Fission track data

Apatite fission tracks are immediately annealed at temperatures $>\sim$110$^o$C (e.g., Wagner and Van den Haute, 1992). At temperatures less than about 60$^o$C, apatite fission tracks are completely preserved. The temperature zone between $\sim$60 and $\sim$110$^o$C is named the apatite fission track partial annealing zone (e.g., Dumitru, 2000). In this zone, fission tracks are not immediately annealed, but gradually shortened with time. The annealing temperature of zircon fission tracks is more controversial, but generally considered to lie between 230 and 310$^o$C (see discussion by Tagami and Dumitru, 1996). In this paper, we will assume the more ``conservative'' value of $\sim$230$^o$C.

Figure: Apatite fission track radial plots (Galbraith, 1990) of the Joaquin Ridge. The gray bands represent depositional ages. The histograms show the fission track length distributions. n = number of grains, f = largest population fraction of older/younger grains that are p=5% likely to have been missed, N = number of confined tracks.

First, we will discuss the apatite fission track data (Figure 2). Of the five Great Valley Group samples, four have exclusively Cretaceous apatite fission track grain-ages, indicating that these grains never reached temperatures greater than 110$^o$C since their deposition in the Great Valley Group. However, sample JR2 has the oldest depositional age of these four samples but the youngest fission track ages, with the latter being even slightly younger than the former. Therefore, JR2 has been partially reset, and saw temperatures less than $\sim$110$^o$C, but well above $\sim$60$^o$C. The fission tracks of sample JR2 are also significantly shorter than those of the other samples, an additional suggestion that JR2 must have been heated to well within the partial annealing zone. Sample JR1, located the nearest to the New Idria serpentite, has completely annealed apatite fission tracks and, therefore, was heated above $\sim$ 110$^o$C. It dates the end of the heating event at $\sim$14 Ma. Sample JR6 from the Miocene Temblor Formation contains two age components: one Cretaceous and one Miocene component (Table 2). There also is a hint of bimodality in the fission track length distribution. A first group of relatively short ($\sim$9-13 $\mu$m) fission tracks formed prior to the mid-Miocene. These tracks preserve Sierran provenance ages but were partially annealed during the mid-Miocene thermal event. A second group of long fission tracks ($\sim$13-17 $\mu$m) formed after this thermal event, and have not been annealed since then. Paleocurrent directions in the Temblor and Big Blue Formations are west-to-east, which is the opposite flow direction as for the Great Valley Group (Casey and Dickinson, 1976; Bate, 1985; Bent, 1985). Therefore, the apatite grains of the Temblor Formation have been redeposited from the underlying Great Valley Group, some of which was thermally annealed during a mid-Miocene thermal event.

Figure: Zircon fission track radial plots. As in Figure 2, the light gray bands represent depositional ages. The dark gray bands mark the crystallization ages, as measured by DeGraaff-Surpless et al. (2002) using the U/Pb method on zircon. n and f as in Figure 2.

The zircon fission track ages for four of the five samples are older than the age of Great Valley Group deposition (Figure 3). Sample JR1, which had completely annealed apatite fission tracks, also has unreset zircon fission track ages. Therefore, sample JR1 was heated to more than $\sim$110$^o$C, but less than $\sim$230$^o$C after its deposition. The lag between crystallization, exhumation and deposition times were short, which means that the source area of these sediments exhumed rapidly. The most surprising observation is that the Middle Miocene sample JR6, which had a bimodal apatite fission track age distribution, also has a bimodal zircon fission track distribution. The oldest mode of Mesozoic ages is compatible with the unreset fission track ages of the Joaquin Ridge samples located away from the serpentinite body (Table 1). The youngest age peak is concordant with the $\sim$14 Ma apatite fission track age of sample JR1, and with the youngest mode of the apatite fission track age distribution of JR6. Because not all the apatite grains in JR6 were reset at $\sim$14 Ma, we know that this sample was not heated to more than $\sim$110$^o$C. In fact, there is ample evidence that the Temblor formation did not see temperatures higher than $\sim$56$^o$C east of Joaquin Ridge (see below). Therefore, the $\sim$14 Ma old zircons must have been been annealed prior to deposition in the Temblor and Big Blue formations. Recalling the eastward paleocurrents of these deposits, this indicates that at least part of the provenance area for the Temblor Formation, which is Joaquin Ridge, reached temperatures as high as $\sim$230$^o$C as recently as $\sim$14 Ma.