INTRODUCTION

The topography and geology of central California are dominated by petrotectonic elements of the Mesozoic convergent margin: the Sierra Nevada magmatic arc, the Great Valley forearc basin, and the Franciscan accretionary prism. These three domains are genetically linked, inasmuch as they were jointly formed by Pacific plate subduction beginning in the Late Jurassic (Dickinson et al., 1996), until the transformation of the subduction zone into a transform margin during the Late Cenozoic (Atwater, 1970). The upper Mesozoic strata of the Great Valley Group (or Sequence: Bailey et al., 1964) filled the forearc basin, and comprise one of the thickest sequences of Cretaceous sediments known (Ingersoll, 1982). These Great Valley Group strata crop out in a homocline along the western margin of California's Central Valley, and are in fault contact with the Franciscan accretionary complex (Dickinson et al., 1996).

Figure: Simplified geologic map of the Great Valley Group near Coalinga with the sample locations. Modified from Bate (1985).

This paper focuses on samples of the Great Valley Group collected in and around Joaquin Ridge near Coalinga (Figure 1). These sedimentary rocks contain geochronological and petrographic information about the evolution of their source region, the Mesozoic Sierra Nevada magmatic arc. They also contain clues about their post-depositional history, and the tectonic evolution of the Diablo Range in which they are now exposed. Conventional petrographic studies of Great Valley Group rocks indicate that, with time, the lithic fraction of the sediments decreased, the percentage of total feldspar increased and the composition of the feldspars became more potassic. These trends reflect increasing chemical and geomorphic maturity of the magmatic arc (Ingersoll, 1979, 1983). Paleocurrents are dominantly west-directed, indicating that the source was the southern Sierra Nevada (Ingersoll, 1979). In addition to conventional petrographic studies, samples in the vicinity of Coalinga were studied by bulk-rock $\epsilon_{Nd}$ and $\epsilon_{Sr}$ analysis (Linn et al., 1991, 1992), confirming the petrographic results. Between Cenomanian and Maastrichtian time, $\epsilon_{Nd}$ systematically decreased from -0.7 to -5.0 (Linn et al., 1991), reflecting an eastward migration of the drainage divide, to where the Sierran magmas are more continental in composition (De Paolo, 1981). Some of the samples of Linn et al. (1991) were used by DeGraaff-Surpless et al. (2002) for SHRIMP single grain zircon U/Pb age measurements, which confirmed that the sediment source for the Great Valley Group in the San Joaquin Valley is the southern Sierra Nevada, as the most frequently observed ages are 102-132 Ma. The spread of the Mesozoic ages and the number of peaks in the grain-age histograms increase with decreasing depositional age. This may reflect an expansion of fluvial drainage basins, resulting in a larger sediment source area. The minimum lag-time between the age of zircon crystallization and deposition (determined by paleontology) is short (3-15 Ma). This constrains the Late Cretaceous exhumation rate of the Sierra Nevada (Surpless, 2001) - constraints that this paper will attempt to refine.

Nevertheless, considerable controversy exists about the exhumation history of the Sierra Nevada batholith. Some workers, based on the observation of uplifted and tilted Cenozoic strata, believe that the development of Sierran topography occurred in the last 10 My (e.g., Unruh, 1991). Others claim that significant topography existed since at least the Early Cenozoic based on old (U-Th)/He cooling ages of the Sierra Nevada granites, and the fact that the spatial distribution of these ages preserves the signature of old topography (House et al., 1998, 2001). Both opinions are based upon data acquired from rocks of the modern Sierra Nevada. They seldom make use of the continuous record of sediments shed from the ancient mountain range since the Late Jurassic. These sediments are excellent ``witnesses'' of the evolution of the Sierra Nevada, for much of the batholith and its volcanic carapace have long since been removed.

Detritus from the Sierra Nevada was deposited in the Great Valley forearc basin. Above these strata was deposited a gradually shoaling-upward succession of Cenozoic deposits (Dibblee, 1971; Bartow, 1991). A total thickness of 6000-8000m of the Upper Jurassic through Cretaceous Great Valley Group is exposed in a homocline on the eastern flanks of the Diablo Range (Dickinson, 2002). Superimposed on this homocline are folds formed as a result of right-lateral transpressional strike-slip deformation along the San Andreas fault (Miller, 1998). The Vallecitos syncline is one of these synclinal folds (Figure 1). Cenozoic strata contain petroleum, although they are buried less than 1500m deep in the syncline (Rentschler, 1985). The heat source for oil generation is enigmatic. Joaquin Ridge forms the anticline adjacent to the Vallecitos syncline. The New Idria serpentinite body is exposed in the core of this anticline. A few small intrusions of syenite crop out in the serpentinite body, and are dated at $\sim$12.8 Ma (no error stated) by $^{40}$Ar/$^{39}$Ar dating on the amphibole barkevikite (Obradovich et al., 2000). Rb-Sr dating of benitoite yielded an age of $\sim$ 12 Ma (no error stated) (Obradovich et al., 2000). The Mendocino triple junction passed the latitude of New Idria at $\sim$12-14 Ma (Johnson and O'Neill, 1984). $\sim$14 Ma is also the age of the spectacular deposits of the Big Blue Formation, which consist almost solely of sedimentary serpentinite (Casey and Dickinson, 1976; Bate, 1985).

In the following sections, new fission track and vitrinite reflectance data are presented, followed by a discussion of implications of these data. To reconstruct the pre-depositional history of sediments, they must not be thermally reset. Therefore, the post-depositional history of the New Idria area will be discussed first. Among other conclusions we demonstrate that, until the rapid exhumation of the serpentinite dome, this area was characterized by low thermal gradients that prevented the buried sediments from being heated very much. The discussion of the pre-depositional history then follows. This paper illustrates the great power of simultaneously using multiple thermochronometers on detrital sediments. Each individual thermochronometer only tells part of the story, but the whole is greater than the sum of the parts. When all evidence is jointly considered, a self-consistent story emerges that traces the sediments from their crystallization in the Cretaceous Sierra Nevada until the final stages of their exhumation on Joaquin Ridge. This story not only has consequences for the regional geology of the Coalinga/New Idria area, but also for the tectonic history of the Sierra Nevada and the petroleum geology of the Vallecitos syncline.