Magmatic and tectonic extension at mid‐ocean ridges: 1. Controls on fault characteristics

Abstract

[1] We use 2-D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off-axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off-axis lithospheric thickening produces long-lived, large-offset faults, similar to oceanic core complexes. Such long-lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across-axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.

Mechanical and geological controls on the long-term evolution of normal faults

“…However, the additional influence of fault rotation will allow sustained slip on normal faults formed in thicker lithosphere than would be permitted if fault dip remained fixed at high angle. This is consistent with previous numerical simulations of fault development at mid-ocean ridges by Behn and Ito [2008], who found that a force balance model could only predict fault life span in an elasto-plastic lithosphere if it incorporated the reduction in fault dip observed in their numerical simulations. This suggests that both the finite yield strength of the lithosphere and the flexural rotation mechanism presented here are important components to the physics of normal fault growth.…”

“…Second, numerous studies, including Chapters 2 and 3 of this thesis have shown that normal fault evolution is controlled by the build-up of elasto-plastic stresses in the faulted layer. This build-up is generally attributed to the flexural readjustment of the footwall and hanging wall blocks in response to fault growth [Buck, 1988;Weissel and Karner, 1989;King et al, 1988], and has consequences for both fault rotation [Olive and Behn, 2014;Olive et al, 2014b] and fault life span, hereafter defined as the amount of horizontal offset that can be accommodated on a fault before it is abandoned in favor of a new fault [Forsyth, 1992;Buck, 1993;Lavier et al, 2000;Behn and Ito, 2008]. It is therefore unclear to what extent a visco-plastic description of the lithosphere can produce behaviors that are relevant for understanding geological systems.…”

“…Broadly speaking, fault life span determines whether the "building block" of a rift system is a half-graben structure (characteristic offset smaller than the faulted layer thickness, typically a few km), a long-lived, low-angle detachment fault (with offset in the 10-50 km range, resulting in surficial exposure of lower crustal units), or a combination of both [e.g., Morley, 1995;Lavier et al, 2000;Lavier and Buck, 2002;Whitney et al, 2013]. Over the past two decades a robust conceptual framework has been assembled to identify the first-order controls on the modes of extensional faulting [e.g., Forsyth, 1992;Buck, 1993;Lavier et al, 2000;Lavier and Buck, 2002;Behn and Ito, 2008;Olive and Behn, 2014]. These authors considered the mechanical cost of sustaining slip on a single normal fault, which requires an increase in tensional force as extension proceeds.…”

Crustal faults exposed in the Pito Deep Rift: Conduits for hydrothermal fluids on the southeast Pacific Rise

“…It is conceivable that the faults observed in the PDR passively rotated into their current orientation. Barring passive rotation, however, they were likely not part of the fault system inferred from seafloor scarps [Bohnenstiehl and Carbotte, 2001] and modeled as a response to flexural forces during seafloor spreading [Buck et al, 2005;Behn and Ito, 2008;Ito and Behn, 2008]. Instead, the faults were part of a set that is driven primarily by magmatic accretionary processes [Karson et al, 2002a;Carbotte et al, 2003] and, as suggested here, varying pressure conditions due to the influence of hydrothermal fluid flow on fault mechanics.…”

“…In contrast, recent numerical models have implicitly questioned the importance of subaxial faults in fast spreading centers. Mechanical models suggest that faulting is less important than dike injection along fast spreading centers [Buck et al, 2005;Behn and Ito, 2008;Ito and Behn, 2008]. Additionally, recent transport models suggest that hydrothermal systems can develop without the planar, anisotropic permeability, and mineral sealing associated with faults [Fontaine and Wilcock, 2007;Coumou et al, 2008].…”