Geology of the Alarcon Rise, Southern Gulf of California

Abstract

Meter‐scale AUV bathymetric mapping and ROV sampling of the entire 47 km‐long Alarcon Rise between the Pescadero and Tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest, hottest, and highest effusion rate lava flows. This shallowest inflated part is located ∼1/3 of the way between the Tamayo and Pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the East Pacific Rise or on the Gorda and Juan de Fuca Ridges. Starting ∼5 km south of the Pescadero transform, 6.5 km of the Alarcon Rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77.3 wt % SiO2. These are the first known rhyolites from the submarine global mid‐ocean ridge system. Silicic lavas range from >11.7 ka, to as young as 1.1 ka. A basalt‐to‐basaltic andesite sequence and an andesite‐to‐dacite‐to‐rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite. Magmatism occurred along the bounding Tamayo and Pescadero transforms as extensive channelized flows. The flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills. The transforms are transtensional to accommodate magma migration from the adjacent Alarcon Rise.

Discovery of Hydrothermal Vent Fields on Alarcón Rise and in Southern Pescadero Basin, Gulf of California

“…However, B concentrations are lower than most basalt hosted systems other than TAG (Figure b) and Cs is significantly enriched (Figure d). These differences could be due to differences in the source rocks, although Alarcón Rise basalts are not particularly Cs‐rich (Clague et al, ), or potentially could relate to differences in water‐rock ratio, as easily leached elements such as the alkalis can be enriched at low water‐rock ratio (Palmer & Edmond, ; Seyfried et al, ; Von Damm, , ). The δ 18 O values of the vent fluids are elevated by about 2‰ relative to local bottom seawater values (Table ), consistent with low water‐rock ratio, with relatively unshifted δD, similar to most basalt‐hosted vent fields (Shanks, ).…”

“…Chemical and isotopic compositions of hydrothermal fluids, precipitates, and sediments were largely determined using established techniques (Text S2). Minimum ages of lava flows underlying the vent fields were determined by 14 C dating of foraminifera shells sieved from the bottoms of sediment cores taken on top of the flows (Clague et al, , , ).…”

“…In 2012 and 2015, clear shimmering fluids were observed venting through cracks in the lava flow at the base of this chimney ( Figure S2b). Chimney fields Ja Sít and Pericú are located 1.7 km and 800 m northeast of the northern end of Meyibó, respectively, and are near the eruptive fissure producing the same young channelized basaltic lava flow ( Figure 2; Clague et al, 2018). Ja Sít consists of two chimney clusters 250 m apart ( Figure 5), 150 to 400 m west of the fissure, and along channels of the lava flow.…”

“…However, B concentrations are lower than most basalt hosted systems other than TAG (Figure 10b) and Cs is significantly enriched (Figure 10d). These differences could be due to differences in the source rocks, although Alarcón Rise basalts are not particularly Cs-rich (Clague et al, 2018), or potentially could relate to differences in water-rock ratio, as easily leached elements such as the alkalis can be enriched at low water-rock ratio (Palmer & Edmond, 1989;Seyfried et al, 1998;. Table S3.…”

“…The process was commonly time-consuming, could miss chimneys outside the field of view, and did not locate inactive chimney fields.Recent advances in high-resolution mapping from autonomous underwater vehicles (AUVs) have streamlined the search process. The bathymetry produced by such vehicles can resolve hydrothermal chimneys and mounds that are only a few meters across and high, whether they are active or inactive (Connelly et al, 2012;Jamieson et al, 2014;Nakamura et al, 2013) and map uplifted sediment hills where the cooling of sills beneath might drive fluid circulation (Clague et al, 2018;Zierenberg et al, 2015). Water property data (e.g., conductivity, temperature, and particles) are collected concurrently with the bathymetric data.…”