Lithospheric Delamination Beneath the Southern Puna Plateau Resolved by Local Earthquake Tomography

Abstract

We present a local earthquake tomography to illuminate the crustal and uppermost mantle structure beneath the southern Puna plateau and to test the delamination hypothesis. Vp and Vp/Vs ratios were obtained using travel time variations recorded by 75 temporary seismic stations between 2007 and 2009. In the upper crust, prominent low Vp anomalies are found beneath the main volcanic centers, indicating the presence of magma and melt beneath the southern Puna plateau. Beneath the Moho at around 90-km depth, a strong high Vp anomaly is detected just west of the giant backarc Cerro Galan ignimbrite caldera. This high Vp anomaly is only resolved if earthquakes with an azimuthal gap up to 300°are included in the inversion. However, we show through data subset and synthetic tests that the anomaly is robust due to our specific station-event geometry and interpret it as a delaminated block of lower crust and uppermost mantle lithosphere under the southern Puna plateau. The low velocities in the crust are interpreted as a product of the delamination event that triggered the rise of fluids and melts into the crust and induced the high topography in this part of the plateau. The tomography also reveals the existence of low-velocity anomalies that link arc magmatism at the Ojos del Salado volcanic center with slab seismicity clusters at depths of about 100 and 150 km and support fluid transport in the mantle wedge due to dehydration reaction within the subducted slab. The northern and southern Puna are separated by the northwest-trending Olacapato-Toro-Lineament (O-T-L) near 24.5°S. To the east, the boundary of the southern Puna is structurally limited by the Santa Barbara system north of 27°S and by the Sierras Pampeanas to the south. In detail, the Santa Barbara system is characterized by predominantly west verging, relatively high-angle thrust faults that are largely inverted Cretaceous normal faults and the Sierras Pampeanas are thickskinned basement uplifts that are bounded by high-angle reverse faults (Allmendinger et al., 1997; Kley & Monaldi, 1998, 2002). To the west, the southern Puna is bounded by the Andean Neogene Central Volcanic Zone (CVZ). Within the region of the southern Puna plateau, there are a series of NW-trending zones of lithospheric weakness along which the largest

Full Waveform Inversion Beneath the Central Andes: Insight Into the Dehydration of the Nazca Slab and Delamination of the Back‐Arc Lithosphere

“…South of 24°S, the thinned lithosphere H2 finally disappears beneath the southern PN and is replaced by the low‐velocity uppermost mantle (Figure 13), possibly representing the upwelling asthenosphere and connected with the mantle wedge (M3–M5) beneath the frontal volcanic arc (Bianchi et al., 2013; J. Chen et al., 2020; Scire, Biryol, et al., 2015; Wang & Currie, 2015). However, in the deeper part of the upper mantle atop of the subducting Nazca plate, a high‐velocity anomaly H5 (Profile KK’–LL’ in Figure 13) is dipping westwards from the boundary of the EC and SB, with its leading edge to the southern PN.…”

“…South of 24°S, the thinned lithosphere H2 finally disappears beneath the southern PN and is replaced by the low-velocity uppermost mantle (Figure 13), possibly representing the upwelling asthenosphere and connected with the mantle wedge (M3-M5) beneath the frontal volcanic arc (Bianchi et al, 2013;J. Chen et al, 2020;Scire, Biryol, et al, 2015;Wang & Currie, 2015).…”

“…In contrast, local and regional earthquake tomography can provide more details for the crust and upper mantle in the selected regions but lacks resolution at larger depths. In some of these aforementioned regional tomographic studies, the upper part of the Nazca slab is visible as a relatively continuous high‐velocity anomaly beneath the Central Andes and various back‐arc seismic structures were also imaged (e.g., Bianchi et al., 2013; J. Chen et al., 2020; Schurr et al., 2006). However, these studies were limited to small specific regions according to the footprints of the temporary seismic arrays, typically differing among each others in many methodological details, which makes margin‐wide comparisons difficult.…”

“…The subduction of the Nazca plate below South America along the Central Andes has resulted in drastic crustal shortening (Oncken et al, 2006) and thickening (Heit et al, 2008;Heit, Sodoudi, et al, 2007;X. Yuan et al, 2000), magmatism (S. M. Kay & Coira, 2009; S. M. Kay & Mpodozis, 2002; S. M. Kay et al, 1994;Wörner et al, 1992Wörner et al, , 2000, and lithospheric delamination (Allmendinger et al, 1997;Beck & Zandt, 2002;Beck et al, 2015;Bianchi et al, 2013;J. Chen et al, 2020;Garzione et al, 2017; R. W. Kay & Kay, 1993;Schurr et al, 2006;Scire, Biryol, et al, 2015;Whitman et al, 1996).…”

“…In contrast, local and regional earthquake tomography can provide more details for the crust and upper mantle in the selected regions but lacks resolution at larger depths. In some of these aforementioned regional tomographic studies, the upper part of the Nazca slab is visible as a relatively continuous high-velocity anomaly beneath the Central Andes and various back-arc seismic structures were also imaged (e.g., Bianchi et al, 2013;J. Chen et al, 2020;Schurr et al, 2006).…”