Evolution of Fine‐Scale Segmentation at Intermediate‐Spreading Rate Ridges

Abstract

Mid‐ocean ridge axes are marked by segmentation of the axes and underlying magmatic systems. Fine‐scale segmentation has mainly been studied along fast‐spreading ridges. Here we offer insight into the third‐ and fourth‐order segmentation of intermediate‐spreading ridges and their temporal evolution. The Alarcón Rise and the Endeavour Segment have similar spreading rates (49 and 52.5 mm/year, respectively) but contrasting morphologies that vary from an axial high with a relatively narrow axial summit trough to an axial valley. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, lava geochemistry, and ages from sediment cores is combined with available seismic reflection profiles to analyze variations in (1) geometry and orientation of the axial summit trough or valley, (2) seafloor depth near the axis, and (3) distribution of hydrothermal vents, (4) lava chemistry, and (5) flow ages between contiguous axes. Along both intermediate‐spreading segments, third‐ and fourth‐order discontinuities and associated segments are similar in dimension to what has been observed along fast‐spreading ridges. The Alarcón Rise and the Endeavour Segment also allow the study of the evolution of fine‐scale segmentation over periods of 300 to 4,000 years. Comparison between old and young axes reveals that the evolution of fine‐scale segmentation depends on the intensity of the magmatic activity. High magmatic periods are associated with rapid evolution of third‐order segments, while low magmatic activity periods, dominated by tectonic deformation and/or hydrothermal activity, are associated with little to no change in segmentation.

Geology of the Alarcon Rise, Southern Gulf of California

“…The shield in AR5 is dated by three cores (Figure ) with statistically overlapping ages suggesting a ∼320 ± 30 years minimum age. This age is more recent than estimated from the spreading rate and extension indicated by the faults and fissures that cut the shield (Le Saout et al, ). Northward, 16 mostly hummocky flows were dated and range from 89 years to 1.3 ka, with two flows yielding modern ages.…”

“…Inactive chimneys are more numerous than active ones along AR and most of the 53 confirmed inactive chimneys are located on AR4 and AR5 clustered in eastern and western groups (Figures B and A) that align with slightly offset fissures (Le Saout et al, ). They occur along about 2.6 km of AR but starting slightly more than 10 km NNE of the northernmost active chimneys.…”

“…The following sections are based on high‐resolution bathymetry, glass compositions, sediment cover detected by the AUV chirp subbottom sonar, sediment cores, and minimum radiocarbon ages of underlying flows. The fine‐scale segmentation of the ridge is described in Le Saout et al () and the faulting and structure of the ridge by Spelz et al ().…”

“…Lava flows are hummocky flows of pillow lava that range from having no fissures and faults to being highly disrupted. A dense network of fissures located near the SE side of the axis, especially in the northern part of the region, is inferred to mark the present spreading axis (Le Saout et al, ).…”

Hydrothermal Chimney Distribution on the Endeavour Segment, Juan de Fuca Ridge

“…The 1-m resolution mapping data, which resolve individual chimney structures, were used to determine the volume and tonnage of the extensive hydrothermal deposits (Jamieson et al, 2014). The general distribution of chimneys and chimney fields was recently used to evaluate models of ridge segmentation (Le Saout et al, 2019;VanderBeek et al, 2016).…”