CRETACEOUS DEFORMATION ALONG THE SHOO FLY-FEATHER RIVER BELT FAULT – EVIDENCE FROM U-PB DATING OF FOLDED DIKES ALONG THE MIDDLE FORK OF THE AMERICAN RIVER, CALIFORNIA

The Feather River Belt (FRB) defines a Jurassic-age tectonic suture that separates outboard terranes of proto-Pacific oceanic basement and island arc affinity from continental-derived rocks of the Shoo Fly Complex. Renewed movement along the fault that separates the FRB and the lower Shoo Fly is indicated by 125 Ma ⁴⁰Ar/³⁹Ar plateau ages on sericite-rich pods that define steep, down-dip stretching lineations (Carlson and others, 1997) and by deformed dikes of Early Cretaceous age that cut the folded Shoo Fly Complex.

The folded Shoo Fly Complex is cut by a NNE-striking swarm of nearly vertical felsic dikes that define up to 20% extension in the hanging wall of the Shoo Fly-FRB fault. The dikes are cut by the fault and locally deformed into tight folds and boudins. Most of the dikes do not show evidence of deformation but may exhibit open flexures and bends that represent relays formed during emplacement. The felsic dikes have been characterized as lamprophyres by Chandra (1961) and Redmond (1966) and are hydrothermally altered by Mother Lode fluids. Near the Shoo Fly-FRB fault, the dike swarm is cut by flat-lying quartz veins that are locally rich in crystalline gold.

Determining unambiguous magmatic ages of the felsic dikes is difficult because of the inheritance of Proterozoic zircons that complicate the U-Pb system. Zircons from a tightly folded dike and an undeformed dike containing a curved relay were analyzed by LA-ICP-MS. Both show evidence of assimilation of sedimentary material from the Shoo Fly Complex. The undeformed dike has zircon ages that range from 2763 to 1958 Ma (n=18) with no evidence for younger magmatic zircon growth. The deformed dike also contains inherited Proterozoic zircons, but preserves some younger magmatic ages. 14 analyses on 4 grains from this sample yield a weighted ²⁰⁶Pb/²³⁸U age of 113.3 ± 6.7 Ma. The higher error results from two grains with ages of ~109 Ma and the other two grains with ages of ~116 Ma.

The deformed ~113 Ma dike and the 125 Ma stretching lineations indicate that deformation was partitioned inboard along the Shoo Fly-FRB fault during the Early Cretaceous.