Abstract Frasnian reef complexes of the southeastern Lennard Shelf (northern Canning Basin) developed on tilt-block highs and their evolution was controlled by fault-related subsidence. Tectonic control on relative sea-level changes was, therefore, a major factor influencing Early–Middle Frasnian palaeogeography of the Lennard Shelf. However, palaeogeographic reconstruction is not consistent with simple landward (northward) backstepping and younging of reef complexes in response to basin extension and subsidence of fault blocks. Using a sequence-stratigraphic approach, in conjunction with sedimentological and biostratigraphic data, we propose that two neighbouring fault blocks (Lawford area on the eastern side of Bugle Gap and the Hull platform to the north) record a similar history and that the reef complexes on those blocks were initiated at similar times. Seven phases of Early–Middle Frasnian platform growth (Fr2–8) are identified. All are bounded by third-order flooding surfaces associated with backstepping of platform margins and three surfaces (defined by conodont Zone 4, Zone 6 and late Zone 6) correlate across the two fault blocks. Only one sequence boundary has been clearly identified and a second relative sea-level fall is proposed based on a major collapse event following progradation and associated coarse siliciclastic facies. We propose that the correlation of flooding events across the SE Lennard Shelf is related to episodes of basin-margin faulting centred on the large, long ( c . 25 km) faults which border these blocks (shelf-parallel faults for the Lawford block and an oblique north-trending transfer zone for the Hull block). There is limited evidence for relative sea-level falls and those recognized most likely resulted from eustatic events. The correlation between the Lawford block and Hull platform suggests linkage between major NW-trending shelf-parallel and oblique transfer faults and an evolved rift system by the Early Frasnian.

The Late Cretaceous Old Woman–Piute Range batholith includes both metaluminous and strongly peraluminous granitoid series that intruded the reactivated craton of southeastern California shortly after the orogenic peak. Whole-rock Sr, Nd, and O, feldspar Pb, and zircon U-Pb isotopic compositions, in combination with major- and trace-element and petrographic data, indicate that although these series are not comagmatic, they both were generated primarily by anatexis of Proterozoic crust. Differences between the two rock types are functions of source compositions: peraluminous granitoids were apparently generated from an intermediate to felsic source, metaluminous granitoids from more mafic igneous material with a possible modest subcrustal contribution. No sedimentary input is required in production of the peraluminous granites, and in fact, chemically mature sedimentary material is ruled out as an important contributor— that is, these are not S-type granites. Lead-isotope data reveal that the crust that yielded both magma series had undergone an ancient high-grade uranium depletion event, but independent evidence indicates that at the time of anatexis this crust was by no means anhydrous.

The Mopah Range volcanic field lies near the western edge of the Whipple detachment fault terrane in southeastern California. The volcanic field formed in early Miocene time. In the Central Mopah Range, a lower succession of basalt-to-rhyolite lavas and tuffs, at least 700 m thick, is capped unconformably by a stack of basalt and andesite flows with a maximum thickness of 70 m. Sedimentary units, which thicken westward toward the crystalline highland of the Turtle Mountains, interfinger with volcanic units. Normal faults with primarily down-to-the-east sense of motion dissect the Mopah Range. The concentration of faults is greater in the lower succession than in the capping succession. Dikes and faults are subparallel in mean surface trend, and show crosscutting relations indicating coeval development. These and other observations demonstrate that the Mopah Range volcanic field formed during detachment faulting. The type of volcanism is most likely to have been low-volume explosive eruptions, dome and dome-fed flow growth, and cinder cone activity at many small vents—similar to the activity described by Bacon and others (1981) in the Quaternary Coso Hills.

The eastern Transverse Ranges provide essentially continuous exposure for >100 km across the strike of the Mesozoic Cordilleran orogen. Thermobarometric calculations based on hornblende and plagioclase compositions in Mesozoic plutonic rocks show that the first-order distribution of rock units resulted from differential Laramide exhumation. Mesozoic supracrustal rocks are preserved in the relatively little exhumed eastern part of the eastern Transverse Ranges and south-central Mojave Desert, and progressively greater rock uplift and exhumation toward the west exposed rocks originating at mid-crustal depths. The eastern Transverse Ranges thus constitute a tilted, nearly continuously exposed crustal section of the Mesozoic magmatic arc and framework rocks from subvolcanic levels to paleodepths as great as ~22 km. The base of this tilted arc section is a moderately east-dipping sheeted magmatic complex >10 km in width by 70 km in length, constructed structurally beneath, yet synchronous with Late Jurassic and Cretaceous upper-crustal plutons. Geochronology and regional structural relations thus suggest that arc magmas generated in the lower crust of this continental arc interacted in a complex mid-crustal zone of crystallization and mixing; products of this zone were parental magmas that formed relatively homogeneous upper crustal felsic plutons and fed lavas and voluminous ignimbrites.

Most granitic batholiths contain plutons which are composed of low-variance mineral assemblages amenable to quantification of the P – T – f O 2 – f H 2 O conditions that characterise emplacement. Some mineral thermometers, such as those based on two feldspars or two Fe–Ti oxides, commonly undergo subsolidus re-equilibration. Others are more robust, including hornblende–plagioclase, hornblende–clinopyroxene, pyroxene–ilmenite, pyroxene–biotite, garnet–hornblende, muscovite–biotite and garnet–biotite. The quality of their calibration is variable and a major challenge resides in the large range of liquidus to solidus crystallisation temperatures that are incompletely preserved in mineral profiles. Further, the addition of components that affect K d relations between non-ideal solutions remains inadequately understood. Estimation of solidus and near-solidus conditions derived from exchange thermometry often yield results > 700°C and above that expected for crystallisation in the presence of an H 2 O-rich volatile phase. These results suggest that the assumption of crystallisation on an H 2 O-saturated solidus may not be an accurate characterisation of some granitic rocks. Vapour undersaturation and volatile phase composition dramatically affect solidus temperatures. Equilibria including hypersthene–biotite–sanidine–quartz, fayalite–sanidine–biotite, and annite–sanidine–magnetite (ASM) allow estimation of f H 2 O . Estimates by the latter assemblage, however, are highly dependent on f O 2 . Oxygen fugacity varies widely (from two or more log units below the QFM buffer to a few log units below the HM buffer) and can have a strong affect on mafic phase composition. Ilmenite–magnetite, quartz–ulvospinel–ilmenite–fayalite (QUILF), annite–sanidine–magnetite, biotite–almandine–muscovite–magnetite (BAMM), and titanite–magnetite–quartz (TMQ) are equilibria providing a basis for the calculation of f O 2 . Granite barometry plays a critical part in constraining tectonic history. Metaluminous granites offer a range of barometers including ferrosilite–fayalite–quartz, garnet–plagioclase–hornblende–quartz and Al-in-hornblende. The latter barometer remains at the developmental stage, but has potential when the effects of temperature are considered. Likewise, peraluminous granites often contain mineral assemblages that enable pressure determinations, including garnet–biotite–muscovite–plagioclase and muscovite–biotite–alkali feldspar–quartz. Limiting pressures can be obtained from the presence of magmatic epidote and, for low-Ca pegmatites or aplites, the presence of subsolvus versus hypersolvus alkali feldspars. As with all barometers, the influence of temperature, f O 2 , and choice of activity model are critical factors. Foremost is the fact that batholiths are not static features. Mineral compositions imperfectly record conditions acquired during ascent and over a range of temperature and pressure and great care must be taken in properly quantifying intensive parameters.

The Peninsular Ranges batholith has been subdivided into two zones based on geochemical, geophysical, and lithologic parameters. Plutons in the eastern zone (La Posta–type) are typically larger and inwardly zoned from hornblende-bearing tonalite margins to muscovite-bearing monzogranite cores. U-Pb ages on zircon are generally in the 100 to 90 Ma range. They tend to be more discordant in the cores of the plutons and have upper concordia intercepts near 1300 Ma. Rb-Sr systematics on mineral separates yield an Sr i range from 0.7030 to 0.7044, although one pluton is reported to have a rim-to-core variation from 0.7043 to 0.7074. Whole-rock δ 18 O is lowest in the hornblende-bearing facies (8.3 to 10.9 per mil) and highest in the muscovite-bearing facies (10.2 to 11.8 per mil); the level of variation is pluton dependent. δ 18 O for quartz separates indicate an eastward-directed asymmetry toward heavier oxygen rather than the facies control observed in the whole-rock data. REE patterns from two plutons have nearly identical LREE enrichment and lack any Eu anomaly. Associated with the La Posta–type plutons are a series of small, compositionally restricted, garnet-bearing monzogranites. They are 1 to 5 m.y. younger than the surrounding La Posta–type plutons and contain zircons inherited from a 1200- to 1300-Ma source. Whole-rock δ 18 O values between 12.5 and 13.2 per mil and Sr i = 0.706 reflect a continental contribution to these magmas. La Posta–type melts were generated by subduction-related anatexis of amphibolite-or eclogite-grade oceanic crust. The relatively short emplacement interval and large size of the plutons suggest rapid separation of large volumes of melt from the source region under elevated P H 2 O. Rise toward the present erosional level occurred along the juncture between oceanic lithosphere and the older (ca. 1300 Ma) continental margin. Interaction with the continental crust produced the present-day eastward bias toward higher δ 18 O and zircon discordance.

The Peninsular Ranges of southern and Baja California are divided into a western, predominantly magnetite-bearing plutonic subprovince and an eastern, predominantly magnetite-free plutonic subprovince. The boundary that separates the two subprovinces corresponds roughly to the southwestern margin of the La Posta superunit, but in some places extends into the La Posta granitic province. Neither the pre–La Posta foliated granitic rocks nor the garnet- or muscovite-bearing rocks of the eastern Peninsular Ranges contain magnetite. The magnetite/ilmenite distinction occurs on three scales: regional variations that appear to be independent of host rock or individual plutons, variations paralleling modal facies within zoned plutons, and contact loss of magnetite in the outer margin of a pluton (from meters to more than a kilometer in width). Observations to date indicate that the regional distribution of magnetite- and ilmenite-series granitic rocks may result from generation of parental magma within the dehydration zone of a subduction plane. The gradation within zoned plutons probably results from a lowering of oxygen fugacity in the magma during progressive crystallization. The contact effect appears to be a consequence of reactions between the cooling pluton, the host rocks, and water-rich fluids from a variety of sources.

Middle Tertiary volcanic rocks of the Lake Mead field are calc-alkalic to alkalic-calcic and vary continuously in composition from basalt to rhyolite. These volcanic rocks formed during Basin-and-Range extension and are spatially and genetically associated with diorite-to-granite intrusions of the Wilson Ridge pluton. Locally, igneous rocks were subjected to potassium metasomatism. Field relations and petrography provide evidence of disequilibrium mineral assemblages and liquid-liquid mixing of basalt and granite magmas to form the intermediate rock types of the Lake Mead volcanic field. Evidence of mixing includes incompatible phase assemblages of euhedral olivine, embayed quartz, and sodic plagioclase within andesite flows. Plagioclase occurs in rounded and partially resorbed clusters of equant crystals and commonly displays oscillatory zoning and outer glass-charged zones (fretted texture). Quartz phenocrysts are commonly surrounded by rims of prismatic augite and glass. Fine-grained spheroidal to ellipsoidal inclusions of basalt are common in dacite flows and dikes. Thus, various mixing ratios of olivine basalt and granite end members may be responsible for the textural variations observed in volcanic rocks of the Lake Mead field. The evolution of the igneous rocks of the Lake Mead field was evaluated by petrogenetic models involving both crystal fractionation and magma mixing. These processes may have operated together to produce the compositional range in volcanic and plutonic rocks of the Lake Mead area. Open-system models provide estimates of the relative importance of the two processes and suggest that mixing was more important in the derivation of andesite and diorite (mass mixed component/mass crystallizing phase R = 0.8 to 2.2) than dacite, quartz monzonite, or granite (R = 0.1 to 0.65). The calc-alkaline-alkalic-calcic nature of the igneous rocks of the Lake Mead field, and possibly other similar rock suites that formed in the Great Basin during regional extension, may result from magma mixing. Basalts and rhyolites may mechanically mix in various proportions to produce intermediate rock types. The classic bimodal assemblages may only occur where mixing is incomplete or in structural situations where different magma types cannot mix.

Metamorphosed silicic volcanic and hypabyssal rocks of middle Cretaceous (110 to 100 Ma) age occur in two roof pendants in the Kings Canyon area of the central Sierra Nevada. The metavolcanic remnants are similar in age to or are only slightly older than the voluminous enclosing batholithic rocks. Thus, high to surface levels of the batholith are implied for this region. This is interesting considering that deep-level (∼25 km) batholithic rocks of the same age as the metavolcanic rocks occur at the southern end of the range. Apparent structural continuity between these two regions suggests that the southern half of the range offers an oblique section through young (˜100 Ma) sialic crust. The middle Cretaceous ages of the two volcanic sequences are indicated by U/Pb zircon and Rb/Sr bulk-rock isochron data. The two isotopic systems agree very closely with one another. Some of the U/Pb systems within the Boyden Cave pendant are discordant due to the inheritance or entrainment of Proterozoic zircon. This is a common phenomenon in volcanic or plutonic rocks erupted or emplaced within the Kings sequence metamorphic framework, a belt of distinct pendants with abundant continent-derived sedimentary protoliths. In conjunction with other petrochemical parameters, lavas and magmas of this framework domain are shown to be contaminated with sedimentary admixtures. The contaminated domain of the batholith reflects the bounds of the Kings sequence framework, which along its eastern margin probably represents a major pre-batholith to early batholith tectonic break. The middle Cretaceous metavolcanic sequences were apparently built on two distinctly different early Mesozoic substrates separated by a major tectonic break. In the Boyden Cave pendant, the substrate may be represented by the shallow to deep-marine Kings sequence; to the east in the Oak Creek pendant, the substrate consists of a thick silicic ignimbrite sequence. In both areas the middle Cretaceous rocks and adjacent sequences share intense ductile deformation fabrics. Earlier views that considered these fabrics as an expression of Jurassic orogenic deformation are in error. Structural and age relations indicate that the fabrics developed between 105 and 100 Ma and during the medial phases of Cretaceous composite batholith growth.

Variations in initial 143 Nd/ 144 Nd in Late Cretaceous plutonic rocks along the South Fork of the Clearwater River (SFCR) supplement results of Sr and O studies, which demonstrate large-scale mixing in magmas forming the western margin of the Idaho batholith. These marginal or border phases of the batholith span the terrane boundary between Proterozoic crust of North America and late Paleozoic-Mesozoic intraoceanic arc terranes (WSD terranes), delineated by the Western Idaho suture zone (or WISZ). ɛ Nd (t) values in Early Cretaceous and older, pre-accretionary plutons of the WSD range from +3 to +7.6, and average +5.7. Proterozoic orthogneisses and metasedimentary rocks range from -7.4 to -13.7 and -10.45 to -15.7, respectively. ɛ Nd (t) in Late Cretaceous plutons of the SFCR decreases abruptly from west to east near the WISZ, varying inversely with ɛSr (t) . Although Sr isotopic evidence (Fleck and Criss, 1985) is consistent with a binary mixing model, Sm-Nd results modify those conclusions, suggesting that SFCR plutons may be divided into three groups. Group 1 plutons occur in a narrow zone (<4 km width) along the suture zone (WISZ). These bodies probably represent at least three-component mixtures of very high-Sr, arc-type magmas, one or more Proterozoic crustal components that may include lower crust, and a high-Nb, high-Zr component. Group 2 plutons are characterized by high ɛSr (t) .and nearly constant, low ɛNd (t) . These bodies are thought to represent mixtures of deep-seated partial melts of two different Proterozoic lithospheric types, possibly representing upper and lower crust. Plutons belonging to Group 3 have ɛNd (t) .values <-14 and probably incorporated substantial amounts of Proterozoic metasedimentary rocks, but mixing components are poorly defined. Trace-element variations in SFCR rocks also reflect the arc terrane-continental crustal boundary as Nb, Zr, and Nd increase dramatically, whereas Sr, Rb/Nb, and Sm/Nd exhibit coincident decreases east of the WISZ. Modeling of these variations with the isotopic variations in Nd and Sr supports mixing, but precludes contamination-bulk-assimilation models. Correlated ɛNd, ɛSr, and δ 18 O within the SFCR favors mixing of crustal and subcrustal magmas rather than derivation of the melts entirely from subcontinental lithosphere.

Anorogenic magnatic activity characterizes much of the late to mid-Proterozoic, from 1030 to 1770 m.y. ago, in a broad belt trending from the southwestern United States, northeastward through Labrador, across southern Greenland, and into the Baltic shield. The association of gabbroic to anorthositic rocks, a separate mangeritic series of primarily intermediate composition, and granite of definite rapakivi affinity comprise an anorogenic “trinity” of world-wide occurrence. With the exception of the 1.76 b.y. Montello batholith (Wisconsin), this episode in North America is restricted to the interval 1.0 to 1.5 b.y. and occurs in three distinct events. Over 70 percent of Proterozoic anorogenic magmatism occurs in a 1.41 to 1.49 b.y. old 600-1000 km wide belt trending from southern California to Labrador that volumetrically and age-wise is totally a North American phenomenon. Renewed anorogenic granite magmatism occurred form 1.34 to 1.41 and from 1.03 to 1.08 b.y. ago in lesser proportions. Although anorthositic and mangeritic rocks are abundant in some provinces (e.g., Labrador), rapakivi granite (in the broad usage of the term) represents by far the most abundant magma-type generated during this nonorogenic period. The modal and mineral composition of these granitic rocks is distinctive and reflects the potassic and iron-enriched character of the magmas and the unique conditions under which crystallization occurred. Principal rock types include biotite ± hornblende granite to adamellite although numerous peraluminous, two-mica (biotite + celadonitic muscovite ± garnet) granites also occur. Crystallization of these epizonal granitic magmas occurred over the range of 640 to 790° C at low total pressures (most less than 2 kb) and at relatively dry conditions. A dramatic difference in crystallization conditions lies with the level of oxygen fugacity which ranges three orders of magnitude from low (ca. QFM) to high (above Ni-NiO), resulting in systematic differences in Fe-Ti oxide mineralogy and mafic silicate composition. Compositionally, the granite magmas are subalkalic and marginally peraluminous (peralkaline varieties are rare to nonexistent). Although some hastingsite or riebeckite-bearing granites may have been derived from fractional crystallization of the mangeritic series, most are primary melts derived from fusion of lower crust material. The high potassium, Fe /Mg, Ba, and rare earth element (REE) composition of the granites is consistent with small degrees of fusion (10-30 percent) of calc-alkaline crust of quartz dioritic, tonalitic, and granodioritic material. Initial Sr isotopic ratios average 0.7051 ± .0025. The relatively low ratios are the result of short residence times (commonly 170 to 340 m.y.) with much of the crustal source being formed in a preceding orogenic event. An earlier melting episode need not have occurred for the source. The dry nature of the magmas is due to vapor under-saturated melting of a metaigneous source with a total water budget less than 1 percent and tied up in relatively stable residual hydrous phases. The derivation of a marginally peraluminous melt from a metaluminous source is a consequence of variable amounts of residual hornblende ± clinopyroxene. The generation of isolated crustal-derived magma under anorogenic conditions is considered to be the result of localized thermal doming in the mantle. The mantle-derived anorthositic and mangeritic magmas may have played an active role in generating the necessary heat of fusion at lower crustal levels. For North America, the 1.4 to 1.5 b.y. event does not have the consistent age progression of a track and is probably an incipient rift that failed to integrate into a world-wide plate system. At a more mature stage, the 1.0 to 1.1 b.y. Keweenawan episode and the midcontinent gravity high represents another unsuccessful rifting attempt of the Proterozoic North American craton. The isolated granite complexes of this age (Pikes Peak, Enchanted Rock, Red Bluff), as well as the earlier, localized 1.76 b.y. and 1.34 to 1.41 b.y. magmatic episodes, may be further representations of a thermal perturbation at mantle depths during this fragile period of crustal “stability.”

Field studies in the Whipple Mountains, southeastern California, and in the Buckskin and Rawhide Mountains, western Arizona, have defined the existence of an O1igocene(?) to middle Miocene gravity slide complex that is at least 100 km across in the direction of its transport (N50° ± 10°E). The regionally developed complex is underlain by a subhorizontal detachment fault, named the Whipple detachment fault in western areas and the Rawhide detachment fault in eastern areas. The fault, which was warped and domed after its formation, separates a lower-plate assemblage of Precambrian to Mesozoic or lower Cenozoic igneous and metamorphic rocks and their deeper, mylonitic equivalents from an allochthonous, lithologically varied upper plate. Most lower-plate crystalline rocks were subjected to regional Late Cretaceous and / or early Tertiary mylonitization and metamorphism. The abrupt (3- to 30-m-wide) upper limit of mylonitization, the Whipple “mylonitic front,” is a mappable zone of high strain and, presumably, high thermal gradient. In parts of the Whipple Mountains, mylonitization was accompanied by the intrusion of subhorizontal sheets or sills of adamellite to tonalite up to a few hundreds of metres thick, although elsewhere thick sections of mylonitic rocks are devoid of such sills. The sills include both peraluminous and metaluminous varieties and are compositionally distinct relative to plutons in the overlying upper plate, being richer in Al, Mg, Ca, Na, and Sr and depleted in K and Rb. The compositions of most of the minerals in the mylonitized sills and their country rock gneisses did not reequilibrate during metamorphism. However, reequilibrated phases do occur in the ultrafine-grained mylonitic matrix and in tension gashes developed perpendicularly to mylonitic lineation. As a result of incomplete reequilibration, bimodal compositional ranges exist for plagioclase, epidote, celadonitic muscovite, and biotite. The minimum depth for intrusion and mylonitization is estimated to be 9.6 km from consideration of the interaction of compositionally corrected curves of muscovite stability and the adamellite solidus. Metamorphic mineral assemblages and feldspar thermometry indicate that mylonitization occurred from solidus temperatures of the plutonic sills down to middle greenschist grade. Allochthonous (upper-plate) units in the detachment complex include Precambrian to Mesozoic crystalline rocks, Paleozoic and Mesozoic metasedimentary rocks, Mesozoic metavolcanic rocks, and Tertiary volcanic and sedimentary rocks. The oldest Tertiary rocks are debris flows (some containing mylonitic rocks), fanglomerates, lacustrine sediments, and volcanic rocks, all of the Oligocene(?) to lower Miocene Gene Canyon Formation. Red beds and volcanic rocks of the Copper Basin Formation overlie Gene Canyon rocks unconformably and are tilted less steeply than the older Tertiary rocks along northeast-dipping listric normal faults that occur widely within the upper plate. In the Whipple Wash area of the eastern Whipple Mountains, volcanic rocks of the Copper Basin Formation sit unconformably on brecciated lower-plate mylonitic rocks in a channel cut ~70 m below the Whipple detachment fault. These volcanic rocks were themselves involved in renewed detachment faulting along that fault. Collectively, these stratigraphic-structural relations indicate that detachment faulting occurred during Tertiary sedimentation over a significant period of time, and was therefore of growth-fault rather than catastrophic nature. Upper Miocene valley-fill sediments and alkali basalts unconformably overlie upper-plate structures and tilted strata, thus providing an upper age limit for the detachment faulting. Northeastward movement of the thin (<5 km) upper plate is believed to have occurred under the influence of gravity, although the Whipple-Rawhide detachment fault could not have originally dipped more than a few degrees. The head, or breakaway zone, of the crustal slide is apparently defined by northeast-dipping normal faults in the Mopah Range, just west of the Whipple Mountains. Central areas of the slide complex in the vicinity of the Colorado River (Whipple and Buckskin Mountains) are characterized by extreme distension of the detached slab along northwest-striking, northeast-dipping, listric normal faults. There is telescoping of allochthonous units in distal, or toe, portions of the slide complex in the Rawhide and Artillery Mountains of western Arizona, where thrust faulting of older rocks over rocks as young as middle Miocene is common. Northeastward displacements of allochthonous units in excess of several tens of kilometres are indicated by field relations in the Buckskin and Rawhide Mountains.

Abstract This field guide describes a two-and-one-half day transect, from east to west across southern California, from the Colorado River to the San Andreas fault. Recent geochronologic results for rocks along the transect indicate the spatial and temporal relationships between subarc and retroarc shortening and Cordilleran arc magmatism. The transect begins in the Jurassic(?) and Cretaceous Maria retroarc fold-and-thrust belt, and continues westward and structurally downward into the Triassic to Cretaceous magmatic arc. At the deepest structural levels exposed in the southwestern part of the transect, the lower crust of the Mesozoic arc has been replaced during underthrusting by the Maastrichtian and/or Paleocene Orocopia schist.