RECONSTRUCTING THE WESTERN MARGIN OF PRECAMBRIAN LAURENTIA IN THE PACIFIC NORTHWEST: REMOVING THE EFFECTS OF LATE CRETACEOUS-PALEOGENE ROTATION

The rifted Precambrian margin of western North America is hypothesized to consist of a series of ~330-oriented rift-segments and ~060-oriented transform segments. One difficulty is that in Idaho, the abrupt boundary between cratonic North America and the accreted terranes – denoted by the 87Sr/86Sri = 0.706 isopleth – is oriented ~NS along the western edge of the Idaho batholith and EW in northern Idaho and eastern Washington. Three lines of evidence suggest that the rift/transform system in Idaho also had an original 330/060 orientation, but was rotated ~30° by clockwise, vertical-axis rotation during the Late Cretaceous-Paleogene. First, ~90 Ma plutons along the EW segment in northern Idaho show paleomagnetic evidence for ~30° clockwise, vertical axis rotation. Second, geochemical trends in the Idaho batholith – determined using zircon xenocrysts in the Idaho batholith – are rotated about ~30° from their trend to the northeast. Third, shear wave splitting underneath the Idaho batholith is oriented ~EW, rotated ~30° clockwise from its regional trend to the east and west. Thus, we propose that the “dog leg” of the Sri = 0.706 isopleth in the Syringa embayment reflects the original Precambrian rift-transform geometry, rotated 30 degrees from its original orientation; the rift-transform intersection occurs near Orofino, ID. This geometry requires SW-directed extension for Precambrian rifting. The rotation is accommodated by sinistral transpression on the shear zone along Lewis & Clark line, and the timing of sinistral deformation on that zone (~88-55 Ma) is consistent with our constraints on the timing of rotation. In a separate study, new detrital zircon data from the western Owyhee Mountains in SW Idaho suggests Precambrian Laurentia extends to almost the Oregon border in that location. These data provides a geological constraint on the southernmost point of the rotating block. By reconstructing the Cretaceous-Paleogene deformation and rotation, we update the Precambrian rifting geometry for this section of the western Laurentian margin.