/Devonian relative plate motion rates of 30 mma-1 and 67 mma-1 based upon geological data are similar to present rates.