The Eastern belt of the Sierra Nevada comprises an Ordovician(?) to Devonian(?) succession of psammites and pelites belonging to the Shoo Fly Complex, and is overlain by three Paleozoic to Mesozoic arc volcanic sequences. The northern part of the belt, the subject of this chapter, is divided into a series of discrete blocks by steeply dipping faults, considered to be eastward-directed thrusts. The metamorphic history of this region has been little investigated previously. It has been argued that low-grade metamorphism of the Eastern belt is a Nevadan orogenic effect; in contrast, it has also been suggested that metamorphism of the arc volcanic rocks was a result of burial effects in the arc environment. In this study the metamorphic grade of the area has been established using mineral assemblages in metabasites and pelites, combined with illite crystallinity and b 0 data from pelitic rocks. The Shoo Fly Complex underwent epizonal metamorphism under Barrovian-type conditions prior to the earliest arc volcanism. Metamorphic grade in the overlying arc volcanic rocks ranges from pumpellyite-actinolite facies in the strongly foliated rocks of the (westernmost) Butt Valley and Hough blocks, through prehnite-pumpellyite facies in the Keddie Ridge and Genesee blocks, to low anchizone to diagenetic grade in Jurassic rocks of the (easternmost) Mt. Jura and Kettle Rock blocks. There is evidence for at least three discrete regional metamorphic events in these arc rocks; one is interpreted as being related to the burial of the arc volcanic rocks, which reached prehnite-pumpellyite facies; this event was followed by deformation and pumpellyite-actinolite facies metamorphism during the Nevadan orogeny; a final episode of static, low-grade metamorphism, possibly due to tectonic loading effects, probably also resulted in pumpellyite-actinolite facies. Subsequently, rocks exposed in the extreme east of the region were affected by contact metamorphism during the emplacement of Sierra Nevada batholith granitoids.

Detailed vitrinite reflectance and illite crystallinity paleothermal data from a portion of the Eocene-Oligocene Shimanto accretionary prism in southwest Japan show that: (1) peak paleotemperatures range from ∼200°C (diagenetic zone) in the northern part of the study area to ∼300°C (anchizone) 10 km to the south. Preliminary b o data from illite (Underwood and others, this volume) suggest that these rocks were buried no more than 10 km. This in turn implies that the Eocene-Oligocene Shimanto Belt experienced a relatively high paleogeothermal gradient of at least 30 to 40°C/km. (2) Shales with the best-developed pressure solution cleavages also record the highest temperatures suggesting that cleavage formation and peak heating were coeval. Structural differences with younger rocks and K-Ar dates on cleavage from correlative rocks in neighboring Ashizuri Peninsula that range from 43 to 18 Ma, with most falling in the range 34 to 26 Ma, suggest that major penetrative deformation of these rocks occurred in the middle to late Oligocene. Peak heating may have commenced in the late Eocene to earliest Oligocene when plate reconstructions suggest subduction of young crust (<10 m.y. old) of the fused Kula/Pacific Plate. An elevated paleogeothermal gradient may also have been maintained during the period of cleavage formation (34 to 26 Ma) by the decrease in subduction rates documented for the middle Oligocene. (3) Significant late Miocene or younger block-faulting, uplift, and flexure of the study area postdated the attainment of peak temperatures and substantially modified patterns of peak paleotemperatures. The most important displacement occurred along the subvertical contact fault between the Eocene-Oligocene (4%R m ) and Oligocene-Miocene (1.5%R m ) Shimanto Belts.