Tectonic significance of serpentinite mobility and ophiolitic melange

Serpentinite is known to be an important constituent of oceanic crust and is particularly abundant along zones of primary structural weakness. Such primary weaknesses occur along fracture zones where large ultramafic massifs are commonly exposed, and along ridge-parallel serpentinite belts that are emplaced near slow-spreading ridges. Tectonic disruption of newly formed crust along transform segments of fracture zones in conjunction with, and followed by, the solid state injection of serpentinitic rocks leads to the development of serpentinite-matrix melange as a primary constituent of oceanic crust. Slumping of rocks along transform axial deep walls and ponding of pelagic and, locally, turbiditic sediments produce an array of additional chaotic rocks, which also belong to the fracture zone regime. Inasmuch as the total length of fracture zone crust on the ocean floors far exceeds the length of subduction and collision zones, and whereas fracture zones are commonly on the order of 50 km wide, geologists working in melange belts of orogenic zones should be aware of fracture zone processes and products. Geological relations in a number of on-land ophiolitic melange and nappe belts, and deformation patterns in modern oceanic environments, where changes in rotational poles have occurred over the recent geologic past, suggest that ophiolite obduction and perhaps intra-oceanic subduction zones develop along primary structural weaknesses, particularly fracture zones. The initial imbricate structural sequences in such environments consist of ophiolitic melange, large ultramafic slices and ophiolite sheets along fracture zones, and ophiolite sheets with basal dynamothermal aureoles where thrusting initiates along or near spreading ridge segments. Such imbricate sequences constitute oceanic orogenic welts that may experience a considerable oceanic residence time prior to accretion within a collisional suture belt or along a subducting trench or that may slide by gravitational forces onto deeply submerged continental shelves or oceanic plateaus. Oceanic orogenic welts, along with large unmodified fracture zone massifs, should be considered similar to oceanic plateaus in regard to their susceptibility for tectonic accretion. Tectonic accretion of the welts often disrupts the initial imbricate sequence, leaving high temperature metamorphic blocks and slices of ophiolitic melange scattered along the base of ultramafic or ophiolitic nappes. In many instances, it is clear that dynamothermal metamorphism and ophiolitic melange mixing pre-date continental margin obduction and the related suturing event. Ophiolitic melange may constitute an important element in uplifted subduction complexes, but it is not clear that “normal” ridge-crest ocean crust is technically mixed into melange within inner trench wall environments. Pre-mixed fracture zone assemblages are highly susceptible to accretion in such environments, because of their pre-weakened state, bathymetric expression, and their gravitational instability with respect to “normal” oceanic crust. In addition to en masse accretion of fracture zone crust, ophioutic melange components in subduction complexes are probably also derived by the diapiric movement of oceanic serpentinites away from the downgoing slab. Such rising serpentinites may carry with them deep-seated metamorphic blocks and are susceptible to intermixing with higher-level components of the accretionary prism by solid state injection, surface breachment, and olistostromal recycling. The point stressed in this paper is that many serpentinitic assemblages of orogenic zones are the result of polystage developmental histories with heritages that commonly stem from primary serpentinitic assemblages of the ocean floor.