Mafic Terranes of Northern Alaska
Much of my work has focused on the geochemistry and tectonics of the Angayucham Large Igneous Province (LIP) in Northern/Interior Alaska. The Angayucham LIP plays a central role in the assembly of Alaska, but its origins, age, and tectonic history are poorly constrained. This sequence of mafic-ultramafic rocks is equivalent in total area to the country of Switzerland, but fewer than ten modern ICP-MS trace element analyses were published from this LIP. I am currently increasing the number of high-quality analyses by an order of magnitude, including detailed petrography, bulk geochemistry, mineral chemistry, and petrogenetic modeling. Based on my results and previous mapping, I found that portions of the LIP are an accreted oceanic arc/forearc, while the majority of the LIP has an N/E-MORB affinity. These portions seem to be genetically unrelated, and therefore the Angayucham LIP should be split into two separate LIPs. These results may help to resolve several outstanding issues regarding the assembly of Alaska and timing of orogenic events in the Arctic.
Oregon Coast Range Rotation
The Coast Range of Oregon is thought to have rotated ~60 degrees into its current position since the Eocene. This is largely based on paleomagnetic data gathered in the 1960s and 70s, which are not up to modern standards (no demagnetization was performed in many cases). I collected and analyzed ~600 paleomagnetic cores from the Eocene Tyee Formation of the Oregon Coast Range to test the rotation hypothesis. I was unable to reproduce the results of earlier studies, despite using more sensitive magnetometers and more modern demagnetization techniques. My results suggest that these sediments do not preserve a primary paleomagnetic direction and should not be relied upon for any tectonic interpretations.
Uplift of Steens Mountain
This is a tectonic geomorphology project focusing on Steens Mountain in southeastern Oregon. This large shield volcano was of comparable size to Mauna Loa, Hawaii, before being dissected by Basin and Range extension. The timing of this extension is not known, but has profound implications for the onset of Basin and Range faulting in the Pacific Northwest. This project uses a novel combination of structural geology, thermochronology, volcanology, paleomagnetism, and mapping to determine the age of the modern topography. Apatite fission track data from the base of Steens Mountain yield a pooled age of 10.5 +5.9/-3.8 (2s). These results indicate that these units remained at greater than 100℃ until c. 10 Ma. We also conducted a paleomagnetic fold-test on the 9.7 Ma Devine Canyon Tuff, which sits on the western flank of Steens Mountain. Results from this test indicate that the tuff was originally flat-lying and tilted by 5 degrees after deposition. Collectively, our results indicate that motion along the range-bounding normal fault began ~10.5 Ma but significant topographic relief was not present until after 9.7 Ma. These results provide a new constraint on the onset of Basin and Range extension in the Pacific Northwest.
Contact:
biasi@dartmouth.edu
Joe Biasi 