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Fluid release (H2O) modelling in the lithosphere

This series of models features a 70km-thick lithosphere profile with varying hydration setups within the weak zone (post-collisional suture) and surrounding lithospheric blocks to evaluate fluid release behaviour under horizontal bulk shortening. 

F_1_1, F_1_2 and F_1_3 

Model F_1_1 shows a homogeneous 70km-thick lithosphere subject to horizontal bulk shortening.  During bulk shortening, preferential zones of vertical lithospheric thickening develop, with fluid preferentially released from each of these two from ~29.4Ma onwards.  By contrast, fluid is not released from the surrounding lithospheric areas that have not been thickened.  Two basins develop after ~16Ma, which are in-filled with sedimentation.  As the sediments are hydrated, they also release water.  Model F_1_2 is the same lithosphere setup as F_1_1, here with a 100km-wide central hydrated weak suture zone confined to the upper and lower crust.  Subjected to the same shortening conditions as F_1_1, this model produces more rapid vertical lithospheric thickening.  Shortly after model initiation (~2.5Ma) a basin is developed which in-fills with sedimentation.  Fluid release is localised within the suture zone, occurs far more rapid than that in F_1_1 (commencing after ~13Ma) and is substantially more voluminous.  Model F_1_3 features a similar setup to F_1_2, with the suture zone extending through the mantle lithosphere.  Fluid release occurs in the suture zone far more rapidly than in the other two models, after ~5.5Ma.  Patterns of fluid release may be observed in the lower window.  Initially, fluid release initiates in the suture zone’s lower crust, but also propagates into the lower crust of the standard-composition terrane immediately to the right of the suture.  Continued horizontal bulk shortening leads to extensive fluid release in the terrane immediately to the left of the suture. 

F_1_3a 

Here, the central weak suture zone is composed of hydrated material extending through the crust and the lithosphere, while the surrounding terranes are composed of a dry mantle lithosphere rheology but hydrated crust material.  The upper window shows the evolution of the lithologies, with propagation of the H2O through the crust and the mantle lithosphere under horizontal bulk shortening; the lower window shows the positions and volumes of fluid released in the model above.  During bulk shortening, the primary H2O pipe initiates in the mantle lithosphere of the suture zone, followed by significant fluid release from the lower crust of the both the suture zone and adjacent terranes after ~3Ma.  The position of voluminous H2O generation from the lower crust on the left flank of the suture after ~3.5Ma corresponds to the primary fault zone developed through the crust and the lithosphere.  This is accompanied by pronounced lithosphere and isotherm asymmetry, creating a ‘step’ in the lithosphere.  A wide basin develops to the left of the model after ~3Ma, which in-fills with sediment.  Substantial fluid release from the suture zone mantle lithosphere continues throughout.  Fluid release on the right flank of the suture (both within and adjacent to it) progressively intensifies as the model evolves.  After ~15Ma, fluid release from the lower crust associated with the fault to the left ceases, and that from the lower crust on the right flank becomes dominant.  Red in the lower windows indicates H2O formed in the lower crust of the suture zone; green marks H2O formed in the lower crust of the adjacent terrain; black marks fluid released from the mantle lithosphere.

F_1_4

This model shows fluid release in a weak suture zone confined to a section of the mantle lithosphere.  The material above the mantle lithosphere is uniform throughout.  Fluid release in this model occurs rapidly after ~2Ma onwards, and is localised within sections of the lower crust immediately adjacent to and above the mantle lithosphere weak zone.  A sediment in-filled basin develops on the right flank of the suture zone after ~3Ma.  A significant proportion of release is associated with the fault developed on the right flank of the suture.