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

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

“…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) [19,23,50]. 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 [46,47]. 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].…”

“…It lies structurally between two accretionary units (AUs) that contain broken formational OPS masses. The overlying unit, AU5 of Raymond et al [46,47], 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).…”

“…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 [46,47] 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 [46,47]). Here, I adopt the interpretation of Raymond et al [46,47].…”

“…Both Meneghini and Moore [23] and Raymond et al [46,47] 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 [46,47]). Here, I adopt the interpretation of Raymond et al [46,47]. The zone of mélange east of the lagoon is thicker and has a wider map dimension than exposures at Rodeo Cove (Figure 5).…”

“…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][46][47]. The Ingram Canyon unit may be correlative with the Garzas Mélange described to the south ( [11,[47][48][49]85,86], and see below), but the latter has been studied in less detail than the Ingram Canyon Mélange.…”