Magma Reservoir Below Laguna del Maule Volcanic Field, Chile, Imaged With Surface‐Wave Tomography

Abstract

The Laguna del Maule (LdM) volcanic field comprises the greatest concentration of postglacial rhyolite in the Andes and includes the products of ~40 km3 of explosive and effusive eruptions. Recent observations at LdM by interferometric synthetic aperture radar and global navigation satellite system geodesy have revealed inflation at rates exceeding 20 cm/year since 2007, capturing an ongoing period of growth of a potentially large upper crustal magma reservoir. Moreover, magnetotelluric and gravity studies indicate the presence of fluids and/or partial melt in the upper crust near the center of inflation. Petrologic observations imply repeated, rapid extraction of rhyolitic melt from crystal mush stored at depths of 4–6 km during at least the past 26 ka. We utilize multiple types of surface‐wave observations to constrain the location and geometry of low‐velocity domains beneath LdM. We present a three‐dimensional shear‐wave velocity model that delineates a ~450‐km3 shallow magma reservoir ~2 to 8 km below surface with an average melt fraction of ~5%. Interpretation of the seismic tomography in light of existing gravity, magnetotelluric, and geodetic observations supports this model and reveals variations in melt content and a deeper magma system feeding the shallow reservoir in greater detail than any of the geophysical methods alone. Geophysical imaging of the LdM magma system today is consistent with the petrologic inferences of the reservoir structure and growth during the past 20–60 kyr. Taken together with the ongoing unrest, a future rhyolite eruption of at least the scale of those common during the Holocene is a reasonable possibility.

Repeated Rhyolite Eruption From Heterogeneous Hot Zones Embedded Within a Cool, Shallow Magma Reservoir

“…Surface wave tomography has revealed a 450‐km 3 domain of attenuated seismic velocities below LdM that is interpreted as a highly crystalline magma body (5% interstitial melt) consistent with cold storage (Wespestad et al, ). Several features in the petrochronologic data also support storage at near‐solidus conditions.…”

“…The arrow marks the center of ongoing uplift. The dashed fields show the approximate footprints and cross sections of the geophysical anomalies—InSAR: the growing sill associated with the ongoing inflation (Feigl et al, ), G1: the 115‐km 3 Bouguer gravity anomaly consistent with crystal‐rich (>50%) magma (Miller, Williams‐Jones, et al, ), G2: the very low‐density (<1,900 kg/m 3 ) body interpreted as magma and exsolved fluid (Miller, Williams‐Jones, et al, ), Vs: the 450‐km 3 low Vs body that largely extends west of the cross section (Wespestad et al, ), MTC3: the C3 conductor identified by Cordell et al () that extends west and north of the cross section, and MTC4: the C4 conductor of Cordell et al (), which is located to the north and deeper (>10 km) than the area depicted. The gravity, conductivity, and Vs anomalies are broadly consistent with a several hundreds of cubic kilometers, crystal‐rich magma body beneath the center of inflation.…”

“…The gravity, conductivity, and Vs anomalies are broadly consistent with a several hundreds of cubic kilometers, crystal‐rich magma body beneath the center of inflation. The G2 anomaly is indicative of extractable mush, exsolved fluid, and possibly crystal‐poor rhyolite but is too small to be resolved by the other geophysical techniques (Cordell et al, ; Miller, Williams‐Jones, et al, ; Wespestad et al, ). The southeastern basin is not well‐resolved by these surveys, and thus the current state is uncertain.…”

“…Although seismic tomography is consistent with an extensive crystal‐rich reservoir, gravity and MT surveys suggest that smaller (<10 km 3 ), higher melt fraction magma domains may exist but cannot be resolved by the S wave velocity model (Cordell et al, ; Miller, Williams‐Jones, et al, ; Wespestad et al, ). Ongoing efforts to reconcile minor spatial offsets in the geophysical models and quantitatively integrate the results of these methods, including joint inversion of these data sets, should better delineate variations in the crystal content and composition of modern magma reservoir.…”

“…The source of inflation has been interpreted as magma emplacement at a rate of 0.03–0.05 km 3 /year at ~5‐km depth (Le Mével et al, ). This growing sill resides within a >450‐km 3 extremely crystal‐rich (~5% melt) magma system that extends between 2‐ and 8‐km depth imaged by surface wave seismic tomography that may also host smaller melt‐rich bodies inferred from magnetotelluric (MT) and Bouguer gravity data (Cordell et al, ; Miller, Williams‐Jones, et al, ; Wespestad et al, ). Deformation of an early Holocene highstand paleoshoreline and the ubiquitous presence of mafic inclusions in the rhyodacite lavas suggest the modern unrest is only the most recent increment of magma recharge since the late Pleistocene (Andersen et al, ; Hildreth et al, ; Singer et al, ).…”