Amanda B. Clarke | Geology Reports

USGS2026

The eruptive behavior of distributed volcanism forming low shield edifices—A case study of Sentinel-Arlington volcanic field, U.S.A.

Distributed volcanic fields are present in various tectonic settings worldwide, and their characteristics reflect differing influences from magmatic and tectonic processes. In the southwestern United States alone, there are 37 Quaternary distributed volcanic fields. After the primary period of extensional tectonics in the southern Basin and Range 15–5 million years ago, the Sentinel-Arlington volcanic field developed in southwestern Arizona between 4 and 1 million years ago. The Sentinel-Arlington volcanic field consists primarily of low relief shield volcanoes, a type of distributed volcanism with poorly understood temporal evolution. The Sentinel-Arlington volcanic field is less than 200 kilometers (km) from the Colorado Plateau, Gulf of California, and southern San Andreas Fault system. This work identifies and examines controls on the emplacement of the Sentinel-Arlington volcanic field by documenting shallow and surficial structures as well as eruption characteristics and style through time. The Sentinel-Arlington volcanic field consists of 21 volcanoes with a total of 33 vents over an area of about 770 square kilometers (km 2 ). The prominence of low relief shield volcanoes may be explained by ascent of basaltic magmas through thin Basin and Range crust, without much crustal contamination, and low viscosities common to mafic compositions. Typical eruption characteristics involve the construction of low relief shield volcanoes followed by Strombolian fissure eruptions at the summits or near-summit medial areas that produce scoria lapilli, which may weld to form agglomerate. The total lifetime erupted volume of about 4.3 cubic kilometers (km 3 ) represents an average eruptive flux of approximately 2x10 -3 km 3 per thousand years (k.y.). This erupted volume is low relative to Neogene basaltic intraplate distributed volcanic fields worldwide, which typically range from 10 -3 to 1 km 3 k.y. -1 . Sentinel-Arlington volcanic field eruptions were likely triggered by intermittent rejuvenation of transient magmatic zones that exist in thinned crust. Instantaneous flux from point sources feeding the lava flows is estimated to be on the order of 10 -1 to 10 cubic meters per second.

Arizona↗

USGS2026

Self-similar vent clustering in the Pinacate volcanic field (Sonora, Mexico and Arizona, United States)

The spatial distribution of vents in a distributed volcanic field may provide clues about the geometry of the volcanic field’s plumbing system. Distributed volcanism is characterized by magma feeders that are independent of one another, scattered over wide areas, and active for relatively short times (typically less than one hundred years). On the other hand, focused volcanism results in large volcanic edifices, forming central or shield volcanoes that are active for tens of thousands of years or longer. Whether distributed or focused, the form of volcanism may depend on several factors: magma fluxes at depth; magma composition, temperature, and viscosity; and crustal stress and strain states. Over time, some volcanic fields may shift from focused to distributed volcanism. Located in northwestern Sonora, Mexico, and southwestern Arizona, United States, the Pinacate volcanic field is made up of lava flows, 8 maars, and more than 400 cinder cones. It has been active for approximately 2 million years. The oldest features in the field are large-volume lava flows that constructed Sierra El Pinacate (known locally and referred to herein as the Santa Clara shield volcano) and formed the base of the field. Later activity took the form of monogenetic cones and maars, produced by high-alkali basalts with an ocean-island basalt chemical signature. The plumbing system of the Pinacate volcanic field, along with its relations with the structural setting of the area, are analyzed here in terms of the spatial distribution of vents, vent morphology, and main structural trends of the morphologic lineaments in the area. The spatial analysis of vents identified three clusters. The largest cluster (cluster 1) is in the north-northwest portion of the volcanic field, whereas the smallest cluster (cluster 2), formed by younger cones, is on the east side of the volcanic field. The southern cluster (cluster 3) is mainly located over the Santa Clara shield volcano. Self-similar clustering analysis was performed on the whole volcanic field, as well as on the two larger clusters (clusters 1 and 3); cluster 2 was not analyzed individually due to the small number of vents. The self-similar clustering analysis indicated that the dataset containing all the vents in the volcanic field and the dataset containing only the vents in cluster 1 (the largest cluster) have magma storage at approximately 17 kilometers (km) depth. The vents of cluster 3 indicate storage at approximately 9 km depth. The fractal exponent characterizing the self-similar clustering of cluster 3 (1.5905) is lower than the fractal exponent of cluster 1 (1.7252), implying that cluster 1 formed after cluster 3. The available absolute ages of volcanic products in the Pinacate volcanic field indicate a middle to early Pleistocene age for the vents in cluster 3, and younger ages (late Pleistocene to Holocene) for cluster 2. The cone morphology is equivocal for defining relative ages of the clusters; nevertheless, the spatial distribution of the best-preserved cones implies that northwest-southeast, north-south, and northeast-southwest trends are linked to the development of the volcanic field. Lineament mapping, as well as the shape, elongation, and orientation of inferred feeder dikes, indicates that the Pinacate volcanic field formed along a northwest-southeast, regional-scale dextral transtensional shear zone and southwestward shallowing of the Mohorovičić discontinuity to 18–20 km.

Arizona, Sonora↗