Magmatism along strike-slip faults is the process of rock melting, magma ascent and emplacement, associated with the tectonics and geometry of various strike-slip settings, most commonly occurring along transform boundaries at mid-ocean ridge spreading centres and at strike-slip systems parallel to oblique subduction zones. Strike-slip faults have a direct effect on magmatism. They can either induce magmatism, act as a conduit to magmatism and magmatic flow, or block magmatic flow. In contrast, magmatism can also directly impact on strike-slip faults by determining fault formation, propagation and slip. Both magma and strike-slip faults coexist and affect one another.
Strike-slip faults – overview
Strike-slip faults are commonly almost vertically inclined faults, where the main displacement and slip is in the horizontal direction, parallel to the strike of the fault. Depending on the movement of the fault blocks relative to the fault plane, strike-slip faults can be classified as either sinistral (left-lateral displacement) or dextral (right-lateral displacement). There are different kinds of strike-slip fault and settings in which they occur: continental and oceanic transform faults form in areas of plate divergence and sea floor spreading, strain-partitioned strike-slip faults can occur in both oceanic and continental crust at zones of oblique subduction.
Typically, magmatism is not commonly associated with strike-slip regimes. It is more frequently associated with extensional and compressional regimes, which give rise to normal and reverse/thrust faults, respectively. In most cases, strike-slip faults only play a role in the migration of magma along their planes of weakness, rather than being directly responsible for the origin of magmatism. However, there are certain scenarios in which strike-slip movement and its associated features can induce magmatism. Examples include the Dead Sea Transform in the Middle East and the San Andreas Fault in California.
Mechanisms of magmatism in areas of strike-slip tectonics
Magmatism is the process by which rock is heated deep in the earth's mantle or crust via different types of melting, forming magma, liquid or semi-liquid molten rock which buoyantly rises towards the earth's surface to either be intruded as an igneous body or extruded as lava. There are various mechanisms of rock melting that can be achieved in different stress regimes dependent on the tectonics of the region. Flux and decompression melting processes are typical of subduction zones or mid-ocean ridges, in compressional and extensional stress regimes. Strike-slip stress regimes can include components of extension and/or shortening, for example in transtensional or transpressional situations, which ultimately allow for the creation and accumulation of magma along strike-slip shear zones. Magma in several field studies is seen to exploit strike-slip faults of a continental scale, which begin during either collisional or extensional tectonics.
Transpression
Transpressional environments involve strike slip with a minor shortening/compressional component. Transpression is seen along bends in strike slip fault zones where the land is forced into each other and compressed. These bends, known as restraining bends, allow for contraction and transpression; they can cause topographic uplift, crustal shortening and exhumation of basement rocks. This geometry can result in localised volcanism along the strike-slip fault.
Subduction and Flux Melting – Where subduction of oceanic lithosphere is occurring, magma forms via flux melting. Flux melting starts with the subduction of the hydrated oceanic slab. Hydrated minerals within the subducting lithosphere increase in temperature. This causes the hydrated minerals to emit volatiles – water vapour and gases – which are driven upward and dissolved into the mantle which lowers the melting temperature of constituent minerals, allowing them to melt to form magma. Compression and rock melting - In a high strain compressional environment, additional strain energy can increase melting of rock by overstepping the activation energy required to melt constituent minerals. Due to the thickening of Earth's crust that is related to transpression, granitic melts can be generated.
Activation energy is the minimum energy that must be exceeded in order for the rock minerals to melt., Encyclopedia Britannica. (n.d.). Activation energy | chemistry. [online]. Activation energy breaks this barrier when there is significant kinetic energy, for example from the friction of rock collision. This kinetic energy can be converted into thermal energy resulting in frictional heating of the rock and melting of constituent minerals. This rock melting behaviour is usually associated with continental-continental collisions.
Transtension
Transtensional environments involve majority strike-slip with a minor extensional component. Slight bends in the strike-slip fault, called releasing bends, can accommodate transtension, and induce the formation of pull apart basins or transtensional basins along the fault plane. This geometry and the shearing motion of the two fault blocks causes extension of the crust.
Extension and Decompression Melting - Decompression melting occurs mainly in extensional regimes, where the crust thins, allowing the mantle to upwell to an area of lower pressure where the melting point of the minerals is lower. The formation of pull-apart basins along strike-slip faults causes decompression melting. Advection of melts and heat flux into the lower crust results in partial melting of crustal rock. The buoyant magma rises due to its lower density and is able to exploit weaknesses such as fault planes, using strike-slips as conduits for motion.
Heat induced melting
Magmatism can occur in instances unrelated to the regional tectonics and stresses. This occurs due to exposure of the rock to higher temperatures, for example at mantle plumes. Mantle plumes are areas of the lower mantle significantly hotter than the mantle around it, which upwell towards the surface due to density contrasts. At the surface, the extremely high temperatures cause a rapid increase in the geothermal gradient, and so the rocks near the plume cross the solidus and melt. This type of magmatism is only related to strike-slip faults if they are present at the hotspot; for example magmatism is associated with the transform faults in Iceland, and magma injection from the hot spot can also trigger strike-slip motion and formation.
Types of Strike-Slip Settings
Mid ocean ridge fault zones
A type of strike-slip fault called transform faults are abundant at mid-ocean ridge settings. These differ to normal strike slip faults as they occur only at plate boundaries, and are conservative boundaries: at these boundaries no crust is created or destroyed. Transform faults literally 'transform' to extensional rifting features, this way features are no terminated but interconnected globally. Oceanic lithospheric plates converge away from each other in an extensional setting, driven by Harry Hess's sea floor spreading model subduction slab-pull and ridge-push mechanism, however some areas of the ridge will spread at different rates due to differing factors, for example faster spreading rates are found to correspond with a lower Mg/Fe concentration. Transform faults begin to propagate perpendicular to the mid-ocean ridge, to accommodate the difference in spreading rates. Nucleation of the transforms always begins after spreading, and can occur at a range of spreading velocities from 12-20mm/yr to 145mm/yr. Displacement of the transform faults occurs centrally in a zone known as the principal transform displacement zone and slip along this displacement zone occurs via slower subseismic mechanisms rather than fast rupturing as seen in continental strike-slip faults. Across the mid ocean ridge, transform faults are areas of greater seismicity, however this does not correlate to magmatism.
Magmatism is abundant at the spreading centre, along the diverging plate boundary, due to decompression melting as the lithosphere thins. However, the formation of transform faults does not result in volcanic activity. Transform faults are thought to be a dam to magmatism flow, blocking the migration of partial melts away from mid oceanic ridge hot spots so they cannot reach adjacent spreading centres. This occurs because transform faults juxtapose the position of cooler, older plates with the warmer ridge, and so magma cannot reach a level of hydrostatic equilibrium as it cools quickly when exposed to the older plate. This mechanism effects the topography of the axial ridge, and generally at the intersection between the ridge and the transform, the topography is much deeper, becoming shallower towards the mid-way point between two transforms. Although magmatism induction and flow does not occur along transform faults, they are sites of hydrothermal systems (second-order phenomena of magmatism) which require active magma generation and high crustal permeability, both components that can be found at tectonically weakened, ridge-transform intersections.I.M Poroshina, S.G Krasnov. Tectonic control of hydrothermal activity in the Mid-Atlantic Ridge. Trans. (Doklady) Russ. Acad. Sci., 351 (1996), pp. 1245-1249 In this respect, at mid-ocean ridges, magmatism occurs as a result of sea-floor spreading, and influences the morphology and structure of the strike-slip transforms. Rather than strike-slip faulting having a causal relationship to magmatism, it is an effect of magmatism.
Oblique convergent margins
Oblique convergent margins are subduction zones where subduction occurs not completely orthogonal to the plate boundary, and instead the lithospheric slab subducts at an angle, commonly between 15°-30°. Strike-slip faults can occur as a result of strain partitioning, which is characteristic of oblique convergent margins, as the oblique plate motion can be split into stress components normal to the margin (thrust faulting) and parallel to the margin (strike-slip faults). Initially, a single strike-slip fault may form to accommodate strain partitioning, but as deformation continues, more strike-slip faults propagate and the strain is distributed among multiple strike-slip faults., Fossen H, Tikoff B, et al. (1994). Strain modelling of transpressional and transtensional deformation. "Norsk Geologisk Tidsskrift." 74:134-145. Oblique convergent margins are therefore transpressive tectonic environments.
There a two types of magmatic arcs formed at oblique convergent margins:
Intraoceanic Arcs – Subduction involves dense oceanic lithosphere subducting under less dense oceanic lithosphere. Characteristically forms a volcanic island arc that produces primitive magmas of a mafic composition, such as basalts, basaltic andesite and dacite. Continental Arcs – Dense oceanic crust subducts below less dense continental crust. Produces more evolved magmas of a felsic composition such as granitic melts.
Collisions and subduction between two continental plates – arc collision – produces collisional orogens and associated strike-slip fault systems, however these zones do not produce magmatic arcs as the crust and mountain ranges formed are too thick to accommodate magma flow. Crustal melting occurs due to the intense compressional pressure, and granitic melt intrusions can be found enriched in Sr–Nd–Hf isotope compositions, but do not reach the surface nor associate with the strike-slip faulting in this region.