SUMMARY AND CONCLUSIONS

Figure: The history of Joaquin Ridge sediments summarized on a single fission track radial plot, compiled from all the apatite fission track data.

Figure 7 summarizes the history of the Great Valley Group near Coalinga and New Idria:

• The U/Pb, zircon and apatite fission track ages of unreset apatite and zircon show that the southern Sierra Nevada underwent rapid and steady exhumation during the Late Cretaceous. As the exhuming mountain range was eroded, it shed sediments in the Great Valley forearc basin. These sediments are presently exposed in a homocline on the eastern flank of the Diablo Range.
• The Great Valley Group turbidites were buried to great depths, but under refrigerated thermal gradients, produced by Franciscan subduction (Dumitru, 1988).
• During the Middle to Late Miocene ($\sim$12-14 Ma), the Mendocino triple junction migrated from south to north to the west of Coalinga (Johnson and O'Neil, 1984). Its passage was associated with minor igneous activity, dated at $\sim$12.8 Ma (Obradovich et al., 2000).
• The faulting and folding caused by the San Andreas fault introduced fluids in the ophiolitic crust that underlies the Great Valley Group. These fluids reacted with the peridotitic rocks and formed serpentine minerals. Because of its low density and the relative ease with which it deformed, the serpentinite body rose rapidly. The combined effect of the folding and the rise of the serpentinite diapir caused substantial uplift of Joaquin Ridge, and secondary folding and faulting in the adjacent Vallecitos Syncline (Rentschler, 1985). Paleocurrent directions reversed, and re-sedimentation of Great Valley Group detritus formed the Middle Miocene Temblor and younger formations. The Big Blue came to the surface as a rising hot protrusion, heating the Great Valley Group on the way up and spreading at once over the countryside.
• Because it formed at great depth or because the serpentinization reactions are exothermic, the New Idria serpentinite body was hot when it approached the surface, forming a thermal halo. The heating was sufficient to completely anneal apatites in the surrounding country rock and explain the relatively high vitrinite reflectance values that are observed near the contact between the Great Valley Group and the serpentinite body.
• The heat released by the serpentinite produced diagenetic alteration of cristobalitic to quartzose silica in the Moreno Shale, which underlies the Vallecitos syncline (Murata et al., 1979), pushing the source rocks of this basin into the thermal oil window.
• Alternatively, it is possible that syenitic intrusions in the serpentinite body provided the ``missing'' heat source. However, we think that the serpentinite body itself is a more realistic alternative because it is size rather than temperature that determines the amount of time it takes for a hot body to cool, and the size of the resulting thermal halo. A small igneous intrusion may have been hotter than the serpentinite protrusion, but it would not be felt as long, and as far into the country rock.
• Deformation and folding continue in the Joaquin Ridge area today, witnessed by the 1983 Coalinga earthquake. However, gravity data suggesting that the root of the New Idria body is only a few kilometers deep, imply that the exhumation of the serpentinite diapir has almost stopped (Byerly, 1966; Casey and Dickinson, 1976).