Numerical modelling of non-transform discontinuity geometry: Implications for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends

Abstract

Ocean ridge discontinuities partition and offset spreading centres at a range of scales. Large scale discontinuities (10's-100's km) are synonymous with first-order transform faults, which have well defined linear fault zone valleys. In contrast, NonTransform Discontinuities (NTDs) are diffuse, smaller scale offsets (0 to b 20 km), characterised by central basins or topographic highs. The geometry of NTD offsets can be categorised by the sense of offset, either right-stepping or left-stepping, and by the relative positions of the segment tips. The segment tip configurations include under-lapping, over-lapping or simple across-axis jumps or stepping in the ridge axis. In this study finite difference software is used to model segment geometry at a slow-spreading ridge under a normal tensile-stress within a homogeneous and isotropic medium. Along-and across-axis segment separations were varied incrementally for left-and right-stepping senses. The results show that the ratio of along-axis to across-axis segment tip separation is a dominant control of stress field rotation within an NTD. Features which most clearly show rotation within an NTD include basins and tectonically controlled constructional ridges. The obliquity of these features along with measurements of the surrounding fault fabrics are used as a way of observing and determining stress rotations within NTDs along the Central Indian Ridge (CIR). These rotations were used to obtain segment geometries from models where the central tensor showed an equivalent rotation. The results show that geometry has a profound effect on stress field rotation under which large-and small-scale volcanotectonic fabrics form. In addition, a shortfall of the predicted model tip relative to interpreted positions, along with morphology and observation of the ridge fabrics at the terminations to some segments, suggests the existence of a zone, broadly analogous to the process zone observed in fracture mechanics, which we call a damage zone. Given the criteria for the promotion of hydrothermal circulation, this damage zone would have a greater potential for hosting hydrothermal activity.

Initiation of a Proto‐transform Fault Prior to Seafloor Spreading

“…(b) Bookshelf faulting accommodates transfer of extension between overlapping oceanic spreading segments (Green et al, 2014). (c) Extension is transferred between overlapping spreading centers through a region of rotational deformation (Macdonald & Fox, 1983;Tyler et al, 2007). (d) Centimeter-scale faulting observed in bedrock that accommodates extensional transfer between microcracks (Willemse et al, 1997).…”

“…Another style of nontransform ocean ridge discontinuity is found where magmatic segments overlap by tens of kilometers. This results in a focused region of rotational deformation, which results in a volcanically active, elevated terrain (Macdonald & Fox, ; Tyler et al, ; Figure c). This central domain is the focus for shear and rotational deformation and is thought to be unstable with time.…”

Magmatic and tectonic segmentation of the intermediate-spreading Costa Rica Rift—a fine balance between magma supply rate, faulting and hydrothermal circulation

“…However, it lies at the lower end of the size range for similar features observed along the slow-spreading MAR (Spencer et al, 1997) and Central Indian Ridge (Tyler et al, 2007), the intermediate-spreading…”

Hydrothermal Activity at the Ultraslow‐Spreading Mohns Ridge: New Insights From Near‐Seafloor Magnetics

“…The deviations of volcanic lineaments and faults in the studied segment (31°NE) from the expected axial trend (39°NE) manifest the obliquity of the rifting, which is common in slow‐spreading nontransform offsets and is explained by the oblique shear stress. Curved and sigmoidal faults also suggest the rotation of stresses between the offset spreading segments (Tyler et al, ). The stresses surrounding discontinuities and the rotation in the volcanic crust can create more complex cross‐cutting fault populations that will grow in both horizontal and vertical direction forming soft‐link relay structures, or evolving into hard‐link relay structures at the later stages, promoting hydrothermal circulation.…”