Franciscan Complex Of California Research Articles

Silicon isotope compositions of metaperidotites from the Franciscan Complex of California- implications for Si isotope fractionation during subduction dehydration

Slab-derived fluids have a great significance in material transfer in subduction zones and exert a great influence on element and isotope characteristics of the overlying mantle wedge. To investigate Si isotope fractionations during fluid-rock reactions in subduction zones, and to constrain Si isotope compositions of the residual slab after metamorphic dehydration, we studied a set of metaperidotite samples (four groups) with good characteristics from the Franciscan Complex of California, USA. Due to multistage fluid metasomatism, a series of fluid-mineral reactions including serpentinization, talcification, and tremolitization, occurred in the subduction channel. Mantle wedge peridotites successively reacted with slab-derived fluids to produce partially and completely serpentinized peridotites (Group 1 & 2), talc-dominated metaperidotites (Group 3) and tremolite-dominated ones (Group 4). All of these four groups of samples exhibit relatively homogeneous Si isotope compositions, and δ30Si values vary from −0.39‰ to −0.26‰, with an average value of −0.32 ± 0.08‰ (2SD, n = 15). This observation demonstrates that the Si isotope compositions of mantle peridotites may be not modified significantly during metasomatism by slab-derived fluids. Mass balance calculations indicate that the slab-derived fluids in the Franciscan Complex have Si isotope compositions close to those of mantle peridotites. The Si isotope compositions of the residual subducting slab may also not be changed significantly during metamorphic dehydration processes.

Accretionary unit formats in subduction complexes: examples from the Franciscan and Miura-Boso complexes

ABSTRACT Subduction accretionary complexes are composed of major and minor structural, stratigraphic, and tectonostratigraphic units. The major architectural units, Accretionary Units (AUs), are bounded by major faults, but differ from terranes in that they may contain units stratigraphically correlative with units in other AUs and they are smaller than the largest terranes. AUs occur in three basic formats – (1) singular sheets or blocks of stratigraphic layers, dismembered formation, or mélange, (2) folded units composed of one or more stratigraphic or block-in-matrix units, and (3) faulted stratigraphic masses (i.e., broken formations). Composite AUs with multiple units and multiple attributes are common. Multiple suites of structures may arise in AUs from progressive early deformation or later superimposed deformational events. AUs may be subdivided through detailed mapping into sub-units such as fault blocks, mélanges, dismembered formations, broken formations, intact formations, and members. Each AU should be defined on the basis of unique characters that derive from a thorough description of the AU, including its distinct rock types and character; and where possible, lithofacies, metamorphic facies, structures, and unit history. Descriptions of partially described AUs from the Franciscan Complex of California and the Miura-Boso area of Japan provide examples of the character of AUs. Ideally, the architecture and history of a subduction complex can be reconstructed by assembling detailed map and text descriptions of the constituent AUs, their 3D-positions, their relationships, and their histories.

Origin of Mélanges of the Franciscan Complex, Diablo Range and Northern California: An Analysis and Review

The Franciscan Complex of California is characterized in part by the presence of mélanges. In general, mélange origins are attributed to sedimentary, tectonic, or diapiric processes—or a combination of these. Published reviews list the main features of mélanges characteristic of each type of origin. In this review, particular diagnostic features typical of sedimentary, tectonic, and diapiric mélanges are used to assess 15 specific mélanges, which in some cases have been interpreted in contrasting ways in the literature. The data do not support the view that most Franciscan mélanges were formed by sedimentary processes, but rather that both tectonic and sedimentary processes are important. There is little evidence that diapirism contributed significantly to Franciscan mélange genesis. Tectonic features present in most mélanges of subduction accretionary complexes create challenges in assessing mélange-forming processes. Notably, although tectonic overprints commonly mask the primary diagnostic fabric of sedimentary mélanges, some diagnostic features—such as depositional contacts, fossils in mélange matrix, and interlayering of mélange and non-mélange units—are critical to recognition of mélanges of sedimentary origin.

Magnesium isotope record of fluid metasomatism along the slab-mantle interface in subduction zones

Fluids released from subducting slabs play an important role in mantle metasomatism and arc magmatism. Magnesium could be mobile in subduction-zone fluids and Mg isotopic signatures of mélange rocks may provide information on crust-mantle interactions at subduction zones. To investigate the behaviour of Mg isotopes during fluid metasomatism along the slab-mantle interface, we report Mg isotopic compositions of a set of well-characterized samples from the ultramafic blocks in the Franciscan Complex of California. The partially serpentinized and completely serpentinized peridotites have δ26Mg values (from −0.26 to −0.14) clustered around the mantle value, indicating small Mg isotope fractionation during peridotite serpentinization. Samples with serpentine completely replaced by talc have lower MgO contents and heavier Mg isotopic compositions (δ26Mg of −0.13 to −0.01) than serpentinized peridotites, implying that Mg was non-conservative during this metasomatic replacement. The shift toward higher δ26Mg values of talc-dominated metaperidotites reflects the preferential loss of light Mg isotopes into fluids via Rayleigh distillation during the dehydration reactions that produced talc from serpentine. On the other hand, samples with talc completely replaced by tremolite have lighter Mg isotopic compositions (δ26Mg of −0.50 to −0.41) and higher CaO contents than the talc-dominated samples. These features suggest a metasomatic process involving isotopically light carbonate-rich fluids derived from the subducting slab. Collectively, large Mg isotope fractionation occurs during multistage fluid-rock interactions within the Franciscan subduction channel, which provides direct evidence for significant Mg isotopic variation during fluid metasomatism along the slab-mantle interface. This may account for the observed heterogeneous Mg isotopic compositions of arc lavas. Magnesium isotopes thus could be a potentially useful tracer of crust-mantle interactions at subduction zones.

Whither the megathrust? Localization of large-scale subduction slip along the contact of a mélange

Long-lived subduction complexes, such as the Franciscan Complex of California, include tectonic contacts that represent exhumed megathrust horizons that collectively accommodated thousands of kilometres of slip. The chaotic nature of mélanges in subduction complexes has spawned proposals that these mélanges form as a result of megathrust displacement. Detailed field and petrographic relationships, however, show that most Franciscan mélanges with exotic blocks formed by submarine landsliding. Field relationships at El Cerrito Quarry in the eastern San Francisco Bay area suggest that subduction slip may have been accommodated between the blueschist facies metagreywacke of the Angel Island nappe above and the prehnite-pumpellyite facies metagreywacke of the Alcatraz nappe below. Although a 100–200 m-thick mélange zone separates the nappes, this mélange is a variably deformed, prehnite-pumpellyite facies sedimentary breccia and conglomerate deposited on the underlying coherent sandstone, so the mélange is part of the lower nappe. A 20–30 m-thick fault zone between the top of the mélange, and the base of the Angel Island nappe displays an inverted metamorphic gradient with jadeite-glaucophane-lawsonite above lawsonite-albite assemblages. This zone has a strong seaward (SW)-vergent shear fabric and hosts ultracataclasite and pseudotachylite. These relationships suggest that significant subduction megathrust displacement at depths of 15–30 km was accommodated within the 20–30 m-thick fault zone. Field studies elsewhere in the Franciscan Complex suggest similar localization of megathrust slip, with some examples lacking mélanges. The narrow megathrust zone at El Cerrito Quarry, its uniform sense-of-shear, and the localization of slip along the contact of, rather than within a mélange, contrast sharply with the predictions of numerical models for subduction channels.

Designating tectonostratigraphic terranes versus mapping rock units in subduction complexes: perspectives from the Franciscan Complex of California, USA

Accretionary complex histories are broadly understood. Sedimentation in seafloor and trench environments on drifting subducting plates and in associated trenches, followed by (1) deformation and metamorphism in the subduction zone and (2) subsequent uplift at the overriding plate edge, result in complicated stratigraphic and structural sequences in accretionary complexes. Recognizing, defining, and designating individual terranes in subduction complexes clarify some of these complicated relationships within the resulting continent-scale orogenic belts. Terrane designation does not substitute for detailed stratigraphic and structural mapping. Stratigraphic and structural mapping, combined with radiometric and palaeontologic dating, are necessary for delineation of coherent, broken, and dismembered formations, and various mélange units, and for clarification of the details of subduction complex architecture and history. The Franciscan Complex is a representative subduction complex that has evolved through sedimentation, faulting, folding, and low-temperature metamorphism, followed by uplift, associated deformation, and later overprinted deformation. Many belts of Franciscan rocks are offset by strike-slip faults associated with the dextral San Andreas Fault System. In the Franciscan Complex, among the terrane names applied widely, are the ‘Yolla Bolly Terrane’ and the ‘Central Terrane’. Where detailed mapping and detrital zircon ages exist, data reveal that the two names have been applied to rocks of similar general character and age. In the northeastern Diablo Range, Franciscan Complex rocks include coherent units, broken and dismembered formations, and various types of mélanges, all assigned at various times to the Yolla Bolly and other terranes. The details of stratigraphic and structural history revealed by large-scale mapping and radiometric dating prove to be more useful in clarifying the accretionary complex history than assigning a terrane name to the rocks. That history will assist in resolving terrane assignment issues and allow discrimination of subduction-associated and post-subduction events, essential for understanding the overall history of the orogen.

Petrogenesis of serpentinites from the Franciscan Complex, western California, USA

Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major and trace element compositions, relict mineral (spinel and pyroxene) compositions, and stable isotope (O, H, Cl) compositions with the goal of determining protolith origin and subsequent serpentinizing fluid sources in order to decipher the tectonic setting of serpentinization. We focused on serpentinite bodies found in the Franciscan Complex (west of Cuesta Ridge; south of San Francisco; Tiburon Peninsula; Healdsburg) (n=12). Three samples from Cuesta Ridge (part of the Coast Range ophiolite) were also analyzed for comparison. Serpentinites from Cuesta Ridge have flat to U-shaped chondrite-normalized REE patterns and spinels with Cr# values >0.60 implying a supra-subduction zone origin. In contrast, Franciscan serpentinites west of Cuesta Ridge and Tiburon Peninsula have positive-sloped REE patterns. This depletion in LREE is typical of abyssal peridotites. Most relict spinels have low Cr# values (<0.3) and relict clinopyroxenes from Tiburon Peninsula have high HREE concentrations, also supporting an abyssal origin. Franciscan serpentinite samples from south of San Francisco and near Healdsburg have U-shaped REE patterns and spinel compositions that lie within the forearc peridotite field with some overlap into the abyssal field and are of more ambiguous origin. All samples are high in fluid-mobile elements with remarkable positive Ce and Y anomalies. We speculate that these anomalies may be due to involvement of highly oxidizing fluids resulting in the preferential scavenging of Ce and Y by ferromanganese oxyhydroxides during serpentinization. All samples (except those south of San Francisco) have δ18O values of +5.4 to +7.9‰, typical values for oceanic serpentinites formed via low-T seawater hydration on the seafloor. δD values of all samples are extremely low (−107 to −90‰), likely the result of post-serpentinization, post-emplacement interaction with meteoric water at low temperature. Samples south of San Francisco lie on the San Andreas fault and have high δ18O values (+7.2 to +9.5‰) likely due to low-T interaction with meteoric water at high fluid–rock ratios. Most of the serpentinites have δ37Cl values between +0.2 and +0.9‰, typical values for serpentinites formed by interaction with seawater. Exceptions are those from the San Andreas fault and one sample from Cuesta Ridge with a high δ37Cl value (+1.7‰) possibly from interaction with a slab-derived fluid.

First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

The Franciscan Complex of California is a type example of an accretionary prism with widespread high-P/T subduction-zone metamorphism. Low-T, fine-grained, lawsonite-bearing mafic blueschists encased in a shale-matrix mélange near San Simeon have been dated for the first time. 40Ar-39Ar ages were obtained for phengite separated from four blocks and from an actinolitic rind on the margin of one of the blocks. The three blocks with lawsonite + epidote yield ages of between 154 and Ma, while the actinolitic rind yields an age of Ma. These ages are from the part of the Franciscan Complex west of the San Andreas Fault that moved northward at least 300 km with respect to the extensive Franciscan exposures east of the fault. The ages obtained in this study show that some fine-grained, low-T (lawsonite + epidote) Franciscan blueschist blocks formed at the same time as coarse-grained, high-T (garnet ± epidote) Franciscan blueschist blocks. These dated rocks indicate that both high- and low-T mafic blueschists are coeval and probably formed along 1000+ km of the North American Plate margin at ∼160–155 Ma. The episode of dynamic blueschist metamorphism was soon followed by high-Mg rind formation and a long period of nearly static high-P/T conditions.

The processes of underthrusting and underplating in the geologic record: structural diversity between the Franciscan Complex (California), the Kodiak Complex (Alaska) and the Internal Ligurian Units (Italy)

AbstractExisting studies on active subduction margins have documented the wide diversity in structural style between accretionary prisms, both in space and time. Together with physical boundary conditions of the margins, the thickness of sedimentary successions carried by the lower plate seems to play a key role in controlling the deformation and fluid flow during accretion. We have tested the influence of the subducting sedimentary section by comparing the structural style and fluid‐related structures of four units from three fossil accretionary complexes characterized by similar physical conditions but different subducting sediment thicknesses: (1) the Franciscan Complex of California, (2) the Internal Ligurian Units of Italy and (3) the Kodiak Complex, Alaska.Subducting plates bearing a thick sedimentary cover generally result in coherent accretion through polyphase deformation represented by folding and thin thrusting events, while underplating of sediment‐starved oceanic sections results in diffuse deformation and mélange formation. These two structural styles can alternate through time in a single complex with a long record of accretion such as Kodiak.The parallel analysis of the selected analogues show that although the volume of sediments carried by the lower plate determines different structural styles, deformation is strongly controlled by injection of overpressured fluids during underthrusting and accretion. Transient hydrofracturing occurs through the development of a system of dilatant fractures grossly parallel to the décollement zone. Copyright © 2009 John Wiley &amp; Sons, Ltd.

The b0 lattice parameter and chemistry of phengites from HP/LT metapelites

The statistical b 0 method is based on the fact that the cumulative b 0 values of white K-micas (WKM) from metapelites of a specific bulk composition can be used, in the lowermost T range of metamorphism, as a semiquantitative P indicator: mean b 0 values (and related celadonite content) in WKM increase with increasing P, thus characterizing the baric type of metamorphism. The solid petrological framework of this method includes specific constraints on rock bulk composition and need of other conditions. In this way, a reference b 0 scale for LT metapelites has become available in the literature, covering the whole range of metamorphic P. However, after many publications based on it, which made a large amount of data available, revision of this b 0 scale is now opportune, and the primary need is a better definition of the reference b 0 values and WKM chemistry for HP/LT metapelites. Such a better definition is the aim of this paper. The considered HP/LT metapelites come from various terrains: Schistes Lustres of W Alps, Diablo Range (Franciscan Complex of California), Phyllite-Quartzite Unit of Crete and Peloponnese, UHP gneiss from Dora Maira, Attic-Cycladic Complex of Sifnos, Tavsanli Zone and Nilufer area of NW Turkey. In addition, WKM chemical and b 0 data were used from various sources. The new chemical and b 0 data (EMPA and XRPD) allowed us: ( i ) to establish the reference b 0 and chemistry of WKM from HP/LT metapelites: b 0 turns out to be around 9.040 A; ( ii ) to confirm and extend to HP/LT metapelites the petrological basis of the b 0 method, particularly the control of mineral assemblages on b 0 values; ( iii ) to glimpse more complex relationships between b 0 and Si than those currently known, and particularly the importance of the Fe 2+ /(Mg + Fe 2+ ) ratio; ( iv ) to ascertain the validity of the b 0 method for detecting generic HP/LT conditions; (ν) to ascertain the inability of the b 0 method to discriminate, within the above HP field, differences between various P/T regimes, mainly due to still poorly known crystal-chemical problems, and the fact that comparative b 0 interpretations of various HP/LT sample populations from different belts are impeded by poor understanding of the extent of reequilibration of WKM at higher T and lower P during exhumation.

Chromium geochemistry in serpentinized ultramafic rocks and serpentine soils from the Franciscan complex of California

<h3>BACKGROUND AND PURPOSE:</h3> Optimal diagnosis and management of spontaneous intracranial hypotension remains uncertain. CT-guided blood patching has been described but has not been evaluated in larger case series. We sought to evaluate the efficacy of CT-guided blood patching of observed or potential CSF leaks in spontaneous intracranial hypotension. <h3>MATERIALS AND METHODS:</h3> Patients referred for evaluation of spontaneous intracranial hypotension were retrospectively reviewed. Inclusion criteria were findings of intracranial hypotension on pretreatment brain MR imaging, evaluation and treatment with CT-guided myelography and blood patching, and availability of posttreatment brain MR images. Eight patients met inclusion criteria. Imaging findings, treatment details, and clinical outcomes were assessed. <h3>RESULTS:</h3> Pretreatment imaging findings included dural enhancement, tonsillar ectopia, subdural collections, and syrinx. All findings resolved or significantly improved on posttreatment imaging. Presenting clinical symptoms included positional headache, neck/interscapular/shoulder pain, and tinnitus. Headaches and neck/interscapular/shoulder pain improved in all patients; tinnitus improved in 1 of 2 patients. CSF leak sites were directly visualized in 37% of patients and were targets for patching when seen. When no direct visualization of leaks was seen, irregular spinal nerve root diverticula were targeted as potential leak sites. The average number of blood patching sessions was 3 (range, 1–6) and the average number of individual sites patched per session was 5 (range, 1–10). <h3>CONCLUSIONS:</h3> Our results suggest that CT-guided blood patching targeting observed or potential leak sites can be effective in the treatment of intracranial hypotension. Prospective controlled studies are needed to confirm efficacy and compare outcomes with other treatment options.

Lithic‐volcanic sandstones derived from oceanic crust in the Franciscan Complex of California: ‘sedimental memories’ of source rock geochemistry

ABSTRACTThe Permanente terrane, part of the Franciscan accretionary complex of California, contains an unusual fault‐bounded packet bearing lithic‐volcanic sandstones and conglomerates. Based on petrographical, sedimentological, stratigraphical and geochemical studies, it is concluded that these rocks were deposited on the flank of an oceanic rise or plateau by sediment gravity flows probably below the carbonate compensation depth. The bulk geochemistry of the sandstones is similar to the chemistry of greenstones in the Permanente terrane, suggesting that the greenstones or rocks similar to them represent the source of the lithic‐volcanic sandstones, and that the sandstones geochemically ‘remember’ the chemistry of their source rocks. This ‘sedimental memory’ is impressive: geochemical variation in the sandstones is co‐linear with presumable magmatic variation trends in the greenstones. Sandstone geochemistry, plotted on Zr/Y vs. Y and V vs. Ti diagrams, shows that compositions occupy similar fields as greenstones in the Permanente terrane and oceanic crustal compositions in general.Because oceanic crustal sequences can become accreted to continental margins, oceanic crust should be considered a potential source of sediment, especially in continental margin settings. However, differentiating sandstones derived from oceanic arcs and from oceanic crust is extremely difficult unless discriminating geochemical criteria are used.

Paleomagnetic results from the Upper Cretaceous Maudlow and Livingston formations, southwest Montana

A paleomagnetic investigation of welded tuff units from the Late Cretaceous Livingston and Maudlow Formations was undertaken to refine the Late Cretaceous to Paleocene APWP for cratonic North America. After progressive AF and thermal demagnetization, the Maudlow Formation (46.1°N, 248.9°E) yields a paleomagnetic pole at 70°N, 208°E (11 sites, A95 = 9°, K = 22). Tectonic correction improves the directional grouping, and the change is statistically significant. Two normal polarity intervals bounding a single reversed polarity interval are present. We interpret the reversed interval to be Chron 33r, on the basis of: [1] stratigraphic correlation of the Maudlow Formation with the Campanian Sedan Formation located 40 km to the east, and [2] K‐Ar dates of 83 ± 2 Ma at the middle of the sampled section and 74.9 ± 1 Ma near the top of the sampled section.The Livingston Formation (45.5°N, 250.1°E) yields a high Hc magnetization with a negative fold test, suggesting a post‐folding secondary magnetization. The Livingston mean direction yields a paleomagnetic pole at 72°N, 218°E (10 sites, A95 = 4°, K = 107) and is clearly distinguishable from a co‐existing lower Hc component reflecting a recent viscous overprint. We interpret the high Hc magnetization as Late Cretaceous to Paleocene in age.Neither the Maudlow Formation pole nor the pole derived from the high Hc overprint in the Livingston Formation agree with previously published poles for either the Late Cretaceous or Tertiary, but they do lie on a simple continuation of the APWP between Late Cretaceous and Paleocene mean poles. Secondary magnetizations found in the Franciscan Complex of California also yield poles which lie on a proposed extension of the APWP. If the Franciscan secondary poles represent an overprint event associated with early Tertiary accretion, and if the Livingston secondary pole represents a deformational event related to Laramide uplift of the nearby Beartooth Range, then together these three poles suggest that the North American APWP may continue farther to the east and south to define a Cretaceous‐Tertiary cusp.

Aptian anoxia in the Pacific Basin

Marine strata of Aptian age in the Pacific Basin include two distinct levels that represent episodes during which sediments rich in organic carbon were deposited. Both episodes lasted less than 1 m.y., as revealed in strata deposited atop submerged topographic highs. One unusually widespread episode of early Aptian age (∼117.5 Ma) correlates with coeval units in Europe and thus is analogous to the Cenomanian-Turonian oceanic anoxic event in its short duration and wide geographic extent. The second episode of late Aptian age (∼116.5 Ma) is restricted to allochthonous pelagic deposits in the Franciscan Complex of California. These results support the concept of widespread and narrowly synchronous anoxic events. Further, they show that organic carbon deposition in the Pacific Basin took place in intermediate water oxygen-minimum zones and thus differed in the mode of deposition, and hence paleoceanography, from that in other middle Cretaceous ocean basins.

Thermo-mechanical modelling of arc-trench regions

The thermal structure of the shallow part of a subduction zone, i.e. the region between the trench and the volcanic line, is calculated with a finite difference method. Published heat flow measurements, in combination with rheological arguments and the distribution of interplate thrust earthquakes, are used to constrain the thermal structure of this region and the magnitude of shear stresses acting on the plate contact. A pressure and temperature dependent rheology is used to model shear stresses. From our thermo-mechanical modelling it follows that temperatures at the plate contact and within the upper plate are determined by the subduction of cold material and by frictional heating. For models that satisfy the constraints the average shear stress at the plate contact (between the trench and the volcanic line) varies from about 10 to about 40 MPa and shear stresses during brittle deformation range from 2.5 to 7.5% of the lithostatic pressure. For a wide range of convergence velocities (4–12 cm/yr) and ages of the subducting oceanic lithosphere (30–150 Ma), shear stresses and temperatures at and above the plate contact are essentially independent of these parameters. Temperatures at the upper surface of the slab for the preferred thermal models are in good agreement with pressure-temperature conditions during high-pressure metamorphism inferred from mineral assemblages in the Franciscan Complex of California. Frictional heating has a large influence on the conditions for high-pressure metamorphism.

Temperatures and shear stresses in the upper part of a subduction zone

The temperature structure of the upper part of a subduction zone, i.e. the region between the trench and the volcanic line, has been investigated by means of numerical modelling. Published heat flow measurements are used to constrain the thermal structure of this region and the shear stresses at the plate contact. A pressure and temperature dependent rheology is used to model shear stresses. It is shown that temperatures at the plate contact and within the upper plate are primarily determined by frictional heating. Except for the subduction of very young oceanic lithosphere (less than 30 Myr old), these temperatures, as well as shear stresses, are essentially independent of the age of the subducting oceanic lithosphere and the convergence velocity between the two plates. Pressure‐temperature conditions at the upper surface of the slab for the proposed thermal model are in good agreement with conditions inferred from high‐pressure metamorphic material of regional extent in the Franciscan Complex of California.

Sediment provenance within subduction complexes—An example from the Aleutian forearc

The ultimate composition of any sandstone is affected by a host of primary and secondary factors, including the lithologies present in source terranes, climate, depositional environment and diagenesis. In the case of a subduction complex, however, unequivocal identification of detrital provenance may be impossible because of the cumulative effects of tectonic and sedimentary transport. Long-distance sedimentary transport (> 1000 km) is common within trenches, and abyssal-plain turbidites can be tectonically transported for long distances as the underlying oceanic basement drifts towards a subduction front. Post-accretionary displacement can occur as a consequence of strike-slip faulting, and the total distance of tectonic dislocation may reach several thousand kilometers. The present-day Aleutian forearc region (North Pacific Ocean) illustrates many of the “problems” which typify subduction zones. Several petrologic suites can be identified, and there are significant variations in detrital modes in both time and space. The Aleutian region serves as a sobering modern analog for accreted rock units such as the Franciscan Complex of California, where intercalations of discrete sandstone suites have been noted. In the absence of paleomagnetic control, interpretations of sediment provenance within ancient subduction complexes probably should be restricted to the generic level.

Thermal evolution of convergent plate margins: Thermal modeling and reevaluation of isotopic AR‐ages for Blueschists in the Franciscan Complex of California

Modeling of heat flow through a wedge‐shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of Ar‐isotopic ages and metamorphic progressions across blueschist belts. If convergence rates are maintained at rates of a few centimeters per year or more, material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where Ar‐loss by diffusion and recrystallization effectively ceases. Maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider. Because of this difference in thermal history, K‐Ar and 40Ar/39Ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth. Application of this thermal modeling to the Franciscan subduction complex of northern California indicates that the coherent blueschists in the Eastern Belt (typically with 115 to 125 m.y. Ar‐isotopic ages) and the high‐grade blueschist blocks in the mud matrix melanges of the western portion of the Central Belt (typically with 140 to 150 m.y. Ar‐isotopic ages) could both have formed during a single, prolonged period of continuous convergence.

Mineral chemistry of some Franciscan blueschist facies metasedimentary rocks from the Diablo Range, California: Summary

In recent years it has been demonstrated that much of the Franciscan Complex of California has undergone zeolite or blueschist facies metamorphism (McKee, 1962a, 1962b; Coleman and Lee, 1962, 1963; Ernst, 1965, 1971a, 1971b, 1971c; Bloxam, 1956, 1960, among others). Observed mineral assemblages are consistent with experimentally derived mineral stabilities and are suggestive of progressive metamorphism in a subduction-zone environment (Ernst, 1971a, 1971b). The mineral chemistry of some of these Franciscan metamorphic rocks has been studied; most investigations have centered on individual minerals-sodic amphibole (Coleman and Papike, 1968), sodic pyroxene (Essene and Fyfe, 1967; Coleman and Clark, 1968), and garnet (Lee and others, 1963)-with particular emphasis on high-grade exotic blocks. A few studies have been conducted to characterize specific metamorphic terrances within the Franciscan (Crawford, 1975) or to compare. Franciscan metamorphism with that of other subduction-zone complexes (Ernst and Seki, 1967; Ernst and others, 1970). Thus far, however, little work has been done to compare the composition of minerals crystallized at different grades of metamorphism in Franciscan metasedimentary terrances.

Mesozoic-cenozoic tectonic history of the southwestern Oregon coast in relation to cordilleran orogenesis

Mesozoic rocks of the Cordilleran mobile belt extend to the continental margin on the southwestern Oregon coast. The region has been eugeosynclinal since at least the early Paleozoic, but dominance of continental-type andesites suggests the presence of essentially continental crust throughout its known history. Difficulties in understanding the history of the central Pacific coast have centered around problems of the subdivision, dating, and correlation of seemingly homogeneous graywacke terranes. Detailed mapping, sedimentary petrography, and isotopic dating provide considerable clarification in Oregon. The Jurassic Dothan formation (about 150 m.y. old) now appears clearly older than the Rogue and Galice formations. All three units were involved in the late Jurassic Nevadan orogeny which occurred here 135 to 145 m.y. ago. None of the three is equivalent to any paleontologically dated part of the largely post-Nevadan Franciscan complex of California (latest Jurassic to late Cretaceous) some unfossiliferous older strata, however, are almost certainly included in the Franciscan. It appears that during the Cenozoic the Franciscan and the older Oregon rocks were structurally juxtaposed by shearing in northwest California where the California Coast Ranges abut against the Klamath province. Diastrophism has been nearly continuous in southwestern Oregon. Plutonic and metamorphic activity culminated in the late Jurassic between 135 and 145 m.y. ago (Nevadan), but more volcanism and severe orogenic disturbances soon followed. Deformation in the westwardly-convex Klamath structural arc continued, though diminishing, into the early Tertiary. Many specific, short orogenic events in the Cordillera have been named, but it is suggested that Cordilleran orogeny or revolution be adopted for the entire sequence of late Mesozoic through early Cenozoic orogenesis in western North America. The most striking subsequent Cordilleran structural discontinuity occurred in the mid-Cenozoic. Cascadan orogeny is suggested for a late Cenozoic diastrophic sequence during which the entire continental margin has been increasingly fragmented by intense faulting accompanied by widespread volcanism. Great northwest-trending shear zones were superimposed upon the coastal edge of the Klamath arc during this youngest orogenesis.