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GeoScienceWorld Lithosphere Volume 2020, Article ID 8853351, 19 pages https://doi.org/10.2113/2020/8853351

Research Article Metamorphic Temperatures and Pressures across the Eastern Franciscan: Implications for Underplating and Exhumation

William L. Schmidt and John P. Platt

University of Southern California, Earth Sciences Department, 3651 Trousdale Pkwy, Los Angeles, CA 90089, USA

Correspondence should be addressed to William L. Schmidt; [email protected]

Received 23 July 2019; Revised 1 March 2020; Accepted 10 August 2020; Published 9 November 2020

Academic Editor: Patricia Durance

The Eastern Belt of the Franciscan Complex in the northern California Coast Ranges consists of coherent thrust sheets predominately made up of ocean floor sediments subducted in the Early Cretaceous and then accreted to the overriding plate at depths of 25-40 km. Progressive packet accretion resulted in the juxtaposition of a series of thrust sheets of differing metamorphic grades. This study utilizes laser Raman analysis of carbonaceous material to determine peak metamorphic temperatures across the Eastern Belt and phengite barometry to determine peak metamorphic pressures. Locating faults that separate packets in the field is difficult, but they can be accurately located based on differences in peak metamorphic temperature revealed by Raman analysis. The Taliaferro Metamorphic Complex in the west reached 323-336°C at a minimum pressure of ~11 kbar; the surrounding Yolla Bolly Unit 215–290°C; the Valentine Springs Unit 282-288°C at 7:8 ± 0:7 kbar; the South Fork Mountain Schist 314–349°C at 8.6–9.5 kbar, a thin slice in the eastern portion of the SFMS, identified here for the first time, was metamorphosed at ~365°C and 9:7 ± 0:7 kbar; and a slice attributed to the Galice Formation of the Western Klamath Mountains at 281 ± 13°C. Temperatures in the Yolla Bolly Unit and Galice slice were too low for the application of phengite barometry. Microfossil fragments in the South Fork Mountain Schist are smaller and less abundant than in the underlying Valentine Springs Unit, providing an additional method of identifying the boundary between the two units. Faults that record a temperature difference across them were active after peak metamorphism while faults that do not were active prior to peak metamorphism, allowing for the location of packet bounding faults at the time of accretion. The South Fork Mountain Schist consists of two accreted packets with thicknesses of 300 m and 3.5 km. The existence of imbricate thrust faults both with and without differences in peak metamorphic temperature across them provides evidence for synconvergent exhumation.

1. Introduction thus of both societal and geotectonic importance. Despite many decades of investigation, however, we still have a very Accretionary wedges such as the Franciscan Complex of Cal- poor understanding of some fundamental questions about ifornia represent the near-surface expression of the subduc- these complexes. The thermal structure is in a state of tion zone interface, and as such have considerable extreme disequilibrium, with very low gradients parallel to geodynamic significance. The rheology of the accreted mate- the subduction zone, and at certain times and places, very rial may affect rates of subduction and the transfer of stress high inverted gradients normal to the subduction zone inter- from the subducting to the upper plate [1]; and on shorter face (e.g., [10, 11]). Mechanisms of accretion at depth time-scales controls the frequency and magnitude of subduc- (underplating) are debated, in part because of uncertainties tion zone seismicity [2]. Both solid and fluid materials are in the thermal structure, fluid pressure, metamorphic reac- recycled on various scales in subduction zones by mecha- tions, and the distribution and rheologies of the various rock nisms that are still very much under investigation [3–9]. types involved [12, 13]. The mechanisms by which rocks bur- The thermal and mechanical structure of accretionary com- ied to depths of 50 km or more in subduction zones are plexes, and the kinematics of deformation within them, are exhumed are even more strongly debated, particularly as they

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are likely to disturb or fundamentally modify the internal [25] and to the Tehama-Colusa Melange, both deformed structure of the accretionary complex [14]. For all these rea- ocean floor units that are in fault contact with each other sons, further structural, petrological, and geochemical inves- [26]. These ocean floor rocks have an uncertain origin and tigation of accretionary complexes is required. are overlain by the basinal sediments of the Great Valley The Franciscan Complex of California was one of the first Group to the east [27]. In some locations, there are small to be recognized as a fossil accretionary wedge [15], yet its slices of low-grade metasediments between the Coast Range internal structure and the distribution of metamorphic tem- Fault and the ophiolitic rocks (Figure 1); these have been ten- perature and pressure are still very poorly defined. It consists tatively attributed to the Jurassic Galice Formation of the primarily of ocean-floor sediments and lesser amounts of Western Klamath Mountains [28]. The Coast Range Fault mafic material that were accreted both by large-scale frontal was originally interpreted to be the subduction zone mega- accretion and by underplating along the convergent margin thrust [15], but most workers have since recognized it to have of North America in Cretaceous to Paleogene time [16]. been reactivated with a normal sense of motion, contributing The underplated material experienced significant exhuma- to the exhumation of the underlying Franciscan [8, 29–32]. tion between 100 and 70 Ma [17, 18]. High pressure/low tem- Contrary to this view is the argument that exhumation was perature metamorphism is widespread and, while extensive driven by erosion, rather than by extensional faulting [33– coherent terranes are present, especially in the eastern por- 35]. tion of the Franciscan, there are also significant bodies of In the Coast Ranges of northern California and southern mélange [19]. Structural analysis of the Franciscan is difficult Oregon, the Franciscan has traditionally been divided into due to the combination of complex structure and the monot- three belts, the Coastal Belt, the Central Belt, and the Eastern onous lithological sequence of predominately metapelites Belt (Figure 1), based on the timing of accretion, the grade of and metagreywackes. metamorphism, and the overall structural style [36, 37]. With The recent development of multiple geothermometers the exception of the younger King Range terrane, the Coastal based on laser Raman analysis of carbonaceous material Belt was accreted between 65 and 45 Ma [38] and is only (LRCM) has extended the lower bound of the temperature lightly metamorphosed [19]. It experienced a maximum tem- range that can be investigated with this method, making it a perature close to the closure temperature for fission tracks in viable method for determining peak metamorphic tempera- apatite and cooled between 20 and 10 Ma [18]. Much of the tures in the low temperature Franciscan Complex. This study Central Belt is composed of deformed pelitic material carry- uses the geothermometer of Kouketsu et al. [20] together ing distributed exotic blocks [36], fitting the definition of a with phengite barometry to better constrain peak metamor- mélange as proposed by Hsü [39] and, after being largely phic pressure and temperature attained across multiple tran- accreted between 95 and 88 Ma [38], attained metamorphic sects in the coherent terranes of the Eastern Franciscan Belt. temperatures of 100-250°C and pressures of 3-10 kbar [40]. This allows a much more detailed definition of the metamor- The Eastern Belt was accreted and metamorphosed in the phic zonation in this part of the Complex. In a region that Early Cretaceous; it is the oldest and highest grade of the consists of imbricate thrust packets of differing metamorphic three belts [17] and is described in more detail below. The grade, peak metamorphic temperatures can also be used to peak temperatures of the three belts reveal a pattern of a reveal the location of tectonic boundaries. The combination metamorphic grade that increases to the east across the Fran- of improved structural constraints and improved tempera- ciscan as a whole. The Eastern and Central Belts both experi- ture and pressure determinations also allows for more enced significant synsubduction exhumation between 100 focused discussion of the processes of underplating and and 70 Ma, as determined by zircon fission track analysis exhumation that have led to the present large-scale distribu- [18, 41]. tion of these units. 2.2. Eastern Belt. The Eastern Belt consists mainly of coherent 2. Geologic Setting thrust sheets with some intercalated mélange units. Meta- greywackes and metapelites are the dominant lithologies 2.1. The Mesozoic Accretionary Margin in California. Califor- and are accompanied by lesser amounts of metabasalt and nia is dominated by three NW-SE trending tectonic regions, chert. Metamorphic grade is thought to increase to the east the Franciscan Complex, the Great Valley Group, and the internally across the Eastern Belt, as evidenced by changes Sierra Nevada batholith. Created by the subduction of thou- in mineral assemblage and degree of quartz recrystallization. sands of kilometers of sea floor beneath North America [21], The change in mineral assemblage is predominately reflected these three zones are, respectively, the accretionary prism, by the presence of pumpellyite only in the west, and the pres- forearc basin, and magmatic arc of the subduction zone ence of progressively coarser-grained lawsonite and sodic [15, 22]. Subduction began in the Middle Jurassic (Waka- amphibole towards the east [42–44]. The presence of epidote bayashi, 1992; [23]) and ended with a transition to a trans- in only the easternmost margin of the Eastern Belt has also form boundary during the Neogene [24]. The E-dipping been noted as an indication of varied metamorphic grade Coast Range Fault, which approximates the position of the [45]. Suppe [45] identified one unit, the Taliaferro Metamor- paleosubduction zone, separates the Franciscan from essen- phic Complex (TMC), that does not conform to the pattern tially unmetamorphosed ophiolitic rocks and the Great Val- of eastwardly increasing metamorphic grade. This is a fault- ley Group to the east [15]. The ophiolitic rocks to the east bounded body of blueschist-facies metasedimentary and of the Coast Range Fault belong to the Coast Range Ophiolite metabasaltic rocks that is intercalated with lower grade

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Figure 1: Study area, adapted from Schmidt and Platt [57] and Jayko and Blake [28]. Studied portions of stream beds are marked in red. Inset: sketch of tectonic map of northern California Coast Ranges. TMC = Taliaferro Metamorphic Complex; SFMS = South Fork Mountain Schist.

lawsonite-albite facies metasediments. There are no reliable while metamorphic grade increases across the Franciscan as a age data available for the TMC [17]. In contrast to the view whole, it does not vary signiﬁcantly within the Eastern Belt. that metamorphic grade increases regularly to the east, The Eastern Belt has been subdivided into the Yolla Bolly Bröcker and Day [46] have argued that, save for the epidote terrane to the west and the Pickett Peak terrane to the east in the eastern SFMS and the unusual TMC, all variations in [43]. Both terranes were metamorphosed under high- mineral assemblage within the Eastern Belt can be explained pressure low-temperature conditions as evidenced by the as a result of diﬀerences in composition. This view holds that presence of lawsonite, blue amphibole, and sporadic jadeite

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[45, 47–51], but the Yolla Bolly terrane is thought to be of The metamorphic age was determined by whole rock somewhat lower grade (lawsonite albite facies) than the over- 40Ar/39Ar analysis, yielding an average age of 117 Ma [58]. lying Pickett Peak terrane (blueschist facies). The lowest esti- Peak temperatures in the Valentine Springs Formation are mated temperatures are 100-187°C ([43, 46], and references thought to be 240–280°C [43, 46], while peak pressures are therein). The TMC lies within the Yolla Bolly terrane and thought to be ~5.5–7 kbar [35, 43]. was subjected to higher temperatures than the main body The SFMS is the oldest and highest-grade coherent unit of lawsonite-albite facies rocks; its peak metamorphic tem- in the Pickett Peak terrane and in the Franciscan. It is made perature was estimated to be 300°C by Suppe [45], while up of pelites metamorphosed into quartz-mica schist as well Bröcker and Day [46] calculated 240°C–280°C based on as metagraywacke, metabasalt, and chert. Estimates for peak phase equilibria. Peak pressures are thought to have been temperatures range from <200°C to 350°C, while peak pres- ~7 kbar [43] and 8.5-10 kbar ([46], and references therein) sure is estimated at ~7 kbar with upper pressure limits in for the main body of the Yolla Bolly and the TMC, respec- the range of 8–10 kbar, based on the absence of jadeitic clin- tively. Ar-Ar dating of metamorphic white mica has placed opyroxene ([43, 46, 48], and references therein). Detrital zir- the age of metamorphism of the main body of the Yolla Bolly con data places deposition between 131 and 137 Ma while at ~110 Ma, while the age of its deposition is interpreted to be metamorphic ages for the SFMS were determined by ~111 in northern California based on detrital zircons [17, 40Ar/39Ar step heating of white micas, yielding a weighted 52]; depositional ages are seen to decrease to ~100 Ma in average age of 120:8±1:2Ma(2σ error) [17]. The white mica the San Francisco Bay area and to ~89 Ma in the Nacimiento ages are interpreted as dating the growth of metamorphic block further to the southeast [51, 53, 54]. white mica, with the differences in ages between the Valen- The Pickett Peak terrane has been further subdivided into tine Springs and the SFMS being due to the progressive sub- the Valentine Springs Unit in the west and the South Fork duction and accretion of each unit, creating the current Mountain Schist (SFMS) in the east [43]. Both units consist imbricate thrust geometry. predominantly of metagreywacke and pelitic schist, but the SFMS includes a body of metabasalt ~1 km thick, which 3. Methods was named the Chinquapin Member by Blake et al. [42]. Unit divisions within the Pickett Peak terrane, and within the 3.1. Temperature. Amorphous carbon becomes progressively Eastern Belt as a whole, were defined based on the textural more graphitized with increasing temperature, and this characteristics of medium-grained greywackes. Blake et al. increase in crystallinity can be measured. Because the process [42, 47] defined three textural zones, ranging from an unme- is not reversed when the carbonaceous material (CM) is tamorphosed appearance in hand sample to totally recrystal- cooled, peak metamorphic temperature is recorded [59]. To lized. The South Fork Mountain Schist (SFMS), the measure the degree of crystallinity, a laser is focused on the easternmost unit, was defined as belonging to textural zone sample. A portion of the incident photons induce vibrations three, totally recrystallized. In those papers, no faults were in the molecular bonds of the sample, and the wavelength of described within the Eastern Belt, an interpretation which the scattered photons is changed as a result. The change in favored a gradational contact between the SFMS and under- wavelength is measured and different wavelength changes lying units. Subsequent work identified a fault along the basal correspond to different vibrational modes. Measurement of contact of the SFMS [44, 45, 55]. Suppe [45] named this fault amorphous carbon results in a larger number of vibrational the Log Spring Thrust (LST) and identified an abrupt change modes than does measurement of graphite. When graphi- in textural grade across the contact. Worrall [44] then identi- cally plotted, these additional wavelength changes are termed fied additional faults and described a series of eastward dip- defect bands and are numbered, e.g., D1, D2, etc. The wave- ping imbricate thrust faults as the defining structures of the length change corresponding to graphite is termed the G Eastern Belt (Figure 2), also finding an abrupt change in tex- band (Figure 3) [60], and references therein). Laser Raman tural grade across the LST. Suppe [45], Worrall [44], and analysis of CM was used to assess peak metamorphic temper- Jayko et al. [43] all used subtly different definitions of the tex- atures across the Eastern Belt. tural zones, and Jayko et al. [43] did not find an abrupt tran- Samples of metapelite and metagreywacke were collected sition in textural grade across the LST. Disagreements over from streambeds across the Eastern Belt following the struc- textural grade classifications can arise because different scales tural transects shown in Figure 1 and described by Schmidt are used by different workers, because classifications are and Platt [57]. The main focus of this study is the Thomes qualitative, and because the degree of quartz recrystallization Creek transect and the Middle Fork Eel River transect; the depends not only on metamorphic grade but also on proto- Cottonwood Creek, Grindstone Creek, and Salt Creek tran- lith grain size and intensity of deformation. In addition to sects sample the same units at different latitudes and were disagreements over the nature of the basal contact of the used to establish the regional applicability of our results. SFMS, there are also disagreements over its location; within These supplemental transects were studied at a reconnais- Thomes Creek (Figure 1) different workers have identified sance level, rather than in detail. The extent of the Thomes the LST in locations separated by as much as two kilometers Creek transect was designed to capture the differences in [45, 56, 57]. metamorphic temperature between the Galice, the SFMS, The Valentine Springs Formation in the area of this study and the Valentine Springs Units. The crucial boundaries that is dominated by metagraywacke sandstones that were depos- were investigated were the CRF, which separates the Galice ited ~123 Ma, as determined by detrital zircon analysis [17]. from the SFMS, and the LST, as mapped by Schmidt and

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W E Taliaferro Metamorphic South Fork Complex Coast Great Mountain Range Valley Schist Ophiolite Group

Undiﬀerentiated lawsonite-albite facies rocks 10 km

Figure 2: Simpliﬁed cross-section of the Eastern Belt Franciscan, from Schmidt and Platt [57].

the samples. All analysis points were selected to be slightly below the surface of the thin section, to avoid analyzing CM damaged by polishing [62]. Analyses were carried out on a Renishaw M1000 Micro Raman Spectrometer System at the California Institute of Technology using a 2400 lines/mm grating and a 514 nm green laser, with collection D1 D2 times of 30 seconds. Laser power on the sample surface was D4 D3 G 2-3 mW. Peaks were then deconvolved using the computer program PeakFit 4.12 (SeaSolve Software Inc.). All samples 1100 1300 1500 1700 in this study had a minimum of 12 analysis spots. Absolute error for Raman analysis is typically taken as ~50°C (e.g., Raman shi (cm–1) [60]). Errors reported in this paper are measurement errors, reported at 1σ based on the ~12 measurements per sample, Figure 3: Raman spectra (black) deconvolved into G and D bands. and errors reported in figures are as described in captions. This study processed the Raman spectra following the Platt [57], which separates the SFMS from the Valentine method developed by Kouketsu et al. [20]. It was important Springs. The Middle Eel transect was designed to reveal tem- for this study to use a single geothermometer to assess the perature differences between the TMC and the surrounding entire range of recorded temperatures, as a major goal of lawsonite-albite facies rocks. Samples were selected based our work is locating abrupt changes in recorded temperature on CM content and on location; multiple samples were col- across faults. If separate geothermometers are used on either lected from every identifiable fault-bounded tectonic block, side of a fault, it would be possible to ascribe the difference in with enough sample density to capture differences in peak recorded temperature to differences between competing cali- metamorphic temperature across the unit. brations. When using a single calibration, the relative differ- As part of their work calibrating a geothermometer for ence in temperature between two areas is independent of the contact metamorphism, Aoya et al. [61] explored the differ- accuracy of the absolute temperature and, as such, is a more ences between two calibrated Raman geothermometers, one reliable indicator of a real difference in peak metamorphic meant for use on deformed, regionally metamorphosed rocks temperature. There are four regional metamorphism calibra- and one for use on weakly or undeformed rocks that had tions to consider. The original Raman thermometer devel- experienced contact metamorphism. They found that, while oped by Beyssac in 2002 is applicable in the range of there are meaningful differences between the calibrations 330°C–650°C and is inappropriate for the lower temperature for highly ordered CM, corresponding to higher peak meta- rocks of the Eastern Belt. The calibration of Lahfid et al. [63] morphic temperatures, this is not true for low temperature, is useful in the range of 200°C–320°C, a range that is too small disordered CM with R2 ratios (R2 = D1/ðG + D1 + D2Þ to capture the higher temperature portions of the Eastern greater than 0.6. The two calibrations produce the same Belt. Rahl [64] developed a calibration for the range 100°C– results at ~345°C, and differences between them are nearly 700°C. Because there is not a unique solution when deconvol- negligible for disordered CM corresponding to investigated ving the G and D2 bands with this method, it is inappropriate temperatures below ~375°C. A full study of the effect of for some samples and was not used. In contrast, Kouketsu deformation on Raman spectra is beyond the scope of this et al.’s [20] thermometer is applicable in the range of paper. As this previous work suggests that the effect of defor- 150°C–400°C, sufficient for capturing expected Eastern Belt mation is negligible for rocks that have experienced peak temperature variation. It provides two methods for deter- metamorphic temperatures consistent with those found in mining temperature, one based on the full width at half max- the Eastern Belt of the Franciscan, no consideration was imum height (FWHM) of the D1 band and one based on the given to the degree of deformation when collecting samples. FWHM of the D2 band. FWHM-D1 is the preferred method Samples were cut perpendicular to foliation and parallel for temperatures between 200°C and 400°C, and it was used to lineation before being investigated using LRCM within for all calculations in this study.

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3.2. Pressure. The Si content of white mica generally increases 4. Results with increasing pressure from 3 Si atoms per formula unit (apfu) in muscovite to 4 Si pfu in celadonite via the Tscher- 4.1. Temperature ð Þ VI IV mak substitution Fe, Mg SiAl−1Al−1, allowing pressure to be determined based on the Si content of phengite [65]. 4.1.1. Thomes Creek. Peak metamorphic temperatures vary The Si content of white mica was analyzed at the University spatially across the transect. The highest temperatures of California Los Angeles, using a JEOL JXA-8200 electron obtained are in the eastern portion of the transect, while the fi lowest temperatures are in the western portion; the full tem- microprobe equipped with ve wavelength-dispersive spec- ° ff trometers. Analyses were performed using the following con- perature range is 282-377 C (Figure 4). The di erences in temperature are also seen qualitatively in changes in Raman ditions: an accelerating voltage of 15 kV, a sample current of ff 15 nA, and a spot size of 5 μm. Correction of measured inten- spectra across the transect (Figure 5). Di erences in peak sities used the ZAF method. Natural and synthetic standards metamorphic temperature occur across some mapped faults, but not across all faults. The CRF separates the Galice slice, were used to calibrate the microprobe; for major oxides (>0.2 ° wt %), standard deviations were ≤8.65% of the total mea- which attained a temperature of 281 ± 13 C, from a small slice of SFMS which reached temperatures as high as 377°C. sured value. Quantitative compositional X-ray maps were fi made of phengite grains to check for the presence of compo- An unnamed thrust zone that duplicates a thin slice of ma c sitional zoning, and were made on the same machine using schist separates this small, high-temperature slice from the the following analytical conditions: 15 kV accelerating volt- main body of the SFMS, which experienced temperatures of μ 314-349°C. The previously estimated range for SFMS tem- age, 160 ms dwell time, 2 m step size, and a 95-150 nA sam- ° ple current. Data analysis and calibration were done using peratures was 200-350 C (Bröcker and Day, and references the program XMapTools 3.3.1 [66, 67]. Standardization was therein). Our temperature results that are higher than these fi done using high-quality measurements performed within previous estimates are con ned to the small, high- the X-ray map area [68]. All Si apfu reported or discussed temperature slice of the SFMS in the eastern portion of the in this study are based on 11 oxygen atoms. transect. Our results for the main body of the SFMS, 314- ° To calculate a pressure from a measured Si apfu, Si apfu 349 C, are entirely within the upper limits of the previously isopleths were plotted on pseudosections along with temper- estimated range. atures determined by laser Raman; the intersection of a cal- The lower boundary of the SFMS in Thomes Creek is a culated temperature with a calculated Si apfu isopleth in P- fault oriented at 56/025 (dip/dip direction) that separates the SFMS from underlying Valentine Springs rocks with very T space then gives a pressure that is based on an assumption ° ° that peak temperature and peak pressure occurred simulta- consistent peak temperature estimates of 282 -288 C, in good neously. Pseudosections were constructed with the software agreement with the upper limit of the previously estimated range of 240-280°C [43, 46]. Schmidt and Platt [57] inter- package Perple_X 6.8.3 ([69, 70], downloaded from the interpreted the fault as a normal fault that cuts the original LST, net site http://www.perplex.ethz.ch/) for the system MnO- based on its steep orientation and the omission of a SFMS Na O-CaO-K O-FeO-MgO-Al O -SiO -H O-TiO . The 2 2 2 3 2 2 2 mafic blueschist body at this point in Thomes Creek. Folia- thermodynamic data set of Holland and Powell ([71], with fl tion orientations in the vicinity of the fault are highly vari- 2002 updates) was used, as was the uid equation of state able, and broken formation is present in the disrupted zone (CORK) of Holland and Powell [72]. The following thermo- on both sides, but the rocks structurally above the fault are dynamic solution models were used: for clinoamphibole, distinguishable from the rocks below it by the more strongly GlTrTsPg [73, 74]; for biotite, Bio (HP) [75]; for chlorite, differentiated foliation, produced by pressure solution [57]. Chl (HP) [76]; for chloritoid, Ctd (HP) [77]; for clinopyrox- Further below the LST, to the west, the peak metamorphic ene, Omph (HP); for orthopyroxene, Opx (HP) [78]; for feld- temperature does not vary significantly within the Thomes spar, feldspar [79]; for garnet, Gt (HP); for phengite, Pheng Creek transect, although Schmidt and Platt [57] noted signif- (HP); and for staurolite, St (HP) [71]. An ideal solution icant changes in structural style, and a decrease in overall model was used for ilmenite (IlGkPy). Excluded components intensity of the deformational fabric. The western four sam- were as follows: diopside (di) and stilpnomelane (stlp, msnp, ples in the Thomes Creek transect produced Raman spectra fi fi and mstl). H2O was treated as existing in excess. The that could not be accurately t with the D4 band xed at obtained pseudosections were contoured with Si apfu iso- 1245 cm-1 as prescribed by Kouketsu et al. [20], raising the pleths using the Perple_X subprograms werami and pstable. possibility that these four temperatures may be erroneously Construction of pseudosections required whole rock bulk high despite the fact that they closely match nearby temper- compositions for each of the six analyzed samples. Whole atures to the east. rock bulk compositions were measured using a Bruker M4 Two of the major faults mapped by Schmidt and Platt Tornado Micro-XRF Spectrometer at California Institute of [57] do not separate rocks of different peak metamorphic Technology. Bulk composition data was slightly modified temperatures. There is no recorded change in temperature prior to pseudosection calculations. P2O5 was ignored and, across the Tomhead Fault, which bisects the Chinquapin because all P was assumed to be bound to apatite, a corre- Member, or across the fault at the base of the Chinquapin, sponding amount of CaO was also removed. All Fe was which places Chinquapin metabasalts structurally above the treated as divalent. Bulk compositions are reported in ocean floor sediments of the remainder of the SFMS. The Table 1. temperature structurally above the Chinquapin is 321 ± 12°

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Table 1: XRF bulk rock analysis for all samples used to determine pressures, in oxide wt %. Values input to the thermodynamic modeling program were modified from the original measurements as described in the text. Both modified and unmodified values are given.

SiO TiO Al O FeO MgO CaO MnO Na O K2O Fe O P O Sample 2 2 2 3 2 2 3 2 5 (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) SS69 unmod. 66.090 0.605 13.435 0.000 2.915 1.464 0.054 2.631 1.578 7.257 0.118 SS69 mod. 69.461 0.636 14.121 6.863 3.063 1.376 0.057 2.765 1.658 0.000 0.000 SS1 unmod. 67.991 0.566 13.383 0.000 2.632 0.707 0.041 3.348 1.620 6.592 0.187 SS1 mod. 70.844 0.590 13.944 6.180 2.742 0.479 0.044 3.488 1.688 0.000 0.000 SS10 unmod. 67.478 0.514 12.597 0.000 2.892 1.853 0.053 3.028 1.302 6.857 0.134 SS10 mod. 70.502 0.537 13.161 6.447 3.021 1.752 0.056 3.163 1.360 0.000 0.000 SS53 unmod. 64.818 0.677 14.312 0.000 3.100 1.421 0.062 2.259 1.723 7.890 0.228 SS53 mod. 68.106 0.711 15.038 7.459 3.257 1.178 0.065 2.373 1.813 0.000 0.000 SS118 unmod. 65.700 0.603 13.914 0.000 2.762 0.270 0.057 2.087 2.092 7.223 0.155 SS118 mod. 70.056 0.643 14.837 6.931 2.945 0.071 0.061 2.225 2.231 0.000 0.000 Note: unmod. = unmodiﬁed XRF data; mod. = XRF data modiﬁed as described in the text.

C, while the temperature just below the Chinquapin is observed faults. One sample was also analyzed from Salt ~320°C. Creek, to the south of Grindstone Creek. The sample was collected from just west of the Coast Range Fault (39°38.07′N, 4.1.2. Middle Fork of the Eel River. The Middle Eel transect 122°36.33′W) and it yields a temperature of 306 ± 9°C. crosses the TMC and captures the temperature difference between it and the surrounding lawsonite-albite facies rocks of the Yolla Bolly Unit (Figure 6). The TMC reached a peak 4.2. Pressure. Microprobe analyses of white mica oxides add metamorphic temperature of 323-336°C, while rocks to the up to ~95 weight percent, while chlorite oxides add up to south reached temperatures in the range of 282-293°C. Rocks ~88 weight percent. Measurements of white mica with totals to the north reached temperatures ranging from >280°Cat below 93 weight percent were interpreted to have been con- the boundary with the TMC to ~217°C a short distance north taminated with intergrown chlorite and were discarded. of the boundary. One additional sample was collected from Some measurements also revealed a Cr component in the metasediments structurally overlying a slice of TMC in Bea- white mica. Cr can exist in a variety of valence states includ- 2+ 3+ 2+ ver Creek (39°56.23′N, 122°59.23′W), a tributary of the Mid- ing Cr and Cr .Cr has the same charge and a similar 2+ 2+ dle Eel, ~11 km north of the Middle Eel transect. It records a ionic radius as Fe and Mg , making it likely to also partic- temperature of 219 ± 21°C, similar to the lowest tempera- ipate in a substitution similar to the Tschermak substitution, VI IV 3+ 3+ tures from the Yolla Bolly unit in the main transect. Previ- CrSiAl−1Al−1.Cr shares the same valence state as Al and ously estimated temperatures for the lawsonite-albite facies so may be able to substitute directly for Al3+. Because the role rocks of the Yolla Bolly and the TMC were both slightly lower of Cr in white mica substitution is somewhat unclear, mea- than our temperatures, at 187°C and 300°C, respectively [43, surements including Cr in excess of 0.2 weight percent were 45, 46]. discarded, consisting of three measurements of sample SS10 and four measurements of sample SS53. The remainder of 4.1.3. Cottonwood Creek. The Cottonwood Creek transect the probe data results are reported in Table 2 with standard (Figure 7) examined the extreme eastern and western por- deviations. tions of the section and yielded results similar to those seen Quantitative compositional X-ray maps were also in Thomes Creek. Peak temperatures in the easternmost Cot- affected by the intergrowths of other phases, especially chlo- tonwood Creek SFMS are ~370°C, similar to the thin, high- rite. Where intergrowths of other phases in phengite are too temperature slice on the eastern margin of the SFMS in small, the XMapTools software interprets the measurement Thomes Creek. Metamafic and metasedimentary rocks are as lower or higher Si apfu phengite, without recognizing the intercalated by minor thrust faults a short distance west of coexisting second phase (Figure 9). As a result, the composi- the high-temperature zone in Cottonwood Creek, but we tional maps must be compared to a backscatter image to have not examined these for temperature differences. The avoid erroneously interpreting intergrowths of other phases western limit of the Cottonwood Creek transect records tem- as compositional zoning. As an example, the map of SS118 peratures of ~280°C, similar to those determined for the Val- captures a phengite grain which is parallel to the main folia- entine Springs Unit in Thomes Creek. tion of the Valentine Springs Unit and which has been slightly folded. Small intergrowths visible in the backscatter 4.1.4. Grindstone Creek and Salt Creek. Grindstone Creek image show as pixels of phengite with a lower or higher Si samples range in temperature from 266 to 304°C, but the apfu, depending on the composition of the additional phase. temperatures do not regularly increase or decrease to the east There are no variations in Si apfu away from the inclusions, or west (Figure 8). This transect did not cross any known or and there is no overall compositional zoning (Figure 9).

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Figure 4: Peak metamorphic temperatures and structural cross-section of the Thomes Creek transect. Map view shows temperatures, pressures, fossils, and sample locations. Below it are temperatures projected onto an E-W line and plotted with error bars equal to 2σ of the measurements. On this projection, faults separating units with different peak metamorphic temperatures are marked in red, while faults separating units with the same peak metamorphic temperature are marked in blue. TF = Tomhead Fault (39°51.18′N, 122°41.74′ W); CRF = Coast Range Fault (39°51.05′N, 122°40.96′W); LST = Log Spring Thrust (39°52.24′N, 122°45.43′W). Cross-section based on field observations and modified from Schmidt and Platt [57].

Similar results are seen for samples SS69 (Figure 10), SS53 Calculated pseudosections closely match the dominant (Figure 11), and SS1 (Figure 12). mineral assemblages for each sample. Because S and P The map of SS10 shows multiple phengites, aligned with were not included in the modeled system, pyrite and apa- both a folded S2 and with an incipient S3. The phengite tite are not predicted by the modeling, despite being com- grains have varying amounts of additional included phases. mon in the thin section. A pressure was not obtained for There is no compositional variation across phengites of the Galice formation, as the majority of Galice white micas either population, and Si apfu values are the same for S2 visible in the thin section were equal to or smaller than and S3 aligned phengites (Figure 13). These two foliations the microprobe spot size and we were unable to obtain were described by Schmidt and Platt [57] and interpreted reliable compositional analyses as a result. We were also to have formed during or shortly after accretion, based on unable to calculate a reliable pseudosection for the TMC. lawsonite grains which formed before or during the creation Calculated and observed assemblages do not match, sug- of the main S2 foliation and on lawsonite porphyroblasts gesting disequilibrium, and exploratory phengite measure- with rotational inclusions which indicated continued growth ments revealed two populations with Si apfu of 3.56 and during D2. This, in conjunction with the lack of composi- 3.61. An estimate of minimum pressure is possible based tional zoning in all SFMS and Valentine Springs samples, on the presence of jadeite in the TMC, which at the mea- leads us to interpret their measured phengite compositions sured temperature of 330°C indicates a minimum pressure as reﬂecting peak or near peak pressure conditions. of ~11 kbar.

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East Middle Eel transect

Galice

High temperature 1 km SFMS

12.5 kbar PF38 Forest Hwy. FH7

A Indian Dick Rd.

Lawsonite-albite facies rocks Taliaferro Metamorphic Complex Main body ick-bedded greywacke Metagreywacke and slate Greywacke, slate, and Metabasalt and metachert SFMS broken formation fault with sense of motion AB

SW NE

Reverse fault Normal fault observed Normal fault inferred Middle Eel streambed Valentine Road Springs Figure 6: Peak metamorphic temperatures of the Middle Eel transect. The Taliaferro Metamorphic Complex (TMC) is the fault-bounded syncline in the middle of the section. The bounding West fault was active prior to folding and now shows a normal sense of 1100 1300 1500 1700 motion on one limb and a reverse sense of motion on the opposite −1 Raman shi (cm ) limb. Figure 5: Representative Raman spectra from west (bottom) to east (top) across the Thomes Creek transect. phengite, albite, lawsonite, sphene, and pyrite. The corresponding pseudosection (Figure 15) predicts a matching 4.2.1. Sample SS118, Upper Valentine Springs. The pseudo- mineral assemblage, minus pyrite and with the addition of section produced for SS118 (Figure 14) accurately predicts paragonite, as discussed above. The resulting pressure is 9:2 the dominant mineral assemblage. SS118 contains quartz, ±0:6 kbar. Sample SS53 contains the same mineral assem- chlorite, phengite, albite, sphene, pyrite, and apatite. Phases blage as SS69 and also has a pseudosection (Figure 16) that with constituents that were not modeled, pyrite, and apatite, predicts the observed assemblage minus pyrite and plus para- are necessarily absent from the pseudosection. Paragonite gonite. SS53 reached a pressure of 8:7±0:8 kbar. SS10 has Si and rutile were predicted, but not observed. The prediction apfu isopleths that can produce multiple intersection points of paragonite is seen in all other modeled samples in this with the Raman temperatures; however, all but the lowest study as well. In all cases, it is predicted as an accessory phase pressure of these plot within the jadeite stability ﬁeld and, while it may have been erroneously predicted, it is also (Figure 17). As SS10 was not observed to contain jadeite, possible that it was not observed because it exists as a minor and jadeite has never been reported in the SFMS, we consider amount of small crystals that are optically quite similar to the the lower pressure intercept to produce the most accurate ubiquitous phengite. The result is a calculated pressure of pressure and to indicate the calculated mineral assemblage 7:8±0:7 kbar, from just beneath the Log Spring Thrust, a to which the actual mineral assemblage should be compared. result that is somewhat higher than the previously estimated Sample SS10 contains quartz, chlorite, phengite, albite, law- pressures of 5 – 6:7 kbar for the Pickett Peak terrane [43]. sonite, sphene, and pumpellyite. The pumpellyite exists as a ﬁne-grained replacement of an earlier phase and is present 4.2.2. Samples SS69, SS53, and SS10, Main Body South Fork on the pseudosection at lower pressures; as such, it is inter- Mountain Schist. Sample SS69 contains quartz, chlorite, preted as resulting from retrograde growth and is not part

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smaller than the fossils in the underlying Valentine Springs Cottonwood Creek Transect Unit (Figures 19(a)–19(c)). Additionally, while fossils were observed in ~58% of Thomes Creek Valentine Springs samples, they were found in only ~14% of SFMS samples in the same transect. This change in size and abundance coincides exactly with the discontinuity in peak metamorphic temperature revealed by laser Raman analysis (Figure 4). Fossil abundance was also examined within the Grindstone Creek transect and is similar to that seen in the Valentine Springs, as fossils are large and are present in ~42% of the examined LST samples (Figures 4 and 8). 1 km 5. Discussion N 5.1. Temperature. Our results from Grindstone Creek and Salt Creek are somewhat surprising, as these locations recorded temperatures lower than those seen in the main body of the SFMS in Thomes Creek, despite being mapped as a continuation of that unit. It is possible that this is the Reverse Fault, approximately located result of a north-south variation in peak metamorphic tem- Cottonwood Creek Streambed perature within the SFMS, but it is also possible that a lower Figure 7: Peak metamorphic temperatures of the Cottonwood grade unit, possibly the Valentine Springs, has been incor- Creek transect. LST = Log Spring Thrust. rectly mapped as SFMS. Our sampling transect was designed to cross the boundary between the SFMS and the Valentine of the peak assemblage. Actinolite was predicted as an acces- Springs, but we neither observed a fault in Grindstone Creek sory phase, at 1.46 weight percent, yet was not observed. The nor discovered a jump in peak metamorphic temperature resulting pressure for SS10 is 8:4±1:5 kbar. Because no similar to the difference observed across the LST in Thomes major faults separate these samples, we interpret them to Creek. In addition to the similarity between the Grindstone have experienced the same peak pressure and take the pres- Creek temperatures and the temperatures recorded by the sure of the main body of the SFMS to lie within the overlap- Valentine Springs in Thomes Creek, the two units have ping error of each of the three samples, 8.6–9.5 kbar. Our microfossils of similar size and abundance. These two simi- results exceed the pressure of 6.5-7.5 determined by Bröcker larities lead us to suggest that the area of Grindstone Creek and Day [46]. mapped as SFMS may be more appropriately ascribed to the Valentine Springs, but a full analysis of this unit designa- 4.2.3. Sample SS1, High Temperature South Fork Mountain tion is beyond the scope of this paper. Schist. The observed mineral assemblage of SS1 is quartz, Our Cottonwood Creek transect assessed temperatures chlorite, phengite, albite, sphene, and apatite. The pseudosec- near the eastern and western limits of the SFMS. The temper- tion (Figure 18) predicts this assemblage, minus apatite as atures of ~280°C in the west match those determined for the discussed above. It also predicts paragonite and a trivial Valentine Springs in Thomes Creek; it is likely that the LST amount of lawsonite, 0.35 weight percent, neither of which lies east of these samples. The high temperature of 375 ± 5° was observed. Sample SS1 reached pressures of 9:8±0:7 C in the east of Cottonwood Creek is nearly identical to the kbar. There are no previous estimates of pressure for this measured temperature of 377 ± 4°C at the eastern limit of small, fault-bounded slice of metasediments in the eastern the SFMS in Thomes Creek. A zone of faulting, which juxta- portion of the SFMS. This study is the first to identify it poses metasediments and metamafics, was discovered to the and to determine that it is metamorphically distinct from west of this location and, as it defines a small, high- the main body of the SFMS. temperature slice in the easternmost portion of the transect, it is likely that this is the continuation of the fault in Thomes 4.3. Fossils. Microfossils of unknown identity, composed Creek that separates a thin slice of high-temperature SFMS exclusively of carbon, and only observable in thin sections from the main body of the unit (Figure 19(d)). These results under reflected light, are common in the metasediments of suggest that temperature patterns revealed in the main the Pickett Peak terrane. These fossils exist as fragments Thomes Creek transect extend for tens of kilometers to the ranging in size from ~50 to 1200 μm, and their primary fea- north. tures are regularly spaced holes, giving them an appearance At the northern boundary of the TMC, the temperature similar to that of some diatoms. However, the microfossils decreases abruptly from >320°Cto287 ± 7°C and then to are made of C, rather than of silica, as determined optically 215 ± 15°C in the lawsonite-albite facies rocks over a hori- and by laser Raman analysis. Some fossil fragments have zontal distance of less than half a kilometer. The systematic quartz growing in pressure shadows along their margins. decrease in peak temperature away from the TMC boundary The size and abundance of these fossils are different in adja- could be explained as a result of conductive heat transfer cent tectonic units. Fossils found in SFMS samples tend to be from the TMC into colder Yolla Bolly rocks emplaced

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Grindstone Creek Transect

CRF

1 km

Microfossils Observed Microfossils Not Observed Grindstone Creek Streambed Coast Range Fault

Figure 8: Peak metamorphic temperatures of the Grindstone Creek transect. CRF = Coast Range Fault.

Table 2: Microprobe measurements of white mica in wt % with standard deviations, temperatures, Si apfu with standard deviations, and pressures.

T P SiO2 TiO2 Al2O3 Cr2O3 FeO MgO CaO MnO Na2O K2O Total Si Sample ° (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (apfu) ( C) (kbar) South Fork Mountain Schist SS10 (n =35) 51.54 0.05 23.94 0.28 2.93 4.02 0.02 0.03 0.09 10.90 93.80 3.51 327 8:4±1:5 S.D. SS10 0.30 0.04 0.54 0.80 0.14 0.13 0.02 0.02 0.02 0.15 0.45 0.01 28.44 SS69 (n =33) 50.40 0.12 26.88 0.05 2.55 3.21 0.02 0.03 0.18 10.80 94.24 3.41 338 9:2±0:6 S.D. SS69 0.33 0.05 0.33 0.04 0.18 0.08 0.03 0.02 0.04 0.17 0.38 0.02 12.38 SS53 (n =9) 49.97 0.08 26.97 0.95 2.21 3.09 0.03 0.02 0.21 10.40 93.95 3.39 331 8:7±0:8 S.D. SS53 0.39 0.05 1.08 1.10 0.17 0.12 0.02 0.03 0.06 0.15 0.52 0.03 16.36 SS1 (n =31) 49.09 0.12 28.64 0.49 2.26 2.86 0.03 0.02 0.53 9.53 93.57 3.41 364 9:8±0:7 S.D. SS1 0.76 0.09 0.69 0.04 0.55 0.32 0.04 0.02 0.16 0.30 0.55 0.03 10.54 Valentine Springs SS118 (n =21) 50.48 0.11 27.72 0.03 2.26 2.92 0.02 0.02 0.29 10.08 93.94 3.40 286 7:8±0:7 S.D. SS118 0.54 0.07 0.60 0.03 0.34 0.22 0.02 0.02 0.18 0.41 0.43 0.03 14.19 Note: pressures were determined as described in the text and as seen in Figure 9.

beneath it. In contrast, at the southern boundary, the peak used returned higher temperature values than competing cal- temperature of the lawsonite-albite facies rocks was deter- ibrations, the results were considered maximum estimates. mined to be 293 ± 11°C a full 0.75 km from the boundary Underwood [81] reported an average temperature of 190°C and temperatures in the 215-219°C range were not observed. for the Central Belt and an average temperature of 140°C Temperatures elsewhere in the Franciscan are generally for the Coastal Belt, not including the anomalously higher in good agreement with our results. Lahfid et al. [80] used temperature of the King Range terrane, which was inter- LRCM to determine peak metamorphic temperatures of the preted to have experienced a later hydrothermal overprint. Lucia subterrane near San Simeon, finding that temperatures Underwood [81] also found that, within the Central Belt, ranged from 220 to 315°C. In northern California, Under- temperatures were higher in the east and lower in the west. wood [81] used vitrinite reflectance to assess temperatures The Central Belt was seen to have experienced temperatures over multiple Franciscan terranes. Because the calibration of ~250°C near the border with the Eastern Belt and ~150°C

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3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Si (apfu) phengite Si (apfu) phengite

S2 S

20 �m 20 �m Ap Ab

Chl Py Chl Ph Spn Ph Qz Ab Qz

Figure 9: SS118 quantitative X-ray map and backscatter image, Figure 11: SS53 quantitative X-ray map and backscatter image. with mineral abbreviations from Whitney and Evans [105]. The Multiple examples of intergrowths which show as phengite with smallest intergrowths of chlorite show as low Si apfu phengite; one variable Si apfu are marked (red arrows). Mineral abbreviations example is marked (red arrows). from Whitney and Evans [105].

3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55 3.6

3.1 3.2 3.3 3.4 3.5 3.6

Si (apfu) phengite

Si (apfu) phengite � 20 m S 20 �m Spn Qz Ph Ab Chl Ap Chl Py Ph Spn Qz Ap

Figure 10: SS69 quantitative X-ray map and backscatter image. Figure 12: SS1 quantitative X-ray map and backscatter image. One Mineral abbreviations from Whitney and Evans [105]. example of an intergrowth is marked (red arrows). Mineral abbreviations from Whitney and Evans [105]. near the border between it and the Coastal Belt. The Coastal Belt results were somewhat more complicated, but much of peak pressure and peak temperature coincided with each the Coastal Belt was seen to have experienced temperatures ° ° other. Previous studies have revealed that high-grade blocks of 100-150 C. The temperatures of 215-219 C determined in the Franciscan have experienced counter-clockwise P-T-t for the western portion of the Eastern Belt in this study are ° paths, some of which have experienced widely separated peak quite similar to the temperature of ~250 C determined by temperatures and pressures [82–84]. However, these blocks Underwood [81] for the eastern limit of the Central Belt. are likely to have experienced a different thermal history than The slight discrepancy between these two results may be ff the coherent blueschist facies metasediments, as they were due to the di erent methods used, or to the fact that Under- emplaced within a subduction zone that was still cooling wood [81] employed a calibration that produced a maximum slowly due to a low convergence rate [85]. In contrast to this, temperature estimate. Our results clearly show that tempera- the metasediments were emplaced long enough after subduc- tures increase to the east across the Eastern Belt and a com- tion initiation for the zone to be in a thermally mature steady parison of them with previously determine temperatures state. Ernst [86] argues that the preservation of a blueschist provides additional evidence for an eastward increase in tem- facies mineral assemblage without a significant retrograde perature across the northern Franciscan as a whole. overprint requires that the retrograde path approximately retraced the prograde path. A P-T-t path such as this pro- 5.2. Pressure. An important assumption made when calculat- duces a peak temperature and peak pressure that occur ing pressures from Si apfu isopleths for this paper was that simultaneously or near simultaneously. Evidence for such a

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13 1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 All ﬁelds 3 4 Chl Ph Gl Gt Jd Law Sph Pa 1 +qtz, H20 2 Chl Ph Gl Gt Jd Zo Sph Pa Chl Ph Gl 5 3 Chl Ph Gl Gt Jd Bio Sph Pa Gt Jd 6 Sph Pa Chl Ftr 4 Chl Gl Gt Jd Bio Sph Pa 12 9 Gt Bio 5 Chl Gl Ftr Gt Jd Bio Sph Pa Zo Sph Chl Ph Gl Jd 10 6 Chl Gl Gt Jd Bio Zo Sph Pa Chl Ph Jd Law Pa Ru Law Sph Pa Pa Ab 7 7 Chl Gl Ftr Gt Bio Sph Pa Ab Chl Ph Gt S2 8 Chl Gl Gt Bio Zo Sph Pa Ab Zo Sph 8 11 11 Pa Ab 9 Chl Ph Gl Jd Sph Pa Ab 2 Chl Gt Bio 10 Chl Ph Gl Gt Zo Sph Pa Ab Chl Ph Jd Law Sph Pa Zo Sph Pa Ab 11 Chl Ph Gl Law Sph Pa Ab 12 Chl Ph Gt Bio Zo Sph Pa Ab 12 10 3.43 Si pfu 13 Chl Bio Zo Sph Pa Ab

3.41 Si pfu 14 Chl Gt Bio Zo Sph Pa Ab 15 Chl Ph Bio Zo Sph Ab S3 16 Chl Ph Bio Zo Sph Ab 9 3.39 Si pfu 17 Chl Ph Bio Sph Ab Chl Ph Zo Sph Pa Ab 18 Chl Ph Bio Sph Ab

Pressure (kbar) Pressure 19 Chl Ph Bio Sph Ab Ru 20 Chl Ph Gt Bio Zo Ab Ru 13 14 21 Chl Ph Bio Zo Ab Ru 8 Chl Ph Law � Si (apfu) phengite Pa Ab Ru 22 Chl Ph Bio Ab Ru 20 m Chl Ph Cz 23 Chl Ph Ilm Bio Ab Ru 16 Sph Pa Ab 24 Chl Ph Ilm Bio Ab 20 17 25 Chl Ph Cz Law Sph Ab Spn 7 15 21 Chl Ph Law Sph Pa Ab Ab = albite, Bio = biotite, Chl Ph Cz 19 Chl = chlorite, Cz = Sph Ab clinozoisite, Ftr = ferroactinolite, Ph 6 Gl = glaucophane, Chl 25 18 22 23 Gt = garnet, Ilm = ilmenite, Chl Ph Zo Sph Ab Jd = jadeite, Law = lawsonite, 24 Pa = paragonite, Chl Ph Ph = phengite, Ru = rutile, Law Sph Ab Sph = sphene, Zo = zoisite 5

200 250 300 350 400 450 500 Ab Temperature (°C) Figure Qz 15: Pseudosection used to calculate pressure for sample Lws SS69. Temperature determined from LRCM in blue with errors equal to 1σ of the measurements as dashed lines. Si apfu in red Figure 13: SS10 quantitative X-ray map and backscatter image. with errors equal to 1σ of the measurements as dashed lines. Black White boxes mark areas enlarged to show detail. Note the lines at intersection points of Si apfu isopleths and temperature uniformity of the large phengite grains and the relationship give calculated pressure, with corresponding errors as dashed between intergrowths and nonuniform Si apfu. One example of an lines. Darker colors correspond to higher variances. Bulk intergrowth is marked (red arrows). Mineral abbreviations from composition data used in pseudosection construction is available Whitney and Evans [105]. in Table 1.

13 1 Chl Ph Gl Jd Bio Pa Ru 13 2 4 1 Chl Ph Gl Gt Jd Law Sph Pa Chl Ph Jd 2 Chl Ph Jd Bio Pa Ru All ﬁelds 1 3 + qtz, H20 Gl Pa Ru + qtz, H20 Chl Ph Gl 5 2 Chl Ph Gl Gt Jd Zo Sph Pa 3 Chl Ph Gt Ab Bio Pa Ru Gt Jd 3 Chl Ph Gl Gt Jd Bio Sph Pa 1 Chl Ph Gt 7 Sph Pa 4 Chl Gl Gt Jd Bio Sph Pa Bio Pa 4 Chl Ph Ab Bio Pa Ru 15 11 6 5 Chl Gl Ftr Gt Jd Bio Sph Pa 12 Sph Ru 5 Chl Ph Ab Ilm Bio Pa Ru 12 9 13 Chl Ftr Gt Bio 6 Chl Gt Jd Bio Zo Sph Pa Chl Ph Jd Pa Ru Chl Ph Bio Chl Ph Ab Ilm Pa 10 Sph Pa 6 Chl Ph Jd Law Pa Ru Chl Gl Gt Jd Bio Sph Pa Pa Sph Ru Chl Ph Gl Jd 2 8 Ab 7 Law Sph Pa Chl Ph Gt 8 Chl Ftr Gt Bio Zo Sph Pa Ab Zo Sph 9 Chl Ph Gl Gt Jd Sph Pa Ab Chl Ph Jd Law Pa Ru 11 Pa Ab Chl Ph Gl Ab Chl Gt Bio 10 Chl Ph Gl Gt Zo Sph Pa Ab Zo Sph 11 Chl Gl Gt Bio Sph Pa 11 Pa Sph Ru Chl Ph Jd Law Pa Sph Pa Ab Chl Ph Jd 3.38 Si pfu 12 Chl Ph Gt Bio Zo Sph Pa Ab Gl Ab 13 Chl Gl Ftr Gt Bio Sph Pa 12 Pa Ru 14 Chl Gt Bio Zo Sph Pa Ab 10 15 Chl Ftr Gt Bio Sph Pa Ab 3.41 Si pfu 3.35 Si pfu 16 Chl Ph Bio Sph Pa Ab 10 17 Chl Ph Gt Bio Sph Pa Ab Chl Ph Jd Pa Sph Ru Chl Ph Jd Ab Pa Ru Chl Ph Bio Ab 18 Chl Ph Sph Pa Ab Ru Pa Sph Ru 9 19 Chl Ph Gt Bio Ab Zo Ru 20 Chl Ph Ab Zo Sph Ru

3.43 Si pfu Pressure (kbar) 21 Chl Ph Ab Ilm Bio Zo Ru Chl Ph Zo Sph Pa Ab Chl Ph Zo Sph Pa Ab 22 Chl Ph Ab Bio Zo Ru 3.40 Si pfu 14 9 23 Chl Ph Ab Ilm Bio Ru 8 Chl Ph Law Pa Ab Ru 17 3.37 Si pfu 24 Chl Ph Ab Sph Ru 16 25 Pressure (kbar) Chl Ph Ab Bio Sph Ru 3 Chl Ph Sph Pa Ab 19 26 Chl Ph Ab Bio Ru 27 Chl Ph Ab Ilm Bio Ru 20 4 28 Chl Ph Ab Ilm Bio 8 7 Chl Ph Sph Pa Ab 18

22 21 Ab = albite, Bio = biotite, Chl Ph Ab Pa Sph Ru Chl Ph Law Sph Pa Ab 23 Chl = chlorite, Cz = clinozoisite, Ftr = ferroactinolite, 5 6 Chl Ph Cz Gl = glaucophane, Sph Pa Ab 27 Gt = garnet, Ilm = ilmenite, 7 25 Jd = jadeite, Law = lawsonite, 28 Pa = paragonite, 26 24 Ph = phengite, Ru = rutile, Sph = sphene, Zo = zoisite Ab = albite, Bio = biotite, 5 Chl = chlorite, Cz = 200 250 300 350 400 450 500 Chl Ph Law Chl Ph Ab 6 Ilm Pa Ru clinozoisite, Ab Pa Ru Gl = glaucophane, Temperature (°C) Gt = garnet, Ilm = ilmenite, Chl Ph Ab Jd = jadeite, Law = lawsonite, Pa Ru 6 Pa = paragonite, Ph = phengite, Ru = rutile, Figure Sph = sphene, Zo = zoisite 16: Pseudosection used to calculate pressure for sample 5 200 250 300 350 400 450 500 Temperature (°C) SS53. Temperature determined from LRCM in blue with errors σ Figure 14: Pseudosection used to calculate pressure for sample equal to 1 of the measurements as dashed lines. Si apfu in red σ SS118. Temperature determined from LRCM in blue with errors with errors equal to 1 of the measurements as dashed lines. Black equal to 1σ of the measurements as dashed lines. Si apfu in red with lines at intersection points of Si apfu isopleths and temperature errors equal to 1σ of the measurements as dashed lines. Black lines give calculated pressure, with corresponding errors as dashed lines. at intersection points of Si apfu isopleths and temperature give Darker colors correspond to higher variances. Bulk composition calculated pressure, with corresponding errors as dashed lines. data used in pseudosection construction is available in Table 1. Darker colors correspond to higher variances. Bulk composition data used in pseudosection construction is available in Table 1. as having a small amount of offset. The LST, the easternmost P-T-t path has been found by Radvanec et al. [87], giving mapped thrust within the SFMS, and the Coast Range Fault support to our assumption that peak temperature and pres- were all active after peak metamorphism in the SFMS. The sure were nearly synchronous. Tomhead Fault has duplicated the originally ~450 m thick Chinquapin metabasalt member [57] and appears to have 5.3. Exhumation. The existence of differences in peak meta- accommodated significant offset, and yet records no differ- morphic temperature across mapped faults provides infor- ences in peak metamorphic temperature on either side. It is mation on the relative timing of the faulting, as these faults therefore interpreted as having been active prior to peak must have been active after peak metamorphism. Units of metamorphism, as is the fault that places the Chinquapin different temperatures that were juxtaposed with each other structurally above the main bulk of the SFMS sediments. prior to peak metamorphism would have thermally equili- The coexistence of these subparallel, imbricate thrust faults brated during peak metamorphism. Using this as a guide, that were active both prior to and after peak metamorphism faults that record different temperatures on either side are provides strong evidence for synconvergent exhumation that interpreted as having been active after peak metamorphism proceeded without altering the local kinematics of thrusting on the higher grade side of the fault, while faults that do and underplating, suggesting a widespread regional exhuma- not record such differences in temperature are interpreted tion mechanism. Continual underplating during regional as either having been active prior to peak metamorphism or exhumation is consistent with the lack of a greenschist facies

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13 1 Chl Ph Gl Gt Jd Zo Sph Pa All ﬁelds 3 5 extensional faulting within the Franciscan Eastern Belt [57], + qtz, H20 2 Chl Ph Gl Gt Jd Sph Pa Chl Ph Gl 3 Chl Ph Gl Gt Jd Bio Sph Pa 4 Jd Law Sph 2 4 Chl Gl Ftr Gt Jd Bio Sph Pa Chl Ph Gl 3.50 Si pfu 6 and so we favor interpretations that include synconvergent 12 Law Jd Ru 7 5 Chl Gl Gt Jd Bio Sph Pa Chl Ph Gl 1 6 Chl Ftr Gt Bio Sph Pa Ab Gt Jd Law 3.51 Si pfu 12 7 Chl Gl Gt Jd Bio Sph Pa Ab Sph Pa 8 8 Chl Ftr Gt Bio Zo Sph Pa Ab extension. Platt [8] suggested that underplating directly 9 13 9 Chl Ph Gl Gt Bio Zo Sph Ab 11 Chl Ph Gl Chl Ph Gl 14 Chl Ph 10 Chl Ph Gl Gt Bio Zo Sph Ab Law Sph Jd Law 15 Gt Bio Jd Ru 11 Chl Gt Bio 11 Chl Ph Gl Ftr Gt Zo Sph Ab Sph Ab Zo Sph Ab Zo Sph beneath the wedge increases its thickness and taper, resulting 12 Chl Ph Gl Gt Jd Zo Sph Ab 10 Pa Ab 3.52 Si pfu 13 Chl Ph Gl Gt Zo Sph Ab 10 14 Chl Ph Gl Gt Jd Law Sph Ab in extension in the upper rear of the wedge. Warren et al. [9] 15 Chl Ph Gl Gt Law Sph Ab Acti Chl Ph Gt 16 Chl Ph Gl Law Sph Acti Ab Zo Sph 16 17 17 Chl Ph Gt Law Sph Acti Ab ﬂ Ab Acti Chl Ph Gl 18 Chl Ph Gt Cz Acti Sph Ab advocated buoyancy-driven ow leading to high rates of 9 Jd Law Sph Chl Bio Zo 19 Chl Ph Cz Sph Ab Acti Chl Ph Zo 22 Sph Pa Ab Sph Ab Acti 20 Chl Ph Pump Sph Ab Pressure (kbar) 21 Chl Ph Cz Pump Sph Ab return from deep within the subduction zone (~100 km); Chl Bio Zo 18 22 Chl Gt Bio Zo Sph Pa Ab Sph Pa Ab 23 Chl Ph Bio Zo Ru Ab Chl Ph Gl 8 Chl Ph Bio Law Sph Ab 24 Chl Ph Ilm Bio Ru Zo Sph Ab the exhumed material then reaches the base of the accretion- 23 19 Chl Ph Bio Zo Sph Ab

7 ary wedge and triggers extension at shallower levels. Both of Chl Ph Bio Chl Ph Law 21 Acti = actinolite, Ab = albite, Sph Ru Ab Sph Ab Chl Ph Law Bio = biotite, Chl = chlorite, Sph Ab Acti Cz = clinozoisite, these models can explain the observed synconvergent Chl Ph Bio Ftr= ferroactinolite 6 Ru Ab Gl = glaucophane, Gt = garnet, Ilm = ilmenite, Jd = jadeite, Law = lawsonite, Pa = Chl Ph Bio extension. Chl Ph Bio paragonite, Ph = phengite, Chl Ph Law Sph Ab Cz Sph Ab pump = pumpellyite, Ru = rutile, Pump Sph Ab 24 20 Sph = sphene, Zo = zoisite 5 200 250 300 350 400 450 500 Temperature (°C) 5.4. The Subduction Interface. The nature and thickness of Figure 17: Pseudosection used to calculate pressure for sample the subduction interface vary with a number of factors, SS10. Temperature determined from LRCM in blue with errors including depth, types of lithologies present, ﬂuid migration, equal to 1σ of the measurements as dashed lines. Si apfu in red with errors equal to 1σ of the measurements as dashed lines. Black and subduction geometries and rates ([12], and references lines at intersection points of Si apfu isopleths and temperature give therein). At shallow levels, the interface has been observed calculated pressure, with corresponding errors as dashed lines. both directly via drilling [93] and indirectly in exhumed Darker colors correspond to higher variances. Bulk composition complexes [16, 94], revealing it to generally be ≤~300 m data used in pseudosection construction is available in Table 1. thick. An extensive compilation of underplated slab thicknesses shows that this thickness is common even at greater

13 Chl Ph Gl Jd Bio Pa Ru All ﬁelds Chl Ph Jd 2 3 1 depths [12], though some workers have reported interfaces 1 Gl Pa Ru 2 Chl Gl Jd Bio Pa Ru + qtz, H20 4 3 Chl Gl Jd Bio Ab Pa Ru Chl Ph Jd 5 4 Chl Gl Gt Jd Bio Ab Pa Ru – – Gl Pa 7 on the order of 2 10 km thick [89, 95 97] and these greater 12 Ab Ru 6 9 5 Chl Gl Jd Bio Sph Pa Ab 10 8 6 Chl Jd Bio Ab Sph Pa Chl Ph Jd Chl Ph Jd 12 7 Chl Gt Jd Bio Sph Pa Ab Gl Pa Sph Ru 11 Law Pa Ru 8 Chl Ftr Gt Bio Sph Pa Ab thicknesses are consistent with the interface as revealed by 9 Chl Gt Bio Sph Pa Ab 11 Chl Ph Gl Law Pa Chl Ph Gl 10 Chl Ph Gl Gt Jd Sph Pa Ab – Sph Ab Jd Pa 11 Chl Ph Gl Law Sph Pa Ab Sph Ab some seismic imaging (e.g., [98 101]). Chl Gt Bio Chl Ph Gt 12 Chl Ph Gt Bio Zo Sph Pa Ab Zo Pa Zo Pa Chl Ph Jd Sph Ab Sph Ab 13 Chl Ph Bio Zo Sph Pa Ab Pa Sph Ru 10 14 Chl Bio Zo Sph Pa Ab For the purpose of this discussion, we assume that the 15 Chl Gt Bio Sph Pa Ab 16 Chl Ph Sph Pa Ab 17 Chl Bio Pa Ab Ru 13 18 Chl Bio Sph Pa Ab subduction interface width in the SFMS is roughly equal to 9 15 19 Chl Ph Sph Ab 20 Chl Bio Ilm Ab Pa Ru Pressure (kbar) 14 the thickness of the most recently accreted packet. Previous Chl Ph Zo 18 3.44 Si pfu Sph Pa Ab 16 8 3.41 Si pfu 17 3.38 Si pfu 19 workers have found that subduction interfaces are bordered

20 7 by basal and roof shears ([102], and references therein), Chl Ph Law Chl Ph Law Sph Pa Ab Pa Ab Ru Ab = albite, Bio = biotite, fi Chl Ph Cz Chl Ph Bio Chl Ph Bio Chl = chlorite, Cz = and the packet bounding faults identi ed in this study are Sph Pa Ab Sph Ab Ilm Ab Ru clinozoisite, Ftr = ferroactinolite, 6 Gl = glaucophane, Gt = garnet, Ilm = ilmenite, Chl Ph Cz Chl Ph Zo Chl Ph Bio Jd = jadeite, Law = lawsonite, good candidates for these shears as they accommodated Sph Ab Sph Ab Ab Ru Pa = paragonite, Ph = phengite, Ru = rutile, Sph = sphene, Zo = zoisite 5 subduction-related deformation at the time of accretion. 200 250 300 350 400 450 500 Temperature (°C) Figure We have no constraints on whether or not motion ceased 18: Pseudosection used to calculate pressure for sample SS1. on structurally higher bounding faults after the activation Temperature determined from LRCM in blue with errors equal to of structurally lower faults, and it is likely that deformation 1σ of the measurements as dashed lines. Si apfu in red with errors σ extended for some distance beyond the bounding faults of a equal to 1 of the measurements as dashed lines. Black lines at fi intersection points of Si apfu isopleths and temperature give given accreted packet, but this assumption allows for a rst calculated pressure, with corresponding errors as dashed lines. order estimation of the interface thickness. Packets that were Darker colors correspond to higher variances. Bulk composition both emplaced within an actively exhuming complex and data used in pseudosection construction is available in Table 1. that experienced near synchronous peak pressures and temperatures must have experienced these peak conditions at overprint in the Eastern Franciscan, as subduction refrigera- or immediately after the time of accretion. As such, faults that tion maintained depressed geothermal gradients during lack a temperature difference across them are seen to have exhumation. ceased activity prior to accretion and are eliminated as candi- Exhumation of deeply subducted material could have dates for packet bounding faults at the time of accretion; this been driven by a number of different mechanisms, including includes the Tomhead Fault and the fault which places the extension of the accretionary wedge caused by underplating Chinquapin structurally above the bulk of the SFMS metase- at its base [8]; steady state return flow in a parallel-sided sub- diments. The remaining faults, the CRF, the eastern most duction channel, driven by buoyancy [6, 88] or topographic fault within the SFMS, and the LST, are interpreted as packet gradients in the forearc [89]; forced return flow in a bounding faults at the time of accretion. This divides the downward-closing subduction channel [5, 6, 90]; and buoy- SFMS at this latitude into two packets, an ~300 m thick ancy contrasts between subducted material and overlying packet to the east and an ~3.5 km thick packet to the west. rock [9, 91, 92]. The evidence for episodic accretion of dis- The high-temperature slice of the SFMS, at ~300 m structural crete thrust slices in the Eastern Belt, and the absence of evi- thickness, closely matches those most commonly reported by dence for a reversal of the sense of shear across the SFMS, for Agard et al. [12], while the western packet is significantly example, as observed by Xia and Platt [89] in the Pelona thicker. It is possible that the thin eastern packet was origi- Schist, suggest that viscous return flow was not the primary nally thicker before being subjected to subduction erosion, mechanism of exhumation. Previous work has identified but in the absence of evidence for this, we interpret it to have

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(a) (b)

mb ms ms mb

(c) (d) Figure 19: (a) The largest observed fossil from beneath the Valentine Springs. Reflected light (39°52.31′N, 122°48.75′W). (b) A fossil from the Valentine Springs showing a typical size and degree of deformation. Reflected light (39°52.07′N, 122°45.80′W). (c) A South Fork Mountain Schist (SFMS) fossil showing typical size and degree of deformation. Reflected light (39°51.16′N, 122°42.12′W). (d) Photo taken looking S at the thrust fault zone that separates the thin, high-temperature slice of the eastern SFMS from the lower temperature SFMS to the west. The mafic schist is seen to have been duplicated by the thrust (39°51.06′N, 122°41.20′W). Out of frame to the right (W) is a second thrust fault that has placed the mafic schist structurally above the pelitic schist. mb = metabasalt; ms = metasediments.

simply been a narrower interface. Our results indicate that ally lower level, was accreted under lower temperature and subduction interface thickness can vary from a few hundred pressure conditions than the preceding packet. This model meters to a few kilometers, even within a single subduction adequately explains the stepwise variations in metamorphic zone. This is somewhat striking, given that the two packets grade, metamorphic age, and depositional age across the were accreted under quite similar conditions. Lithologic con- eastern Franciscan, and this pattern of accretion is supported trol over different accretion styles is suggested by the proxim- by analyses of the Central and Coastal Belts as well [104]. ity of the ~1 km thick Chinquapin metabasalt member to the How the TMC was emplaced within the lawsonite-albite boundary between the two slices as well as by the lack of any facies rocks is an outstanding question, and it appears that it significant metabasalt or coarse-grained metagreywacke must have been exhumed, part way at least, before or during component in the high-temperature slice. It is possible that the subduction and accretion of the Yolla Bolly unit. At pres- the difference in accreted packet thickness could be due to ent, we lack reliable data on both the protolith age and the some combination of the rheological differences caused by timing of metamorphism of the TMC, which hinders further the presence or absence of the metabasalts and greywackes discussion. or by differing fluid content at the time of accretion. 6. Conclusions 5.5. Underplating. Underplating is thought to be facilitated by mantle scale changes in plate dynamics and transient changes Peak metamorphic temperatures revealed by laser Raman to mechanical coupling ([12] and references therein), espe- analysis of carbonaceous material can be used to identify cially by dewatering of subducted sediments leading to a major faults and tectonic boundaries within the Franciscan. lower rheological contrast between them and the overriding Metamorphic temperatures in the Eastern Belt decrease from plate [90, 102, 103]. While extensive veining is present in east to west, and abrupt changes in peak metamorphic tem- the SFMS, indicating large scale fluid flow consistent with a perature are observed across mapped faults. The timing of dewatering model, this study does not provide the observa- faulting can be constrained by the existence or lack of a dif- tions needed to differentiate between different underplating ference in peak metamorphic temperature on either side of mechanisms. After underplating, Franciscan Eastern Belt the fault. The Log Spring Thrust, the unnamed thrust fault sediments occupied the base of an actively exhuming wedge zone in the eastern SFMS, and the Coast Range Fault were and so peak metamorphic conditions were experienced close all active after peak metamorphism and are the bounding to the time of accretion. Due to progressing exhumation, faults of accreted sediment packets, while faults without a each successive packet, despite being accreted at a structur- temperature difference on either side likely ceased activity

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