Magma mixing was an important process in the genesis of plutonic suites of the Peninsular Ranges batholith, San Diego County transect. Contrary to expectations, minimum Hf arc mantle model ages (Hf TAM ) calculated from Lu-Hf spot analyses of zircon from 15 granite samples and one gabbro sample indicate a Neoproterozoic component in granites from the western zone of the batholith and even older crustal components, including a Paleoproterozoic component, in those from the eastern zone. The delineation between western and eastern zones in the San Diego County transect of the batholith corresponds closely with a rapidly formed suture zone marked by the western limit of Jurassic S- and transitional I-S-type granites, magnetic and gravity anomalies, and the δ 1 8 O gradient. Zircon U-Pb ages, many reported herein for the first time, indicate that Early Cretaceous I-type plutons were emplaced into the western zone of the batholith and stitched across both the suture zone and the central belt of deformed Jurassic S-type and I-S-type granites. I-type plutons that intruded east of the suture zone are mainly Late Cretaceous in age. Zircon U-Pb ages, measured as much as possible from the same grains used for 176 Hf/ 177 Hf analyses, not only provide a record of crystallization ages but also of the degree of zircon inheritance—of which there is little for Cretaceous western-zone I-type granites. The variation in 176 Hf/ 177 Hf (εHf (t) ) values for the population of zircon grains from each plutonic sample is therefore interpreted to reflect the degree of magma mixing between crustal- and mantle-derived components between the time of melt generation and final pluton construction, a process that can only be reconciled with open-system chemical behavior. We consider the process of formation of the short-lived suture zone and the S-type granites of the Peninsular Ranges to be examples analogous to the short lived Bundarra Supersuite of the New England batholith (Jeon et al., 2012). The new Hf data of this study are compared to published Nd-Sm model age data for the Peninsular Ranges batholith and to new zircon Hf data for the Tuolumne intrusive suite of the Sierra Nevada batholith.

In the Peninsular Ranges batholith of southern California, a central belt of Jurassic metagranites was intruded by a Cretaceous magmatic arc that migrated from west to east across the belt. The Cretaceous batholith has been divided into western and eastern zones, zones that correspond to age, lithologic, geochemical, and geophysical zonations. In this study, density and magnetic susceptibility measurements performed on ~960 hand samples show that, in the eastern zone of the Peninsular Ranges batholith, values of magnetic susceptibility are uniformly low (<0.5 × 10 −3 cgs [centimeter-gram-second] units), while density values are in general lower and have less scatter than those in the western zone. A relatively sharp break between western and eastern zones indicates the existence of two crustal types separated by a tectonic suture: on the west, oceanic crust (mainly Mesozoic and older mantle and mantle-derived rocks) and on the east, continental crust (Neoproterozoic, Paleozoic, and early Mesozoic rocks). Previous studies in the San Diego County segment of the Peninsular Ranges batholith revealed petrologic distinctions between two Jurassic metagranite suites (S-type and transitional I-S type) and nine Cretaceous granite suites (exclusively I-type). The results of electron microprobe (EM) analyses of mafic minerals from Jurassic and Cretaceous plutonic rocks in general confirm plutonic suite subdivision. On biotite and hornblende variation diagrams, Early Cretaceous plutons tend to plot in distinct fields/trends that are characteristic of their various plutonic suites. Hornblende from three Early Cretaceous tonalite suites is Mg enriched, as expected from melts of mafic-intermediate composition that were H 2 O rich and contained hornblende as an early-crystallizing phase. Hornblende from gabbro plutons (Cuyamaca Gabbro) shows the greatest Mg enrichment for a given whole-rock SiO 2 value, reflecting cumulate processes in the evolution of gabbroic magmas. Biotite and hornblende from highly evolved leucomonzogranite-leucogranodiorite plutons assigned to three leucogranite suites have the most Fe- and Mn-rich compositions. Hornblende compositions of two Late Cretaceous tonalite suites overlap those of the Early Cretaceous tonalite suites, but, in general, Late Cretaceous hornblende does not show the extreme fractionation shown by hornblende of Early Cretaceous suites with similar SiO 2 contents. Biotite of two Jurassic plutonic suites has the most aluminous compositions of all Peninsular Ranges batholith suites, with biotite of the S-type Harper Creek suite markedly more Al rich than that of the transitional I-S–type Cuyamaca Reservoir suite. Complete overlap of Harper Creek biotite compositions with those of metasedimentary rocks of the Triassic–Jurassic Julian Schist indicates that partial melting of the latter was an appropriate source for Harper Creek magma. The existence of two Cuyamaca Reservoir biotite trends suggests that its parental magma originated by fractionation and contamination of an I-type magma by aluminous metasedimentary material, thus producing transitional I-S characteristics. All but one sample of hornblende from the Cuyamaca Reservoir suite falls in the subaluminous compositional range.

Abstract The San Luis Rey River comprises a 1,450-km2 (560-mi2) watershed in northern San Diego County, California. Construction aggregate has been mined along a 22.5-km (14-mi) reach of the river. Cumulative extraction volumes of eight operators peaked in the late 1980s, with few restrictions or coordinated oversight by local, state, or federal agencies. The river channel was deepened and widened as sand was removed at rates far in excess of natural replenishment. The 1992-1993 floods caused headward erosion of the mined pit boundaries, interruption of sediment transport continuity, and downstream scour. Lowering the base level in the mined portions triggered rapid erosional adjustments in nonmined portions of the river, affecting infrastructure, adjacent property, and wildlife habitat. During the 1992-1993 storms, the riverbed degraded 2.4 to 3.7 m (8 to 12 ft) under the old Route 395 bridge, causing structural instability that closed the bridge to traffic and necessitated a $4.5 million bridge replacement project. The Route 76 bridge over a tributary to the San Luis Rey River failed as the tributary headcut upstream, lowering the bed in the mainstem. The exposure of aqueduct crossings, sewage lines, natural gas conduits, and bridge foundations prompted a comprehensive evaluation of instream mining activity, initiated by the San Diego County Water Authority in 1990. Concurrently, the Environmental Protection Agency authorized funding a watershed management plan to preserve or replace habitat critical to rare, threatened, and endangered species. The lessons learned from the San Luis Rey River include: (1) the cumulative impacts of sand removal should be quantified, and potential offsite impact areas identified; (2) the effects of bed lowering on infrastructure can be quantified and used to limit mining depths and locations; and (3) the loss of riparian habitat can be minimized by identifying affected areas, preserving critical areas, and promptly implementing aquatic habitat and wildlife enhancement programs to restore impacted areas.

Abstract A series of 16 marine terraces and associated landforms and sedimentary deposits provides a record of Quaternary paleo-geographic and tectonic history of coastal San Diego County, California. The width of individual terrace platforms varies dramatically, from several kilometers in places where the platforms are cut into poorly lithified sedimentary strata, to a few meters or tens of meters where the platforms are cut in more resistant rocks. Beach ridges formed of dune sands lie just landward of most of the terrace shorelines. Radiometric ages and amino-acid racemization ratios, together with shoreline elevations, have been used to calculate average rate of uplift and to construct a chronology of the 16 shorelines, which range in age from 80 ka to perhaps as old as 1.29 ma. Shoreline-angle elevations suggest that nearly the entire length of coastal San Diego County has been uplifted at a rate of 0.13 to 0.14 m/ka during the Quaternary. Both higher and lower rates are recorded in areas deformed by the Rose Canyon fault zone. Changes in configuration of successive shorelines indicate that rocks on one side of the fault have been rising through middle and late Quaternary time and that the Rose Canyon fault has been active for at least the past 1 my.