Rifting is a fundamental geodynamic process associated with thinning of the Earth’s lithosphere. These zones give rise to high heat flow, enhanced geothermal gradients and decompression melting, promoting melt generation. In its extreme form, protracted periods of rifting lead to continental (and super-continental) breakup via the development of oceanic basins, passive margins, oceanic crust production and seafloor spreading. Alternatively, bulk extension may cease in advance of seafloor spreading, resulting in a ‘failed rift’. In many geodynamic settings (such as the Zambian copperbelt of West Africa) phases of extension and rifting follow prior episodes of continental collision and amalgamation.
This series explores the influence of post-collisional suture zones upon intra-continental geodynamics during subsequent episodes of horizontal bulk extension. This series of models use a bulk extension rate of 1cm/a, however a few (specified) use a lower rate of 0.5cm/a. A diverse range of post-collisional suture architectures were constructed for testing, in order to evaluate their evolution and which are most representative of what is observed in nature. All models presented in this section feature a standard 150km-thick lithosphere (20km-thick upper crust, 20km-thick lower crust and 100km-thick mantle lithosphere) with a 10km air column overhead. The series also explores the influence of different crustal geotherms upon the evolution of key models using Moho temperatures of 550°C, 600°C, 650°C, 700°C and 750°C. Therefore, the base of the lower crust is partially molten at the start of a number of models, having already exceeded the solidus temperature. All models use an initial temperature of 1400°C for the base of the lithosphere. The mantle lithosphere portions of the suture zone in most models are shear zone material, however some models use an alternate hydrous mantle lithosphere material. The two are assigned different colour codes for easy distinction.
Suture zone architectures fall into two initial categories:
- Vertical suture zones
- Inclined suture zones
The vertical suture zone series (1) is further sub-divided into:
- Solid (homogeneous) vertical suture zones that extend through the entire lithospheric column (weak upper crust, weak lower crust and weak mantle lithosphere)
- Homogeneous vertical suture zones with weakness confined to the mantle lithosphere
- Homogeneous vertical suture zones with weakness confined to the upper and/or lower crust
- Discontinuous (heterogeneous) vertical suture zones confined to the mantle lithosphere
Examples of the initial inclined suture geometries are shown below in Figure 2. Models with the suture zone extending throughout the entire suture zone column represent significant mantle lithosphere metasomatism of both the crust (upper and lower) and the mantle lithosphere during terrane convergence and amalgamation. Those with weak zones confined to the mantle lithosphere (i.e. the upper and lower crust above have the same properties as elsewhere throughout the terranes) consider that metasomatism of the crust was minimal during suture zone development and terrane amalgamation. Suture zones with weaknesses confined to the upper and lower crust consider that metasomatism of the mantle lithosphere developed during prior collision has since been subject to devolitization, with only that in the crust remaining.
The inclined suture zone series (2) is composed of continuous (homogeneous) weak zones, either:
- confined to the mantle lithosphere
- confined to the upper and lower crust, or
- extending through the entire lithospheric column
Examples of the initial inclined suture geometries are shown below in Figure 3. 10km-wide sutures were constructed at angles of 75°, 60°, 45° and 25° to the horizontal. A horizontal bulk extension rate of 1cm/a was applied to all inclined suture models (unless specified). Evolution of inclined suture scenarios
This series presents the results of inclined suture zone scenarios including those extending throughout the entire lithospheric column (upper crust, lower crust and mantle lithosphere) inclined suture zones confined to within the mantle lithosphere. Bulk extension rates of 1cm/a were applied to each model, with active bulk extension terminated immediately prior to the onset of asthenospheric decompression melting to allow the model to continue under quiescence. All models use a Moho temperature of 650°C.
Under these conditions, scenarios with suture zones extending through the entire lithosphere, and also those confined to the mantle lithosphere, show similar structural and magmatic evolutions. In each case, lithospheric thinning is preferentially laterally partitioned into the section of the model marked by the interface between the inclined suture and the base of the lower crust (i.e. the Moho). Lithospheric thinning is accommodated in both the crust and mantle lithosphere, associated with localised upwards inflection of the isotherm and enhanced geothermal gradients. Upwards inflection of the isotherms is asymmetric, elevating local Moho temperatures at the suture zone-lower crust interface zone to >700°C – leading to localised lower crustal melting in the hanging wall immediately above the suture zone. In turn, the local interaction of lower crustal melting with the shear zone results in local mafic melt production – which is emplaced in the mid and upper crust. Importantly, these initial mafic melts are generated in advance of asthenospheric decompression melting. Therefore, post-collisional suture zones may promote the generation of mafic melts under horizontal bulk extension even in scenarios where thinning is not sufficient to achieve decompression melting of the asthenosphere.
Continued lithospheric thinning leads to asthenospheric decompression melting, associated with an emplacement of coarsely crystalline re-molten mafics in the lower to mid crust and dolerite/basalt in the upper crust. Active extension is ceased immediately prior to the onset of decompression melting. These subsequent mafics are more voluminous than those produced prior to decompression melting, and are associated with in-situ melting of upper crustal sediments. The zone of decompression progressively propagates laterally, forming a ~250km-wide ‘mushroom’. The system evolves into a ~200km-wide zone of widespread re-molten mafics and basalt, with melt emplacement propagating preferentially along the upper and lower crust boundary. New zones of lower crustal melt are produced along the base of the gabbroic plutons. Geothermal gradients within the central magma province in the crust are enhanced to >700°C at shallow depths of ~10km.
Similar structural and magmatic evolution relative timing chronologies occur when suture zone dip is varied (75°, 60°, 45° and 25°) in both the mantle-lithosphere confined suture zone and entire lithospheric column suture zone scenarios. However, the absolute timeframe of key processes varies with shear zone orientation. Steeper shear zone orientations are associated with more rapid lithospheric thinning and more rapid onsets of decompression melting.
Once the localization of lithospheric thinning is established at the shear zone-lower crust interface, the section of the shear zone in the crust above exerts no further influence upon the subsequent evolution of the region. Where shear zones are moderate to steeply dipping (those ≥ 45°) the flank of the rift with the lower portion of the shear zone delaminates as the zone of decompression melting expands. This is subsequently followed by delamination of the other flank. Where the shear zone is shallowly dipping (25°) delamination of mantle lithosphere on both flanks is simultaneous. The volume of lower crustal melt produced at the shear zone-lower crust interface is slightly greater where the shear zone is shallower.
Inclined suture zones with a given dip value that are confined to the mantle lithosphere produce the same evolution as their counterparts that extend up through the entire lithosphere. This is true of both the absolute timeframe of lithospheric thinning, and both the relative and absolute timing of the magmatic and thermal evolution that follows the cessation of bulk extension.
Models ss168a_a, ss170a_a and ss173a_a illustrate the evolution of a system with a 60°-dipping suture zone where subsequent horizontal bulk extension is more transient and is sufficient to induce a degree of lithospheric thinning, but not asthenospheric decompression melting. These models use the same 650°C Moho temperature as other inclined suture zone models. These models produce the same pattern of lower crustal melting in the hanging wall immediately above the suture zone, followed by the generation of mafic magma – which is emplaced in the mid and upper crust. Delamination of the rift flank containing the lower section of the shear zone occurs during relaxation following the cessation of bulk extension. Importantly, ss170a_a features a significantly narrower (1km-wide) suture zone and illustrates that narrow inclined suture zones:
1) also localise strain at the suture zone-lower crust interface, and
2) produce the same volumes of lower crustal melt as broader 10km suture zones
However, lithospheric thinning and the onset of lower crustal melting occur more rapidly in the system featuring the 10km-wide shear zone than with the equivalent 1km-wide zone (ss168a_a).
Where the 60°-dipping suture zone is wider (e.g. 40km - ss180a_a) deformation localised at the centre of the suture zone-lower crust interface is symmetric, associated with symmetric upwards inflection of the isotherms in the deformation zone. The lateral step in the isotherms is not as pronounced as that within models featuring narrow shear zones, and therefore lower crust melt generation is negligible because Moho temperatures do not exceed the solidus.
Model ss169a_a presents a similar suture zone scenario; here with the mantle lithosphere shear zone material replaced by hydrous mantle lithosphere. Under the same horizontal bulk extension rate of 1cm/a, the model produces a number of similar features including localised lower crustal melting at the suture zone-lower crust interface, followed by the generation and emplacement of mafic magmas into the mid and upper crust. However, this model produces a significant volume of early upper crustal melts, which are absent from the shear zone material models. Under the same imposed extension rate, lithospheric thinning is more rapid in models with hydrated mantle lithosphere that those with shear zone mantle lithosphere. This is caused by the partially molten mantle lithosphere. Further, delamination of the mantle lithosphere on the rift flank occurs much more rapidly earlier in this model, due to the partially molten mantle lithosphere.
Inclined sutures – varying crustal geothermal gradient
Models ss166a_a, ss162a_a, ss163a_a and ss164a_a present 60°-dipping, lithosphere-confined, suture zones that are 1, 5, 10 and 20km-wide, respectively, with an increased Moho temperature of 700°C. Here, a thin layer at the base of the crust is already molten at the start of the model. Lithospheric thinning during the initial onset of horizontal bulk extension is associated with a reduction in Moho temperature – alleviating the zone of lower crustal melting. From there, each scenario has the same structural and magmatic evolution as per its 650°C Moho counterpart. Volumes of lower crustal melting are slightly greater in models with the 700°C Moho (compare ss168a_a to ss163a_a). The rates of lithospheric thinning between models with a 650°C and 700°C Moho are essentially the same.