San Francisco State University Petrology — Spring 2006

Geology of the central coast

April 21-22, 2006

Field trip guide compiled by Mary Leech

Name: Score (out of 100):

1 COAST RANGES PROVINCE

The Coast Ranges extend from the southern and western edge of the near the Oregon border, to the in , a distance of more than 1000 km; in addition, the found in the Coast Ranges continues as through the western Transverse Ranges, then offshore from the Peninsular Ranges and Baja California, finally to reappear on land at Cedros Island, offshore Mexico. The Coast Ranges got their name because of their proximity to the Pacific Ocean, and the interface between shoreline processes and the geology of the province dominates the scenery found in the province, as seen in the view of a wave-carved sea tunnel near Mendocino in , eroded into .

In the Coast Ranges Province proper, three linear belts are recognized: (1) an eastern belt composed of Franciscan Assemblage (Franciscan Complex)rocks (graywacke and in coherent units, often metamorphosed to facies, together with melanges composed not only of sheared shale and graywacke blocks but also exotic blocks of some or all of the following types: , , greenstone (as in the photo to the right), , and blueschist, overlain by the (in thrust contact), which is in turn depositionally overlain by the Jurassic to of and fan deposits; (2) a central belt composed of plutons that had intruded metamorphic basement rocks, overlain by younger sediments, and called the ; and (3) a western belt which consists once again of the sequence of Franciscan Assemblage, Jurassic Coast Range ophiolite, and remnants of the Great Valley Sequence, but with some differences -- less blueschist and more age restriction of Franciscan sediments (Jurassic to in the northern part of the eastern belt, but lack of Paleocene-age sediments in the western belt), and more restricted Jurassic age for Great Valley sequence equivalent rocks. The three belts are separated by major faults, the eastern belt from the Salinian Block by the active San Andreas strike-slip system, and the Salinian Block from the western belt by the more enigmatic Sur-Naciemento fault system, a complex system of apparent strike-slip faults (e.g., Rinconada fault), but with some faults that have reverse components and are thrusts (Page, 1981). These boundary fault systems have been responsible for several hundreds of kilometers of right lateral motion in the , but paleomagnetic and micropaleontologic evidence in studies of Franciscan suggests that these rocks have travelled from low latitudes, more than 1000 km (Champion, 198?; Wahrhaftig, 1987). While the San Andreas system of faults is clearly active, and has caused numerous historical earthquakes in the Coast Ranges (the most famous being the large 1906 event), the Sur-Nacimiento is considered to be inactive (Alt and Hyndman, 2000).

The origin of the Franciscan Complex has been assumed to be related to processes. A simplified version of the geological history was synthesized by Blake and Jones (1981):

"The close temporal and spatial relations of Franciscan rocks with rocks of the structurally overlying Great Valley Sequence and the volcanoplutonic rocks that lie to the east in the Klamath Mountains and the have led to the widely accepted hypothesis that rocks of the Franciscan assemblage were deposited in an active trench contemporaneously with deposition of the Great Valley sequence in a forearc basin adjacent to the active Klamath-Sierra arc complex. These three elements comprise the arc, arc-trench gap, and trench setting . . . typical of convergent plate boundaries (Dickinson, 1971). A large component of subduction generally has been thought to be eastward normal to the trend of the (e.g., Hamilton, 1969)." Blake and Jones (1981) go on to explain that, although meritorious in a general sense, the hypothesis is too simplistic to explain some anomalous relationships, including the nearby presence of additional arc , and sedimentological problems (compositional characteristics and correlations).

GREAT VALLEY PROVINCE

The Great Valley Province consists of the San Joaquin and Sacramento Valleys that together extend about 700 km from the Klamath Mountains in the north to the Transverse Ranges in the south; the Great Valley spans a width of about 150 km from the Sierra Nevada in the east to the Coast Ranges in the west. The valley reaches depths of about 10,000 m at its southern end, and is filled with a large volume of sediments of Mesozoic through Recent age, reflecting a variety of types and provenances; for example, inland shallow seas deposited with more siliceous clasts than coastal Monterey exposures due to their origin from the Coast Range batholith, and Tertiary through Recent alluvium has been largely derived from the adjacent, well-watered Sierra Nevada. The older alluvium sometimes is exposed as inverted topography on the eastern edge of the valley, where older river channels, filled with fluvial debris, were covered with more resistant sediments or volcanics, and adjacent materials were eroded away following uplift; an example of this type of topography northnortheast of Fresno is shown in the photo to the right. Recent alluvium covers nearly the entire valley floor, so that the underlying geology has largely been deduced from exposures in the adjoining mountain ranges, from geophysical studies, and from deep drill holes, particularly along the western margin of the valley where oil is found in a series of domes and other types of traps. So voluminous has been the deposition from Recent alluvium that the surface topography has little expression, consisting mostly of the inverted topography (discussed above) locally exposed along the eastern margin, gently sloping fan deposits along both margins, and the , a Tertiary volcanic feature of older Cascades origin exposed west of Yuba City. The alluvial materials have such small gradients and are sufficiently unconsolidated that river systems easily erode this material and reach old age, creating extensive meandering systems, particularly in the Sacramento area. The Recent alluvium is also extremely fertile, making the Great Valley a major source of agricultural products for the United States -- the agriculturally-based economy for Fresno County, for example, is larger than those for all but five of the states in the United States.

Based on the geophysical and well data available, a major suture exists along the length of the west side of the valley, between Sierran and Franciscan-type basements. In addition, the valley is subdivided into sub-basins separated by uplifts (e.g., the Stockton Arch). San Francisco State University Petrology — Spring 2006

Driving directions and brief stop descriptions — Day 1

Time Mileage Directions 10:00 0.0 Meet in the COSE van parking lot behind Thornton Hall, SFSU 0.5 L on Lake Merced Drive 1.2 L on Brotherhood Way 2.0 R onto southbound Hwy 1 to Hwy 280 south 40.2 Exit Hwy 85 south 48.1 Exit Hwy 17 south (get in R lane) 70.2 Exit Hwy 1 south toward Watsonville & Monterey 93.2 Pass Elkhorn Slough (good kayaking — lots of seals, otters, birds) 116.4 Pass Rio Road shopping center, town of Carmel 118.6 Monastery Beach/San Jose Creek — one head of the Carmel Canyon 12:30 119.3 R into State Park from Hwy 1

STOP 1: Point Lobos — The Salinian and the sub-marine Monterey Canyon We will have lunch here — when you’re finished, wander around the coastal outcrops and check out the rocks.

You just drove past Monterey Bay and Monterey Canyon, one of the largest submarine canyons along the contiguous U.S.; it cuts deeply into the Cretaceous granitic rocks of the Salinian Block. It is about 15-21 m.y. old and originated a considerable distance to the south, around Santa Barbara. This canyon has been moved to its present location by transform motion along the system.

One of several heads of the Monterey Canyon forms the beach at Monastery Beach. Two other heads of the canyon are located offshore at Stillwater Cove and off Point Lobos Reserve. The canyon head off Point Lobos Reserve is fault-controlled by the Carmel Canyon fault segment of the Palo Colorado-San Gregorio fault zone.

At Point Lobos, Mid-Cretaceous porphyritic Santa Lucia granodiorite of the Monterey Peninsula crops out. The granodiorite contains large euhedral K-feldspar crystals and is similar to the Cathedral Peak quartz monzonite of Yosemite Park and Sonora Pass (but of course, is not related). It is unconformably overlain by gravels and sandstones of the Carmello Formation.

List the modal percentages of minerals in the Santa Lucia granodiorite:

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Time Mileage Directions 3:00 0.0 Leave STOP 1 at Point Lobos — Zero your odometer at Hwy 1 7.2 Doud Creek

STOP 2: The San Gregorio fault and the western Salinian block The Palo Colorado-San Gregorio fault zone intersects the shore at this location; this fault is part of the paleo-San Andreas fault system. Nearer San Francisco, the San Gregorio fault accounts for a small part of the motion between the Pacific and North American tectonic plates. Along the beach, you will find a good fault contact between folded Cretaceous and granitic rocks. Walk south along the beach, looking at the rocks, and find the fault. Approximately 2 km of faulted and sheared rocks is exposed in the cliffs. This fault zone is the western margin of the granitic Salinian Block.

Describe the fault zone. How did you identify it?

Time Mileage Directions 4:00 7.5 Leave STOP 2 at Garrapata Creek 8.7 Rocky Point (park near the roadcut)

STOP 3: The Great Valley sequence NE-dipping Upper Cretaceous, brown-weathering Great Valley coarse sandstone, shale, and siltstone. Well-bedded clastic sedimentary strata are derived from the Klamath-Sierran plutonic- volcanic arc.

Time Mileage Directions 4:30 8.7 Leave STOP 3 at Rocky Point 11.4 Cross Bixby Bridge 12.5 Hurricane Point

STOP 4: Hurricane Point and Jurassic A long coast vista allows observation of the steep tectonically uplifted and faulted seaward margin of the . Roof pendants of Jurassic limestone are visible on the road cut to the east; the tombolo at Point Sur seen to the south is comprised of resistant Franciscan rocks (dominantly greenstone).

What is a roof pendant?

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Time Mileage Directions 5:00 12.5 Leave STOP 4 at Hurricane Point 13.4-14.2 Look left: Light-colored graphitic marbles of the Sur Series (Coast Ridge Belt), striking N-S, dipping about 60°E. The south end of the outcrop contains a poorly exposed Great Valley arkosic sandstone layer. This is followed by conglomeratic Upper Cretaceous Great Valley sandstone. Notice sand dunes that contain material derived from the abundant nearby granitoids and Great Valley sediments. Here, we cross from Salinia to the Nacimiento block. 16.9 Road to Point Sur — Point Sur is a former sea stack 19.6 Entrance to . Directly opposite, on the east side of the road, is the southern entrance to the old Coast Road, a beautiful 10.4 mi-long gravel road drive through bucollic alpine meadows and valley redwood groves. Serpentinite is exposed at 1.4 mi, but exposures of Salinian block granitoids and metamorphics to north are intensely weathered. This is an optional route on return north. 22.6 Chevron gas station, café, bar and grill 5:15 24.0 Pfeiffer State Park. Hills to west of Big Sur State Park entrance are in the Franciscan Complex, but are poorly exposed. How typical! This is where we camp.

STOP 5: Pfeiffer Big Sur State Park We will camp here for the night. Set up your tents first, then we’ll drive to the Redwood Grill for dinner (about 1 mile north of the park on the west side of Hwy 1 [831-667-2129]). Stop at the store in the Lodge or in one of the stores ~1 mile north or ~1.5 miles south to buy any supplies you forgot. There is wireless internet access at the store in the middle of the state park campground.

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Driving directions and brief stop descriptions — Day 2

Time Mileage Directions 9:00 0.0 Leave camp — Zero your odometers at the Park entrance 1.7 Small gas station, grocery, and deli 2.2 Look right: Franciscan graywacke and a greenstone pod crop out along west side of Hwy 1 near the crest at Post Ranch Inn (west) and Ventana Inn (east), and ~0.2 mi north of . Beyond, cross from the Nacimiento to Salinian block. 3.3 Look left and right: Sheared serpentinite on west, dark Franciscan shale across road on east. 4.2 Smallish parking area on curve

STOP 6: Sur Series, Coast Ridge belt Sur Series marbles, quartzites, and feldspathic gneisses of the Coast Ridge Belt. Mineralogy includes uralite after clinopyroxene, red , graphite, and rare, unaltered clinopyroxene.

Describe the rock(s) that you see:

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Time Mileage Directions 10:00 4.2 Leave STOP 6 parking area 8.6 Pull out on the west side of the road and start walking south along the road studying the rocks

STOP 7: Charnockitic tonalite Coarse, dark gray charnockite with stringers of white alaskite. Charnockitic tonalite contains a large, fine-grained, plate-like mafic granulite body consisting chiefly of biotite + hornblende + plagioclase, representing a wall rock inclusion or an early, mafic igneous border phase or relict mafic dike. It is clearly intruded by dark tonalite and by even later, pale alaskite. Well-exposed, banded charnockite crops out southward to bridge at 32.8 miles.

Describe the rock(s) and any igneous textures or other features you see:

Time Mileage Directions 11:15 8.6 Leave STOP 7 parking area 9.9 Mafic enclaves in granitoid

STOP 8: More charnockitic tonalite We are traveling through brown, weathered charnockitic tonalite. Hornblendic charnockite contains felsic layers and stringers (migmatitic sweatouts or Cretaceous granitoids?). Sur Series marble float suggests that the intrusive contact with the Coast Ridge Belt is nearby. Notice mafic enclaves.

Describe the rock(s) and any igneous textures or other features you see:

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Define “migmatite”:

Define “enclave”:

Define “dike”:

Time Mileage Directions 12:00 9.9 Leave STOP 8 parking area 11.2 Julia Pfeiffer Burns State Park — grab your lunch and take the path under Hwy 1 to the beach

STOP 9: Julia Pfeiffer Burns State Park Take the ocean trail from the parking lot to the beach; be sure to note the exposure of Miocene Monterey Formation porcellanite.

Describe and name the rock(s) found around the beach:

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Time Mileage Directions 2:00 11.2 Leave STOP 9 at Julia Pfeiffer Burns State Park 12.4 Look left: Coarse conglomerate of the Great Valley Series on the east side of road. Mostly granitic clasts, this unit is probably proximal to the Salinian granite source terrane. 14.6 Park in a safe place along the road

STOP 10: Great Valley Series conglomerate Fantastic exposure of heavy conglomerate of the Great Valley Series.

Describe the conglomerate. What rocks comprise the clasts? What are the likely source rocks?

Time Mileage Directions 2:30 14.6 Leave STOP 10 15.4 Cross from the Salinian to the Nacimiento block. 17.8 Roadside parking

STOP 11: Franciscan Complex greenstone Franciscan greenstone knocker, somewhat weathered, with vague pillows and more obvious pillow breccia. Calcite-epidote veins transect the pod.

Where must this rock have originally formed?

Time Mileage Directions 3:00 17.8 Leave STOP 11 18.1 Roadside parking

STOP 12: Franciscan Complex mélange Very well endurated, disrupted Franciscan graywacke, siltstone, and dark shale. Some would describe this as “beautiful” tectonic mélange; compared to the Bodega Bay mélange, this is fairly craptacular. We’ll spend just a few minutes here.

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Time Mileage Directions 3:15 18.1 Leave STOP 12 19.3 Pull out at a safe place along the road

STOP 13: Franciscan Complex greenstone in mélange Large Franciscan greenstone lenses in Franciscan mélange, approximately 2 km long. Subhorizontal flow layering is well-displayed. Abundant calcite + epidote veins and hydrothermal alteration. Pillows are present ~0.3 mi east of this stop.

Time Mileage Directions 3:45 19.3 Leave STOP 13 19.5 Look left: Hummocky hillsides; rough, patched pavement; and heavy-duty netting are indications of major landslide problems along this stretch of coastline. This is an area of alternating Cretaceous sandstone and Franciscan Formation rocks that are deformed and fractured along the Sur-Nacimiento fault zone. 20.1 Big Creek Bridge — the concrete used to make this bridge incorporated sand taken from the beach below. There is evidently enough greenstone in the beach sand to give this bridge a slightly greenish cast. 21.2 Tectonic block of Franciscan chert 21.7 Look left: Small outcrop of serpentinite (California’s state rock) 22.8 Look left: Franciscan shaley mélange 47.8 Leave pull-out 48.9 Franciscan shaley mélange. 50.4 Pass through the metropolis of Lucia 52.0 Lime Kiln Canyon Landslide—a truly big honker slide 52.4 Limekiln Canyon State Park.

STOP 14: Limekiln Canyon State Park Park and walk down to the beach and check out the boulders and cobbles there.

What rock types do you see here? Limekiln Creek cuts far into the Santa Lucia Range — What are the likely sources for these rocks (i.e., Franciscan Complex, Salinian, something else?)?

Time Mileage Directions 4:00 Leave STOP 14 — Drive back to SFSU

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What is the Salinian Terrane? What is its age and tectonic affinity? Where did it originate?

What is the Franciscan Complex? What is its age and tectonic affinity? What rock types comprise typical Franciscan Complex rocks

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Summary of the geology of the coast by Gary Ernst

Introduction

The Central contain a telescoped lithotectonic complex representing stages in the geologic evolution of the Mesozoic margin of California. Important maps and cross- sections have been published by Ross (1976a, b; 1977; 1983), Ross and McCullock (1979), and Hall (1991). Physiographic provinces and rock assemblages include: (1) the landward Andean (Sierran) calcalkaline plutionic-volcanic arc + Northwest Foothills upper Paleozoic-lower Mesozoic metamorphic belt; (2) the Upper Jurassic-Cretaceous Central Valley (Great Valley forearc basin turbiditic strata); and (3), the contemporaneous deep-water trench deposits (Franciscan Complex). Neogene dextral slip along the San Andreas system transected the old continental margin from an internal, landward position on the south in the present , to an external, oceanward position near Cape Mendocino on the north, reflecting the fact that the North American -capped plate progressively overran the outboard East Pacific Rise spreading center during the late time. Right-lateral motion along the San Andreas thus has duplicated the Mesozoic continental margin along the Big Sur coast — (1) Salinian silicic, calcalkaline granitoids + Paleozoic Sur Series metamorphics, (2) Great Valley first-cycle detrital sediments depositionally resting on the Salinian basement, and tectonically thrust over the outboard Franciscan Complex, and (3), Nacimiento graywackes, greenstones, and cherts of the Franciscan mélange. The nature of the Neogene strike-slip is quite apparent, but he antecedent history of rock sections disposed along the Sur-Nacimiento fault is not.

What is clear is that the Sur Series + Salinian granitic rocks share petrochemical and geochronologic characteristics with inboard granitoids of the western and the (miogeoclinal upper Paleozoic- Calaveras Complex = platform strata of the Paleozoic Sur Series?). In sharp contrast, the Franciscan is everywhere a dog’s breakfast of oceanic affinity (far-traveled + hemipelagic chert) overlain by voluminous masses of calcalkaline arc-derived first-cycle clastics, all intensely tectonized within the North American/paleo--boundary subduction zone. The facts that Salinian tonalities have been recrystallized to pyroxene(s) ± garnet-bearing phase assemblages (i.e., charnockites) and associated aluminous metapelitic rocks contain sillimanite suggest high temperatures and considerable depths of origin. Somewhat similar, deep-seated, garnetiferous tonalities in the eastern Transverse Ranges have been described by Sams and Saleeby (1987); therefore, a palinspastic restoration (approximately 310 km) of the Big Sur region against the farther inboard Tehachapis + western Mojave is plausible. Eocene to lower Miocene strata of the northern Santa Lucia Range were deposited in a basin adjacent to analogous rocks exposed in the San Emigdio area of the Transverse Ranges just east of the San Andreas (Nilsen and Link, 1975), supporting such a Neogene offset. The more oceanic, deep-sea Franciscan assemblages along the Big Sur coast have been exhumed from much shallower subduction depths compared with along- strike metagraywacke terranes cropping out farther south in the San Luis Obispo area (Ernst, 1980), but comparable rocks crop out from the Oregon border at least as far south as central Baja.

Various scenarios regarding the plate-tectonic assembly of the Salinia/Nacimiento amalgamated terrane have been proposed. At the end of this road log, abstracts are presented by Page (1981),

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Dickinson (1983), Hall (1991), and Dickinson and Butler (1998). Interpretations for the origin and evolution of the Coast range ophiolite are summarized by Dickinson et al. (1996). In addition, handouts describing the Salinian (Ross, 1983; James and Mattinson, 1987) and Ncimiento (Ernst, 1980) blocks are presented for further study and hours of boundless enjoyment. Field trip stops and points of reference are presented below.

References Cited

Dickinson, W.R., 1983), Cretaceous sinistral strike slip along Nacimiento fault in , Bull. Am. Assoc. Petrol. Geol., 67, 624-645.

Dickinson, W.R. and Butler, R.F., 1998, Coastal and Baja California paleomagnetism reconsidered, Geol. Soc. Am. Bull., 110, 1268-1280.

Dickinson, W.R., Hopson, C.A. and Saleeby, J.B., 1996, Alternate origins of the Coast Range Ophiolite (California)): Introduction and implications, Geology Today, 6, 1-10.

Ernst, W.G., 1980, Mineral paragenesis in Franciscan metagraywackes of the Nacimiento block, a subduction complex of the southern California Coast Ranges, J. Geophys. Res., 85, 7045- 7055.

Hall, C.A., Jr., 1991, Geology of the Point Sur-Lopez point region, Coast Ranges, California: A part of the Southern California allochthon, Geol. Soc. Am. Spec. Paper 266, 40 p.

James, E.W. and Mattinson, J.M., 1987, Metamorphic history of the Salinian Block: an isotopic reconnaissance: in Ernst, W.G. (ed.), Metamorphism and Crustal Evolution of the Western United States, Prentice Hall, Englewood Cliffs, NJ, 938-952.

Nilsen, T.H. and Link, M.H., 1975, , sedimentology and offset along the San Andreas fault of Eocene to lower Miocene strata of the northern Santa Lucia Range and the San Emigdio Mountains, Coast Ranges, central California: in Weaver, D.W. (ed.), Paleogene Symposium, Pacific Section, Am. Assoc. Petrol. Geol., Long Beach, CA, 367-400.

Page, B.M., 1981, The southern Coast Ranges: in Ernst W.G. (ed.), The Geotectonic Development of California, Prentice Hall, Englewood Cliffs, NJ, 329-417.

Ross, D.C., 1976a, Reconnaissance geologic map of the pre- basement rocks, northern Santa Lucia Range, Monterey County, California, U.S.G.S. Misc. Field Studies Map MF-750.

Ross, D.C., 1976b, Distribution of metamorphic rocks and mineral occurrences, Santa Lucia Range, Calif., U.S.G.S. Misc. Field Studies Map MF-791.

Ross, D.C., 1977, Maps showing sample localities and ternary plots and graphs showing modal and chemical data for granitic rocks of the Santa Lucia Range, Salinian Block, California Coast Ranges, Calif., U.S.G.S. Misc. Field Studies Map MF-799.

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Ross, D.C. and McCulloch, D.S., 1979, Cross section of the Southern Coast Ranges and from offshore Point Sur to Madera, California, Geol. Soc. Am. Map and Chart Ser. MC-28H.

Ross, D.C., 1983, The Salinian Block — a structurally displaced granitic block in the California Coast Ranges, Geol. Soc. Am. Memoir 159, 255-264.

Sams, D.B. and Saleeby, J.B., 1987, Geology and petrotectonic significance of the crystalline rocks of the southernmost Sierra Nevada, California: in Ernst, W.G. (ed.), Metamorphism and Crustal Evolution of the Western United States, Prentice Hall, Englewood Cliffs, NJ, 865-894.

13 Regional Tectonics And Structural Evolution Offshore Monterey Bay Region By H. Gary Greene The tectonic and structural evolution of the Monterey Bay region of central California is complex and diverse. The region has been subjected to at least two different types of tectonic forces; to a pre-Neogene orthogonal converging plate (subduction) and a Neogene-Quaternary obliquely converging plate (transform) tectonic influence. Present-day structural fabric, however, appears to have formed during the translation from a subducting regime to a transform regime and since has been modified by both strike-slip and thrust movement. Monterey Bay region is part of an exotic allochthonous structural feature known as the Salinian block or Salinia tectonistratigraphic terrane. This block is proposed to have originated as part of a volcanic arc a considerable distance south of its present location, somewhere in the vicinity of the southern Sierra-Nevada Mountain Range. It consists of Cretaceous granodiorite basement with an incomplete cover of Tertiary strata. Paleocene rocks are scarce, evidently stripped from the block during a time of emergence in the Oligocene time. The Ascension-Monterey Canyon system, one of the largest systems in the world, is located on and adjacent to the Salinian block. The system is composed of two parts which contain a total of six canyons: 1) the Ascension part to the north, which includes Ascension, Año Nuevo and Cabrillo canyons, and 2) the Monterey part to the south, which includes Monterey Canyon and its distributaries, Soquel and Carmel canyons. The ancestral Monterey Canyon originated in early Miocene time, cutting east-west into the crystalline basement rocks. Since that time (~21 Ma), the Salinian block, riding on the Pacific Plate, moved northward along the San Andreas fault zone. During this period of transport the Monterey Bay region was subjected to several episodes of submergence (sedimentation) and emergence () that alternately caused sedimentary infilling and exhumation. The present configuration of the Ascension-Monterey canyon system is the result of tectonic displacement of a long-lived Monterey Canyon, with associated canyons representing the faulted offsets of past Monterey Canyon channels. Slivering of the Salinian block along several fault zones trending parallel or sub-parallel to the San Andreas fault zone (i.e., the Palo Colorado-San Gregorio fault zone) displaced to the north the westerly parts of Monterey Canyon. In this manner Monterey Canyon “fathered” many of the canyons to the north (i.e., Pioneer and Ascension canyons). Tectonics continues to dictate the morphology and processes active in the canyon system today. Erosion has formed a seafloor physiography that is significantly greater in relief than onshore. The Palo Colorado-San Gregorio fault zone marks the continental shelf boundary in the Monterey Bay region and divides the canyon system into two parts, the Ascension and Monterey parts. The Monterey canyon part is youthful with heads that lie at, or close to, the shoreline and its present morphology is the product of active erosion originating near the canyon heads. This canyon system is the main regional conduit for the transport of terrestrial sediments to the . In contrast, the Ascension Canyon part heads far out on the continental shelf, far removed from the littoral drift, but still subjected to erosion from mass wasting, some possibly fluid induced.

Structure–1 Fluid Flow In The Offshore Monterey Bay Region By H. Gary Greene Moss Landing Marine Laboratories and Monterey Bay Aquarium Research Institute Moss Landing, CA 95039, Norman Maher, Thomas H. Naehr Monterey Bay Aquarium Research Institute Moss Landing, CA 95039 and Daniel L. Orange Department of Earth Sciences Univ. California, Santa Cruz Santa Cruz, CA 95064 Abstract Fluid flow out of the seafloor offshore Monterey Bay region is extensive. To date 16 major active and ancient, or dormant, seep sites have been identified and many of these sites are composed of smaller sites too numerous to map at a regional scale. These seeps have been identified by the presence of chemosynthetic communities that are primarily composed of chemoautotrophic organisms or by carbonate deposition and buildups. Of the 17 identified sites, 9 active sites support living chemosynthethic communities. Seven major dormant seep sites have been identified based upon the presence of carbonate deposits or buildups. Identified seep sites are primarily concentrated along fault trends associated with the boundary of the Salinian block or Palo Colorado-San Gregorio fault zone, and along the lower flanks and crests of tectonically uplifting slopes. A combination of transpressional squeezing and overburden pressures, vertical advection through hydrocarbon and organic-rich sediment, and seaward flow of meteoric waters supply fluids to the seep sites. Introduction Monterey Bay is located within the active transform boundary that separates the Pacific Plate from the (Fig. 1). In central California this boundary is over 100 km wide and includes offshore faults of the Palo Colorado-San Gregorio and Monterey Bay fault zones (Fig. 2). These fault zones are seismically active and in many places offset the seafloor or Quaternary sedimentary rocks (Greene et al., 1973, 1989; Greene, 1977, 1990; McCulloch and Greene, 1990). The Palo Colorado-San Gregorio fault zone is a 200 km long fault zone that trends nearly N300W and defines the western boundary of the Salinian block in the Monterey Bay region (Page, 1970,; Page and Engerbretsen, 1984; Greene, 1977, 1990). The Salinian block is a sliver of southern Sierran granitic rocks that is being carried northward on the Pacific Plate, sliding along the San Andreas fault proper (Page and Engerbretsen, 1984). The Monterey Bay region can be divided into two major physiographic and tectonic provinces; (1) an eastern allochthonous (Salinian) block and strike-slip fault sheared and slivered province and (2), a western allochthonous (San Simeon) block and transpressionally faulted and deformed or continental slope accretionary province (Greene et al., 1997). These provinces are separated by

Structure–2 the Palo Colorado-San Gregorio fault zone, the local western boundary of the Salinian block (Fig. 1).

Figure 1. Generalized sketch map showing the allochthonous Salinian block of Sierran granitic basement rocks. Modified after Greene (1990).

The Palo Colorado-San Gregorio fault zone juxtaposes the Tertiary marine sedimentary rocks and their underlying Mesozoic basement units of the two allochthoneous blocks. West of the fault zone continental slope sediments are subjected to transpressional forces associated with the oblique convergence of the Pacific Plate against the North American Plate (Nagel and Mullins, 1983; Greene, 1990). Here Greene et al. (1997) and Orange et al. (1993, 1995, in press) interpret that two areas (Smooth ridge and Sur slope) are being uplifted by the oblique fault motion associated with the Palo Colorado-San Gregorio fault zone. This compression is probably causing interstitial fluids to migrate up through the sediments and seep out along the surface trace of the faults where extensive areas of carbonate slabs have been found. Within the Salinian block, the Monterey Bay fault zone is comprised primarily of short (2-3 km long), discontinuous, en echelon faults oriented primarily NW-SE (Greene et al., 1973; Greene, 1977, 1990; Gardner-Taggart et al., 1993). Two longer faults within the Monterey Bay fault zone, the offshore extensions of the Chupines and Navy faults (25-30 km long offshore), mapped onshore near the towns of Seaside and Monterey (Rosenberg and Clark, 1994), are exceptions to this. These two faults generally define the boundaries of this fault zone which is restricted to Monterey Bay and the onshore area to the southeast, in the northern Santa Lucia Range. The Monterey Bay fault zone merges with the Palo Colorado-San Gregorio fault zone offshore of Santa Cruz and southward along the trend of Carmel Canyon (Fig. 2). Gardner-Taggart et al.

Structure–3 (1993) reports that two types of faults occur in the southern part of the Monterey Bay fault zone, strike-slip and thrust. The primary NW-SE oriented faults appear as right-lateral strike-slip faults, whereas conjugate faults are thrust faults that generally trend east-west. Rosenberg and Clark (1994) reported similar fault relationships onshore.

Figure 2. Physiographic map of the Monterey Bay region showing generalized geologic structure and sites of past, present and potential fluid flow and gas concentration on and in the seafloor. Offshore shaded relief map constructed from Simrad EM300 (30 kHz) multibeam bathymetric data collected by MBARI and the USGS; contour interval is 1000 m. Onshore topography from USGS DEM’s. Black lines are active faults: solid lines where well defined and dashed lines where inferred. Long dashed black lines on mid-slope are surface expressions of inactive faults Structure–4 Cretaceous granitic basement rocks of the Salinian block (Fig. 1) lie adjacent to the Franciscan complex west of the San Andreas fault (Jennings and Burnett, 1961) and are thought to underlie the Tertiary marine and Quaternary continental slope deposits west of the Palo Colorado-San Gregorio fault zone (Greene, 1977, 1990; Mullins and Nagel, 1981; Nagel et al., 1986). Offshore in the Monterey Bay region, approximately 1,790 m of Tertiary strata overlie the Cretaceous basement rocks and about 570 m of Quaternary sediments overlie the Tertiary strata, totaling about 2,360 m of sedimentary rocks overlying basement (Greene, 1977). East of the Palo Colorado-San Gregorio fault zone in northern Monterey Bay about 550 m of the Monterey Formation unconformably overlie Cretaceous granitic basement rocks (Greene, 1977; 1990). Unconformably overlying Monterey is about 370 m of upper Miocene Santa Margarita sands and 200 m of the Santa Cruz Mudstone, a well layered diatomaceous mudstone of middle Miocene age. Unconformably overlying the Santa Cruz Mudstone is approximately 670 m of the Purisima Formation which, in turn, is either exposed on the seafloor or covered by deltaic and alluvial deposits or shelf deposits that can total up to 670 m thick (Fig. 3).

Figure 3. Composite stratigraphic section of the northern Monterey Bay region, east of the Palo Colorado-San Gregorio fault zone. Thickness based on continuous single channel seismic reflec- tion profiler data. Modified after Greene (1977, 1990) and on ROV and submersible observations.

East of the Palo Colorado-San Gregorio fault zone in southern Monterey Bay as much as 850 m of Neogene sedimentary rocks and 630 m of Quaternary sediments are piled an average of 1,480 m above the basement (Fig. 4). The sedimentary units of this sequence total approximately 640 m of the Monterey Formation, a porcelaneous and diatomaceous mudstone sequence of Miocene age rich in hydrocarbons. This formation is either exposed on the seafloor or is unconformably overlain by up to 210 m of the Purisima Formation, a nearshore marine sandstone of Late Mi-

Structure–5 ocene to Pliocene age (Fig. 4). Overlying the Purisima Formation are local deposits of Pleis- tocene deltaic, aeolian, alluvial and Holocene shelf sediments that total more than 630 m.

Figure 4. Composite stratigraphic section of the southern Monterey Bay region, east of the Palo Colorado-San Gregorio fault zone. Thickness based on continuous single channel seismic reflec- tion profiler data. After Greene (1977).

During the summer of 1998, MBARI in conjunction with the USGS undertook an extensive bathymetric survey of the Monterey Bay offshore region using a Simrad EM300 (30 kHz) multibeam system mounted aboard the M/V Ocean Alert. The purpose of this survey was to define the seafloor physiography and geomorphology at a resolution that allows identification and investigation of geologic, biologic and chemical features using MBARI’s ROVs Ventana and Tiburon. Over 17,000 km2 of continental shelf, slope and rise were covered. Evidence Of Fluid Seeps In the Monterey Bay region 16 major seafloor sites, many composed of several scattered smaller sites, of fluid seeps have been identified by the presence of chemosynthetic communities primarily composed of chemoautotrophic organisms that are dependent upon thiotrophic symbionts or by carbonate deposition and buildups (Fig. 2). The chemosynthetic communities consist of vesicomyid clams, vestimentiferan worms and free-living bacteria that are dependent upon sulfide-rich fluids for life support and thus indicate present-day seep activity. Carbonate deposits typically represent ancient seeps, although some carbonates were found in the vicinity of current chemosynthetic communities and may be forming today (Orange et al., in press; Stakes et al, in press). Cold-seeps, both and active are being discovered on a regular basis today along active convergent plate margins. Deep water chemosynthetic communities were first noticed during the discovery of hydrothermal vents along the Galapagos Ridge in 1977 (Corliss et al., 1979). Chemosynthetic communities similar to those found at hydrothermal vents, but associated with “cold” seeps, have been reported offshore of Louisiana (Bright et al., 1980; Kennicutt et al., 1985), along the Florida Escarpment (Paull et al., 1984), within Sagami Bay, Japan (Okutani and Egawa, 1985; Hashimoto et al., 1987, 1989), offshore Oregon (Suess et al., 1985; Kulm et al., 1986; Ritger et al., 1987), and in the Japan Trench (Laubier et al., 1986; Le Pichon et al., 1987, Structure–6 Pautot et al., 1987; Cadet et al., 1987; Ohta and Laubier, 1987), to mention a few studies. Many cold seep communities appear to be associated with dynamic geological processes such as tectonically induced high fluid pressures in compressional regimes (Kulm et al., 1986, 1990), artesian springs (Paull et al., 1984; Robison and Greene, 1992), hydrocarbon or natural biogenic seeps (Brooks et al., 1987; Kennicutt et al., 1989; Hovland and Judd, 1988), or mass wasting (Mayer et al., 1988). Chemosynthetic Communities – Indicators of Active Seeps Cold seep communities were discovered in the Monterey Bay region during Alvin dives and bottom-camera tows in Monterey and Ascension Fan Valleys in 1988 (Embley et al., 1990;

Figure 5. Shaded relief map of Smooth Ridge constructed from MBARI EM300 bathymetry showing seafloor morphology, faults and seep sites. Triangles represent active seeps; squares indicate ancient seeps.

Structure–7 McHugh et al., 1997). Since then several additional cold seep sites have been observed and sampled in Monterey Canyon and along the offshore fault zones and continental slope using the Monterey Bay Aquarium Research Institute’s (MBARI) remotely operated vehicle (ROV) Ventana (Barry et al., 1993).

Figure 6. Shaded relief map of the Monterey meander and spur located within the middle part of Monterey Canyon. Seafloor morphology, faults, and active seep sites are shown and labeled. Map constructed from MBARI/USGS EM300 mulltibeam bathymetry.

Structure–8 Barry et al. (1993, 1996, 1997) and Greene et al. (1997) divide the biota inhabiting cold seeps of the Monterey Bay region as including ‘obligate’ species, restricted to sites in direct proximity to fluids rich in sulfide, methane, or perhaps other reduced inorganic compounds (e.g. ammonia; Fisher 1990), and ‘regional’ species that are found at seeps, as well as local non-seep habitats. Obligate species may be chemoautotrophic (e.g. Beggiatoa), or have thiotrophic or methanotrophic symbionts (e.g. vesicomyid clams, mytilid mussels, and vestimentiferan worms), but also may be heterotrophic and rely nearly exclusively on chemosynthetic fauna (e.g. galatheid crabs [Munidopsis? sp.], gastropods [Mitrella sp.], limpets [Pyropelta sp.]). Regional fauna may forage on chemosynthetic biota (e.g. Neptunia amianta, lithode crabs), but range throughout regional benthic environments and are clearly not dependent trophically on chemosynthetic production. Distribution of Seeps West of the Salinian Block West of the Salinian block, west of the Palo Colorado-San Gregorio fault zone and within the eastern compressional province, four well defined cold seep sites which support active chemosynthetic communities are known (Fig. 2). The deepest site is located within the proximal Monterey Fan Valley at a water depth of ~3,200 m (Plate 1, A). Here the source of the sulfide- rich fluids that sustain the biota appear to come from either compressional dewatering of sediment under convergent compression or from buried organic sources deposited in channel fill (Embley et al., 1990; Greene et al., 1997). Three distinct cold seep sites (Clam Flat, Horseshoe Scarp-South, Tubeworm Slump in Fig. 5) that support chemosynthetic communities have been identified on Smooth Ridge (Barry et al., 1993; Greene et al. 1997; Orange et al, in press), a smooth sediment covered ridge that separates the Monterey Canyon system from the Ascension Canyon system and makes up the continental slope immediately west of Moss Landing (Figs. 2 & 5). The site known as “Clam Flat” (Barry et al. 1996, 1997; Greene et al., 1997; Orange et al., in press) is located between 980 and 1,010 m deep along the crest of Smooth Ridge. Barry et al. (1996) described the vesicomyid clam Calyptogena kilmeri as the dominant obligate taxa at Clam Flat (Plate 1, B). Tectonic compression at this site results in “squeezing” of the sediment package and outflow of CO -saturated interstitial fluid according to Barry et al. (1996), Greene et al. (1993), Orange et al. (1993)2 and Martin et al. (1997). Apparently fluid expulsion in this area promotes surficial carbonate precipitation and the release of sulfide and methane-rich fluids at the sediment-water interface, in close proximity to where aggregations of thousands of live clams are located (Barry et al., 1996) Pore water analyses of push core sediment samples taken on Smooth Ridge at Clam Flat showed elevated levels of sulfide and methane (Orange et al., in press). Isotopic analyses of authogenic carbonate precipitates by Stakes et al. (in press) from carbonate samples indicate the presence of methane and report isotopic values of –48.8 ‰ to -52.6 ‰ (PDB) for 13C and of +4.05 ‰ to +5.19 ‰ (PDB) for 18O whereas oxygen isotopes indicate precipitation at or near ambient seafloor temperatures. Martin et al. (1997) interprets the presence of the fluids at Smooth Ridge as being derived from both shallow and deep sources based on the presence of higher order hydrocarbons. These authors state that pore fluids outside the Clam Flat seeps include thermogenic methane and within the seeps fluids are of a mixed biogenic-thermogenic origin.

Structure–9 The two other active cold seep sites located west of the Palo Colorado-San Gregorio fault zone lie along the eastern and southern flank of Smooth Ridge (Fig. 5; Orange et al., in press). One of these two sites is known as “Horseshoe Scarp-South” and is located near the upper eastern edge of the ridge at about 800 m deep along an area faulted and deformed by the Palo Colorado-San Gregorio fault zone. At this locality, Orange et al. (in press) reports cold seep chemosynthetic clams and surface parallel authigenic carbonate deposits. The second seep site is located in the secondary head scarp of a recent slump near the lower southern flank of Smooth Ridge in 2,310 m of water and is known as “Tubeworm Slump” (Fig. 5). Here the existence of Vestimentiferan tubeworms and authogenic barite deposits indicate an active cold seep (Naehr et al., 1998). All of the active seep sites on Smooth Ridge appear to result from dewatering of accretionary- like sedimentary units squeezed against the Salinian block by motion of the Pacific Plate (Figs. 2 & 5; Nagel and Mullins, 1983; Greene et al, 1990; Orange et al., 1994, in press; Barry et al, 1996). A combination of uplift and compression leading to dewatering appears to be responsible for fluid-induced mass wasting along the flanks of Smooth Ridge. Distribution of Seeps on Salinia East of the Palo Colorado-San Gregorio fault zone distinct evidence of fluid seepage along faults of the Monterey Bay fault zone and bedding planes of the Purisima Formation exposed along the northern wall of Monterey Canyon consists of sites where metal oxidizing bacterial mats and/or chemosynthetic communities occur (Figs. 2 & 6). Greene (1997) and Orange et al. (in press) proposed that these fluids could be sulfide-rich aquifer waters in the Purisima Formation that originate in the northeast of Monterey Canyon and/or fluids that circulate through the hydrocarbon-rich Monterey Formation. Oxygen isotopic analyses of carbonate deposits sampled at some of these seeps do not indicate fresh water flow (Stakes et al., in press). They conclude that hydraulic connectivity to fresh water aquifers does not force fluid flow at the seeps and that the carbon isotopic values indicate that the carbon source is sedimentary, and that lateral transport of particulate organic carbon dominates fluid flow at the canyon head sites. Of the five confirmed active seep sites found on the Salinian block, three (Mt. Crushmore, Tubeworm City, Clam Field; Fig. 6) are located along the northern wall of Monterey Canyon within Monterey meander and opposite the Monterey meander spur, within the Monterey Bay fault zone (Fig. 6). The fourth site ( Cliff) is located within the extensive mass wasting field and canyon complex that has formed between the Monterey Bay and Palo Colorado-San Gregorio fault zones (Fig. 2). The shallowest seep site is at a depth of 550-700 m and is located at the confluence of Monterey and Soquel submarine canyons, in a region of intense deformation associated with movement along faults within the Monterey Bay fault zone (Greene et al., 1997; Barry et al., 1996). The site is known as “Mount Crushmore”, a name that reflects the shatter ridge-like structure that has been produced from cross-faulting and thrusting within the fault zone. Here bacterial mats and clams buried deeply within black hydrogen sulfide-bearing mud form 0.25-3 m diameter patches of seep communities that stretch for approximately 1 km along the NW-SE trend of the faults in this location (Plate 1, C). Gray bacterial crusts or authigenic carbonate precipitates are common to all seep patches (Barry et al., 1996). The vesicomyid clam Calyptogena pacifica and bacterial mats are the most conspicuous biota found at this site (Barry et al., 1996) (Plate 1, D). Stakes et al. (in press) and Orange et al. (in press) report that pore waters contain moderate sulfide and Structure–10 extremely low methane concentrations. The second site is known as “Tubeworm City,” named after Vestimetifera tubeworms found there. This site is structurally similar to the Mount Crushmore site and is located in water depths of 610 to 810 m along the outer western wall of the Monterey meander. Barry et al. (1996), Greene et al. (1997) and Orange et al. (in press) show that active cold seeps are scattered within crevices and fault gullies in the Monterey Canyon walls that ring the apex of the Monterey meander (Fig. 6). This is an area where several faults of the Monterey Bay fault zone, including the Navy and Chupines faults, cut through the walls of the canyon. Two other active seep sites exist on the outer western wall of the Monterey meander and are aligned along the trend of the Navy fault within the Monterey Bay fault zone (Fig. 6). The third site is known as “Clamfield” and is located along the outer wall of the Monterey meander in 875- 920 m of water, centered at 896 m, is 2 m wide and 150 m long trending E-W. This site lies along the Navy fault trend and is composed of a dense aggregation of Calyptogena sp. clams concentrated in muds overlying the Monterey Formation (Barry et al., 1996). At Clamfield the communities may depend upon a sulfide source within the Miocene Monterey Formation. Here Ferioli (1997) found that the fluids are salt water derived. The fourth site is called “Invertebrate Cliff” named after clams found there. This site lies in water depths of approximately 900 to 1,100 m (Fig. 6). Because the site has just recently been discovered, no detailed descriptions have been reported. We interpret the community to be sustained by fluids seeping out along the offshore extension of the Navy fault. The fifth seep site (842 in Fig. 6) is located along the southern wall of Monterey Canyon, just south of the eastern flank of the Monterey meander spur. Stakes et al. (in press) report that faulted granitic rocks exposed along the southern wall of Monterey Canyon in this area is occasionally covered with bacterial mats. Ancient Carbonate Deposits - Indicators of Past Fluid Flow Seven major fossil or dormant seep sites have been identified based on the existence of carbonate deposits or buildups. Five of these sites either lie on, or in close proximity to, the western margin of the Salinian block, along the Palo Colorado-San Gregorio fault zone, or on Smooth Ridge and the upper flank of Sur slope (Fig. 2). Two other sites are located on the Salinian block, on the shelf of southern Monterey Bay. West of Salinia Using side scan sonar data along with in situ observations and sampling from MBARI’s ROV Ventana, three major areas of fluid-produced carbonate deposits have been identified near the head of Smooth Ridge (Fig. 5; McHugh et al., 1997; Orange et al., in press; Stakes et al., in press). These deposits occur along the trend of the offshore extension of the San Gregroio fault zone, both east and west of the main fault strands. Although many areas are devoid of living chemosynthetic biota, they are underlain by carbonate cemented sediment. Orange et al. (in press) describe carbonate layers on the seafloor along the western margin of the San Gregorio fault zone and large (5 m x 2 m x 1 m thick) rectangular blocks of carbonates along the eastern margin of the fault zone that are randomly scattered and oriented. In other areas along the Palo Colorado-San Gregorio fault zone (Fig. 5), on the outer continental

Structure–11 Figure 7. Shaded relief map of the Sur Ridge area showing seafloor geomorphology and seep sites. Image constructed from MBARI/USGS EM300 multibeam bathymetry. Ancient seep sites indicated by squares, potential seep sites by astricks and possible carbonate deposits by circles. shelf NE of Smooth Ridge, Orange et al. (in press) describe en echelon carbonate ridges a few centimeters wide and high and about 10 m long and trending N-S. Brachiopods are often attached to the carbonate deposits. At sites where no carbonate deposits crop out on the seafloor yet extensive patches of brachiopods occur, we found hard carbonate substrate about 8-10 cm beneath the seafloor. Petrographic analyses reported by Orange et al. (in press) indicate that the carbonates in this area are composed in part of micritic to sparitic calcite and brachiopod shell hash with framboids of sulfide and inclusions of hydrocarbons. Push cores obtained with the ROV Ventana contained minor amounts of hydrocarbons. Orange et al. (in press) concluded that methane seepage is probably dormant, although earlier methane seeps did produce the carbonate deposits. Structure–12 Figure 8. Shaded relief map of the headward part of Monterey Canyon and the Monterey Bay shelf showing seafloor geomorphology, faults and locations of carbonate deposits (squares). Map constructed from USGS EM1000 and MBARI/USGS EM300 multibeam bathymetry.

Two other areas of past fluid seepage were identified on Smooth Ridge. One site, known as “Chimney Field”, is located on the upper northern flank of the ridge, at the base of a slump head scarp (Fig. 5). Broken carbonate chimneys (as large as 1.5 m long x 0.6 m in diameter and in the form of doughnut-like features) lying on their sides are concentrated in this area. Orange et al. (in press) and Stakes et al. (in press) report that the isotopic composition of the carbonates range Structure–13 from -55.9 ‰ to -11.8 ‰ (PDB) for 13C and +3.44 ‰ to +6.82 ‰ (PDB) for 18O. Oxygen isotopic analyses indicate that precipitation took place at near ambient seafloor temperatures. The other site is known as “Horseshoe Scarp-North” and is located along the eastern margin of Smooth Ridge, a fault truncated boundary (Fig. 5). Authigenic slope parallel carbonate slabs are found near the base of the head scarp of the slump here. No biologic communities indicative of present-day fluid flow were found (Orange et al., in press). A Fourth site is located in about 400 m of water along the upper northern flank of Sur slope (Figs. 2 & 7). Carbonate “patties” (Plate 1, E) are scattered about the slope in this locality and suggest that past fluid flow has occurred in this area (Waldo W. Wakefield, pers. Commun. 1998). On the Salinian Block Three major sites of past fluid flow are located on the Salinian block and two are found on the southern Monterey Bay shelf (Fig. 2). One of the three sites lies within the Monterey Bay fault zone, between the offshore extensions of the Navy and Chupines faults (Fig. 8). Side scan sonar and seismic reflection profile data, as well as submersible diving observations and sampling, indicate that a carbonate mound (80 m x 40 m x 4 m high), that incorporates Pleistocene gravels, formed on top of faulted and tilted beds of the Monterey Formation since the last rise in sea level (Plate 1, F). This mound is located in a major fishing ground known as Portuguese Ledge in water depths of 90 m. Simrad EM1000 swath bathymetry data collected by the U.S. Geological Survey (Ettreim et al., 1997; Edwards et al., 1997) indicate that Portuguese Ledge and nearby Italian Ledge, are partially composed of carbonate mounds and angular blocks forming “hard ground” critical to rockfish habitat (Fig. 8). Oxygen and carbon isotopic analyses of the carbonate sampled from the mound at Portuguese Ledge yielded 18O values of -6.01 ‰ to -6.06 ‰ (PDB) and 13C values of -9.77 ‰ to -10.04 ‰ (PDB) (K.C. Lohman, Univ. of Michigan, Written Commun., 1996). Stakes et al. (in press) obtained similar values from a sample collected in the same area, which range from values of -5.81 ‰ to -5.68 ‰ (PDB) for 18O and values of -10.01 ‰ to 10.21 ‰ (PDB) for 13C. These data suggest meteoric water sources and based on this, along with interpretation of geophysical data and in situ seafloor observations, we speculate that fresh water fluids flowed offshore along a fault and then along fault-tilted bedding planes to the seafloor where the carbonate mound formed. Because the carbonate mounds found at this location contain Pleistocene gravel, we speculate that seeping occurred before or shortly after the last transgression started, ca 7000 BP. The second site is located along the shelf break at the top of a slump head scarp, at the top of the southern headward wall of Monterey Canyon (Fig. 8). The EM300 multibeam bathymetry data show several rounded mounds that stand about 4 m above an otherwise flat seafloor 90-100 m deep (Fig. 8). In addition, 100 kHz side scan sonar data show a high reflectivity bottom composed of concentric reflectors that dip away from a gently uplifted center indicating that the seafloor here is domed and composed of thin sheets of hard ground. Dredge haul samples of well lithified gravel cemented by spary calcite (Robert E. Garrison, Pers. Commun., 1998) were collected from this locality. Based on the exposures of fresh water aquifers (180 foot and 400 foot aquifers in the Aromas Red Sands and the deep aquifer in the Purisma Formation) exposed along the slump scarp, we suspect that the carbonate precipitation resulted from the flow of fresh water. Also, just Structure–14 below the shelf break in the slump head scarp, side scan sonar and EM 300 bathymetric data show a series of linear reflectors and ROV observations show that these reflectors correspond with extensive flat lying tabular beds of CaCO exposed in the upper wall of the canyon. These3 beds form overhangs and ledges. Stakes et al. (in press) described an area (site 842 in Fig. 6) in northern Monterey Bay, along the western edge of the Monterey Bay fault zone, where crusts of CaCO were 3 identified. These authors also noted that a. sometimes bacterial mats are found here as well and that several other minor associated sites in this area contain carbonates that were not occupied by chemosynthetic communities and, therefore may represent past fluid flow. EVIDENCE OF GAS EXPULSION In the Monterey Bay region only two gas sites have been identified, and these are found in Tertiary sedimentary rocks at the heads of submarine canyons (Fig. 2). One site is located on the Salinian block east of the Palo Colorado-San Gregorio fault zone, at the head of Soquel Canyon which incises b. the northern Monterey Bay shelf. The Figure 9. Geopulse seismic reflection profiles (a., canyon cuts the Purisima Formation and line 64 and b., line 68) across the shelf immedi- seismic reflection profiles collected across ately north of the head of Soquel Canyon show- the shelf just north of the canyon head ing acoustic anomalies (bright spots) characteris- exhibit acoustic anomalies or “bright spots” tic of gas charged sediments. See Figure 8 for indicative of gas charged sediments (Figs. 8 location. After Sullivan (1994). & 9; Sullivan, 1994). Sullivan (1994) reported finding water column anomalies which she interpreted as gas. We speculate that the block glides and other slump deposits found in the head of Soquel Canyon (Sullivan, 1994) result from elevated pore pressures associated with periodic venting of gas in the headwalls of the canyon. A second gas site lies along the shelf at the head of Año Nuevo Canyon (Fig. 10). Data collected from a single channel 1 kJ sparker and a 300 J Uniboom seismic reflection profiling systems along the Año Nuevo Point to Santa Cruz shelf defined an area of acoustic anomalies (“bright spots”) on the shelf immediately adjacent to the canyon head (Mullins and Nagel, 1982; Nagel et

Structure–15 Figure 10. Shaded relief map of Ascension Canyon slope showing inferred gas induced canyon head collapse at Año Nuevo Canyon. Dashed line represents inferred structural lineament, possi- bly a fault or lithologic contact. Map constructed from MBARI EM300 multibeam bathymetry. Slashes and dashed symbols indicate the location of interpreted gas occurrences near the head of Año Nuevo Canyon by Mullins and Nagel (1982). al., 1986). Similar to Soquel Canyon, Año Nuevo Canyon notches the continental shelf and has eroded Pliocene sandstones equivalent in age and lithologies to the Purisima Formation. Mullins and Nagel (1982) also reported finding several water column anomalies that they interpreted as gas venting from the seafloor. The recently collected MBARI EM300 swath bathymetry shows Año Nuevo Canyon to have a crecentic to circular collapsed head (Fig. 11). We speculate that this collapse is primarily the results of high pore pressures produced by gas forcing from Structure–16 Figure 11. Shaded relief map of the northern Ascension slope showing inferred groundwater geomorphology and seep sites. Map constructed from MBARI EM300 multibeam bathymetry. Circle shows location of possible carbonate mounds. A denotes scallop, B marks thin sediment flow, and C’s indicate areas of incipient canyon formation. hydrocarbon sources at depth with the gas traveling to the headwalls of the canyon where mass wasting and headward encroachment is stimulated EVIDENCE OF POTENTIAL FLUID SEEPS The EM300 multibeam data revealed a seafloor morphology that suggests deformation and alteration through transpressional tectonic processes, fluid flow and submarine canyon erosion. Much of the region exhibits fluid induced mass wasting that is especially prominent in the north along the Ascension Canyon slope. Parts of the Sur slope also exhibit a seafloor morphology that

Structure–17 may result from fluid induced mass wasting. These two slopes, the Ascension Canyon (including Smooth Ridge) and Sur slopes, are separated by Monterey Canyon and Fan Valley (Fig. 2) Ascension Canyon Slope and Smooth Ridge The Ascension Canyon slope (Fig. 11) exhibits features such as rills, pits and gullies associated with piping, fluid induced thin sediment flows, concentric- to circular-shaped small (meters to 10’s of meters in diameter) slump scarps we call “scallops” or “cusps” associated with fluid sapping, and fluid induced rotational slumps (Greene et al., 1998; Maher et al., 1998; Naehr et al., 1998). These features are remarkably similar to those described on land by Parker et al. (1990), Jones (1990), Higgins et al. (1990) and Baker et al. (1990). In addition, areas along the distal edge of the shelf and upper slope (100-300 m deep) where rills and pipes terminate correspond to incipient canyon formation (C in Fig. 11). The term pipes as used here refers to narrow conduits through which fluids flow in sedimentary deposits and where granular material is removed forming linear collapsed structures that can be identified as rills and aligned pits on the surface of the seafloor. Past, and perhaps present, fluid flow is suggested in the EM300 bathymetry by the presence of possible carbonate mounds scattered about the flat shelf floor (Fig. 11). The base of the slope is similar to outwash plains. We speculate that fluid and gas migration from the hydrocarbon source in the Monterey Formation of the Outer Santa Cruz Basin (Hoskins and Griffiths, 1971; McCulloch and Greene, 1990) may be responsible for the shelf and slope morphology. We have identified several geomorphic features along the upper slope and outer shelf where we suspect that fluids and perhaps gases are seeping out into the water column (Figs. 2 and 11). The EM300 data also shows that the southern flank of Smooth Ridge is shedding its sediment cover, which appears the result of fluid induced mass wasting. We have identified a few of these areas in Figures 2 and 11. Young to incipient cold seep sites aligned N-S along the eastern crest of the ridge, just west of the Horseshoe Scarps, may be forming from compressional squeezing, a result of the buttressing effect the Salinian block creates where the ridge is obliquely converging along the western boundary of the block, along the Palo Colorado-San Gregorio fault zone. This compression is forcing fluid flow. Sur Slope Similar to the Ascension Canyon slope and Smooth Ridge, the Sur slope is being tectonically uplifted as indicated by the uplifted terraces in the coastal part of the Santa Lucia Mountains adjacent to Point Sur and the shedding of the sedimentary cover at the base of Sur slope. This uplift has led to the initiation of denudation of the upper slope and the formation of gullies and canyon heads (Fig. 7). Apparent fluid induced mass wasting occurs here as suggested by rills, mounds that may be constructed of carbonates and scallops. The EM300 data collected along the northern flank of Sur slope shows two possible carbonate sites where mounds have been identified associated with canyon heads on an otherwise smooth area of seafloor (Figs. 2 & 7). One site is located at the head of an unnamed canyon just offshore of Garrapata Beach, in the area where the Palo Colorado fault zone extends offshore to connect with the southern extension of the Carmel Canyon fault zone, the southern extension of the Palo Colorado-San Gregorio fault zone. The other site is to the south of this unnamed canyon on the upper slope and distal outer shelf where distinct mound-like topography is mapped (Figs. 2 & 7).

Structure–18 The entire western base of Sur slope is undergoing mass wasting and, based on the geomorphology, we speculate that many of the landslides in this area are fluid induced. The very large and extensive Sur slide, identified by Hess et al. (1979) and Normark and Gutmacher (1988) is composed of a number of retrogressive slumps (Greene et al., 1989) which probably result from an increase in slope due to tectonic uplift of the Sur slope and platform to the east, and increased fluid flow resulting from compressional squeezing and overburden pressure. The Monterey Formation exists at depth in this area and gas expulsion may also be taking place. The identification of an extensive pockmark field south of the Sur slope, near the lower part of Lucia Canyon, suggests that gas venting occurred in this area in the past (Maher et al., 1998). We have identified a series of sites along the base of Sur slope that we think are potential fluid seep sites (Figs. 2 & 7). These sites by no means represent the total number of seeps we believe are present, but are identified to indicate symbolically that fluid induced mass wasting is most likely occurring around the base of the lower Sur Ridge. CONCLUSIONS At least 16 major active and past fluid seep sites are identified in the Monterey Bay region. Of the 16 identified sites, 9 are confirmed active and 7 are fossilized or dormant. The active seeps are based on the presence of chemosynthethic communities composed of vesicomyid clams, vestimentiferan worms and free-living bacterial mats. The existence of ancient seeps are based on the presence of slabs, pavements, chimneys and other buildups that are composed of authigenic carbonate. Seeps are generally concentrated along faults, particularly along the Palo Colorado-San Gregorio fault zone that marks the western boundary of the Salinian block in the Monterey Bay region. This boundary locally separates two major tectonic provinces; 1) the eastern fault sheared and slivered allochthonous Salinian block province and 2), the western transpressional accretionary province. Expelled fluids in the eastern province appears to derive from several different sources. Ancient or dormant seep sites on the southern Monterey Bay shelf may have resulted from the expulsion of aquifer driven meteoric waters. In the Portuguese Ledge and Italian Ledge areas we suggest that these waters traveled along faults within the Monterey Bay fault zone to sites on the seafloor where the water vented. Along the eastern margin of the western province, west of the Palo Colorado-San Gregorio fault zone and in the area where the faults of the Monterey Bay fault zone merge with the Palo Colorado-San Gregorio fault zone, many active seeps are located. Cold-seep communities of metal oxidizing bacterial mats and chemosynthetic clams (Vesicomya) along the northern wall of Monterey Canyon indicate that sulfide-rich fluids are seeping out of faults in the Monterey Bay fault zone (Barry et al., 1993, 1996, 1997). The fluid chemistry suggests the mixing of fluids from several sources (Stakes et al., in press; Orange et al., in press). Some fluids may result from advection through the hydrocarbon-rich Monterey Formation whereas other fluids may be transported along fault conduits. Other fluids travel through freshwater aquifers within the Purisima Formation that extend from the Santa Cruz Mountains to Monterey Bay and surface along the walls of Monterey Canyon (Greene et al.,

Structure–19 1997). Artesian conditions exist, although flow rates at the faults are not known. At Mount Crushmore the chemosynthetic communities may obtain sulfide-rich fluids from artesian flow of aquifer waters as the biota are all concentrated in black hydrogen-sulfide mud in fault and fractured crevices that cut deeply into the Pliocene Purisima Formation, a shallow water marine sandstone and the best recharged aquifer of the Santa Cruz Mountains (Muir, 1972). Some fluids may be methane-rich resulting from gas overpressures in the Monterey Formation and migration to overlying permeable sandstones such as the Purisima Formation exposed at the heads of Soquel and Año Nuevo canyons. In the western province expelled fluids originate from several sources and processes. Compression of the sedimentary slope west of the Palo Colorado-San Gregorio fault zone due to transpressional movement may cause dewatering. This dewatering is shown by the existence of cold seep chemosynthetic communities and carbonate crust formation. Carbon and oxygen isotopic analyses indicate several sources for these fluids (Orange et al., in press; Stakes et al., in press). One source is from the hydrocarbon-rich Monterey Formation of Miocene age whereas the other is from carbon buried in Quaternary sediments. The Monterey Bay region is a major upwelling area and high organic production occurs here. Organic-rich sediment accumulates on the slope and produces considerable biogenic methane. Other fluids flow along faults and have a source deep in the sedimentary column. In Monterey Fan Valley, chemosynthic seeps are reported to be the result of biogenic fluids that originate from buried organic material in filled channels (Greene et al., 1997). No chemical analyses have been made of these seeps so the origin of fluids is still speculative. The recently collected EM300 multibeam bathymetry data indicates extensive areas of seafloor fluid flow and gas expulsion north and south of Monterey Bay. The Ascension Canyon slope exhibits distinct groundwater geomorpholgy with numerous fluid-induced mass wasting features. Fluid induced rills, grooves, pipes and pits are common in this area. We speculate that these features may be forming from gas and fluid expulsion originating from deep hydrocarbon sources in the Outer Santa Cruz Basin. EM300 data show that the Sur slope offshore of Point Sur is undergoing similar mass wasting processes. In addition, these data revealed an extensive pockmark field to the south of the Sur slope (Maher et al., 1989) that may be the result of past gas expulsion associated with of gas escaping from the Monterey Formation at depth here, possibly initiated by seismic activity. ACKNOWLEDGEMENTS We wish to thank the USGS for use of the shallow water EM300 and EM1000 multibeam bathymetry collected on the southern Monterey Bay and Carmel to Point Sur shelves. This manuscript was substantially improved by the reviews of David Howell and Steve Eittreim of the USGS and David Clague and Charlie Paull of MBARI. Special thanks goes to Bob Garrison for his encouragement, review and patience in working with us on this paper.

Structure–20 REFERENCES CITED Baker, V. R., Kochel, C. R., Laity, J. E., and Howard, A. D., 1990, Spring sapping and valley network development, in Higgins, C. G. and Coats, D. R., Groundwater Geomorphology: The Role of Subsurface Water in Earth-Surface Processes and Landforms: Geol. Soc. Amer. Special Paper 252, p.235-266. Barry, J. P., Robison, B. H., Greene, H. G., Baxter, C. H., Harrold, C., Kochevar, R. E., Orange, D., Lisin, S., Whaling, P. J., and Nybakken, J., 1993, Investigations of cold seep communities in Monterey Bay, California, using a remotely operated vehicle: Amer. Acad. Underwater Sci., Symposium volume, p. 17-32. Barry, J. P., Greene, H. G., Orange, D. L., Baxter, C. H. Robison, B. H. Kochevar, R. E. Nybakken, J. W., Reed, D. L. and McHugh C. M., 1996, Biologic and geologic characteristics of cold seeps in Monterey Bay, California: Deep-Sea Research I, v. 43, n. 11- 12, p. 1739-1762. Barry, J. P., Kochevar, R. E., and Baxter, C. H., 1997, The influence of pore-water chemistry and physiology on the distribution of vesicomyid clams at cold seeps in Monterey Bay: implications for patterns of chemosynthetic community organization: Limnology and Oceanography, v. 42, p. 318-328. Bright, T. J., LaRock, P. A., Lauer, R. D. and Brooks, J. M., 1980, A brine seep at the East Flower Garden Bank, northwestern Gulf of Mexico: Int. Rev. Gesamten Hydrobiol., p. 65, 535-549. Brooks, J. M., Kennicutt II, M. C., Fisher, C. R., Macko, S. N., Cole, K., Childress, J. J., Bidigare, R. R. and Vetter, R. D., 1987, Deep-sea hydrocarbon seep communities: evidence for energy and nutritional carbon sources: Science, v. 238, p. 1073-1083. Cadet, J.-P., Kobayashi, K., Lallemand, S., Jolivet, L., Aubouin, J., Boulegue, J., Dubois, J., Hotta, H., Ishii, T., Konishi, K., Niitsuma, N. and Shimamura, H., 1987, Deep scientific dives in the Japan and Kuril Trenches: Earth Planet. Sci. Letters, v. 83, p. 313-328. Clark, J. C., 1966, Tertiary stratigraphy of the Felton-Santa Cruz area, Santa Cruz Mountains, California: Ph.D. Thesis, Stanford Univ., Stanford, Calif., 184 p. Corliss, J., Dymond, B., J., Gordon, L. I., Edmond, J. M., von Herzen, R. P., Ballard, R. D., Green, K., Williams, D., Bainbridge, A., Crane, K. and van Andel, T. H., 1979, Submarine thermal springs on the Galapogos Rift: Science, v. 203, p. 1073. Edwards, B. D., Gardner, J. V., Medrano, M. D., 1997, Grain size, organic carbon, and CaCO of surface sediments from the southern Monterey Bay continental shelf seafloor, in Eittreim,3 S.L. (ed.), Southern Monterey Bay Continental Shelf Investigation: Former Fort Ord Restricted Zone, U.S. Geol. Survey Open File Rept. 97-450, p. 22-51. Eittreim, S. L., Stevenson, A. .J., Mayer, L. A., Oakden, J., Malzone, C., and Kvitek, R., 1997. Multibeam bathymetry and acoustic backscatter imagery of the southern Monterey Bay shelf, in Eittreim, S.L. (ed.), Southern Monterey Bay Continental Shelf Investigation: Former Fort Ord Restrcted Zone: U.S. Geol. Survey. Open-File Rept. 97-450, p. 1-21. Embley, R. W., Eittreim, S. L., McHugh, C. H., Normark, W. R. Rhu, G. H. Hecker, B. DeVevoise, A. E., Greene, H. G., Ryan, W. B. F., Harrold, C. and Baxter, C., 1990, Geological setting of chemosynthetic communities in the Monterey Fan Valley system: Deep Sea Res., v. 37, p. 1651-1667.

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Structure–25

TECTONICS, VOL. 22, NO. 5, 1058, doi:10.1029/2002TC001409, 2003

Tectonic and magmatic development of the Salinian Coast Ridge Belt, California

Steven Kidder, Mihai Ducea, George Gehrels, P. Jonathan Patchett, and Jeffrey Vervoort Department of Geosciences, University of Arizona, Tucson, Arizona, USA Received 6 May 2002; revised 13 May 2003; accepted 9 July 2003; published 24 October 2003.

[1] We present new field, structural, petrographic, Coast Ridge Belt, California, Tectonics, 22(5), 1058, doi:10.1029/ and geochronologic data on a rare midcrustal (25 km) 2002TC001409, 2003. exposure of a Cordilleran arc, the Coast Ridge Belt, located in the Santa Lucia Mountains of central California. The study area is composed primarily of 1. Introduction a deformed suite of upper to granulite [2] Cordilleran batholiths are extensive belts of interme- facies rocks (the ‘‘Sur Series’’), which is dominated by diate calc-alkalic plutons formed in the continental crust metaigneous tonalites, diorites, and gabbros with above subduction zones. Understanding the petrology and subordinate metasedimentary quartzite and marble. tectonic framework of these granitic batholiths has stirred Inherited zircons in magmatic rocks suggest that the great geologic controversies and continues to pose several provenance of framework rocks is drawn heavily from major problems in modern geology such as quantifying the miogeoclinal formations and that sedimentation rates and processes of crustal growth versus recycling in occurred in the late Paleozoic or later. Minor arc environments [e.g., Hamilton, 1988]. The key ques- tions are centered around understanding the mechanisms magmatism in the Coast Ridge Belt began in the by which continental magmatic arcs may contribute to the Early or Middle Cretaceous, but magmatic activity production of on average andesitic continental crust, while was most intense during a short period time from 93 to melt additions from the mantle are basaltic [Rudnick, 81 Ma, based on U-Pb zircon ages of a felsic gneiss 1995]. Determination of the composition of the crust and and two less-deformed diorites. The time period 93– upper mantle beneath arcs can provide first-order con- 81 Ma also brackets a period of extensive thickening straints on defining the processes of magmatic addition and high-temperature ductile deformation. While a and batholith formation and help resolve the crustal thrusting cause for ductile deformation cannot be ruled compositional paradox. out, we favor the hypothesis that the exposed rocks [3] One of the major limitations in deciphering large- correspond to a zone of return flow of supracrustal scale magmatic and deformational features in arcs is a rocks locally displaced by granitoid plutons in the poor knowledge of their vertical dimension. Most major active or recent continental arcs are located around the shallower crust. Magmatism ended throughout Salinia Pacific, the largest ones being found along the western between 81 and 76 Ma, coincident with the attainment of margins of North and South America. Exposures of arcs to peak pressure and temperature conditions of 0.75 GPa paleodepths in excess of 20 km are virtually absent in and 800C. Exhumation followed immediately, South America, and rare in the North American Cordillera. bringing the Coast Ridge Belt to the surface within North American exposures are not deeper than some 8 My at a rate of at least 2–3 mm/yr. Exhumation was 30 km, although seismic [Graeber and Asch, 1999] and coincident with an episode of extensional collapse that xenolith [Ducea and Saleeby, 1996] data suggest that has been documented elsewhere in the southern Cordilleran arc crustal thickness is commonly about California arc during the early Laramide orogeny 75 km. With very limited arc-related middle or deep- and that may be related to underthrusting of the crustal exposures available around the Pacific Rim, seismic velocity has been used with various degrees of success forearc at that time. INDEX TERMS: 8102 Tectonophysics: to estimate crustal composition [Smithson et al., 1981; Continental contractional orogenic belts; 8035 Structural Geology: Christensen and Mooney, 1995; Rudnick and Fountain, Pluton emplacement; 3640 Mineralogy and Petrology: Igneous 1995]. Much less is known about the interplay between petrology; 8030 Structural Geology: Microstructures; 8124 deformation and magmatism at middle to deep-crustal levels Tectonophysics: Earth’s interior—composition and state (1212); in arcs [Paterson and Miller, 1998]. One of the most KEYWORDS: Salinia, Salinian block, Santa Lucia Mountains, important issues in studies of convergent margins is the deep arc exposures, amphibolite-granulite, Coast Ridge Belt. question of the degree to which crustal thickening at conti- Citation: Kidder, S., M. Ducea, G. Gehrels, P. J. Patchett, and nental arcs is ‘‘magmatic’’ versus ‘‘tectonic’’. Magmatic J. Vervoort, Tectonic and magmatic development of the Salinian thickening refers to addition of melts from the Earth’s mantle during the buildup of the arc, whereas tectonic thickening Copyright 2003 by the American Geophysical Union. requires crustal shortening, intracrustal deformation, and 0278-7407/03/2002TC001409$12.00 crustal melting to be major process in continental arc

12 - 1 12 - 2 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT

Figure 1. Map of central and southern California showing geologic features discussed in the text. Mesozoic granitic and related metamorphic rocks are shaded lightly. Fine-gridded areas are mainly Jurassic Franciscan formation. Mesozoic amphibolite-granulite terranes are outlined and labeled. The restored location of the Santa Lucia Mountains is based on palinspastic reconstruction prior to disruption by the San Andreas Fault system (20 Ma; Powell [1993]). Restoration is relative to the southern Sierra Nevada. environments. Are the well-established magmatic flare ups metamorphism in the section and (5) the exhumation in major continental arcs largely determined by episodes of history of the arc. crustal shortening [e.g., Ducea, 2001], or are they predom- inantly driven by fluctuations of melt productivity in the 2. Geologic Background mantle wedge? [4] We address the above questions regarding the com- [5] The Salinian block or Salinian ‘‘composite terrane’’ position and evolution of arc crust with new geologic [Vedder et al., 1982] is located west of the San Andreas results from a midcrustal exposure of the Cretaceous fault, east of the Sur and Nacimiento faults, and north of the continental arc located in the Santa Lucia Mountains. Big fault (Figure 1). On the basis of the ages and The Santa Lucia Mountains are located in the California isotopic characteristics of widespread calc-alkaline to calcic Coast Ranges and comprise predominantly upper crustal, tonalites and granodiorites characterized by Ross [1978], Mesozoic, out of place arc-related rocks of the allochtho- Mattinson [1990] suggested an origin for the Salinian nous Salinian terrane (Figure 1). Rocks exposed however basement as a middle to lower crustal exposure of a west in the Coast Ridge Belt, a narrow zone along the south- facing Cretaceous arc straddling the cratonic margin. Mag- western edge of the Santa Lucia mountains, were signif- matic activity in the Salinian arc most likely began between icantly deeper based on the presence of upper amphibolite 100 and 110 Ma, and continued until 76 Ma [Mattinson, and granulite facies rocks [Compton, 1966b; Hansen and 1990], coincident with the major pulse of magmatism that Stuk, 1993]. We present here a detailed map of a transect generated the main segments of the California arc, the Sierra across the Coast Ridge Belt, a geologic and petrographic Nevada and Peninsular Ranges batholiths [Ducea, 2001; description of the metamorphic framework and four pre- Coleman and Glazner, 1998]. viously undocumented intrusions, and U/Pb and Sm/Nd [6] Heterogeneous gneisses and found in the data constraining ages of deformation, intrusion, metamor- Santa Lucia Mountains and elsewhere in the Salinian phism and uplift in the Coast Ridge Belt. Specifically, we composite terrane are known collectively as the ‘‘Sur address the following questions in this study: (1) the origin Series’’ [Trask, 1926]. The Sur Series forms the metamor- of the framework rocks in the area, (2) the timing relation- phic framework for most of the Cretaceous intrusions and is ships between deformation and magmatism, (3) the origin composed predominantly of quartzofeldspathic gneiss and of ductile fabrics, (4) the conditions and timing of peak granofels, quartz biotite schists, marbles and KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 3

Figure 2. Geologic map showing a portion of the Salinian composite terrane near Big Sur, California. The Coast Ridge Belt is bounded by the Coast Ridge fault to the east and by Franciscan rocks on the west. Figure 2 is based on Seiders et al. [1983] and Ross [1976]. See Figure 1 for its location. Courtesy of the U.S. Geological Survey.

[Ross, 1977]. These lithologies have been interpreted to granitic rocks by thousands of km relative to the North represent metamorphosed medium to fine-grained clastic American Cordillera [e.g., Champion et al., 1984; Kanter sedimentary material [Ross, 1977] deposited perhaps as a and Debiche, 1985]. However, recent paleomagnetic data ‘‘shallow, platformal sequence of continental margin ori- [Dickinson and Butler, 1998] strongly argue against the gin’’ [Mattinson, 1990]. Folding and metamorphism related large transport hypothesis for Salinia. to numerous intrusions have thoroughly obscured the orig- [8] While uplift exposed much of the inal sedimentary sequence or sequences, and have pro- Salinian arc to shallow or mesozonal depths of up to hibited descriptions of stratigraphy and estimations of 0.4–0.6 GPa [Wiebe, 1970a], limited 0.7–0.8 GPa expo- sedimentary ages [Wiebe, 1970a; Ross, 1977; James and sures [Hansen and Stuk, 1993] surfaced along what is now Mattinson, 1988]. Although it has no stratigraphic mean- the western edge of the Santa Lucia Mountains. The current ing, the term Sur Series has remained in use. study encompasses a transect of these rocks from the Sur fault [7] The Salinian rocks are currently juxtaposed to the to the area (Figure 2). These deeper rocks have west and east against the Mesozoic accretion-related Fran- been referred to as the Coast Ridge section [Reich, 1937], ciscan assemblage and are thus tectonically ‘‘out of place’’ Coast Ridge Belt [Ross, 1976] and Salinian Western block (Figure 1). There is little argument that the Salinian com- [Ross, 1978]. Ross [1976] linked the Coast Ridge fault with posite terrane has traveled northward some 330 km during the Palo Colorado fault (Figure 2), thereby severing the the Late Cenozoic along the San Andreas fault system deeper ‘‘Salinian Western block’’ rocks from the Salinian (Figure 1; Powell [1993]), and petrologic, isotopic, struc- Central block. In other maps of the area the Coast Ridge fault tural and sedimentologic evidence have placed it near the has not been considered a through-going structure [Jennings southern end of the Sierra Nevada Batholith during the and Strand, 1959; Hall, 1991; L. I. Rosenberg, unpublished Cretaceous as well [Hill and Dibblee, 1953; Page, 1981; data, 2001]. In either case there is little reason to invoke large Dickinson, 1983; Ross, 1984; Silver and Mattinson, 1986; offsets between the deeper rocks exposed along the Coast Hall, 1991; Grove, 1993; Schott and Johnson, 1998]. This Ridge and those in the central part of the Santa Lucia Range. hypothesis has been questioned based on paleomagnetic Metamorphic grade increases gradually as the Coast Ridge evidence for right-lateral offset of Salinian sedimentary and Belt is approached from the east [Wiebe, 1970b; Compton, 12 - 4 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT

Table 1. U-Pb Analyses by ID-TIMSa

Age, Ma Number of Weight, U, Pb, 206Pb/238Pb 207Pb/235U 207Pb/206Pb Grains mg ppm ppm 206Pb/204Pb 208Pb/206Pb (Error) (Error) (Error) 206Pb/238Pb 207Pb/235U 207Pb/206Pb

4 86 65 1.2 1041 0.21 0.01706 (1.8) 0.14833 (2.5) 0.06305 (1.7) 109.1 140.4 710 4 72 43 0.7 471 0.15 0.01442 (3.4) 0.09689 (5.6) 0.04873 (4.2) 92.3 93.9 134.8 4 78 132 2.2 456 0.15 0.01467 (1.7) 0.09589 (5) 0.0474 (4.5) 93.9 93 69.6 4 65 111 2.1 701 0.16 0.01683 (1.5) 0.12321 (3.1) 0.0531 (2.6) 107.6 118 333.3 1 21 122 2 369 0.22 0.01445 (4.8) 0.09884 (7.6) 0.04961 (5.6) 92.5 95.7 177 1 26 112 2.1 275 0.17 0.01569 (3.5) 0.11168 (7) 0.05161 (5.9) 100.4 107.5 268.3 1 30 57 1 158 0.13 0.01431 (5.8) 0.09645 (12.4) 0.04889 (10.5) 91.6 93.5 142.4 1 18 91 1.5 209 0.14 0.01449 (6.4) 0.09111 (11.2) 0.0456 (8.9) 92.8 88.5 23.7

aAll grains analyzed were highly translucent euhedral crystals, 250 m in length. 206Pb/204Pb is the measured ratio, uncorrected for blank, spike, or fractionation. 208Pb/206Pb is corrected for blank, spike, and fractionation. Concentrations have an uncertainty of up to 25% due to the uncertainty of weight of grain. Constants used: l235 = 9.8485 1010; l238 = 1.55125 1010; 238U/235U = 137.88. All uncertainties are at the 95% confidence level. Isotope ratios are adjusted as follows: (1) mass-dependent corrections factors of 0.14 ± 0.06%/amu for Pb and 0.04 ± 0.04%/amu for UO2; (2) Pb ratios corrected for 0.005 ± 0.003 ng blank with 206Pb/204Pb = 18.6 ± 0.3, 207Pb/204Pb = 15.5 ± 0.3, and 208Pb/204Pb = 38.0 ± 0.8; (3) U has been adjusted for 0.0005 ± 0.0005 ng blank; (4) initial Pb from Stacey and Kramers [1975], with uncertainties of 1.0 for 206Pb/204Pb, 0.3 for 207Pb/204Pb, and 2.0 for 208Pb/204Pb.

1966b], and rocks found on both sides of the fault are similar that U, Th, and Pb isotopes are measured simultaneously. petrographically [Compton, 1966b]. For these reasons we All measurements are made in static mode, using Faraday consider the rocks exposed along the Coast Ridge a deeper detectors for 238U, 232Th, 208 –206Pb, and an ion-counting extension of the Salinian Central block rather than a separate channel for 204Pb. Ion yields are 1 mv per ppm. Each tectonic entity. In this view the basement rocks exposed near analysis consists of one 20-s integration on peaks with the Cone Peak and continuing east of the Coast Ridge represent a laser off (for backgrounds), 20 1-s integrations with the tilted exposure of a Cordilleran magmatic arc with exposure laser firing, and a 30 s delay to purge the previous sample depths ranging from 25 km in the west to some 10 km in the and prepare for the next analysis. The ablation pit is east. 20 microns in depth. [12] Common Pb correction is performed by using the 204 3. Analytical Techniques measured Pb and assuming an initial Pb composition from Stacey and Kramers [1975] (with uncertainties of 1.0 206 204 207 204 [9] Electron microprobe analysis on minerals were carried for Pb/ Pb and 0.3 for Pb/ Pb). Measurement of out at the University of Arizona using the Cameca SX50 204Pb is unaffected by the presence of 204Hg because back- microprobe equipped with 5 LiF, PET and TAP spectrom- grounds are measured on peaks (thereby subtracting any eters. Counting times for each element were 30 s at an background 204Hg and 204Pb), and because very little Hg is accelerating potential of 15 kV and a beam current of present in the argon gas. 206 10 nA. Measurements with oxide totals outside of the range [13] For each analysis, the errors in determining Pb/ 100 ± 1% were discarded with the exception of clinopyrox- 238U and 206Pb/204Pb result in a measurement error of ene in sample 730-4 and clinopyroxene and plagioclase in several percent (at 2s level) in the 206Pb/238U age (Table 4). 710–5 for which sample totals between 98.5% and 99% were The errors in measurement of 206Pb/207Pb are substantially also used. larger for younger grains due to low intensity of the 207Pb 207 235 206 207 [10] Three samples were processed for U-Pb analyses. signal. The Pb/ U and Pb/ Pb ages for younger Zircons from one sample were analyzed successfully by grains accordingly have large uncertainties (Table 4). conventional ID-TIMS using a VG-354 multicollector mass Interelement fractionation of Pb/U is generally <20%, spectrometer using techniques described by Gehrels [2000]. whereas isotopic fractionation of Pb is generally <5%. Run The results from ID-TIMS analyses are reported in Table 1 analyses of fragments of a large zircon crystal (generally and shown on a conventional concordia diagram (Figure 7). every fifth measurement) with known age of 564 ± 4 Ma ID-TIMS analyses of the other two samples were not (2s error) are used to correct for this fractionation. The possible due to the very low U concentration (generally uncertainty resulting from the calibration correction is <10 ppm U) in most grains and the presence of inherited generally 3% (2s) for both 207Pb/206Pb and 206Pb/238U components. Zircons from these samples were accordingly ages. also analyzed by laser ablation multicollector inductively [14] The ages interpreted from the ICPMS analyses are coupled plasma mass spectrometry (LA-MC-ICPMS). based on 206Pb/238U ratios because errors of the 207Pb/ 235 206 207 [11] The laser ICPMS analyses involve ablation of U and Pb/ Pb ratios are significantly greater. This zircon with a New Wave DUV193 Excimer laser (operat- is due primarily to the low intensity (commonly <1 mV) of ing at a wavelength of 193 nm) using a spot diameter of the 207Pb signal from these young, U-poor grains. A 25–35 microns. The ablated material is carried in argon comparison between ID-TIMS ages and laser ICPMS ages gas into the plasma source of a Micromass Isoprobe, is available from sample 630-5, which was analyzed by which is equipped with a flight tube of sufficient width both methods, and by a second zircon standard (00–87) KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 5 that was analyzed during the same session. The ID-TIMS 143Nd/144Nd = 0.001%. The isochron standard error and age for 630-5 is 93 ± 3 Ma, whereas the laser ICPMS mean the Mean Squared Weighted Deviation (MSWD) were age is 92.0 ± 4.1 Ma (Figures 7 and 8). For sample 00–87, calculated using Ludwig [2001]. the ID-TIMS age is 99.2 ± 1.0 Ma, and the ICPMS 206Pb/238U age is 99.9 ± 3.3 Ma. We accordingly conclude that our reported 206Pb/238U ages are reliable indicators of 4. Geology of the Cone Peak Transect crystallization age. [18] While the Sur Series rocks exposed in the study area [15] Whole-rock sample 710-5 was broken down by have not previously been mapped in detail, they have been hammer and crushed in a jaw crusher to about 1/3 of its considered to be predominantly metasedimentary based on average grain size. The sample was then homogenized and their banded appearance, the presence of marble and quartz- split, roughly two thirds for mineral separation, and a third ite layers, and their proximity to the less metamorphosed for whole rock analysis. Both splits were initially washed rocks of the Salinian Central block. The rocks are predom- in deionized water and then acid leached for 25–30 min in inantly in the amphibolite facies, with typical assemblages warm 1 N distilled HCl while in an ultrasonic bath. The comprising plagioclase + hornblende + biotite ± quartz (in whole-rock sample was then ground to a fine powder order of decreasing abundance). Granulite facies rocks and using a ceramic Al2O3 shatter box prior to dissolution. The veins are also present, with typical assemblages comprising split for mineral analyses remained relatively coarse plagioclase + clinopyroxene ± biotite ± garnet ± quartz ± throughout the separation procedure, although for some orthopyroxene. Small mappable gabbro and diorite intru- samples rich in composite grains, a few additional grinding sions were also found and comprise a significant portion of steps were necessary. Separation of minerals was accom- the study area. The igneous rocks typically share the above plished by a Frantz magnetic separator and handpicking in mineral assemblages with the gneisses and are distinguished alcohol under a binocular microscope. Two garnet frac- based on remnant igneous textures and the near absence of tions were used, one corresponding optically to the slightly marble and quartzite within the intrusions. Fine grained more orange colored mineral rims (gar-1), and the other mafic rocks or ‘‘amphibolites’’ are also common in the one representing a mixture of the red and orange colors of gneiss and igneous rocks. We noted no differences between the Cone Peak . The samples were then ultrasoni- the fine-grained mafic rocks within the metamorphic frame- cally cleaned, rinsed multiple times with ultrapure water work and those within the igneous rocks. The metamorphic and dried in methanol. 147 150 framework, igneous rocks, and fine-grained mafic rocks are [16] The samples were spiked with mixed Sm- Nd discussed separately below. A detailed geologic map show- tracers described by Ducea et al. [2003c]. Dissolution of the ing the study area and locations of the various rock types is spiked samples for isotopic analyses was performed in shown in Figure 3. screw cap Teflon beakers using HF-HNO3 (on hot plates) and HF-HClO4 mixtures (in open beakers at room temper- ature). A few garnet separates were subjected to up to 4.1. Sur Series 5 dissolution steps before becoming residue free. The [19] Metamorphic framework rocks in the study area vary samples were taken in 1 N HCl and any undissolved residue greatly in composition and appearance. At outcrop scales, was attacked using the HF-HClO4 mixture digestion tech- interlayered felsic gneisses, amphibolites, marbles and nique. Separation of the bulk of the REE was achieved via quartzites give the framework an overall banded appearance HCl elution in cation columns. Separation of Sm and Nd (Figure 4). measurements are variable (Figure 3b), 1 was carried out using a LNSpec resin following the showing significant scatter about an average WNW strike procedures in Ducea et al. [2003c]. and 30 NE dip. Small outcrops and hand samples are often [17] Mass spectrometric analyses were carried out on massive or show only a weak foliation due to low mica two VG Sector multicollector instruments (VG54 and content, and mica-rich layers are negligible in the section. VG354) fitted with adjustable 1011 Faraday collectors Veins are found in nearly all outcrops, and occasionally are and Daly photomultipliers [Patchett and Ruiz, 1987]. concentrated enough to lend outcrops a migmatitic appear- Concentrations of Sm and Nd were determined by isotope ance. Felsic gneisses are the most common framework rocks dilution. An off-line manipulation program was used for and are composed primarily of plagioclase ± quartz ± biotite isotope dilution calculations. Typical runs consisted of 100 ± alkali feldspar ± garnet ± clinopyroxene ± hornblende ± isotopic ratios. The mean results of five analyses of the orthopyroxene. Granoblastic fabrics are occasionally found, standard nSmb performed during the course of this study but high-temperature fabrics are more often overprinted by are: 148Sm/147Sm = 0.74880 ± 21, and 148Sm/152Sm = shear bands or weakly mylonitic fabrics. Quartz commonly 0.42110 ± 6. Fifteen measurements of the LaJolla Nd shows undulatory extinction and significant recrystalliza- standard were performed during the course of this study, tion. The felsic gneisses have an average tonalitic compo- yielding the following isotopic ratios: 142Nd/144Nd = sition but are split between two populations, one with 1.14184 ± 2, 143Nd/144Nd = 0.511853 ± 10, 145Nd/144Nd = an average diorite or quartz diorite composition, and the 0.348390 ± 20, and 150Nd/144Nd = 0.23638 ± 2. The Nd other with a tonalitic composition containing typically isotopic ratios were normalized to 146Nd/144Nd = 0.7219. 50% quartz. The only unambiguous sedimentary units in The estimated analytical ±2s uncertainties for samples the section are quartzites and marbles. Pure quartzites analyzed in this study are: 147Sm/144Nd = 0.4%, and (>90% quartz) are quite rare in the gneiss, and are found 12 - 6 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 7

Figure 3. (continued) only as cm-scale to dm-scale bodies. Pure quartzites were dikes, veins, and outcrop-scale to small map-scale bodies found only in the northern portion of the section. Less pure in the gneiss. The contacts of the larger igneous bodies are quartzites, occasionally graphite bearing, appear at meter generally gradational and concordant with folitation. scale here and there throughout the section but were not [22] Two concordant 100- to 200-m-thick gabbro sills mapped due to their scarcity and small size. Some of the found in the southern part of the mapped area comprise impure ‘‘quartzites’’ in the section may be igneous or the most mafic igneous bodies found. The sills are quite hydrothermal in origin, as they are occasionally seen in heterogeneous, but generally comprise a mixture of two crosscutting relations with other rocks. textural end members: a medium to coarse-grained variety [20] Marbles are widely scattered throughout the gneiss at with typically elongate subhedral hornblende crystals, and a variety of scales, from layers a few cm in thickness to two a fine-grained typically granoblastic variety. In places the map-scale layers (Figure 3). The marbles are generally fine-grained material appears as enclaves in the coarser- concordant with foliation, although in one locality a grained material, in other cases the coarser-grained mate- meter-wide marble body is seen to cut foliation. This rial exists as patches and veins in the fine-grained material one marble probably formed during metamorphism from (Figure 6). Both varieties are also seen in cross cutting CO2-rich fluids. The marble is generally a dirty gray color, relationships with the other. While the fine-grained rocks fine grained and porphyroblastic. Occasionally it is found maintain rather even ratios of felsic to mafic minerals, the recrystallized to a pure, white, coarse-grained dolomite or coarser-grained rocks range from nearly 100% hornblende marble where it is typically marked by the presence of to a near total absence of hornblende in some veins. In olivine. Gneissic rocks and fine-grained mafic rocks are thin section, coarse-grained samples generally contain commonly associated with the marble as tubes, blobs, and subhedral or granoblastic hornblende crystals with feld- isolated tight folds (e.g., Figure 5). spars showing granoblastic textures. Contacts between the gabbro and country rock were not observed due to thick 4.2. Plutonic Rocks cover, but mapping leaves generally less than 50 m for a [21] Earlier mapping at larger scales [Reich, 1937; diffuse transition zone (Figure 3). Compton, 1966b; Seiders et al., 1983] did not distinguish [23] A roughly 500-m-thick diorite in the central part of four mafic to intermediate intrusions that occupy in cross- the area comprises the largest igneous body mapped. The sectional thickness about a third of the study area. The diorite is very heterogeneous, containing a multitude of intrusions comprise two hornblende gabbro sills in the fine-grained mafic enclaves and country rock inclusions. It southern portion of the mapped area, a diorite in is generally medium-grained with a granoblastic fabric, and the central portion, and a quartz diorite in the northern typically contains 65–80% plagioclase + hornblende ± portion (Figure 3). Other mafic igneous rocks similar in quartz. The intrusion thins significantly and is more felsic texture and appearance to the larger bodies are found as in the western portion of the mapped area. It is unclear

Figure 3. (opposite) (a) Geologic map of Hare Canyon and Cone Peak area showing location of units and samples discussed in the text. See Figure 2 for location. The gneiss unit is strewn with minor marble and fine-grained mafic layers, lenses, and blobs. The size of the symbols is intended as a rough indication of the size of the bodies; the larger symbols are shown to scale, however, smaller symbols represent bodies that may be as small as a few square m. The geometry of the bodies is commonly obscured by thick cover; thus the true shapes of the minor mafic and marble bodies may differ significantly from the shape of map symbols. Contacts separating the gneiss units from the quartz diorite and diorite bodies are often diffuse over as much as 300 m. (b) Stereoplot showing poles to all measured foliations. (c) Stereoplot showing mineral lineation directions. (d) Cross section from A to A0 in Figure 3a. Units are as shown in Figure 3a. There is no vertical exaggeration. Elevation is shown in m. 12 - 8 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT

Figure 4. Coast Ridge Belt gneiss near Cone Peak. The upper part of the photograph shows the typical well-defined layering of the gneiss, cut by a rare mylonite comprising the lower third of the photograph. A thick quartzofeldspathic Figure 5. Porphyroblastic marble showing z-shaped vein lies between the two foliations in the left-hand side of tightly folded quartzofeldspathic inclusion. The field of the photo. Asymmetric pressure shadows in the mylonite view is 20 cm. The black material is lichen. [Passchier and Trouw, 1998] and thinning in the gneiss suggest top to the right (east) offset. The thick, darker layer content varies in both types between 0 and 20%. Textures intersecting the top of the photograph is a typical small are generally granoblastic or partially granoblastic, with mafic body in the gneiss. The field of view is 3.5 m. some grains showing serrated edges or other evidence of ductile deformation. Within the deformed framework these rocks most commonly comprise concordant layers or whether the thinning is a primary feature of the intrusive body or whether it is offset by a fault along the southern boundary of the intrusion. [24] The unique, light-colored, generally banded to grossly isotropic biotite hornblende gneiss exposed at Cone Peak has been described in detail by Compton [1960] and by Hansen and Stuk [1993]. The body is interlayered at its base with quartzites and marbles, and neither Compton [1960] nor our own work revealed any cross cutting relationships involving the gneiss and country rocks. These and other observations led Compton [1960] to propose a sedimentary origin for the body. Zircons in the rock however are 81 Ma (see below), signifying an igneous origin. This is supported by the quartz diorite composition of the Cone Peak gneiss, based on mineral modes and compositions estimated by us and Hansen and Stuk [1993].

4.3. Fine-Grained Mafic Rocks

[25] Fine-grained mafic rocks ranging in size from a few tens of cm to outcrop scale are widespread throughout all of the above igneous and metamorphic units. We found no systematic differences between samples derived from Figure 6. Photograph showing foliated quartzo-felds- within the different rock types and thus we describe them pathic layers (lower left and middle right side of photo) here together. Most are amphibolites, with a groundmass offset by fine-grained amphibolite material (darker material containing the assemblage plagioclase + hornblende ± in areas between upper left and lower right corners of the biotite. Veins in the amphibolite are widespread and often photo). Such cross cutting relationships are quite unusual in contain clinopyroxene in place of hornblende. Clinopy- the Coast Ridge Belt. The foliated amphibolite is over- roxene replaces hornblende as the dominant mafic mineral printed by a more felsic, coarse-grained vein also running in roughly a quarter of the fine-grained mafic rocks, thus from upper left to lower right across the photograph. The many of them would be better termed pyroxenites. Biotite field of view is about 1 m. KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 9

Table 2. Mineral Compositions for Geothermometry and Geobarometry

Sample 710-5 710-5 730-4 730-4 730-4 730-4 804-2 804-2 804-2 804-2 Location Core Rim Core Rim Included Core Included Rim Core Rim Included Core Included Rim

Garnets SiO2 37.44 37.36 37.98 37.83 37.87 38.34 37.33 37.38 37.33 37.46 TiO2 0.06 0.06 0.1 0.07 0.09 0.07 0.08 0.05 0.1 0.07 Al2O3 21.25 21.11 21.44 21.25 21.38 21.39 21.36 21.34 21.4 21.41 FeO 28.91 29.45 27.56 28.21 27.56 27.69 28.88 29.02 28.47 28.6 MnO 1.37 1.45 0.7 0.77 0.69 0.73 1.49 1.58 1.38 1.33 MgO 3.91 3.31 5.05 4.56 4.99 4.7 4.01 3.51 4.24 4.28 CaO 6.62 6.75 6.72 6.56 6.67 6.72 6.81 6.86 6.89 6.86 Na2O 0 0.01 0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.02 K2O 0 0 0.01 0.01 0.01 0 0 0 0.01 0.01 Total 99.6 99.5 99.6 99.3 99.3 99.7 100 99.8 99.8 100 na 34103 5 8 13161210

Clinopyroxenes SiO2 50.13 50.98 51.94 51.79 52.48 51.45 51.77 51.78 51.01 51.3 TiO2 0.23 0.2 0.23 0.2 0.13 0.03 0.08 0.13 0.24 0.19 Al2O3 1.73 1.43 2.18 1.74 1.64 1.51 0.87 1.04 1.71 1.48 FeO 15.46 15.11 13.03 12.26 12.39 12.54 12.76 13.29 13.47 12.96 MnO 0.33 0.31 0.14 0.14 0.13 0.12 0.23 0.28 0.27 0.25 MgO 9.51 10.23 12.66 11.73 12.14 11.94 11.57 11.56 11.15 11.58 CaO 20.86 20.92 18.18 20.72 20.78 20.72 22.02 21.3 21.1 21.42 Na2O 0.35 0.33 0.3 0.32 0.4 0.37 0.24 0.27 0.37 0.34 K2O 0 0.01 0.02 0.02 0 0.01 0.01 0.01 0 0.01 Total 98.6 99.5 98.7 98.9 100.1 98.7 99.6 99.7 99.3 99.5 na 2338 2 2 78 148

Plagioclasesb SiO2 2.62 2.61 2.59 2.56 2.6 2.59 2.61 2.59 2.61 2.59 Al2O3 1.38 1.38 1.4 1.43 1.4 1.41 1.39 1.41 1.38 1.4 CaO 0.38 0.38 0.41 0.43 0.4 0.41 0.39 0.4 0.38 0.4 Na2O 0.63 0.6 0.58 0.56 0.57 0.57 0.62 0.6 0.62 0.6 K2O 0.01 0.02 0.03 0.02 0.03 0.02 0.01 0.01 0.02 0.03 Total 99 99.2 99.6 99.7 99.5 99.3 99.8 99.8 99.9 99.6 na 236114 2 105 8 5

aNumber of analysis spots. bConcentration of Ti, Fe, Mn, and Mg are <0.01. lenses in the gneiss (e.g., Figure 4), although in places Cone Peak (710-5). Temperature and pressure estimates amphibolite dikes are seen to cut foliation (e.g., Figure 6). were calculated using the rim and core compositions of touching garnet, clinopyroxene and plagioclase grains. 4.4. Garnet Growth Mineral compositions used are given in Table 2. Most of the garnets contain inclusions, and ‘‘core’’ garnet compo- [26] All of the Coast Ridge Belt rocks, with the exception sitions were taken as far from inclusions as possible rather of the carbonates, are commonly garnetiferous. The garnets than strictly in the center of grains. Rim measurements are almandine rich, with XAl ranging from 0.57 to 0.64 in were taken between 10 and 40 microns from interfaces rock types of a wide variety of compositions. They are with corresponding minerals. Temperatures were estimated commonly as large as 15 mm in diameter, although some using the Ellis and Green [1979] and Ganguly et al. [1996] exceptional specimens reach diameters as large as 10 cm. calibrations of the garnet-clinopyroxene thermometer. The garnets are typically poikiloblastic and overprint host Pressures were estimated using the Eckert et al. [1991] rock fabrics, although in thin section foliation is often bent correction to Newton and Perkins [1982] garnet-clinopyr- around garnets. Inclusions of feldspar, quartz, hornblende oxene-plagioclase-quartz barometer. Core compositions and pyroxene are common. Garnet is commonly associated yielded temperatures, with one exception, between 750 in thin section and outcrop with aureoles or small, recrystal- and 870C at pressures of 0.75 GPa (see Table 3). Temper- lized veins in which clinopyroxene and quartz are common. atures using the Ganguly et al. [1996] thermometer for each site are consistently 50–70C higher than those calculated 5. Thermobarometry using the Ellis and Green [1979] thermometer. Calculated pressures on mineral cores range from 0.66 GPa to [27] Geothermometric and geobarometric estimates were 0.79 GPa at 750 or from 0.69 to 0.82 GPa at 800C. made in two samples located near the southern portion of Analyzed garnet and clinopyroxene crystals show a slight the area (730-4, 804-2) and in one sample located near zoning in the outer 50–100 microns, and conditions based 12 - 10 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT

Table 3. Results of Geothermometry and Geobarometrya range from 110 to 283 Ma, presumably due to the

750 800 presence of inherited components. Sample TE TG PE PE 6.1.2. Sample 701-4, Diorite 710-5 core 863 806 0.73 0.76 [30] Thirty grains were analyzed from this sample, with 710-5 rim 790 728 0.65 0.67 23 grains that do not show sign of inheritance (Table 4). The 730-4 core 822 761 0.79 0.82 weighted mean age is 86.4 ± 4.7 Ma (2s error), with a 730-4 rim 793 727 0.74 0.77 standard error of the mean of 3.3 Ma (Figure 9). The 730-4 inclusion core 817 755 0.75 0.78 730-4 inclusion rim 808 747 0.73 0.76 additional seven grains are slightly older, presumably due 804-2 core 771 705 0.66 0.69 to the presence of inherited components. 804-2 rim 749 684 0.62 0.65 6.1.3. Sample 706-4, Quartz Diorite Gneiss 804-2 inclusion core 811 750 0.73 0.76 [31] Fifty-one grains were analyzed from this sample. 804-2 inclusion rim 793 730 0.69 0.72 Twenty-two grains yield a cluster of <90 Ma ages, with a aTemperatures are measured in C; pressures are measured in GPa. weighted mean age of 81.3 ± 3.4 Ma (2s error) and a Superscript indicates temperatures (750C or 800C) at which pressures standard error of the mean of 1.0 Ma (Table 4 and were calculated. Temperatures were calculated at 0.75 GPa. TG, Ganguly et Figure 10). Twenty-nine grains yield mainly Paleozoic and al. [1996]; TE, Ellis and Green [1979]; PE = Eckert et al. [1991]. Proterozoic ages due to inheritance of older components (Figure 11). As shown in Figure 11, the main 207Pb/206Pb on rim compositions are on average 30–40Cand age clusters of these inherited components are 120 Ma, 0.05 GPa below peak conditions. The rim and core results 360 Ma, 1.0–1.25 Ga, 1.4 Ga, 1.6 Ga. The sample was are indistinguishable given the uncertainties of the calibra- taken from an area below the main body of the gneiss where tions. We take ranges of 750–800C and 0.7–0.8 GPa as the quartz diorite is intermingled with the Sur Series. the best estimates of peak metamorphic conditions recorded in the study area. These results are similar to 6.2. Sm-Nd Garnet results reported by Hansen and Stuk [1993] from Cone [32] A six point Sm-Nd mineral and whole-rock isochron Peak as well as to conditions obtained on rocks from the (Figure 12) obtained on a typical sample of the Cone two other California deeper arc exposures, the Tehachapi Peak quartz diorite gneiss (sample 710-5) yielded an age of section [Pickett and Saleeby, 1993] and the Cucamonga 76.5 ± 1.5 Ma, within the errors of the igneous crystallization section [Barth and May, 1992]. Undoubtedly, the intrusion age. Minerals and whole-rock Sm and Nd isotope data are temperatures were higher than these measured values, shown in Table 5. Two garnet fractions (rims ‘‘gar-1’’ and given the intermediate and/or mafic composition of the whole garnets) were analyzed; and both lie on the same investigated rocks. The results correspond to an average isochron, suggesting a fast growth rate (>4 mm/My). The thermal gradient of 30C/km which is typical for active closure temperature for the Sm-Nd system in garnet from this continental arcs [Barton, 1990]. rock is about 760C using diffusion data from Ganguly and Tirone [1999], similar but slightly less than the temperature 6. Geochronology recorded in the same sample by major cation exchange (800–860C). There are two possible explanations for the 6.1. U///Pb Zircon Results relationships between temperatures and the age recorded in this rock: (1) the age records garnet growth, in which case the [28] Zircons from the Coast Ridge Belt samples were analyzed optically under a petrographic microscope and an Cone Peak rocks are shown to have been at depths of 25 km SEM in BSE mode. Zircons were grouped according to the at 76.5 Ma, or (2) the age records cooling through the morphology classification of Pupin [1983]. Almost all 760C isotherm, in which case the age is a minimum for zircons that yielded Cretaceous ages display igneous mor- garnet growth. Given the fast cooling of this rock evidenced phologies with no detectable optical zoning. The older by the lack of significant zoning in garnets, and the over- zircons we found particularly in sample 706-4, are smaller, lapping error bars of the two temperature estimates, we favor rounded grains that indicate a detrital origin and/or corro- the first interpretation. Although foliation in thin section can sion in a magma. For these zircons, we report the core ages be seen to bend slightly around garnets in this rock, the that are always older than rim ages. Rim ages are typically garnets overprint the gross foliation of the sample, suggesting poorly constrained (probably because the new rim growths that garnet growth was late kinematic to postkinematic. This are very narrow), and are not reported in Table 4, but the date therefore brackets in either of the above cases the timing ages are late Cretaceous. of foliation-forming deformation of the Cone Peak quartz 6.1.1. Sample 630-5, Felsic Framework diorite to between crystallization at 81.3 ± 3.4 Ma and 76.5 ± 1.5 Ma. These dates also bracket peak metamorphic [29] Eight single zircon grains were analyzed by ID-TIMS, three of which are discordant and five of which conditions measured on the Cone Peak rocks [Hansen and are apparently concordant (Figure 7). The interpreted age Stuk, 1993] (this study). derived from the concordant grains is 93 ± 3 Ma (2s level). Thirty-nine grains were analyzed by ICPMS, with 6.3. Other Age Constraints 33 analyses that yield a weighted mean age of 92.0 ± 4.1 Ma [33] The Cone Peak rocks were exposed at the surface by (2s error) and a standard error of the mean of 1.7 Ma mid-Maastrichtian (68–69 Ma), as indicated by the age of (Figure 8). The additional grains yield 206Pb/238U ages that sedimentary rocks unconformably overlying the Coast Ridge KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 11

Table 4. U-Pb Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass Spectrometerya

Isotopic Ratios Apparent Ages, Ma

207Pb/235U 206Pb/238U Error 206Pb/238U 207Pb/235U 206Pb/207Pb Sample U, ppm 206Pb/204Pb U/Th (±, %) (±, %) Correlation (±, Ma) (±, Ma) (±, Ma)

Sample 630-5 1 4 325 0.4 0.10044 (272.6) 0.01360 (9.2) 0.03 87.1 (8.1) 97 (246) 353 (3078) 2 4 334 2 0.29122 (40.4) 0.01455 (4.3) 0.11 93.1 (4.0) 260 (113) 2290 (346) 3 6 243 5 0.03161 (70.8) 0.01396 (11.3) 0.16 89.3 (10.1) 32 (23) 4 5 112 11 0.04878 (131.4) 0.01103 (21.1) 0.16 70.7 (15.0) 48 (63) 5 5 229 11 0.02313 (97.7) 0.01229 (22.2) 0.23 78.7 (17.6) 23 (23) 6 5 210 2 0.28004 (110.1) 0.01428 (3.5) 0.03 91.4 (3.2) 251 (273) 2254 (950) 7 12 1120 8 0.36837 (13.0) 0.04076 (3.7) 0.29 257.5 (9.8) 318 (47) 792 (130) 8 4 426 8 0.10031 (72.2) 0.01432 (7.8) 0.11 91.7 (7.2) 97 (71) 232 (828) 9 5 300 2 0.04636 (100.4) 0.01431 (10.9) 0.11 91.6 (10.0) 46 (46) 10 63 6572 99 0.62605 (2.8) 0.04493 (1.2) 0.44 283.3 (3.6) 494 (18) 1644 (23) 11 7 499 2 1.24496 (0.0) 0.01766 (5.4) 0.02 112.9 (6.1) 821 (1359) 4274 (1662) 12 15 1014 7 0.05806 (45.1) 0.01954 (3.4) 0.08 124.7 (4.3) 57 (26) 13 6 378 4 0.02821 (92.9) 0.01530 (8.1) 0.09 97.9 (8.0) 28 (26) 14 4 275 5 0.05365 (149.8) 0.01331 (14.4) 0.10 85.2 (12.3) 53 (79) 15 4 222 5 0.28819 (85.2) 0.01346 (10.2) 0.12 86.2 (8.8) 257 (223) 2405 (719) 16 3 510 12 0.04599 (339.6) 0.01543 (12.9) 0.04 98.7 (12.8) 46 (147) 17 4 580 27 0.06282 (121.5) 0.01475 (2.5) 0.02 94.4 (2.3) 62 (75) 18 18 693 8 0.34999 (116.4) 0.01404 (4.5) 0.04 89.9 (4.1) 305 (347) 2660 (964) 19 17 213 37 0.05883 (24.9) 0.01363 (3.1) 0.13 87.2 (2.8) 58 (15) 20 10 263 9 0.22115 (103.3) 0.01452 (5.0) 0.05 92.9 (4.7) 203 (209) 1807 (938) 21 25 635 9 0.14426 (35.7) 0.01504 (2.0) 0.06 96.2 (2.0) 137 (51) 916 (366) 22 9 265 8 0.02096 (266.6) 0.01364 (7.1) 0.03 87.3 (6.3) 22 (58) 23 17 810 11 0.08917 (21.8) 0.01754 (3.1) 0.14 112.1 (3.5) 87 (20) 24 14 574 5 0.44737 (189.6) 0.01445 (2.5) 0.01 92.5 (2.3) 375 (624) 3014 (1522) 25 18 506 22 0.13621 (50.2) 0.01546 (4.6) 0.09 98.9 (4.6) 130 (67) 738 (529) 26 7 153 4 0.95721 (433.4) 0.01556 (7.7) 0.02 99.5 (7.8) 682 (1664) 4073 (3225) 27 13 1221 7 0.09090 (51.9) 0.01883 (3.0) 0.06 120.3 (3.6) 88 (47) 28 16 547 7 0.09058 (97.3) 0.01393 (3.7) 0.04 89.2 (3.3) 88 (86) 58 (1160) 29 8 173 24 0.02126 (95.5) 0.01266 (6.7) 0.07 81.1 (5.5) 21 (20) 30 10 841 7 0.06497 (32.6) 0.01627 (5.5) 0.17 104.0 (5.8) 64 (21) 31 12 131 13 0.08849 (141.3) 0.01271 (4.1) 0.03 81.4 (3.4) 86 (120) 217 (1635) 32 7 219 6 0.00611 (116.1) 0.01372 (7.2) 0.06 87.9 (6.3) 6(7) 33 14 592 9 0.09075 (10.0) 0.01406 (4.5) 0.45 90.0 (4.1) 88 (9) 40 (107) 34 10 431 6 0.23671 (82.9) 0.01346 (5.9) 0.07 86.2 (5.1) 216 (182) 2064 (729) 35 16 524 8 0.17552 (82.1) 0.01480 (2.3) 0.03 94.7 (2.2) 164 (137) 1339 (793) 36 9 414 16 0.05388 (76.4) 0.01478 (5.4) 0.07 94.6 (5.2) 53 (41) 37 7 155 2 0.13514 (44.7) 0.01335 (8.4) 0.19 85.5 (7.2) 129 (60) 1025 (444) 38 10 329 4 0.31884 (88.6) 0.01507 (6.1) 0.07 96.4 (5.9) 281 (253) 2385 (752) 39 8 642 5 0.37378 (109.9) 0.01221 (27.0) 0.25 78.2 (21.2) 323 (350) 2995 (857)

Sample 701-4 1 3 235 15 0.05656 (102.0) 0.01347 (10.2) 0.10 86.3 (8.8) 56 (57) 2 4 452 2 0.15748 (207.5) 0.01915 (19.3) 0.09 122.3 (23.7) 149 (287) 591 (2240) 3 5 496 1 0.03840 (50.3) 0.01385 (9.8) 0.20 88.7 (8.8) 38 (19) 4 4 522 3 0.20372 (34.9) 0.01338 (8.4) 0.24 85.7 (7.2) 188 (70) 1806 (308) 5 1 553 68 0.21125 (169.8) 0.01845 (27.7) 0.16 117.9 (32.8) 195 (311) 1270 (1635) 6 5 240 52 0.10077 (53.3) 0.01310 (7.8) 0.15 83.9 (6.6) 98 (53) 444 (586) 7 6 344 4 0.06718 (190.4) 0.01298 (9.9) 0.05 83.2 (8.3) 66 (122) 519 (2541) 8 5 251 7 0.14558 (55.2) 0.01693 (11.2) 0.20 108.2 (12.2) 138 (78) 687 (576) 9 7 289 10 0.04085 (50.2) 0.01427 (8.5) 0.17 91.4 (7.8) 41 (21) 10 5 240 5 0.22678 (105.8) 0.01935 (7.5) 0.07 123.6 (9.3) 208 (218) 1315 (1024) 11 4 321 2 0.20869 (47.3) 0.01403 (7.7) 0.16 89.8 (6.9) 193 (96) 1764 (426) 12 8 322 19 0.19355 (60.6) 0.01338 (6.1) 0.10 85.7 (5.2) 180 (113) 1713 (555) 13 4 396 6 0.06872 (111.3) 0.01353 (12.9) 0.12 86.7 (11.3) 68 (75) 14 4 130 7 0.06841 (98.8) 0.01153 (22.0) 0.22 73.9 (16.4) 67 (66) 165 (1199) 15 6 270 3 0.29754 (73.3) 0.01374 (8.5) 0.12 87.9 (7.5) 265 (200) 2425 (617) 16 5 299 11 0.14960 (30.4) 0.03108 (9.0) 0.30 197.3 (18.0) 142 (45) 17 2 702 26 0.07860 (42.4) 0.02280 (11.3) 0.27 145.3 (16.6) 77 (33) 18 5 356 1 0.16716 (59.3) 0.01342 (8.8) 0.15 86.0 (7.6) 157 (96) 1432 (559) 19 5 196 9 0.30266 (70.3) 0.01460 (7.3) 0.10 93.4 (6.9) 269 (196) 2350 (598) 20 4 150 17 0.00005 (41.7) 0.01223 (9.7) 0.23 78.4 (7.6) 21 2 195 22 0.04127 (55.3) 0.01396 (8.3) 0.15 89.4 (7.5) 41 (23) 22 5 185 5 0.33444 (87.6) 0.01775 (6.3) 0.07 113.4 (7.2) 293 (261) 2186 (760) 23 7 306 7 0.11393 (42.4) 0.01286 (11.0) 0.26 82.4 (9.1) 110 (48) 750 (433) 24 8 325 14 0.08520 (97.9) 0.01380 (7.3) 0.07 88.4 (6.5) 83 (81) 12 - 12 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT

Table 4. (continued) Isotopic Ratios Apparent Ages, Ma

207Pb/235U 206Pb/238U Error 206Pb/238U 207Pb/235U 206Pb/207Pb Sample U, ppm 206Pb/204Pb U/Th (±, %) (±, %) Correlation (±, Ma) (±, Ma) (±, Ma)

25 8 307 8 0.56804 (120.5) 0.01299 (10.6) 0.09 83.2 (8.9) 457 (529) 3556 (924) 26 8 222 18 0.15959 (77.8) 0.01335 (7.3) 0.09 85.5 (6.3) 150 (119) 1354 (747) 27 3 201 18 0.07505 (98.5) 0.01281 (16.1) 0.16 82.1 (13.3) 74 (72) 28 2 385 20 0.06942 (104.4) 0.01136 (53.1) 0.51 72.8 (38.8) 68 (71) 29 4 702 88 0.13393 (45.2) 0.01396 (9.0) 0.20 89.4 (8.1) 128 (60) 916 (456) 30 2 268 22 0.06219 (80.3) 0.01248 (30.5) 0.38 79.9 (24.5) 61 (50)

Sample 706-4 1 35 5514 18 1.37402 (3.0) 0.12674 (1.0) 0.34 769.2 (8.3) 878 (41) 1163 (28) 2 7 2085 5 1.53218 (7.9) 0.15177 (1.3) 0.16 910.9 (12.2) 943 (116) 1020 (79) 3 75 1909 30 0.07635 (10.4) 0.01242 (1.0) 0.10 79.6 (0.8) 75 (8) 4 20 6437 8 1.13721 (2.7) 0.11220 (1.2) 0.45 685.5 (8.8) 771 (31) 1028 (24) 5 23 16757 8 3.25940 (1.5) 0.23337 (0.8) 0.55 1352.1 (12.1) 1471 (48) 1648 (11) 6 124 4817 842 0.46022 (8.5) 0.05676 (2.4) 0.28 355.9 (8.8) 384 (39) 560 (89) 7 22 6612 9 2.11498 (3.6) 0.16850 (1.7) 0.47 1003.8 (18.0) 1154 (74) 1447 (30) 8 22 4173 21 1.34860 (3.4) 0.10493 (2.8) 0.83 643.2 (18.9) 867 (45) 1492 (18) 9 67 1809 934 0.14407 (12.9) 0.01872 (1.6) 0.12 119.6 (1.9) 137 (19) 445 (142) 10 25 6549 9 2.00399 (1.9) 0.18427 (1.2) 0.64 1090.3 (14.0) 1117 (37) 1169 (14) 11 8 3180 9 2.14251 (7.6) 0.16566 (1.0) 0.13 988.1 (10.6) 1163 (154) 1504 (71) 12 5 1799 13 1.08937 (18.6) 0.10388 (4.2) 0.23 637.1 (28.3) 748 (188) 1097 (181) 13 90 3360 1961 0.07782 (14.8) 0.01252 (0.8) 0.06 80.2 (0.7) 76 (12) 14 16 615 13 0.09104 (47.7) 0.01203 (3.6) 0.08 77.1 (2.8) 89 (43) 408 (531) 15 111 13696 166 0.08977 (10.0) 0.01297 (0.9) 0.09 83.0 (0.8) 87 (9) 205 (115) 16 30 1280 9 0.08007 (27.5) 0.01220 (2.7) 0.10 78.2 (2.1) 78 (22) 79 (325) 17 29 6249 21 0.13313 (65.5) 0.01803 (1.8) 0.03 115.2 (2.1) 127 (85) 352 (740) 18 31 1797 10 0.10388 (71.9) 0.01360 (1.6) 0.02 87.0 (1.4) 100 (73) 429 (801) 19 32 1066 12 0.08349 (49.5) 0.01288 (2.2) 0.04 82.5 (1.8) 81 (41) 50 (590) 20 9 5602 5 1.89499 (3.7) 0.16902 (3.4) 0.91 1006.7 (36.5) 1079 (69) 1229 (15) 21 99 3415 654 0.16051 (6.2) 0.01908 (1.1) 0.17 121.8 (1.3) 151 (10) 640 (66) 22 7 37107 4 1.79297 (8.3) 0.14673 (1.6) 0.19 882.6 (14.6) 1043 (141) 1396 (78) 23 20 6043 12 1.41734 (3.3) 0.12759 (1.1) 0.35 774.1 (9.2) 896 (46) 1211 (30) 24 26 9068 13 1.49968 (2.7) 0.13538 (1.6) 0.59 818.5 (13.9) 930 (41) 1205 (22) 25 54 5514 192 0.47149 (8.2) 0.05999 (0.7) 0.08 375.6 (2.6) 392 (39) 492 (90) 26 44 16285 13 1.14974 (2.8) 0.09500 (1.2) 0.42 585.0 (7.1) 777 (32) 1378 (24) 27 24 2690 17 0.22569 (111.1) 0.01279 (2.9) 0.03 81.9 (2.4) 207 (227) 2071 (979) 28 25 1573 15 0.11656 (45.9) 0.01329 (3.6) 0.08 85.1 (3.0) 112 (53) 729 (485) 29 26 4300 16 0.08734 (31.4) 0.01265 (2.4) 0.08 81.1 (1.9) 85 (28) 198 (364) 30 19 2872 14 0.18753 (87.0) 0.01375 (4.7) 0.05 88.0 (4.1) 175 (153) 1604 (810) 31 16 1099 16 0.10290 (82.6) 0.01269 (4.5) 0.05 81.3 (3.7) 99 (83) 559 (899) 32 16 753 3 0.07485 (165.2) 0.01271 (4.2) 0.03 81.4 (3.4) 73 (118) 33 52 13620 22 0.14732 (27.3) 0.01507 (1.5) 0.06 96.4 (1.5) 140 (40) 955 (279) 34 18 1585 0 0.09171 (46.1) 0.01284 (3.6) 0.08 82.3 (3.0) 89 (42) 276 (526) 35 39 2379 8 0.12041 (27.7) 0.01497 (1.5) 0.05 95.8 (1.4) 115 (33) 542 (302) 36 14 4986 13 0.18489 (80.3) 0.01909 (2.8) 0.03 121.9 (3.4) 172 (141) 935 (823) 37 54 7565 298 0.16028 (13.4) 0.01566 (1.3) 0.10 100.2 (1.3) 151 (22) 1048 (134) 38 49 5517 210 0.11080 (34.1) 0.01313 (1.6) 0.05 84.1 (1.3) 107 (38) 647 (366) 39 22 1587 119 0.16187 (141.3) 0.01778 (2.5) 0.02 113.6 (2.8) 152 (209) 807 (1479) 40 49 7230 159 0.28380 (6.2) 0.03020 (1.6) 0.27 191.8 (3.2) 254 (18) 873 (61) 41 63 18885 576 0.20397 (8.0) 0.02029 (1.5) 0.19 129.5 (1.9) 189 (16) 1011 (79) 42 35 2813 3 0.13659 (108.3) 0.01458 (1.1) 0.01 93.3 (1.1) 130 (140) 867 (1123) 43 24 23582 4 0.12177 (26.8) 0.01353 (3.3) 0.12 86.6 (2.9) 117 (33) 783 (279) 44 44 2296 108 0.09242 (29.0) 0.01254 (1.8) 0.06 80.3 (1.5) 90 (27) 349 (327) 45 68 2495 227 0.08972 (19.8) 0.01255 (1.6) 0.08 80.4 (1.3) 87 (18) 278 (226) 46 82 6676 353 0.09038 (12.6) 0.01274 (1.0) 0.08 81.6 (0.8) 88 (12) 261 (144) 47 94 3327 320 0.08428 (15.2) 0.01299 (2.9) 0.19 83.2 (2.4) 82 (13) 52 (178) 48 70 4958 133 0.08628 (10.3) 0.01250 (1.1) 0.11 80.1 (0.9) 84 (9) 198 (119) 49 87 17697 776 0.19282 (7.3) 0.01575 (1.3) 0.17 100.7 (1.3) 179 (14) 1400 (69) 50 67 16909 3861 0.23801 (6.3) 0.01705 (2.1) 0.33 109.0 (2.3) 217 (15) 1647 (55) 51 76 10608 1097 0.08861 (17.9) 0.01182 (2.9) 0.17 75.8 (2.2) 86 (16) 386 (198)

aAnalyses in italics are not used in age calculations. All errors are at 1s level and include only uncertainties in the measurement of isotope ratios. Systematic errors would add an additional 2% (1s) to each age. U concentration and U/Th have uncertainty of 25%. Decay constants: 235U = 9.8485 1010; 238U = 1.55125 1010; 238U/235U = 137.88. Isotope ratios are corrected for Pb/U fractionation by comparison with standard zircon with an age of 564 ± 4 Ma (2s). Common Pb correction from measured 206Pb/204Pb, with initial Pb composition interpreted from Stacey and Kramers [1975]. Uncertainties assigned to common Pb are 1.0 for 206Pb/204Pb and 0.3 for 207Pb/204Pb (2s). 206Pb/204Pb is measured ratio. All other ratios are corrected for common Pb. KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 13

Figure 7. Conventional Concordia diagram showing ID- TIMS U/Pb results for sample 630-5.

Belt [Saul, 1986; Seiders, 1986; Sliter, 1986]. As described Figure 9. ICPMS ages and errors for single zircon crystals above, garnet growth at pressures of .75 GPa is bracketed from sample 701-4. between 81 ± 3 and 76 ± 1 Ma, and these results document 25 km of exhumation from between 81 and 76 and 68 Ma, ature brittle deformation everywhere overprints earlier mid- corresponding to a rate of at least 1.9–3.1 km/my. Naeser temperature or high-temperature deformation, and most and Ross [1976] reported a 71 Ma apatite fission track age rocks in the section show some evidence of brittle defor- (closure temperature of 110C) from rocks near Cone Peak. mation. In thin section zones of crushed grains and defor- Compton [1966b] reported a K-Ar age of 75 ± 4 Ma from mation twins in calcite and feldspars are common. Hundreds biotite in a sample collected at Cone Peak (closure temper- of slickensides and slickenlines were also observed in ature 300C). U-Th/He ages on rocks from the area mapped outcrops throughout the study area. Compton [1966a] in this study [Ducea et al., 2003a] are late Cenozoic provided a detailed analysis of the brittle deformation (2–8 Ma), and when considered with the fission track data, features in the Santa Lucia range, and linked them to a indicate that the Coast Ridge Belt was buried by 2–3 km of long history of deformational events throughout the Plio- Cenozoic sediments. Ducea et al. [2003a] attribute exhuma- cene and Pleistocene. tion in the late Cenozoic to transpressional tectonics related to [35] The most common medium temperature features are the northward transport of Salinia. mineral lineations present in nearly all quartzites or quartz- rich rocks, and mylonitic textures found occasionally in felsic 7. Structures rocks. The generally layer-parallel lineations are born pri- marily by elongate quartz grains and show a NNW-SSE [34] The rocks of the Coast Ridge Belt show evidence of preferred orientation (Figure 3c). In thin section, the elongate deformation at a wide range of temperatures. Low-temper- quartz grains typically show undulose extinction and are

Figure 8. ICPMS ages and errors for single zircon crystals Figure 10. ICPMS ages and errors for single zircon from sample 630-5. crystals from sample 706-4. 12 - 14 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT

Figure 12. Six-point Sm-Nd mineral and whole-rock isochron, sample 710-5.

Figure 11. Relative age-probability curve of inherited structural origin of the banding is as follows: (1) Isoclinal zircon ages in sample 706-4 (‘‘Salinia’’). The star indicates folds are found in the section, (2) At both map and outcrop the age of a Concordia diagram upper intercept (1.7 Ma) on scale, foliation is generally defined by lenses or pods of ‘‘Sur Series’’ rocks at Ben Lomond Mountain, interpreted diverse lithologies rather than semicontinuous layers. For by James and Mattinson [1988] to represent inheritance example, two large marble bodies in the Coast Ridge age. The curve for <100 Ma Salinia grains has been Belt have thicknesses of 1 km, but extend only 2–3 km removed so that the other curves are visible. Similar curves in the plane of foliation [see Ross, 1976]. (3) Intrusive, for the Sierra City me´lange (‘‘Northern Sierra’’ [Harding et metaigneous material is common throughout the gneiss al., 2000]), the Wood Canyon formation of southern (see discussion below), however there is a near-complete California (‘‘Mojave’’ [Stewart et al., 2001]), and lower absence of cross cutting relationships. A high-strain Paleozoic strata from Acatlan and Oaxaca (‘‘southern environment is required to create such features. Three Mexico’’ [Gillis et al., 2001; G. Gehrels, unpublished data, observations suggest that this fabric developed at high 2002] are shown for comparison. temperatures: (1) medium-temperature deformation features such as mylonitic fabrics are scarce, (2) many deformed mafic lenses or layers, show near equilibrium textures in rimmed or nearly completely overgrown by small, recrys- thin section, suggesting equilibration of fabrics at high tallized grains. The quartzite textures are comparable to temperatures during or possibly after deformation, (3) dislocation creep regimes 2 and 3 of Hirth and Tullis garnets equilibrated at 700–800C, and associated aur- [1992], which are typical of rocks deformed at midgreen- eoles and veins, commonly overprint the gneissic banding. and higher metamorphic grades [Hirth and Tullis, The quartz-rich aureoles, as noted by Compton [1960], are 1992]. Quartz also shows undulose extinction and strong generally equidimensional about the garnets, suggesting recrystallization when present in less quartz-rich rocks. growth under isobaric postdeformational pressures. Most commonly the quartz fabrics are associated only with Together the above observations suggest that the gneissic minor deformation of other minerals (e.g., deformation banding of the Coast Ridge Belt is primarily a high- twins in feldspars). In a few samples however a weakly mylonitic texture is evident. Strong mylonitic fabrics are rare in the section but a few small patches of mylonitic rocks were found, in one case associated with ductile Table 5. Sm-Nd Isotope Data for Minerals and Whole-Rock, thinning of the layered gneiss outcrop shown in Figure 4. Sample 710-5, Cone Peak No map-scale relationships were evident linking areas of Sample Sm, ppm Nd, ppm 147Sm/144Nda 143Nd/144Nd (2s)b mylonitized rocks. Deformation at medium temperatures may also have been responsible for gentle outcrop-scale Plagioclase 0.979 6.260 0.0945 0.512612 ± 14 folds observed in the study area, although these folds may Biotite 5.000 19.815 0.1525 0.512639 ± 8 Amphibole 11.398 36.304 0.1898 0.512651 ± 9 also have been formed at high temperatures. These ductile Garnet-1 6.485 7.844 0.4998 0.512812 ± 11 folds are partially responsible for the variation in foliation Garnet-2 7.122 7.553 0.5702 0.512850 ± 15 observed in the mapped area and depicted in Figure 3b. Whole-rock 6.259 26.556 0.1425 0.512634 ± 7 [36] We propose that the overall banded nature of the aParent-daughter ratios are precise to 0.4%. Coast Ridge Belt is a structural feature resulting from the b143Nd/144Nd normalized to 146Nd/144Nd = 0.7219. Errors refer to the last high-strain transposition of originally larger and more two digits. Replicate analyses of Nd standard measured during the continuous preexisting units. The evidence for a high-strain course of this study yielded 143Nd/144Nd = 0.511852 ± 8. KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 15 temperature or subsolidus deformation feature, an axial work rocks of the Santa Lucia Mountains in an increased planar foliation developed on isoclinal folds. concentration of granofels, amphibolites and marble, a near absence of biotite schists and a generally less foliated and more massive appearance of outcrops [Ross, 1976]. 8. Interpretation Although framework rocks in both areas are thought to represent a predominantly metasedimentary sequence [e.g., 8.1. Age and Provenance of the Sur Series Ross, 1977], the results of field and geochronologic work [37] The original tectonic location of Salinia and the age presented here suggests that the sedimentary component of and origin of its metasedimentary framework has been the the Coast Ridge Belt is subordinate to metaigneous rocks. subject of much speculation (see Mattinson [1990] for a These interpretations are also supported by major element summary). The debate has centered on whether Salinia is a data [Ducea et al., 2003b] and isotopic work [Kidder et al., far traveled terrane as suggested by some paleomagnetic 2001]. data [e.g., Kanter and Debiche, 1985], or whether it has [40] The original designation as a sedimentary sequence undergone only limited northward transport from an origi- by Trask [1926] was based primarily on the abundance of nal location in the southern California arc [e.g., Dickinson marble and quartzite in the generally layered gneiss, and on and Butler, 1998]. The debate has not benefited from the overall absence of cross cutting relationships. The bulk evidence relating to the depositional age of the Salinian composition of the gneiss is one of a calc-alkaline tonalite, metasedimentary framework, as medium-grade to high- with subordinate granodiorites, based on the modal compo- grade metamorphism has precluded fossil preservation. sition of rocks examined in thin sections. The U/Pb zircon [38] We use the inherited zircon ages in the Cone Peak age of the most deformed gneiss in the area is 93 Ma with intrusion (Table 4, Figure 11) to constrain the origin of the no inheritance, which corresponds to a period of intense framework rocks in the study area. While the zircons may magmatism (‘‘flare up’’) in the California arc [Ducea, 2001, have been derived from multiple sources in deeper parts of and references therein]. Gneissic rocks with the typical the arc, we make the simple assumption that they were part of composition of the Sur Series are seen cross cutting the the original Salinian sedimentary framework. The distribu- marble layers via tubes and dikes, suggesting intrusive tion of inherited ages in the sample is shown in Figure 11 and relationships. does not match any of the relative age-probability curves for [41] Kidder et al. [2001] determined Sr and Nd isotopic major North and Central America stratigraphic units [e.g., ratios on five typical felsic and mafic samples of the Coast Stewart et al., 2001]. We note however the predominance of Ridge Belt gneiss and found them to be isotopically indis- Grenville ages and a group of Paleozoic ages. With the tinguishable from the intrusive rocks in the section. For exception of the ages, the closest match for this comparison, three less common framework rocks were also distribution is a mix of the major Cordilleran miogeocline analyzed, including a quartzite, a quartzofeldspathic vein, formations if dominated by the extensive Late Proterozoic- and a quartzofeldspathic inclusion in a marble. All three were Wood Canyon formation [Stewart et al., 2001]. found to have significantly more evolved or ‘‘metasedimen- Preliminary Nd isotopic analyses of two quartzites from the tary’’ isotope ratios than the gneisses. The 93 Ma U-Pb age Sur Series (eNd = 15 to 17 (M. Ducea, unpublished data, presented here on a separate sample of the gneiss and the 2002)) are within the range of the eNd of the Middle 81 Ma age on the gneiss exposed at Cone Peak clearly Proterozoic to Cambrian section in the southern indicate Cordilleran magmatic origins for these rocks as well. [Farmer and Ball, 1997]. The nearest source of Devonian [42] We interpret therefore that the gneissic framework in zircons is either northern California sources such as the the study area is predominantly metaigneous, arc-related, Bowman Lake batholith in the Sierra City me´lange [Hanson and that framework rocks of sedimentary origin are subor- et al., 1988; Harding et al., 2000] or the terranes of southern dinate and generally limited to quartzites and marbles. Mexico [Gillis et al., 2001; G. Gehrels, unpublished data, 2002]. Our interpretation then is that the Sur series metasedi- 8.3. Igneous and Metamorphic History of the Coast mentary component is a mix of miogeocline-derived detritus Ridge Belt with a Paleozoic magmatic component. This suggests a late Paleozoic or Mesozoic depositional age for rocks. Given the [43] U-Pb data presented here provide a link between overall similarity to the relative age probability distribution magmatic activity in the shallow level exposures dated of the Wood Canyon formation, we speculate that this unit, throughout Salinia and the deeper crustal exposures studied which is very extensive in the Mojave region, is an important here. The first evidence of Mesozoic igneous activity in the source. However, the preliminary single crystal zircon U/Pb Coast Ridge Belt comes from a few inherited zircons of age geochronology data presented here cannot unambiguously 120 Ma found in two of the three analyzed samples distinguish between the various hypothesized sources of (Table 4). Although no plutons of that age have been framework rocks within Salinia (Figure 11). documented in the Coast Ridge Belt, this suggests that at least some magmatic activity had begun by that time. A pyroxene tonalite near Big Sur crystallized at 104 Ma 8.2. A Largely Igneous Origin for the Coast Ridge [Mattinson, 1978] and is the next known magmatic event Belt Framework Gneiss in the Coast Ridge Belt. Within the study area, three bodies [39] The Sur Series metamorphic framework of the Coast intruded within a fairly short period of time in the late Ridge Belt differs from the shallower metamorphic frame- Cretaceous: a 93 Ma body now incorporated into the 12 - 16 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT framework gneiss, an 86 Ma diorite and an 81 Ma quartz diorite. Magmatic ages recorded with U-Pb and whole-rock Rb-Sr geochronology [Kistler and Champion, 2001] on shallower rocks throughout the Santa Lucia Mountains are also focused in the latest Cretaceous. Thus magmatic activity in the Santa Lucia Mountains apparently began with a few sporadic early and middle Cretaceous intrusions followed by a pulse of activity in the late Cretaceous. This pulse of magmatism may have corresponded with an eastward migration of magmatism across the Salinian arc [Mattinson, 1990]. [44] The accumulation of magmas in the Coast Ridge Belt led to a significant thickening of the section during the Late Cretaceous. Recognizable igneous bodies make up 30% of the section and as mentioned above, much of the gneiss is Cretaceous orthogneiss. On the basis of whole rock geo- chemistry, our best estimate is that a conservative 60% and probably closer to 80% of the section is igneous or metaig- neous [Ducea et al., 2003b]. Thus, barring significant removal of material, the Coast Ridge Belt appears to have more than doubled in thickness between 93 and 80 Ma. These intrusions were split fairly evenly between felsic and mafic magmas, contrasting with the overwhelmingly felsic nature Figure 13. Diagrammatic plot of thermal histories for of contributions to the upper 20 km of the Salinian crust rocks near Cone Peak. See the text for details. (>90% by volume). This distribution is presumably the result of magma stalling at levels of neutral density [Glazner, 1994]. [45] Peak recorded pressures and temperatures of restores foliation in the Coast Ridge Belt to horizontal and 0.75 GPa and 800C occurred throughout the section at brings barometric estimates into rough agreement with their the conclusion of this period, between 81 and 76 Ma, depth in a restored Salinian Arc. The same ‘‘untilting’’ also coinciding with the growth of postkinematic garnets and helps to restore overall northeast dipping metasedimentary at least in part postdating igneous activity. Although there is layers in the central Santa Lucia Range to horizontal [Wiebe, no reported pressure-temperature evidence for magma load- 1970a]. ing in the region, the thickening we record in the middle [48] Whether or not a correction for tilt is made, the crustal section combined with voluminous intrusion seen in metamorphic framework in the shallower exposures of the the upper crust are consistent with overall crustal thickening Santa Lucia Mountains to the east of the Coastal Ridge Belt in the Late Cretaceous. differ in a few important ways from those in the Coast Ridge Belt itself. First, the shallower exposures have a generally steeper foliation. A second important distinction between the 8.4. Timing of Deformation and the two areas is the contrast between the overall more uniform Development of Foliation and layered appearance of the deeper rocks versus a more [46] Magmatic activity from 93 to 81 Ma also corre- disorganized collection of plutons separating generally sponded with a period of high-temperature deformation in steeply dipping framework rocks in the shallower exposures. the middle crust. While deformation may have begun some- These differences in appearance may be explained if the what earlier or later than 93 Ma, this age on a strongly middle crustal Coast Ridge Belt exposures represent a deformed outcrop of amalgamated 93 Ma igneous rocks and destination zone of supracrustal rocks displaced by granitic marble in the same section as much less deformed 86 and intrusions into the upper crust. The shallower and more 81 Ma igneous bodies shows that extensive deformation steeply dipping rocks of the shallower exposures in this case occurred during this time period. High-temperature deforma- represent ‘‘metamorphic screens’’ arrested in movement tion ceased shortly after 81 Ma, as evidenced by the presence toward midcrustal levels. Similar upper crustal rocks were of euhedral 76 Ma garnets grown over foliation planes in the described by Saleeby and Busby [1993] in the Sierra Nevada 81 Ma Cone Peak intrusion. We therefore suggest that batholith. This scenario hypothesizes an essentially ‘‘pure the ductile deformation responsible for the development of shear’’ environment for the Coast Ridge Belt consistent with the gneissic banding in the rocks took place between 93 and the lack of consistent shear sense indicators associated with 76 Ma, synchronous with magmatism in the area. the high-temperature fabrics. [47] Foliation of the Coast Ridge Belt dips on average [49] An alternative explanation is that the fabric formed 30 degrees to the NE, and we suggest that this foliation was during ductile shearing of tectonic significance. It has been originally near horizontal, since the simplest explanation for proposed [e.g., Brown and Solar, 1998] that intrusion of overall northeastward decreasing regional metamorphic granitic bodies can take place along crustal-scale shear zones, grade in the Santa Lucia mountains is a simple tilting of the and magmatism here is certainly associated with synchro- original crustal column. A crude 30 degree ‘‘untilting’’ nous ductile deformation. In southern California, May [1989] KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT 12 - 17 proposed that significant westward thrusting of the eastern sion during this period is suggested by mineral lineations Mojave arc took place between 90 and 80 Ma, as evident measured in the study area. Timing of this medium-temper- from ductile thrusting of arc-related rocks in the San Gabriel ature structure is well constrained to this period of time as and San Bernardino Mountains. The coincidence of the we know (1) that the fabric postdates the 93–80 Ma high- timing of ductile deformation in the Coast Ridge Belt and temperature structures, and (2) that temperatures in the Mojave region may be important in understanding the Coast Ridge Belt did not exceed the apatite fission track development of the Cretaceous arc in California, especially closure temperature (110C) since the latest Cretaceous if Salinia originated in southern California. However, the [Naeser and Ross, 1976; Ducea et al., 2003a]. structures observed in the 25 km deep exposure of the Coast [53] Exhumation of the Coast Ridge Belt may have been Ridge Belt are markedly different from the ones described related to late Cretaceous extensional collapse of the entire from shallower exposures in southern California by May section of the arc at the latitude of the present day Mojave [1989]. region [e.g., Wood and Saleeby, 1997]. Late Cretaceous [50] The two hypothesis (thrusting vs. local return flow) extensional collapse of the arc in that region is commonly may only be distinguished by a detailed field investigation of attributed to underthrusting of the forearc at depths as the transition area from midcrustal levels to the shallow shallow as 30 km [e.g., Malin et al., 1995]. In the Salinian exposures in the central Santa Lucia Mountains. However, composite terrane, the schist is the local our limited field evidence for a pure shear mechanism of equivalent of the forearc-related Pelona/Orocopia/Rand deformation in the Coast Ridge Belt, argue against ductile schists found in the Mojave region [see, e.g., Jacobson thrusting. et al., 2000; Schott and Johnson, 1998; Mattinson, 1990]. The Sierra de Salinas schist was emplaced into the 8.5. Exhumation History southern California crust to depths of 25–30 km by 80 Ma [Mattinson, 1990]. Recently proposed tectonic [51] Several lines of evidence suggest that the Cone Peak models suggest that the North American upper plate of section was quickly cooled and exhumed after the cessation this thrust system had collapsed gravitationally by the end of magmatic activity at 81 Ma. Our data indicate that the of the Cretaceous leading to the development of normal section was at high pressure-temperature at 76.5 Ma (from faults, the exposure of core complexes, and a subdued combined garnet geochronology and thermobarometry). A topography [Saleeby et al., 2003]. In contrast, the main 75 Ma K-Ar date [Compton, 1966b] and an apatite fission parts of the Sierra Nevada and Peninsular Ranges batholith track age of 71 Ma [Naeser and Ross, 1976] both obtained have been slowly unroofed to a present-day average of 5– in the Cone Peak area suggest fast cooling following peak 7km[House et al., 2001; Saleeby et al., 2003]. metamorphism at 76 Ma. Mineral cooling ages by K-Ar, [54] If magmatic cessation and extensional collapse Ar-Ar, U/Pb and Rb-Sr techniques from everywhere within are linked to late Cretaceous forearc underthrusting in the the Santa Lucia Mountains show similar results, falling Salinian belt, then the rocks of the Coast Ridge Belt are the everywhere in the narrow range from about 83 to 69 Ma deepest exposures exhumed to the surface during the latest [Kistler and Champion, 2001]. The section was exposed to Cretaceous and may therefore represent the exposures closest the surface and covered in the latest Cretaceous by coarse- to the major structural ramp of the system. In this scenario the grained submarine sediments derived in an extensional existence of mafic to intermediate intrusions with North environment from local basement rocks and volcanic rocks American isotopic signatures [Kidder et al., 2001] intruded of the California magmatic arc [Grove, 1993]. Although as late as 81 Ma places a maximum age on the beginning of originally considered late Campanian [Compton, 1966a; forearc underthrusting. The postkinematic growth of garnet Howell and Vedder, 1978], subsequent investigations have through the section may then have been the result of isobaric shown that the oldest sediments are either mid-Maastrich- cooling due to this thrusting event. tian or early late Maastrichtian [Saul, 1986; Seiders, 1986; Sliter, 1986]. We estimate the time of final exhumation of the Cone Peak section to be 68 Ma, corresponding to 9. Discussion early-late Maastrichtian. The Maastrichtian sediments are in most places unconformably overlying Salinian basement; in [55] The data and interpretations above provide con- a few outcrops, they are in strike-slip fault contact [e.g., straints for understanding the tectonic and magmatic mech- Compton, 1966a], but the age of faulting is probably late anisms that operate in continental arcs. We discuss here the Cenozoic. implications of our observations to deciphering: (1) mag- [52] A temperature-age plot honoring the thermochronol- matic pulses in the Cordillera, and (2) ductile deformation in ogy and thermobarometry data for the Cone Peak section is arcs. shown in Figure 13. The section was unroofed at an average [56] 1. We show that much of the midcrustal magmatism rate of at least 2–3 mm/yr, which is an order of magnitude that led to the development of the Coast Belt Ridge was late higher than unroofing rates calculated for the Sierra Nevada Cretaceous, corresponding to the major Cretaceous flare up arc [House et al., 2001]. Such high unroofing rates require recorded from shallower exposures of the California arc tectonic denudation in an extensional environment, since [e.g., Ducea, 2001]. We observe that at the paleodepths of the area did not represent a topographic high at the time and 25 km of the Coast Ridge Belt, mafic materials are far may even have been submerged for most of the latest more common at the time of the California arc flare up than Cretaceous [Grove, 1993]. A NNW-SSE direction of exten- they are anywhere in the shallow exposures. The mafic sills 12 - 18 KIDDER ET AL.: DEVELOPMENT OF THE SALINIAN COAST RIDGE BELT have been emplaced throughout the magmatic pulse but support the following conclusions relevant to the regional have rarely escaped into the shallower levels of the crust. geology: This observation suggests that the current depth of exposure [60] 1. The metamorphic framework rocks are predomi- in the Coast Ridge Belt represented the neutral level of nantly orthogneisses with only subordinate amounts of buoyancy for mafic arc magmas. quartzite and carbonate metasedimentary assemblages; [57] 2. We infer that the rocks of the Coast Ridge Belt and [61] 2. Inherited zircon ages in the Cone Peak quartz continuing to the east represent a tilted exposure of a diorite suggest a connection between the ‘‘Sur Series’’ and Cordilleran magmatic arc with exposure depths ranging from the Wood Canyon formation in southern California, and 25 km in the west to some 10 km in the east. No major suggest a late Paleozoic or younger sedimentary age for the displacement have been mapped within this section. This Sur Series; section thus provides a transition from shallow ‘‘batholithic’’ [62] 3. Substantial ductile deformation of the framework depths where metamorphic screens (‘‘pendants’’) are vertical, took place between 93 and 81 Ma, concomitant with to the level at which the foliation of the framework was intrusion in the section; most of the section was built by essentially horizontal. We propose that the transition took magmatic additions during that period; place at about 25 km beneath the arc in this case and this [63] 4. Peak metamorphic conditions (800 Cand paleodepth represents the root of large-scale, batholith form- 0.75 GPa) were achieved immediately after the last intrusion ing intrusions. The ductile deformation is, in our interpreta- in the section between 81 and 76 Ma; tion, the result of localized return ductile flow of upper crustal [64] 5. NNW-SSE oriented mineral lineations and mylo- materials (metasedimentary rocks and arc-related metaplu- nitic fabrics were developed at medium temperatures in the tonic and metavolcanic rocks) in response to the upwelling of late Campanian-Maastrichtian during exhumation of the larger plutons [e.g., Paterson and Miller, 1998]. section at a rate of at least 2–3 mm/yr. [58] Taken together, these interpretations are used to [65] Magmatism in the Coast Belt Ridge took place propose here that the roots of the large volume (hundreds within the short period of late Cretaceous flare up of the of km3 or larger) felsic plutons mapped in the North California arc, further substantiating the notion that American Cordilleran batholiths are represented by a mid- magmatism in the North American Cordillera was highly crustal low-strength zone spreading out horizontally as episodic. We propose that the arc midcrust here repre- gravity current [Wernicke and Getty, 1997]. This zone is sents a zone of flattening of downward flowing super- the destination for both mafic magmas ascending from the crustal rocks and their convergence and mixing mantle wedge and localized ductile flow of plutonic rocks with stalling mafic magmas, presumably due to neutral and metamorphic screens from the upper crust [Ducea et al., buoyancy. 2003b]. Therefore this weak and hot midcrustal domain separating large felsic plutons from a mafic lower crust [66] Acknowledgments. Supported by NSF grant EAR-0229470 could: (1) act as a mechanical detachment surface during from the Petrology-Geochemistry Program (Ducea). Fieldwork was synmagmatic or postmagmatic regional thrusting, extension partly supported by the University of Arizona Geostructure Program or convective removal, (2) provide an efficient zone of and a grant from Chevron. Beth Duschatko, Alisa Miller, Elisabeth magma mixing and mingling during arc magmatism. Nadin and Ken Domanik are thanked for their help in the field and lab. A special thanks to John Smiley at the Landels-Hill Big Creek Reserve for their hospitality and assistance. We are thankful to Jason Saleeby and Bill Dickinson for their constant support, enthusiasm, 10. Conclusions and willingness to share their knowledge of Cordilleran geology. Constructive reviews by James Mattinson, editor Brian Wernicke, and [59] Detailed mapping, petrography, thermobarometry an anonymous reviewer have significantly improved the quality of the and geochronology of rocks from the Cone Peak area manuscript.

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STATE OF CALIFORNIA - GRAY DAVIS, GOVERNOR CALIFORNIA GEOLOGICAL SURVEY THE RESOURCES AGENCY - MARY D. NICHOLS, SECRETARY JAMES F. DAVIS, STATE GEOLOGIST CALIFORNIA DEPARTMENT OF CONSERVATION - DARRYL YOUNG, DIRECTOR GEOLOGIC MAP OF CALIFORNIA

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Copyright © 2002 by the California Department of Conservation, California Geological Survey. All rights reserved. No part of this publication may be reproduced without written permission of the California Geological Survey.

The California Department of Conservation makes no warranties as to the suitability of this product for any particular purpose. 0 0 Start PGS field trip at Aptos overlook,Watsonville 0 122 00' 122 45' not shown on map ! 121 30

Peninsula Geological Society field trip to Salina / Nacimento Qal Qal amalgamated terrane, Big Sur coast, central California. 1 Qt May 19 - 21, 2000 SALINAS RIVER Map compilation by R.G. Coleman, Stanford Geological Survey qd using Strand & Jennings, 1958, Santa Cruz 1:250,000 geologic map Qt Salinas CDMG; Dibblee 1999, Geologic Map of the Monterey Peninsula and Vicinty, #DF-71, 1:62,500; Ross,1976, US. Geological Misc. Field Inv. Map MF 750 ,1:125,000. Qs Qal 101 Qs Monterey Qc EXPLANATION Qt Qp pgm Tm Qm Geologic points Seaside pgm Qt Chualar of interest Qp Qt Ch UNCONSOLIDATED Tm upines F pgm ault SEDIMENTS Qp Carmel Start Qal sur Qs Qal Alluvium qd CARMEL RIVER Tm Qal Qs Dune Sand Point sur Lobos Tm pgm 30' Sierra de Salinas 36 Qt Quaternary non-marine pgm CARMEL terrace deposits VALLEY Qal Qt 1 Tm pgm Qc Pleistocene non-marine Tm Tularcitos Fault qd qd S Qp Plio-Pleistocene non-marine A

N Qm Pleistocene marine Terrace sur T sur deposits P A qd alo Tm Co COVER ROCKS lo pgm S ra u do r qd Tm Monterey Formation, mostly -- qm C pgm qm pgm marine biogenic and T sur o h a pgm ru s sur clastic sediments middle to s qdp t t R qd Tm i sur late Miocene in age. dge qm sp qm Tm S Tm FRANCISCAN SUBDUCTION Qs fs i qm er sp ra COMPLEX J-K Point Sur H sur qd L sur sp K i ll F qm K a U Graywacke, deep-sea trench u fs l F C t sur a C h u sur u deposits l r fc t I ch Qm K C qm N A re Deep-ocean bedded radiolarian cherts o e fc fs sp rth k qm Fo qm F fc rk aul 5' P Fa 36 acific Ocean ult t Sheared serpentine derived from K sur sp qdp oceanic depleted harzburgite. sur qm qm FOREARC SEDIMENTS J-K 28 qm Great Valley forearc qm R K PGS sur A sediments qdp Campsite N sur

NEVADAN ISLAND ARC Pfeiffer-Big Sur G K INTRUSIVE ARC ROCK State Park E (78 -150 Ma age of arc) Sur Thrust qm Porphyritic grandiorite of Monterey ISLAND ARC BASEMENT K Sur Series or Sur Complex sur K pgm Quartz monzonite quartzofeldspathic schist, marble granofels, and gneiss. Protolith qdp pg Porphyritic grandiorite age is Precambrian 1.7 Ga and the qd Quartz diorite metamorphic age is 78 to 100 Ma. qdp sur sur qdp Quartz -poor quartz diorite fs (some are recrystallized tonalites 0 10 20 km fs qdp containing granulite facies 0 5 Lucia garnet + opx , Compton's charnockite) 10 miles 0' SCALE 3

0 0 1 122 00' 122 45' REVISED Cross section location CROSS SECTION OF THE SOUTHERN COAST RANGES AND SAN JOAQUIN VALLEY in California FROM OFFSHORE OF POINT SUR TO MADERA, CALIFORNIA Donald C. Ross, U.S. Geological Survey (1979) Geological Society of America, Map & Chart Series v. 28H

San Andreas Revised by R,G, Coleman Stanford Geological Survey April 2000

F ault

G R E A T Madera B

V San Gregorio-Hosgri Fault A L L G Is this a step forward or backward for California Geology ? E Monterey a Sa bila Y R nta n a Ra S n L a g n e u n A c ia g n A e d Sur-Nacimentoreas Fault ult Fa

ck lo ar G F Pacfic Ocean ault

200 km 0 milligals Scale milligals Bouger Gravity Profile (Bishop & Chapman 1967) -40 40 20 20 0 0 -20 -20 -40 -40 Western edge of former -60 -60 -80 -80 Jurassic - Cretaceous subduction zone Coast Range -100 -100 San Andreas Axis & shape of the Great Valley for point Sur Area Thrust A Fault Zone magnetic high from Jachens et al 1995, JGR 100:12769-12790 KM Sur-Nacimento B Santa Lucia Sierra de Salinas Gabilan Fault Zone Panoche Hills San Joaquin Valley Madera 4 Range Salinas Valley Range 4 KM PACIFIC OCEAN Point Sur Diablo Range a Level 0 0 5.2 ? 3.85 -4 2.0 5.4 ? 4.35 -4 Franciscan Subduction Complex 6.0 5.6 5.5 Middle to -8 -8 JKf 6.0 6.25 Franciscan Subduction Complex 6.45 magmatic underplating of west facing -12 "infant arc" Sierra Nevada -12 6.3 6.75 JKf JKf 6.4 -16 MORB stalled underplate 7.2 -16 6.6 6.7 -20 ? Sierra Nevada Basement -20 -24 6.8 -24 -28 8.0 MORB stalled underplate 7.2 -28 Mantle 7.1 -32 8.1 -32 7.9 Begin GVO by slow -36 rifting & magma generation (155-165 Ma) -36 -40 -40 110 100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 KM Gabilan crossection at Gonzales Diablo crossection Great Valley crosssection EXPLANATION Walter & Mooney (1982) Walter & Mooney (1982) Holbrook & Mooney 1987 Bull. Seis. Soc. Amer., Bull. Seis. Soc. Amer., Tectonophysics, 140:49-63 72:1567-1590 72:1567-1590 Nevadan Island Arc Quaternary & Pleistocene Tonalite Intrusive granites 78 - 100 Ma Slow spreading initiates partial melting in the mantle ~ 165 Ma Ter tiary sediments Sur Series continental basement 1.7 Ga Fore Arc Great Valley Dikes Cretaceous-Jurassic sediments Great Valley Ophiolite slow spreading underplating of Sierran Basement Sierra Nevada Basement Coast Range ophiolite generated above suprasuduction zone Cretaceous-Jurassic Mid ocean ridge MORB

Franciscan subduction complex, Mantle, depleted harzburgite Cretaceous-Jurassic