The Early to early Late Jurassic magmatic arc of the lower Colorado River region of southern California and southwest Arizona spanned ∼30 m.y., from ca. 190 to 158 Ma. The arc-type volcanic and plutonic rocks interacted extensively with the Proterozoic Mojave Province crust and show evidence for geographic-based age and compositional changes. The region lies adjacent to an amagmatic gap in the Jurassic arc of the southwest United States, near the western terminus of proposed Late Jurassic basins formed in conjunction with the opening of the Gulf of Mexico, and near, but to the north of, the projection of the trace of the sinistral Late Jurassic Mojave-Sonora megashear where it crossed from northern Mexico into the United States. Quartz-phyric dacitic to rhyolitic pyroclastic and locally hypabyssal rocks of the Dome Rock sequence were emplaced in two broad time periods, one between 190 and 185 Ma and the second between 173 and 158 Ma. Three compositionally expanded pluton units constituting the Kitt Peak–Trigo Peaks superunit were emplaced in the mid- to upper crust between 173 and 158 Ma, broadly contemporaneous with the younger phase of explosive volcanism. The compositionally expanded plutonic rocks consist of three informally named temporally and compositionally distinct magmatic units, from oldest to youngest, the Araz Wash diorite, the Middle Camp porphyritic granodiorite, and the Gold Rock Ranch granite. Each unit was emplaced over 4–6 m.y. periods of time. Dioritic rocks dominate the older Araz Wash diorite unit (173–169 Ma), granodiorite dominates the Middle Camp porphyritic granodiorite unit (167–163 Ma), and granite dominates the Gold Rock Ranch granite unit (163–158 Ma). Shortening and regional metamorphism throughout the lower Colorado River region accompanied emplacement of the Gold Rock Ranch granite unit. A Late Jurassic(?) mafic-felsic dike swarm forms the youngest magmatic unit in the region. The Jurassic magmatic history in the lower Colorado River region ended in the early Late Jurassic at ca. 158 Ma. Termination of magmatism in the Late Jurassic in the lower Colorado River region is distinct from adjacent parts of the arc to northwest in the Mojave Desert region or to the southeast in southern Arizona, where Late Jurassic magmatism continued to at least 146 Ma. At this time in the Late Jurassic, the Mojave-Sonora megashear had cut through the arc to the south of the lower Colorado River region, where degradation of the arc is recorded in sedimentary rocks now composing the lower parts of the McCoy Mountains Formation, the Winter-haven Formation, and informally named rocks of Slumgullion.

The Peninsular Ranges batholith north of latitude 33°N consists of five distinctive longitudinal batholith zones. Four zones are autochthonous—a western zone, western transition zone, eastern transition zone, and an eastern zone. The fifth zone, the upper-plate zone, is allochthonous. The western zone, western transition zone, eastern transition zone, and eastern zone are contiguous products of Cretaceous subduction transitioning from a Mesozoic oceanic-arc setting to continental margin arc setting. Within the autochthonous zones, the nature and geochemistry of plutons record changes reflecting subduction proceeding from west to east over a 35 m.y. period. The allochthonous upper-plate zone is structurally located above the regional Eastern Peninsular Ranges mylonite zone. Host rocks for the western zone, western transition zone, and eastern transition zone are mostly Mesozoic, and host rocks of the eastern zone are Paleozoic. The composition of the plutons reflects changes in magma originating in shallow oceanic crust in the western zone to a deeper continental marginal setting in the eastern zone and upper-plate zone. Several aspects of the upper-plate zone rocks set them apart from the autochthonous batholithic rocks. Western zone magmatism occurred during an extensional subduction phase that involved Mesozoic oceanic crust. Plutons were emplaced passively from 126 Ma to 108 Ma, forming 47.9% of the area of the autochthonous batholith at a rate of 2.7% per million years. Geochemical variation is greater in the western zone than it is in the other zones. Rock compositions range from gabbro to high-SiO 2 granites; plutons in this zone contain magnetite as an accessory mineral. Most plutonic rocks have initial 87 Sr/ 86 Sr (Sr i ) values <0.7045, initial 206 Pb/ 204 Pb (Pb i ) <19, δ 18 O <9‰, and positive initial epsilon Nd (ε Ndi ). By 111 Ma, conditions for pluton emplacement began to change radically from extensional to compressional as subduction encountered older continental crust. The boundary between the western zone and western transition zone is marked clearly by a change in the magnetic properties, which are highly magnetic in the western zone to weakly magnetic in the transition zones. Western transition zone plutons, which have affinities with the western zone plutons, constitute 13.5% by area of the autochthonous batholith and formed over 13 m.y. at a decreased rate of batholith formation, 1% per million years. Plutons of the western transition zone are characterized by Sr i values of 0.7045–0.7050, δ 18 O <9‰, and positive ε Ndi . Deformation of the prebatholithic rocks was intense at 100 Ma, as the plutonism of the western transition zone ended and emplacement in the eastern transition zone began. From 99 to 93 Ma, the rate of magma emplacement accelerated, forming 2.4% per million years by area of the northern part of the autochthonous batholith. The eastern transition zone plutons, having affinities with the eastern zone plutons, have Sr i values of 0.7051–0.7057, δ 18 O >9‰, and negative ε Ndi . Most eastern transition zone plutons were emplaced in a less dynamic setting than the western transition zone plutons. By 98 Ma, subduction had transitioned eastward as plutons were emplaced in continental crust. The rate of magma emplacement increased to form the eastern zone over 7 m.y., or a rate of batholith growth of 3.4% per million years by area. There is considerable temporal overlap in the magma emplacement of the eastern transition zone and the eastern zone. Combined eastern transition zone and eastern zone magmatism produced 39% (by area) of the autochthonous batholith in 8 m.y. at a rate of ~5% per million years. The 102 Ma gabbro body is not considered in this analysis. Eastern zone plutons are characterized by Sr i >0.7060, mostly in the range of 0.7061–0.7076, Pb i >19, δ 18 O >9‰, and a large negative ε Ndi . The allochthonous granitic sheets that constitute the upper-plate zone include batholithic rocks ranging in age from 92 to 75 Ma; most are in the range of 86–75 Ma. These granitic rocks have a more restricted range of geochemistry than those in the other zones; they are magnetite-bearing rocks, unlike the ilmenite-bearing granitic rocks of the transition zones and eastern zone, and they have large negative ε Ndi , and Sr i in the range of 0.7076–0.7084. During the Late Cretaceous, the Eastern Peninsular Ranges mylonite zone developed in the eastern part of the Peninsular Ranges Province, deforming granitic rocks of the eastern part of the eastern zone. Following mylonitization, westward displacement on a series of low-angle thrust faults placed sheets of metamorphic and plutonic rock above the Eastern Peninsular Ranges mylonite zone, forming the upper-plate zone. Compatible elements decrease west to east across the batholith, and incompatible elements increase. Geochemical variation shows that magma forming the western part of the batholith had a shallow and primitive source compared with the eastern part, which had a deeper and more-evolved continental component. The frequency distribution of Sr i in the batholith is bimodal, having a peak of 0.7038 in the western zone, reflecting the oceanic crustal source, and a peak of 0.7072 in the eastern zone, reflecting increased incorporated continental crust sources. Only a small part of the batholith has Sr i values between 0.7055 and 0.7065, indicating a relatively sharp boundary between oceanic and continental crust. Linear arrays on Harker diagram indicate that geochemical variation within the batholith is from magma mixing and not magmatic differentiation. Our data are most simply explained by the Cretaceous arc transitioning from a Mesozoic oceanic-arc setting to a continental margin setting.

Utilizing both sensitive high-resolution ion microprobe (SHRIMP) and conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) methods, crystallization and/or emplacement ages have been obtained for a suite of Cretaceous intermediate-composition plutonic samples collected along a roughly E-W–trending traverse through the northern Peninsular Ranges batholith. Previously noted petrologic, mineralogic, and textural differences delineated four major zonations from west to east and raised the need for detailed geochemical and isotopic work. U-Pb zircon geochronology establishes that these zonations are essentially temporally separate. Mean 206 Pb/ 238 U ages date the three older zones from west to east at 126–107 Ma, 107–98 Ma, and 98–91 Ma. Despite petrologic differences, a relatively smooth progression of magmatism is seen from west to east. A fourth zone is defined by magmatism at ca. 85 Ma, which represents emplacement of deeper-level plutons east of the Eastern Peninsular Ranges mylonite zone in an allochthonous thrust sheet in the northeastern Peninsular Ranges batholith. The age data presented here differ slightly from those presented in earlier work for similar rocks exposed across the middle and southern portions of the Peninsular Ranges batholith in that our data define a relatively smooth progression of magmatism from west to east, and that the transition from western-type to eastern-type plutonism is interpreted to have occurred at ca. 98 Ma and not at ca. 105 Ma. The progressive involvement of older crustal components in the enrichment of eastern Peninsular Ranges batholith–type magma sources is documented by the occurrence of Proterozoic zircon inheritance within samples of the eastern part of the batholith.

Conventional potassium-argon (K-Ar) ages were obtained on biotite from samples of granitic rocks collected at as regular spacing as outcrop and sample suitability permitted across the entire northern exposed part of the Peninsular Ranges batholith. Uranium-lead (U-Pb) ages on zircons range from 6 to 24 m.y. older than roughly corresponding conventional K-Ar biotite ages. The U-Pb zircon ages are considered to be emplacement or near-emplacement ages and provide a basis for using the conventional biotite ages to approximate variations in cooling history. Contouring of the biotite cooling ages shows the same west-to-east younging trend that earlier regional dating studies have shown. Contours generated by these earlier regional studies produced a relatively smooth, even age gradient across the batholith. Biotite cooling age contours generated by the much more closely spaced data set used here suggest a more complicated cooling history and show strong digressions from the smooth, even regional-scale gradient. Along much of their respective traces, the right-lateral strike-slip Elsinore and San Jacinto fault zones cut granitic rocks of the batholith; we have used the age contours as datums to estimate maximum offset across the Elsinore fault and to support a proposed offset based on geologic mapping evidence on the San Jacinto fault. These estimates, based on offsets of the age contours, may differ from true offsets because of uncertainties related to (1) inability to establish the dip of the age contours, (2) possible vertical components of offsets on faults, and (3) paucity of samples in some areas. Cooling age contours are offset 12 km across the Elsinore fault zone, a figure in keeping with offsets of 10–15 km based on detailed geologic mapping. Likewise, alignment of contour features at the north end of the Perris block with those at the north end of the San Jacinto block requires restoration of 29 km of right-lateral displacement, i.e., essentially the same as documented offsets based on geologic mapping.

The thermochronology for several suites of Mesozoic metamorphic and plutonic rocks collected throughout the northern Peninsular Ranges batholith (PRB) was studied as part of a collaborative isotopic study to further our understanding of the magmatic and tectonic history of southern California. These sample suites include: a traverse through the plutonic rocks across the northern PRB ( N = 29), a traverse across a central structural and metamorphic transition zone of mainly metasedimentary rocks at Searl ridge ( N = 20), plutonic samples from several drill cores ( N = 7) and surface samples ( N = 2) from the Los Angeles Basin, a traverse across the Eastern Peninsular Ranges mylonite zone ( N = 6), and a suite of plutonic samples collected across the northern PRB ( N = 13) from which only biotite 40 Ar/ 39 Ar ages were obtained. These geochronologic data help to characterize five major petrologic, geochemical, and isotopic zonations of the PRB (western zone, WZ; western transition zone, WTZ; eastern transition zone, ETZ; eastern zone, EZ; and upper-plate zone, UPZ). Apparent cooling rates were calculated using U-Pb zircon (zr) and titanite (sphene) ages; 40 Ar/ 39 Ar ages from hornblende (hbl), biotite (bi), and K-feldspar (Kf); and apatite fission-track (AFT) ages from the same samples. The apparent cooling rates across the northern PRB vary from relatively rapid in the west (zr-hbl ~210 °C/m.y.; zr-bio ~160 °C/m.y.; zr-Kf ~80 °C/m.y.) to less rapid in the central (zr-hb ~280 °C/m.y.; zr-bio ~90 °C/m.y.; zr-Kf ~60 °C/m.y.) and eastern (zr-hbl ~185 °C/m.y.; zr-bio ~180 °C/m.y.; zr-Kf ~60 °C/m.y.) zones. An exception in the eastern zone, the massive San Jacinto pluton, appears to have cooled very rapidly (zr-bio ~385 °C/m.y.). Apparent cooling rates for the UPZ samples are consistently slower in comparison (~25–45 °C/m.y.), regardless of which geochronometers are used. Notable characteristics of the various ages from different dating methods include: (1) Zircon ages indicate a progressive younging of magmatic activity from west to east between ca. 125 and 90 Ma. (2) Various geochronometers were apparently affected by emplacement of the voluminous (ETZ and EZ) La Posta–type plutons emplaced between 99 and 91 Ma. Those minerals affected include K-feldspar in the western zone rocks, biotite and K-feldspar in the WTZ rocks, and white mica and K-feldspar in rocks from Searl ridge. (3) The AFT ages record the time the rocks cooled through the AFT closure temperature (~100 °C in these rocks), likely due to exhumation. Throughout most of the northern traverse, the apatite data indicate the rocks cooled relatively quickly through the apatite partial annealing zone (PAZ; from ~110 °C to 60 °C) and remained at temperatures less than 60 °C as continued exhumation cooled them to present-day surface temperatures. The ages indicate that the western “arc” terrane of the WZ was being uplifted and cooled at ca. 91 Ma, during or shortly after intrusion of the 99–91 Ma La Posta–type plutons to the east. Uplift and cooling occurred later, between ca. 70 Ma and ca. 55 Ma, in the central WTZ, ETZ, and EZ rocks, possibly as upwarping in response to events in the UPZ. The UPZ experienced differential exhumation at ca. 50–35 Ma: Cooling on the western edge was taking place at about the same time or shortly after cooling in the younger samples in the ETZ and EZ, whereas on the east side of the UPZ, the rocks cooled later (ca. 35 Ma) and spent a prolonged time in the apatite PAZ compared to most northern traverse samples. Apparent cooling rates from Los Angeles Basin drill core samples of plutonic rocks show that four are similar to the WTZ thermal histories, and two are similar to the WTZ histories, indicating that the eastern part of the Los Angeles Basin area is underlain by mainly western zone PRB rocks. Thermal histories revealed by samples from Searl ridge indicate that the WTZ magmatism intruded the metasedimentary rocks prior to their deformation and metamorphism at ca. 97 Ma. Both low-grade schists and metasandstones of the western side of the ridge and high-grade gneisses of the eastern side of the ridge have thermal histories consistent with eastern zone rocks—suggesting a temporal/thermal relationship between the western transition zone and the eastern zones. Limited ages from six samples across the Eastern Peninsular Ranges mylonite zone (EPRMZ) indicate that this zone underwent cooling after emplacement of the youngest UPZ rocks at 85 Ma, suggesting that thrusting along the EPRMZ was either coeval with emplacement of the UPZ plutonic rocks or occurred shortly afterwards (~10–15 m.y.). Alternatively, the EPRMZ thrusting may have occurred at temperatures under ~180 °C at yet a later date. The geochronology presented here differs slightly from previous studies for similar rocks exposed across the middle and southern portions of the PRB, in that our data define a relatively smooth progression of magmatism from west to east, and the transition from western, oceanic-arc plutonism to eastern, continental arc plutonism is interpreted to have occurred at ca. 99–97 Ma and not at ca. 105 Ma.

Within the duration of the U.S. Geological Survey (USGS)–based Southern California Areal Mapping Project (SCAMP), many samples from the northern Peninsular Ranges batholith were studied for their whole-rock radioisotopic systematics (rubidium-strontium [Rb-Sr], uranium-thorium-lead [U-Th-Pb], and samarium-neodymium [Sm-Nd]), as well as oxygen (O), a stable isotope. The results of three main studies are presented separately, but here we combine them (>400 analyses) to produce a very complete Pb-Sr-Nd-O isotopic profile of an arc-continent collisional zone—perhaps the most complete in the world. In addition, because many of these samples have U-Pb zircon as well as argon mineral age determinations, we have good control of the timing for Pb-Sr-Nd-O isotopic variations. The ages and isotopic variations help to delineate at least four zones across the batholith from west to east—an older western zone (126–108 Ma), a transitional zone (111–93 Ma), an eastern zone (94–91 Ma), and a much younger allochthonous thrust sheet (ca. 84 Ma), which is the upper plate of the Eastern Peninsular Ranges mylonite zone. Average initial 87 Sr/ 86 Sr (Sr i ), initial 206 Pb/ 204 Pb ( 206 Pb i ), initial 208 Pb/ 204 Pb (average 208 Pb i ), initial epsilon Nd (average ε Ndi ), and δ 18 O signatures range from 0.704, 18.787, 38.445, +3.1, and 4.0‰–9.0‰, respectively, in the westernmost zone, to 0.7071, 19.199, 38.777, −5, and 9‰–12‰, respectively, in the easternmost zone. The older western zone is therefore the more chemically and isotopically juvenile, characterized mostly by values that are slightly displaced from a mantle array at ca. 115 Ma, and similar to some modern island-arc signatures. In contrast, the isotopic signatures in the eastern zones indicate significant amounts of crustal involvement in the magmatic plumbing of those plutons. These isotopic signatures confirm previously published results that interpreted the Peninsular Ranges batholith as a progressively contaminated magmatic arc. The Peninsular Ranges batholith magmatic arc was initially an oceanic arc built on Panthalassan lithosphere that eventually evolved into a continental margin magmatic arc collision zone, eventually overriding North American cratonic lithosphere. Our Pb-Sr-Nd data further suggest that the western arc rocks represent a nearshore or inboard oceanic arc, as they exhibit isotopic signatures that are more enriched than typical mid-ocean-ridge basalt (MORB). Isotopic signatures from the central zone are transitional and indicate that enriched crustal magma sources were becoming involved in the northern Peninsular Ranges batholith magmatic plumbing. As the oceanic arc–continental margin collision progressed, a mixture of oceanic mantle and continental magmatic sources transpired. Magmatic production in the northern Peninsular Ranges batholith moved eastward and continued to tap enriched crustal magmatic sources. Similar modeling has been previously proposed for two other western margin magmatic arcs, the Sierra Nevada batholith of central California and the Idaho batholith. Calculated initial Nd signatures at ca. 100 Ma for Permian–Jurassic and Proterozoic basement rocks from the nearby San Gabriel Mountains and possible source areas along the southwestern Laurentian margin of southern California, southwestern Arizona, and northern Sonora strongly suggest their involvement with deep crustal magma mixing beneath the eastern zones of the Peninsular Ranges batholith, as well as farther east in continental lithospheric zones. Last, several samples from the allochthonous, easternmost upper-plate zone, which are considerably younger (ca. 84 Ma) than any of the rocks from the northern Peninsular Ranges batholith proper, have even more enriched average Sr i , 206 Pb i , 208 Pb i , and ε Ndi signatures of 0.7079, 19.344, 38.881, and −6.6, respectively, indicative of the most-evolved magma sources in the northern Peninsular Ranges batholith and similar to radioisotopic values for rocks from the nearby Transverse Ranges, suggesting a genetic connection between the two.

We studied the formation of low-initial-Sr felsic plutons by using data from the Early Cretaceous western Peninsular Ranges batholith near Escondido, California. The systematically sampled Escondido plutons have a uniformly low initial 87 Sr/ 86 Sr isotope ratio of Sr i < 0.704, but a wide range of SiO 2 compositions, from 46 to 78 wt%, which fall in three distinct groups: 20% gabbros, 35% tonalites, and 45% granodiorites. These low-Sr i plutons are unique in having undergone one cycle of mantle melting to give basalt composition rocks, and a second cycle of arc basalt melting to give a range of SiO 2 plutons, but no third cycle of melting and contamination by old continental crust to yield high-Sr i rocks. After doing two-cycle partial melting and fractional crystallization calculations, it was recognized that mixing of gabbro and granodiorite magmas was necessary to yield the tonalites. The linear data pattern on Harker diagrams is interpreted as resulting from mixing of mafic magma from partial melting of the mantle and felsic magma from partial melting of the lower crust to form intermediate magma. These plutons provide a simplified two-cycle Phanerozoic example of the petrogenetic process for forming continental crust.

Twenty-four samples were collected from prebatholithic metasedimentary rocks along Searl Ridge, the north rim of the Diamond Valley Reservoir, Domenigoni Valley, centrally located in the northern Peninsular Ranges of southern California. These rocks exhibit progressive metamorphism from west to east across fundamental structural discontinuities now referred to as a “transition zone.” Documented structural and mineralogical changes occur across this metamorphic gradient. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb ages were obtained from detrital zircons from metasedimentary rocks through the transition zone. To the west, metapelitic and minor metasandstone units yielded numerous concordant 206 Pb/ 238 U ages between 210 and 240 Ma, and concordant 207 Pb/ 206 Pb ages at 1075–1125 Ma, 1375–1430 Ma, and 1615–1735 Ma, although distinct differences in provenance were noted between units. A few older 207 Pb/ 206 Pb ages obtained were ca. 2250 Ma and ca. 2800 Ma. Rocks of the eastern part of the transition zone include high-grade paragneisses that yielded numerous concordant 206 Pb/ 238 U ages between 103 and 123 Ma and between 200 and 255 Ma, and concordant 207 Pb/ 206 Pb ages at 1060–1150 Ma, 1375–1435 Ma, and 1595–1710 Ma. Some zircon results from these high-grade gneisses are marked by distinct Pb-loss discordia with lower-intercept ages of ca. 215 Ma and Paleoproterozoic upper-intercept ages. Younger ages between 100 and 105 Ma are mainly obtained from rims of some zircon grains that are characterized by low Th/U values (<0.1) and high U contents (>1000 ppm), indicating the likelihood of metamorphic zircon growth at that time. The similarity of zircon age populations between western and eastern units through the transition zone indicates that this fundamental structure probably dissects sediments of the same basin. This supposition is further supported by initial whole-rock Pb-Sr-Nd isotopic data that show similar average initial 206 Pb/ 204 Pb (18.65 to 18.9), 87 Sr/ 86 Sr (0.713 to 0.718), and ε Nd (−7 to −12) values for both the western and eastern units—values that also indicate the presence of significantly older crustal material in their provenance. Magmatic zircons from a diorite dike that crosscuts the foliation, but is itself subsequently metamorphosed, yielded a SHRIMP-RG concordia age of 103.3 ± 0.73 Ma, which is within agreement of an isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb age of 103.37 ± 0.25 Ma. A postmetamorphic, cross-cutting pegmatite yielded discordant U-Pb zircon age data, but euhedral, glassy monazite from the pegmatite yielded a slightly discordant 207 Pb/ 235 U age of 101.85 ± 0.35 Ma and a Th-Pb age of 97.53 ± 0.18 Ma, suggesting that this pegmatite was injected during or just after deformation ceased. The age and initial Pb-Sr-Nd signature for the dioritic dike indicate it was produced during the transition zone plutonism elsewhere in the northern Peninsular Ranges batholith, whereas the pegmatitic dike was derived from crustal anatexis. Collectively, these results indicate that this sequence of metasedimentary rocks was derived from mainly a Late Permian to Early Triassic igneous provenance that probably intruded Proterozoic crust. The sequence was subsequently metamorphosed during deformation of the Cretaceous continental margin at ca. 105 to 97 Ma.

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.

The Early Cretaceous Ramona plutonic complex is an elliptical-in-plan structure that intruded the mid- to upper crust of the western zone of the Peninsular Ranges batholith. The complex is composed of a dozen concentrically arranged silicic to intermediate plutons with minor gabbro. Lithologically, these plutons represent six regional plutonic suites, each of which consists of multiple plutons in this part of the batholith. Seven new U-Pb zircon ages indicate that plutons are younger toward the center of the complex, from an age possibly as old as ca. 115 Ma to ca. 101 Ma. The composition of the Ramona plutonic complex and a magmatic history conceivably as long as ~14 m.y. indicate that it was constructed as a series of intrusive pulses. Inward from the margin of the Ramona plutonic complex, plutons vary from leucogranodiorite and leucomonzogranite to tonalite–quartz gabbro. Amphibolite-facies country rocks and Middle Jurassic metagranite comprise screens within the complex. External contacts with surrounding country rocks, internal plutonic contacts, and magmatic and metamorphic foliations dip steeply (~60°–90°) both inward and outward. Emplacement mechanisms included forceful intrusion, magmatic wedging or diking, stoping, magma mingling, and downward ductile flow of country rocks. Although emplacement of the major part of the Ramona plutonic complex appears to have postdated western zone volcanic activity (ca. 138–119 Ma), the occurrence within the complex of a metavolcanic inclusion with a zircon U-Pb age of 119 Ma suggests that early intrusions in the area may have fed a preexisting shallow subsidence system. The Ramona plutonic complex intruded and deformed a suture zone that extends for ~800 km along the axis of the batholith. Suturing occurred during a period of contraction along the continental margin between ca. 129 and 101 Ma that was accompanied by (1) accretion of a Late Jurassic fringing volcanic arc and the inception of the overlying Early Cretaceous Santiago Peak volcanic arc and (2) closure of a Middle Jurassic–Early Cretaceous backarc basin. Although plutons of the Ramona plutonic complex show only minor evidence of ductile deformation, pluton ages and concordance of structures of the complex with those in surrounding amphibolite-facies country rocks indicate that the intrusive sequence was emplaced during regional metamorphism.

Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206* Pb/ 238 U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks. The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils. Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

Abstract Airborne dust suppression is of critical importance to military operations conducted in desert environments. Airborne dust is commonly generated in the desert by surface and near-surface operations during operational, testing, and training missions. Currently, there is no standardized procedure for testing dust suppressants, and the U.S. military lacks a specific test operations procedure (TOP) designed to provide realistic testing of the performance and durability of commercial products sold for dust abatement. The primary purpose of this study is to provide recommendations for the future development of a TOP for testing dust suppressants applied to desert soils. Recommendations were developed from the evaluation of a polyvinyl-based synthetic polymer as a dust suppressant, which was tested at four test intervals over a 19–week period in the late spring and summer of 2008 at the U.S. Army Yuma Proving Ground. The dust suppressant was applied at three separate test sites having different surface characteristics and soil properties ranging from loose, sandy gravel to gravelly sand, alluvial-fan soils to soft, sandy-silt, alluvial-plain soils. Each test site was subjected to a variety of traffic impact types consisting of an increasing number of cumulative passes by different vehicle types—including a low-flying helicopter, a light-weight armored tracked vehicle, and heavy-, medium-, and lightweight wheeled vehicles, plus pedestrian foot traffic. In addition to the sites of traffic impact, three types of control plots were concurrently tested to act as reference sites, as well as to evaluate the longevity of the suppressant, which included: disturbed and static (undisturbed) baseline plots and a static benchline plot. Surface soil and dust-suppressant physical properties were measured following each traffic impact in the form of shear strength and bearing capacity, plus dust-emission flux as measured by a Portable In Situ Wind ERosion Laboratory (PI-SWERL). Results from this study show that dust-emission flux and surface-strength measurements from a layout of control and traffic impact test plots provide a quantifiable and repeatable approach in measuring the efficacy of a dust suppressant for a TOP used by the U.S. Army.

Abstract Dryland channel networks share many similarities with channel networks in more humid regions, but they are also unique in having: extreme temporal and spatial variability in rainfall, runoff, and both hillslope and channel processes; poor integration between tributary and main channels; dominantly ephemeral or intermittent flow; and lack of equilibrium between process and form. Floods are likely to be particularly important in dryland channels, and riparian vegetation exerts a strong influence on channel processes and form. Land managers working in dryland channel networks particularly need to answer the following questions: What is stable? What is the role of disturbance? How do ecosystems depend on physical form and process? This paper explores methods for determining thresholds and resiliency within a channel network and suggests metrics that can be used to assess the condition of a channel segment or entire drainage network relative to management goals. The management metrics focus on flow regime, sediment supply, bed grain size, bedform configuration, width/depth ratio, bed gradient, channel planform, and extent and type of riparian vegetation. For each of these metrics, geological, historical, and systematic records can be used to define the natural range of variability for a particular channel form in the absence of direct land-use impacts. The range of variability present under land use such as military training activities can then be compared to the natural range to assess whether these activities are negatively affecting the dryland channel network. The Yuma Wash drainage in the Yuma Proving Ground, Arizona, is used as a case study.

Abstract Military lands are among the last places most people would list as being pristine. Withdrawn for training and testing, they are tucked away from public exploration, and the results of this isolation have been unexpected: The U.S. Department of Defense (DOD) harbors more rare, threatened, and endangered species on its lands than any other landowner. Despite the vast acreage held in other public ownership, it is the combination of the “tragedy of the commons,” too little funding, and a multi-use mandate that have been the downfall of biodiversity on most non-DOD public lands. Simultaneously, private landowners permanently alter remaining lands at increasing rates. By these processes, the U.S. DOD is now in the position of being capstone stewards. Thus begins an exploration into the question: Does wilderness exist on military installations? In the United States, motorized vehicles are prohibited in federally designated wilderness. On military bases, there are areas where no vehicles may go, such as active air-ground impact areas. If a bomb falls where no tires tread, is it wilderness? The Korean demilitarized zone represents another example of the impact of human conflict and use, or nonuse. Here, the legacy of military actions has unexpectedly created an ecological refuge. Closer to home in the Mojave Desert of California, the juxtaposition of the Marine Corps Air Ground Combat Center and Joshua Tree National Park offers direct comparison between a landscape where people are prohibited and a landscape where people are encouraged. This paper is about the way in which we came to such an unexpected place by examining the trade-off between lands off-limits to civilians, and those where people are encouraged, with the benefits realized from keeping “out,” drawing on examples from the United States and abroad. The objective is to stimulate lively discussion without necessarily coming to a right/ wrong determination or judge whether or not credit is due.

This paper focuses on the potential for developing geologic maps using high-resolution aerial imagery and digital elevation data. A key component of geologic mapping discussed herein addresses the determination of geologic layer orientation using a planar approximation (also known as the three-point method). An analytical solution is presented which can be readily implemented in a spreadsheet. The path from initial data collection in a GIS to spreadsheet computation and back to GIS is outlined. The Mecca Hills in southern California serve as a test case, as there is good coverage with high-resolution aerial imagery, a variety of elevation data including airborne LiDAR, and a published geologic map for the area. The comparison between vector-derived strike and dip and traditional field measured strike and dip suggests that remote mapping can successfully capture the regional aspects of the geology in an area. Estimates of rock orientation can be made if the elevation data is accurate and sufficient visual contrast exists in aerial imagery to define mappable features. A comparison shows that United States Geological Survey's National Elevation Data sets and airborne LiDAR can identify regional geologic structures, while Shuttle Radar Topography Mission and ASTER GDEM data yield results that diverge substantially from higher resolution elevation data. Remote mapping using the vector form of the three-point method offers a promising tool for geologic exploration as data sets continue to improve in quality and resolution.