Crustal Magmatic System Beneath the East Pacific Rise (8°20′ to 10°10′N): Implications for Tectonomagmatic Segmentation and Crustal Melt Transport at Fast‐Spreading Ridges

Abstract

Detailed images of the midcrustal magmatic system beneath the East Pacific Rise (8°20′–10°10′N) are obtained from 2‐D and 3‐D‐swath processing of along axis seismic data and are used to characterize properties of the axial crust, cross‐axis variations, and relationships with structural segmentation of the axial zone. Axial magma lens (AML) reflections are imaged beneath much of the ridge axis (mean depth 1,640 ± 185 m), as are deeper sub‐AML (SAML) reflections (brightest events ~100–800 m below AML). Local shallow regions in the AML underlie two regions of shallow seafloor depth from 9°40′–55′N and 8°26′–33′N. Enhanced magma replenishment at present beneath both sites is inferred and may be linked to nearby off‐axis volcanic chains. SAML reflections, which are observed primarily from 9°20′ to 10°05′N, indicate a finely segmented magma reservoir similar to the AML above, composed of subhorizontal, 2‐ to 7 km‐long AML segments, often with stepwise changes in reflector depth from one segment to the next. We infer that these melt bodies are related to short‐lived melt instability zones. In many locations including where seismic constraints are strongest the intermediate scale (~15–40 km) structural segmentation of the ridge axis identified in this region coincides with (1) changes in average thickness of layer 2A (by 10%–15%), (2) changes in average depth of AML (<100 m), and (3) with the spacing of punctuated low velocity zones mapped in the uppermost mantle. The ~6 km dominant length of multiple AML segments within each of the larger structural segments may reflect the spacing of local sites of ascending magma from discrete melt reservoirs pooled beneath the crust.

The 2017 Eruption of Erta 'Ale Volcano, Ethiopia: Insights Into the Shallow Axial Plumbing System of an Incipient Mid‐Ocean Ridge

“…We do not detect a deeper concentrated magma body below the shallow plumbing system, as is observed elsewhere on the EAVS (Pagli et al, ), on Hawaii (e.g., Poland et al, ), and at fast‐spreading MORs (e.g., Marjanović et al, ). Instead, we detect an off‐rift deflating vertically extensive source from ∼1 to 14 km depth, which likely represents a more complex system of smaller stacked sources.…”

“…The presence of a potential shallow reservoir overlying a region of axial melt storage at Erta 'Ale and Alu‐Dalafilla on the EAVS (Pagli et al, ) shows similarities to the stacked sill structure observed at spreading ridges with high melt flux, such as the fast‐spreading EPR (e.g., Carbotte et al, ; Marjanović et al, ; Wanless & Behn, ). An important distinction at Erta 'Ale is the largely dike‐fed plumbing system, defined by the local topographic and regional tectonic stress fields (Xu et al, ; Wadge et al, ), rather than a system of stacked sills observed at MORs, although dikes at MORs may simply be difficult to image with reflection seismology (e.g., Marjanović et al, ). Yet the existence of a reservoir at ∼1 km depth at Erta 'Ale and Alu‐Dalafilla indicates that shallow magma bodies may be supported on the EAVS.…”

“…Extensive studies on mid‐ocean ridge (MOR) systems show that at fast‐spreading ridges, a shallow sill structure is commonly found (Carbotte et al, ; Marjanović et al, ; Wanless & Behn, ). For example, at the East Pacific Rise (EPR) from 8°20 ′ to 10°10 ′ N, an axial magma lens is typically present at ∼1.6 km depth, with a deeper lens found between ∼1.7 and 2.4 km depth (Carbotte et al, ; Marjanović et al, ). While fast‐spreading MOR plumbing systems are well imaged, the movement of magma and dynamics within these plumbing systems are difficult to study (Sigmundsson, ).…”

Distribution of Crustal Melt Bodies at the Hot Spot‐Influenced Section of the Galápagos Spreading Centre From Seismic Reflection Images

“…Our understanding about crustal accretion has been changing during the last decade as improved seismic reflection images from different settings provided evidence for more complex crustal magmatic systems. Recent evidence include segmentation of the AML (Carbotte et al, ; Marjanović et al, ), crustal axial melt sills below the AML (Arnulf et al, ; Marjanović et al, ), lower crustal sills in the near axis region (Canales et al, ), and off‐axis magmatic systems (Aghaei et al, ; Canales et al, ; Han et al, ).…”

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

“…During the last two decades, high‐resolution studies have improved understanding of fine‐scale, third‐ and fourth‐order ridge segmentation. At fast‐spreading rate segments of the East Pacific Rise (EPR) between 27 and 32°S (Hey et al, ), between 8 and 12°N (e.g., Carbotte et al, ; Soule et al, ; White et al, ), and at 16°N (Le Saout et al, ), and along the intermediate‐spreading rate Galápagos Spreading Center (GSC; Sinton et al, ), studies relating magmatic and hydrothermal processes to deep structure support a strong correlation between morpho‐tectonic segmentation and magmatic lens segmentation (Carbotte et al, , ; Marjanović et al, ; White et al, ). Tomography studies have also shown that tectonic discontinuities are associated with a decrease of mantle melt extraction, and thus, third‐order segmentation of ridges has been attributed to mantle processes (e.g., Toomey et al, ; VanderBeek et al, ).…”