Imaging the Laguna del Maule Volcanic Field, central Chile using magnetotellurics: Evidence for crustal melt regions laterally-offset from surface vents and lava flows

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

“…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 smaller domain comprising a higher melt fraction and exsolved fluid suggested by the Bouguer gravity model (Miller, Williams‐Jones, et al, ) is likely extractable mush, potentially capped by crystal‐poor melt, with fluid derived primarily from the intruding magma driving the uplift (Andersen et al, ; Le Mével et al, , ). The N‐NW dip of the Vs and conductivity anomalies is suggestive of magma transported from deeper reservoirs north of the lake (Cordell et al, ; Wespestad et al, ). The deeper conductivity anomalies are consistent with the mafic magma stalling and crustal assimilation inferred from whole‐rock isotope ratios and trace element compositions (Andersen, Singer, et al, ; Cordell et al, ).…”

“…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, ).…”