HEAVY MINERAL PROVENANCE AND PALEOMAGNETIC STUDIES OF MESOZOIC ROCKS IN THE GOLD BEACH TERRANE

Evolution of the western North American Cordillera during the Mesozoic included the accretion of terranes, some of which may have experienced large-scale northward translation as described in the Baja-BC hypothesis. We investigate evidence for northward translation through the analysis of detrital minerals gathered from Late Jurassic – Late Cretaceous sandstones in the Gold Beach terrane of southwestern Oregon. Mesozoic zircon isotope and trace element geochemistry mimic those of titanite, recording a transition in magma chemistry over time from juvenile to evolved crustal signatures. Zircon Hf isotopes in Jurassic grains (200-144 Ma) yield positive ε_Hf(i) values (+15 to +6), while Late Cretaceous grains (100-90 Ma) have a wide range of values spanning 20 epsilon units (+11 to -12) and titanite Nd isotopes show comparable patterns (+8 to +1 and +5 to -9, respectively). Monazites include a prominent Early Cretaceous age population and rutiles yield a unique Permian population not well-recorded by zircons. Detrital garnets include a Mn-rich population of probable magmatic origin and a Ca-rich population derived from calc-silicates and/or skarn deposits. The variety of data presented here suggests a northwestern Mexico to southern California provenance.

Paleomagnetic results from the Late Cretaceous Houstenaden Creek Formation pass both fold and reversals tests, with a tilt-corrected mean of D = 76.4⁰, I = 57.3⁰, N=18, k=18, α₉₅=8.7⁰. The directions have a widely streaked distribution of directions indicating local rotations (Liner, 2003). The tilt corrected, inclination-only mean is I = 55⁰, and yields a Late Cretaceous paleolatitude of 36^o N. Comparing these results with a Late Cretaceous reference for North America indicates an expected paleolatitude of 48⁰ N, resulting in an estimate of 1500 +/- 900 km of displacement. E/I corrections of inclination error are suspect due to the widely variable rotation – 5^o of inclination error reduces the translation to 900 +/- 1000 km. These data are in agreement with the paleogeographic correlation derived from the heavy mineral provenance data, thus supporting northward translation of the Gold Beach terrane from northwestern Mexico during the Late Cretaceous, to southwestern Oregon by Eocene time.