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SUTURE (GEOLOGY)
In structural geology, a suture is the link along a major fault zone that joins together separate terranes and tectonic units that have different histories in terms of plate tectonics, metamorphic and paleogeography. The suture is often represented on the surface by an orogen or mountain range. [1] The term was borrowed from surgery where it describes the sewing together of two pieces of tissue, however the sutures of the skull, where separate plates of bone have fused, may be a better metaphor.

Overview
In plate tectonics, sutures are seen as the remains of subduction zones, and the terranes that are joined together are interpreted as fragments of different paleocontinent or tectonic plates. Outcrops of sutures can vary in width from a few hundred meters to a couple of kilometers. They can be networks of mylonitic shear zones or brittle fault zones but are usually both.

Type of Sutures
In the broad field of geology, the word suture can be used in two vastly different contexts. The first one, which is the primary focus of this page are sutures in structural geology. The other use of the word suture is sourced from the field of paleontology.

Structural Geology Use
Sutures mark the zone where oceanic crust has been completely subducted (Figure 1), resulting in the piecing together of two blocks of continental crust. [3] This process of continental collision happens over geological time and occurs through the formation of an array of sutures at different scales. This creates multiple intracontinental high strain zones, such as orogenic belts. [3]

Terminal suturing refers to the last stage of the complete subduction of the oceanic lithosphere, reflecting a long history of suturing at various location with changing geometries and polarities. [3] The polarity of a subduction zone refers to the identification of the subducting plate and the overriding plate.

Paleontological Use
It is to be noted that the word suture is also used in paleontology in which it bears a completely different meaning. In paleontology, sutures can be referred when describing a fossil exoskeleton. For example, the trilobite exoskeleton has a suture line between the free cheek and the fixed cheek which allowed the it to perform ecdysis.

Formation of Sutures
Sutures are vastly variable in their formation depending on the material involved as well as temperature, proximity, geometry and then the polarity of the subduction. [3] Collisional events can occur in the form of an attempt at the subduction of a sea floor irregularity such as a sea mount (Fig 2G), or a microcontinent (Fig 2H). [3]

One factor that can affect subduction is the impingement of ridges (Fig 2 I, J, K ), which can have an important effect on the leading edge of the subduction zone, such as the emergence or submergence of the front part of the arc, or it could lead to a temporary cessation of arc magmatism. [3]

Magmatic arcs can either be separated from the continent by a rear-arc basin or lie at the continental margin. Some of these rear arc basins that separate continental lithosphere can be due to spreading (Fig 2B). [3] A continual spreading of the arc could lead to the formation of a train of remnant arcs (Fig 2C). [3] This is of interest, because rear arc basins can close shortly after their formation, leading to the formation of sutures through various mechanisms. [3]

In the case where young oceanic lithosphere breaks near the arc, a thin hot ophiolite may be obducted across the continental margin (Fig 2D). However, if the rear arc basin is completely subducted, the suture will form little to none ophiolites (Fig 2E, F). It is important not to confuse Fig 2E where there is obduction of the rear arc basin to Fig 2A where no obduction occurs.

Identifying a Suture
Sutures are usually associated with igneous intrusions and tectonic lenses with varying kinds of lithologies from plutonic rocks to ophiolitic fragments. Many of the following features can be observed in mountain ranges and deformation belts to infer a suture.

Ophiolite Belts
Ophiolites are pieces of oceanic crust that have been uplifted and emplaced on a continental plate. They can be used to infer ancient spreading processes, as well as collisional and subduction processes. [4] This helps to identify sutures as ancient suture zones are subjected to both collisional and spreading events.

Paleomagnetic Information
Paleomagnetic information relates to the reversal of the poles which has occurred 183 times over the last 83 million years. This Geomagnetic reversal is recorded in the rocks when they form, as the orientation of the earth’s magnetic field influences the orientation of growth of some minerals. When these directions are found in a given mountain belt to be different on rocks that are of the same age, it can be inferred that they formed at different location and were then brought together by the movement of plate tectonics. [4]. A problem with this technique of suture identification is that rocks in mountain belts are often deformed which makes them hard to date and their paleomagnetic data hard to interpret. [4]

Radiometric Age
In some particular cases, boundaries between igneous and metamorphic rocks of different ages can be indicative of suture belts. However, this could very well be unrelated and must then be used with caution. [4]

Geophysical Methods
Gravity and magnetic sensing of an area can show gravity or magnetic anomalies which can then indicate the presence of material of different density such as ophiolites or the remains of a subducting slab. [4] These methods can be coupled with seismic sensing which can be used to further identify old pieces of dipping slabs. These methods help to provide a three-dimensional view of the suture zone. [4]

Stratigraphy
The presence of two “blocks” of distinctly different stratigraphy could be indicative of a suture. In that case, the suture would be the separation zone between the microcontinent and the other continental crust. [4]

Structural Discontinuities
Structural discordance separated by a discontinuity can be linked to suture zone. Structures that are studied for that purpose can vary, but one that is prevalent for identifying sutures at substantial depth is strike-discordant trend. [4] In favorable conditions, the strain, the thermal conditions during movement and rate of movement can be determined from the fabric of the rock. [4]

Metamorphic Belts and Island Arc Sequence
The presence of paired metamorphic belts of the same age such as blue schist and green schist belts can indicate a consuming plate margin of which the polarity can be determined. [4] However, due the possible inversion of the polarity of subduction and the superposition of metamorphic rock assemblage that sometimes occur, this can be complicated to determine. [4]

Overthrust Belts
Overthrust belts involving continental marginal sequence can be used to infer suture zones. [4] However, not all of these are found to be related to a collisional event. What is key in these belts is the environment of the continental margin they contained which can then be compared to similar sequences in modern settings to infer whether the sequence is representative of a passive, active or transform margin. [4]

Benthonic Shelf
Shelf benthonic province boundaries are a potential tool for identifying plate boundaries of the past. [4] This comes from the fact that present plate tectonic configuration is reflected by the distribution of such benthonic shelf as they surround the edges of continental plates. [4]

Example of Sutures
Since the formation of continents on earth, there have been multiple configuration of plate tectonics. The scars of the most recent formations are still visible today and can be found around the world. The following examples demonstrate how certain feature can lead to the identification of sutures, as well as determine their geometry.

Iapetus Suture
An example from Great Britain is the Iapetus Suture which had to be determined by geophysical means since it is now concealed beneath younger rocks. [2] The suture runs along a line roughly parallel with the Anglo-Scottish border. It represents the join between the former continent of Laurentia to the north and the former micro-continent of Avalonia to the south. Avalonia is a plane which dips steeply northwestward through the crust, underthrusting Laurentia. This example demonstrates how sutures can be identified and modeled by means of geophysical analysis.

Eastern Indian Suture
The position of the eastern Indian suture was determined through the use of gravity analysis which detected a zone of high gravity. This geophysical knowledge was combined with the identified ophiolite belts to infer a steeply dipping mafic-ultramafic rock and continental metamorphic rock of two closely juxtaposed sutures. [6] This is a perfect example of how different means of analysis can be combined to identify a suture zone.

South Anyui Suture
This suture dates from the Jurassic, which corresponds to the time when the basin between Siberia and North America closed. [5] Studying this suture enabled the construction of a two-pole parallelogram model for the reconstruction of the Amerasia Basin opening. [5] This example shows the complexity behind studying groups of sutures. It also demonstrate the level of information that can be extracted from studying suture zones.