


Abstract: Arc
magmatism, denudation, and ensuing forearc accumulation of detritus are
inexorably linked convergent margin phenomena. We have developed
a model that simulates these processes to determine exhumation histories
of nascent crust through the analysis of detrital closure age distributions
measured from forearc strata. In formulating the approach, we have
regarded the deeply denuded Peninsular Ranges batholith and associated
forearc strata as representative products of this tectonic environment
and have employed results from them to scale spatial and temporal parameters
in our calculations. Detrital closure ages output by heat flow models
simulating coupled intrusion and erosion are sampled in a manner statistically
compatible with observation. Systematic comparison of measured and
synthetic closure age distributions to arrive at bestfit solutions is
facilitated by application of the KolmogorovSmirnov statistic. Provided
that denudation outlasts intrusion, we find that the method is highly sensitive
to the rate of exhumation and relatively indifferent to the history of
intrusion and the initial time of denudation once sufficient erosion has
occurred. Therefore, so long as forearc strata are representatives
of material removed from the denuding batholith, the method has vast potential
to decipher the evolution of juvenile arc crust.

Fig.1: (a) Location of northern Peninsular Ranges batholith (PRB). Inset shows distribution of batholith rocks, pattern of cooling ages [modified after Krummenacher et al., 1975], and erosion depth [modified after Gastil, 1979] throughout southern and Baja California. NSA (northern Santa Ana mountains) and LJPL (La Jolla/Point Loma) denote sample localities (b) Projection of available UPb zircon [Silver and Chappell, 1988; Walawender et al., 1990; and Kimbrough, unpublished.] and KAr biotite and Kfeldspar ages [Krummenacher et al., 1975; Grove, 1994, Grove,unpublished] into XX' in (a).  Fig.2: Stratigraphy of PRB forearc strata in (a) San Diego area and (b) northern Santa Ana Mountains. (c)(i) Histograms of measured detrital Kfeldspar closure age distributions. 
Fig. 3: Schematic of numerical model illustrating scaling and boundary conditions. Light gray shading indicates portion of grid where closure ages are calculated. Samples assigned to a given depositional age (Age_{D}) are from ±0.5 m.y. region. Bold line represents the final (65 Ma) erosion surface.  Fig. 4: Progressive thermal development of the batholith in Model II (Table 5). Net area intruded in each interval indicated in lower lefthand panel corners. (a)(f) Isothermal sections prior to the onset of denudation (115, 113, 111, 109, 107, and 105 Ma); (hk) Isothermal sections for times characterized by simultaneous intrusion and denudation (103, 101, 99, 97, and 95 Ma). Active erosion surfaces represented by bold lines; (l) isothermal distribution at 65 Ma. 
Fig. 5: Contours of emplacement age for final pluton distributions in (a) I1and (b) I2 (Table 3). (c) Mean intrusion age of plutonic rocks vs. horizontal distance. Values calculated for I1 and I2 represented by solid and dashed lines respectively. Superposed symbols represent UPb zircon age determined for PRB plutons (see Figure 1 legend).  Fig. 6: KolmogorovSmirnov (KS) statistical comparison of model and measured closure age distributions. (a) Variation of significance level of KS statistic (PROB) in comparing model and measured distributions for Mustang Spring sample. Depositional age and uncertainty for Mustang Spring sample indicated above. (b)(f) Comparison of model (light gray) and measured results (black) for Age_{D} = 87, 85, 83, 81, and 79 Ma. All data normalized to 100. 
Fig. 7: Model I results: (a) Contour plot of final f Kfeldspar bulk closure age distribution (10 Ma contour interval). Surface positions every 5 m.y. after initial denudation represented by labeled black lines. Final (65 Ma) surface is bold line; (b) basement surface mineral age profiles at indicated times; (c)(h) Histograms of detrital Kfeldspar closure ages at indicated times. For each stratigraphic horizon, we overlay distributions calculated from 32 (black) and 1000 (light gray) random samples. All data normalized to 100.  Fig. 8: Model II results. Figure 7 caption contains explanation of (a)(h). 
Fig. 9: (a) Depositional ages and uncertainties for each samples (see Table 1). (b) (g) Model IVI results (see Figure 6 caption). PROBmax values represented by symbols defined in (a) above (see Table 5). (h). ?t vs. depositional age models based upon (b)(g) above.  Fig. 10: Effect of radiogenic daughter product retentivity upon Model II results (see Table 2). (a) apatite ((UTh)He); (b) Kfeldspar, (c) biotite, (d) hornblende (KAr); and (e) monazite (ThPb) age distributions. (f) Closure ages variation along final (65 Ma) surface. 
Fig. 11: Ability of variable rate denudation models to fit measured detrital closure age distributions. (a) Depositional ages and uncertainties for measured samples. (b)(d) Model VIIIX results (see Figure 9 caption for explanation; Table 5). (e) Deviation of AgeDmax from estimated stratigraphic ages in models VIIIX. (f)(l) Comparison of measured detrital closure age with those of Model IX. All data normalized to 100.  Fig. 12: (a) Mean denudation rate vs. time for indicated models (b) Histogram of bulk closure ages (= total fusion ages) calculated from 40Ar/39Ar stepheating of Kfeldspars from east central PRB (see Figure 1 caption for data sources). (c)(e) Comparison of predicted surface ages from models IIX with measured basement Kfeldspar results vs. horizontal distance. 
Table 1: Depositional Age Constraints (Northern Santa Ana Mountains)
















































Table 2: Diffusion Parameters Used in Model
















































































































Table 5: Summary of Model Results



































































































































































