PALEOMAGNETISM SURVEY OF THE NORTH PARK SYNCLINE (NORTHERN COLORADO): EVALUATION OF TIMING OF VOLCANISM AND FOLDING

The North Park syncline is a post-Laramide, NW gently plunging syncline within the North Park-Middle Park Basin of which timing and mechanism of uplift are unknown. The syncline’s stratigraphy involves volcanic members within the North Park Formation overlying the White River Formation. These volcanic members originate from the Braddock Peak volcanic complex and include older dacite-rhyodacite flows that have been dated by ⁴⁰Ar/³⁹Ar as 29.0 +/- 0.4 Ma and the younger Thunder Mountain rhyolite ash flow tuff dated as 28.06 +/- 0.06 Ma.

We drilled paleomagnetism samples from 8 sites around the syncline within the volcanic members of the North Park Formation to test whether the syncline formed before or after deposition of the beds. One hypothesis was that the beds were deposited first and then deformed by the syncline. The other hypothesis was that the bedding dips were primary, caused by deposition in a preexisting paleovalley guided by earlier formation of the syncline. The Thunder Mountain tuff is known to have been deposited in paleovalleys elsewhere.

Two sites on the north limb that dip 50° S in the middle and upper Thunder Mountain tuff both carry a remnant magnetization (RM) with northerly declination and shallow positive inclination. In contrast, two sites on the south limb that dip 6° and 28° NNE carry steeply inclined positive RM directions. Correcting bedding to horizontal brings these sites together for a positive fold test with a resulting normal-polarity mean direction of Dec. = 10.7°, Inc. = 58.1° (α₉₅ = 5.2°). This test proves that all syncline limb dip is tectonic and occurred after 28 Ma. Four sites from dacite-rhyodacite lavas that underlie the tuff all carry reversed-polarity RMs and thus erupted in an earlier geomagnetic polarity chron, consistent with the 29.0 Ma age. For the two flow sites from the southern fold limb, the tilt corrected RM directions are far from a typical time-averaged reverse-polarity direction, suggesting that these lava flows may have erupted during a short-lived geomagnetic field excursion.

Further constraints on timing of uplift of post-Laramide structures are important to understand how older structures were modified and how the landscape changed during the late Oligocene – early Miocene.