From the late 1960s to the 1980s, many—perhaps most—Franciscan mélanges were thought to be tectonic in origin [8
]. Tectonic mélanges were also thought by some to be massive units representing the entirety of an accreting subduction complex at a particular point in time [78
]. While early arguments for sedimentary origins existed [21
], those arguments were not widely applied to Franciscan rocks until later. Current conversations at meetings and the published observations and comments of researchers such as Cowan [12
], MacPherson et al. [82
], and Wakabayashi [3
] suggest that tectonic mélanges may be less common than previously thought and may be decidedly subordinate to sedimentary mélanges in the Franciscan Complex [43
]. Wakabayashi [3
] even argues that many serpentinite-matrix mélanges had fragmentation and mixing histories that are primarily sedimentary in character.
Tectonic mélanges are still recognized in the Franciscan Complex and some are universally accepted as such. The one tectonic mélange described relatively recently by Wakabayashi [3
] is the Rodeo Cove shear zone. He earlier also attributed fragmentation and mixing of the Hunters Point Mélange to tectonic processes [54
]. The well-known Ring Mountain (serpentinite-matrix) Mélange is considered by Bero [55
] to be tectonic in origin; however, as is discussed below and by Raymond [2
], it has a disputed origin. A similar mélange at Jenner is considered to be of tectonic origin [52
3.2.1. Rodeo Cove Mélange
The Rodeo Cove Mélange is a fault zone mélange (i.e., a tectonic mélange) within the Marin Headlands Block of the Franciscan Complex located a short distance north of San Francisco (RC on Figure 1
]. This mélange was called the Rodeo Cove thrust zone by Meneghini and Moore [23
], the Rodeo Cove mélange shear zone by Meneghini et al. [51
], the Rodeo Cove shear zone by Wakabayashi [3
], and a mélange comprising unit six of Marin Headlands by Raymond et al. [46
]. In the most fully described section of the mélange at Rodeo Cove, the Rodeo Cove Mélange is atypical of tectonic mélanges, because the mélange is zoned, and parts of the described shear zone do not have typical block-in-matrix structure [23
]. The mélange is traceable from west to east across the southern Marin Headlands and appears as a lenticular mass of variable thickness on maps and in sections (Figure 5
). It lies structurally between two accretionary units (AUs) that contain broken formational OPS masses. The overlying unit, AU5 of Raymond et al. [46
], is dominated by oceanic metabasalt but contains some (meta)chert. The underlying unit (AU7) is the Black Sands-Conzelman AU, which contains parts of three OPS components—metabasalt, chert/metachert, and metasandstone (with meta-mudrock).
Inasmuch as the Rodeo Cove Mélange is a fault zone mélange, its boundaries with adjoining units are sheared, as is the matrix. The matrix varies from sheared metabasites to sheared mudrock, and it exhibits scaly foliation and cataclasite, but in the Rodeo Cove exposure, the matrix diminishes from the core towards the margins, especially the structurally lower margin (toward the north) [23
]. Thus, the fault zone at Rodeo Cove is zoned in terms of deformation features. The southern edge of the shear zone, as defined by Meneghini and Moore [23
], consists of structurally interlayered and juxtaposed broken formations of (meta)chert and metasandstone (not mélange). Towards the top of this zone of interlayering, stratal disruption increases and the shear-fractured-matrix derived predominantly from metabasalt and containing clasts of chert (one in excess of 10 m long) becomes the dominant rock of the mélange. The mélange displays P-R and S-C structures, notably in the core zone [23
East of the excellent exposures studied by Meneghini and Moore [23
], the Rodeo Cove Mélange is hidden beneath a lagoon and colluvium. Both Meneghini and Moore [23
] and Raymond et al. [46
] show the mélange extending east from beneath the colluvium and the lagoon, but Raymond et al. interpret the mélange to be unit six, whereas Meneghini and Moore interpret the mélange to be a unit two layers structurally lower (i.e., unit eight of Raymond et al. [46
]). Here, I adopt the interpretation of Raymond et al. [46
The zone of mélange east of the lagoon is thicker and has a wider map dimension than exposures at Rodeo Cove (Figure 5
). Here, the amount of matrix appears to increase, and “black rock” (pseudotachylite?) zones occur within this matrix. Large to small masses of metasandstone and metachert are exposed on steep slopes in the lower middle section of the mélange, and these are separated from one another by zones of scaly matrix containing small phacoids of metabasite, metasandstone, and metachert (Figure 2
D). All of the clasts in the mélange appear to be OPS fragments and hence are metabasites, (meta)chert, or metasandstone ± metamudrock. To date, the largest blocks known are several tens of meters long, but detailed mapping of the entire mélange has not been completed.
The tectonic origin of the Rodeo Cove Mélange is not in dispute. The mélange has 14 of the 16 features characteristic of tectonic mélanges and exhibits all three of those that are definitive (Table 1
). Its structural position, character, and lack of any of the definitive features of diapiric or sedimentary mélanges strongly support a tectonic origin.
3.2.2. The Ingram Canyon Mélange
In contrast to the Rodeo Cove Mélange, the Ingram Canyon Mélange contains a diverse array of clast types. The unit (formerly called the Rocky Point Mélange [48
]) forms an accretionary unit in the northeastern Diablo Range and lies at the structural top of the Franciscan tectonostratigraphy [45
]. The Ingram Canyon unit may be correlative with the Garzas Mélange described to the south ([11
], and see below), but the latter has been studied in less detail than the Ingram Canyon Mélange. Details of the nature and the structural relationships of the Garzas Mélange are not well enough known for a definitive correlation between the two units.
The Ingram Canyon Mélange forms a lenticular unit in map view and is tabular to wedge-shaped in cross section [2
]. All contacts are faulted (sheared), but most faults on the north, the northeast, and the east are late Cenozoic strike-slip faults [2
]. The fault at the structural base of the unit likely had a diachronous history. Following early thrusting beneath the overlying Coast Range Ophiolite (CRO) and later underthrusting by the Gerber Ranch Mélange, subsequent normal-slip movement likely occurred as the core of the Diablo Range rose during the late Cenozoic orogenic episode [87
]. The block-in-matrix fabric is anisotropic and exhibits S-C fabrics and possible P-R fabrics (cf. S-C fabrics in the Garzas Mélange to the south [88
]). Boudinage of clasts is common, and phacoids resulting from plastic extension are characteristic [48
]. Folds occur in many blocks, but no regular mesoscopic folding is known within the mélange matrix. Scaly fabric pervades the matrix. Veins and striated phacoids and mélange “scales” bounded by anastomosing shears are common.
Both native and exotic blocks occur in the mélange and locally can be observed within the matrix, which varies from a serpentinite-mudrock mix in the north to a mudrock matrix in the south (see the map of block distribution for the mélange in [47
]). In general, the scaly fabric of the matrix wraps around blocks, as is typical in tectonic mélanges. Blocks in the mélange range from serpentinite to sandstone and include conglomerate, garnet-glaucophane schist, chlorite-glaucophane metabasites (metagabbro and metabasalt), OPS fragments (including chlorite metabasites, chert and metachert, and metasandstone), plus uncommon siliceous metavolcanic rocks and chlorite schist [46
]. In addition, rare actinolite schist, retrograde metamorphosed and veined eclogite, and aragonite marble occur locally.
The Ingram Canyon Mélange has been studied only by Raymond [45
] and Raymond and Maddock [46
]. It exhibits all of the features of tectonic mélanges, including the three diagnostic features. It has been argued, however, by Wakabayashi [3
] (following MacPherson et al.) [82
] that conglomerates and breccias with clasts of upper plate rocks, such as siliceous volcanic and plutonic (arc) rocks, suggest a sedimentary origin for a mélange. The conglomerates and the breccias are considered to represent remnants of the original sedimentary protoliths of the mélange, whereas the siliceous volcanic and the plutonic rocks, it is argued [3
], must be clasts eroded via surficial processes from hanging wall terranes. While early-formed conglomerates, breccias, and diamictites may be overprinted by deformation and fragmented to form mélanges, as is clearly indicated by observational data [3
], the existence of arc-type rocks is not absolute evidence that all such rocks found in a mélange require sedimentary processes for the formation of the mélange. One alternative, a variant of the protolith argument, is that conglomerate and breccia fragments in mélanges may just be individual components of the trench sedimentary sequence that later becomes a tectonic mélange beneath the mid- to inner accretionary complex. Both versions of the protolith argument are especially tenable if the arc rocks occur as relatively small, well rounded clasts.
A second alternative to the argument that arc-like plutonic and volcanic rocks must
represent upper plate rocks surficially eroded from an upper plate source and deposited in the trench to become parts of the protolith of a sedimentary mélange is a tectonic alternative. Angular or tectonically rounded clasts and large blocks of arc-like plutonic and volcanic rocks may represent components of the igneous forearc, off-scraped during subduction erosion and subsequently mixed with other rocks to form tectonic mélanges [19
]. It seems unlikely, however, that more siliceous varieties or arc rock, which occur in mélanges such as the Ingram Canyon Mélange, could be produced by subduction erosion in an abscherungzone setting [19
] during initiation of subduction. Such a process would contribute clasts to a tectonic mélange that most likely would be basic to ultrabasic lower arc rocks. For upper arc (more fractionated) rocks to be tectonically eroded at the subduction interface, the arc would need to be extended and thinned, exposing abbreviated sections of arc rocks, including upper arc rocks, to tectonic erosion. Such thinned sections of suprasubduction zone arc rocks are present above the fault separating the Ingram Canyon Mélange from the overlying Coast Range Ophiolite (CRO) forearc section [45
]. Thin and incomplete, 200 to 350 m thick sections of the CRO forearc that include the siliceous Lotta Creek Tuff structurally overlie—above a sub-ophiolite fault—the Ingram Canyon Mélange.
A similar configuration exists southwest of King Ridge Road near Occidental, California, where siliceous volcanic rocks in a highly thinned incomplete CRO section are juxtaposed with serpentinite-matrix and mudrock/sandstone-matrix mélange, the latter assigned to the “Central Belt” [95
]. The presence of thinned forearc arc sections with siliceous components structurally overlying mélange that contains arc-like rocks provides a setting in which subduction erosion of ophiolite could yield arc rock-bearing mélange. The geometry is not conclusive evidence but is supportive of an alternative mode of mélange origin that should be considered.
The Ingram Canyon Mélange contains only seven features of sedimentary mélanges and none of those most definitive of that origin. In contrast, it contains 15 of the 16 features of tectonic mélanges, including three of the most indicative features. Thus, in spite of the presence of siliceous volcanic blocks in the mélange, the strong evidence of deformation suggests that this mélange formed via tectonic fragmentation and mixing.
3.2.3. The Jenner Headlands Mélange
At Jenner, there are two mélanges exposed in the sea cliffs/near-shore area and on surrounding hillsides (Figure 1
, labels HB and JH; Figure 6
]. The cliffs expose the polygenetic Heaven’s Beach Mélange [32
], whereas areas on the hills near the ridge crest to the east of the cliffs (and north of the Russian River) have exposures of a serpentinite-matrix mélange [7
], here called the Jenner Headlands Mélange. The Jenner Headlands Mélange is structurally overlain by serpentinized peridotite and locally overlies, above a thrust fault boundary, either an unnamed jadeitized, foliated metawacke unit (on the north in Figure 6
) or the Wren Rock unit (an olistolithic broken formation or sandstone-matrix mélange) [7
In map view, the serpentinite-matrix Jenner Headlands Mélange forms several irregular to linear masses scattered across the terrain north of Jenner, California. It is a thin tabular to irregular unit in cross section [2
]. The contacts are not well exposed but in all cases appear to be sheared, as are the exposed contacts between matrix and blocks.
The serpentinite matrix of the Jenner Headlands Mélange exhibits a scaly fabric that contains a variety of exotic blocks and clasts [53
]. The matrix is anisotropic in structure with anastomosing shear planes, and it wraps around clasts and blocks (Figure 7
A). The clasts and blocks range up to several tens of meters in long dimension and are rounded to lensoidal in shape [53
]. Block rock types include typical metamorphosed OPS rocks [metabasites (metabasalt and metagabbro), metachert, and metasandstone] and conglomerate plus hornblende and glaucophane schist with chlorite-actinolite rims and veins, chlorite and serpentinite schists, and rare eclogite. Essentially, all of the blocks are exotic.
The fault-bounded Jenner Headlands Mélange is decidedly tectonic in origin, exhibiting 14 of the 16 features of tectonic mélanges, including two of the three definitive features. With the possible exception of some rounded blocks, the mélange has none of the definitive features of diapiric or sedimentary mélanges and only modest numbers of features that occur in two or more mélange types.
Of note here is that the tectonostratigraphy present at Jenner Headlands is repeated elsewhere in the region. The uppermost trectonostratigraphy of the Franciscan Complex includes a mélange—typically a serpentinite-matrix mélange that structurally underlies a blocky serpentinized to massive peridotite unit usually assigned to the Coast Range Ophiolite. Beneath the mélange, the common units that occur successively downward are a foliated blueschist facies unit and a structurally underlying prehnite-pumpellyite facies, metasandstone-metamudrock unit. This sequence occurs at Jenner, at Freestone to the southeast, and on the Tiburon Peninsula near San Francisco (RM on Figure 1
]. Partial or similar sequences with the same order occur near Occidental (southeast of Jenner) and at El Cerrito (HM on Figure 1
]. In central Marin County, blocky serpentinized peridotite is underlain by a sheared serpentinite unit that is arguably a serpentinite-matrix mélange, and that unit is underlain by an olistostrome-bearing, prehnite-pumpellyite facies, metasandstone-metamudrock unit [32
]. A blueschist facies unit has not been recognized in central Marin County. Repetition of a tectonostratigraphy across the northern San Francisco Bay region argues for a similar tectonic accretion history for the region and a similar history for the serpentinite-matrix mélanges.
3.2.4. Other Tectonic Mélanges
Several other mélanges that have been designated as tectonic in origin are listed in Table 1
. These include the Garzas Mélange, the Gerber Ranch Mélange, and the Hunters Point Mélange. The Garzas Mélange (GZ on Figure 1
) is a shale-matrix mélange that is exposed over a wide area in the northeastern Diablo Range [11
]. Preliminary regional structural analysis suggests that the mélange may not be as thick as it seems, inasmuch as the folding patterns shown by Raymond [2
] for the region result in repetition of mélange sections. The mudrock-matrix Garzas Mélange has a relatively diverse range of native and exotic block types, including blocks of so-called “high-grade” glaucophane schist [11
]. Scaly and brecciated textures are common in the matrix (Figure 2
E) and define an anisotropic fabric that is also reflected by S-C fabrics in metawackes [88
The Gerber Ranch Mélange structurally underlies the Ingram Canyon Mélange [45
]. It is distinct from the latter in its lack of both serpentinite bodies and blocks of “high-grade” glaucophane schist and related rock types. Numerous blocks of lower grade glaucophanized metabasites occur in the Gerber Ranch Mélange, and these are accompanied by other OPS fragments of metachert and metasandstone, all metamorphosed under jadeite blueschist facies conditions [45
]. Other clasts and blocks include conglomerate plus rare marble and actinolite schist. The blocks tend to be elliptical, reflecting elongation parallel to the strike of the scaly mudrock fabric of the mélange.
The Hunters Point Mélange on the San Francisco Peninsula (HP, Figure 1
) is a complex body that is not well understood, in part because of the limited outcrops available within a city environment. The unit was mapped by Schlocker [103
] and Bonilla [104
] and designated by Schlocker as the Fort Point-Potrero Hill-Hunters Point Shear Zone, shortened to Hunters Point Shear Zone by Wakabayashi [3
]. Wakabayashi [54
] describes a layered/zoned map pattern with a large serpentinite mass between regional layers of shale-matrix mélange. In detail, however, the zones of sheared (scaly) matrix in the mélange vary spatially from serpentinite to serpentinite+ mudrock to mudrock [103
]. The matrix contains rounded to elongate blocks and clasts of various rock types, including sandstone, chert, metabasites (including gabbro), serpentinite, and metamorphic rocks (e.g., hornblende schist) [54
]. The largest block is approximately 1 km in length, but most are substantially smaller. The Mélange of Hunters Point exhibits two of the definitive features of tectonic mélanges and 11 features of such mélanges overall.