A. Meltzer’s research while affiliated with Lehigh University and other places

The Ecuadorian Andes are a complex region characterized by accreted oceanic terranes driven by the ongoing subduction of the oceanic Nazca plate beneath South America. Present-day tectonics in Ecuador are linked to the downgoing plate geometry featuring the subduction of the aseismic, oceanic Carnegie Ridge, which is currently entering the trench. Using seismic tomography, we jointly invert arrival times of P and S waves from local and teleseismic earthquakes with surface wave dispersion curves to image the structure of the forearc and magmatic arc of the Ecuadorian Andes. Our data set includes > 100 000 traveltimes recorded at 294 stations across Ecuador. Our images show the basement of the central forearc is composed of accreted oceanic terranes with high elastic wave speeds. Inboard of the Carnegie Ridge, the westernmost forearc and coastal cordilleras display relatively low Vp and Vs and high Vp/Vs values, which we attribute to the increased hydration and fracturing of the overriding plate due to the subduction of the thick oceanic crust of the Carnegie Ridge. We additionally image across-arc differences in magmatic architecture. The frontal volcanic arc overlies accreted terranes and is characterized by low velocities and high Vp/Vs indicative of partial melt reservoirs which are limited to the upper crust. In contrast, the main arc displays regions of partial melt across a wider range of depths. The Subandean zone of Ecuador has two active volcanoes built on continental crust suggesting the arc is expanding eastwards. The mid to lower crust does not show indications of being modified from the magmatic process. We infer that the slab is in the process of flattening as a consequence of early-stage subduction of the buoyant Carnegie Ridge.

The Nazca-South America subduction zone in Ecuador is characterized by a complicated along-strike geometry as the slab transitions from flat slab subduction in the south, with the Peruvian flat slab, to what has been characterized as “normal” dipping subduction beneath central Ecuador. Plate convergence additionally changes south to north as the trench takes on a convex shape. Highly heterogenous bathymetry at the trench, including the aseismic oceanic Carnegie Ridge, and sparse intermediate depth seismicity has led many to speculate about the behavior of the down-going plate at depth. In this study we present a finite-frequency teleseismic P-wave tomography model of the northern Andes beneath Ecuador and Colombia from 90 – 1200 km depth. Our model builds on prior tomography models in South America by adding relative travel-time residuals recorded at stations in Ecuador. The complete dataset is comprised of 114,096 relative travel-time residuals from 1,133 stations across South America, with the added data serving to refine the morphology of the Nazca slab in the mantle beneath the northern Andes. Our tomography model shows a Nazca slab with a fragmented along-strike geometry and the first teleseismic images of several proposed slab tears in this region. At the northern edge of the Peruvian flat slab in southern Ecuador, we image a shallow tear at 95-200 km depth that appears to connect mantle flow from beneath the flat slab to the Ecuadorian Arc. Beneath central Ecuador at the latitudes of the Carnegie Ridge, the Nazca slab is continuous into the lower mantle. Beneath southern Colombia, the Malpelo Tear breaks the Nazca slab below ∼200 km depth.

The timing of surface uplift of the Altai Mountains in northern Central Asia-and the climatic consequences-remains controversial. Today, the Altai Mountains cast a substantial rain shadow, effectively separating the western Gobi Desert and steppe from the Siberian Taiga. We take advantage of this stark climatic gradient to trace the interaction of climate and topography in the lee of the Altai. First, we present new water stable isotope data that demonstrate that-along with this climatic gradient-the Altai modify the d18O of precipitation via rainout on the leeward side of the range. Second, we present a new paleosol carbonate clumped isotope (D47) record that spans much of the Neogene from the immediate lee of the Altai in western Mongolia to address how surface temperatures may have responded to potential uplift during the Neogene. We find that D47-derived temperatures have, overall, declined by approximately 7°C over the course of the Neogene, though the precise timing of this decrease remains uncertain. Third, we pair our D47 record with previously published stable isotope data to demonstrate that the timing of decreasing temperatures corresponds with long-term stability in paleosol carbonate d13C values. In contrast, increases in paleosol carbonate d13C values-linked to declining vegetation productivity-are correlated with intervals of increasing temperatures. We speculate that declines in vegetation biomass and leaf area changed the partitioning of latent and sensible heat, resulting in rising surface temperatures during Altai uplift. In contrast, long-term Neogene cooling drove the overall decline in surface temperatures. Reconstructed soil water d18O values (based on carbonate d18O and D47 values) remain surprisingly stable over our Neogene record, differing from our expectation of decreasing d18O values due to progressive uplift of the Altai Mountains and Neogene cooling. We demonstrate that the shift in precipitation seasonality that likely accompanied Altai uplift obscured any change in lee-side precipitation d18O that would be expected from surface elevation change alone.

Repeating earthquakes repeatedly rupture the same seismic asperity and are strongly linked to aseismic slip. Here, we study the repeating aftershocks of the April 16, 2016 MW 7.8 Pedernales earthquake in Ecuador, which generated a large amount of afterslip. Using temporary and permanent stations, we correlate waveforms from a one‐year catalog of aftershocks. We sort events with a minimum correlation coefficient of 0.95 into preliminary families, which are then expanded using template‐matching to include events from April 2015 to June 2017. In total, 376 repeaters are classified into 62 families of 4–15 events. They are relocated, first using manual picks, and then using a double difference method. We find repeating earthquakes during the whole period, occurring primarily within large aftershock clusters on the edges of the areas of largest afterslip release. Their recurrence times, shortened by the mainshock, subsequently increase following an Omori‐type law, providing a timeframe for the afterslip's deceleration. Although they are linked temporally to the afterslip, repeater‐derived estimates of slip differ significantly from GPS‐based models. Combined with the fact that repeaters appear more spatially correlated with the afterslip gradient than with the afterslip maxima, we suggest that stress accumulation at the edge of the afterslip may guide repeater behavior.

The Northern Andes of Ecuador contain some of the most active volcanic systems in the Andes and extend over a broad region from the Western Cordillera to the Subandean Zone. While it is known that the arc straddles a range of basement compositions, from accreted mafic oceanic terranes in the west to silicic continental terranes in the east, the details of the crustal structure beneath the arc is unclear despite being critical for understanding magmatic and tectonic processes in this portion of the Andes. To gain insight into these processes, we create two 3D models of crustal and upper mantle seismic properties throughout the region. The first highlights the discontinuity structure using receiver functions, which allows for the recovery of crustal thickness beneath the Ecuadorian Andes. We observe a range from ∼50 to 65 km under the high elevations, with thicker crust beneath the lower elevation Western Cordillera compared to the higher elevation Eastern Cordillera. This can largely be explained by density variations within the crust that are consistent with observed terranes at the surface, implying these terranes extend to depth. The second model combines our receiver functions with Rayleigh wave dispersion data from ambient noise measurements in a joint inversion to construct a 3-D shear wave velocity model. This model shows several mid-crustal (5–20 km below sea-level) low velocity zones beneath Ecuadorian arc volcanoes that contain a maximum of ∼14% melt. These low velocity zones likely represent zones of long-term magma storage in predominantly crystalline reservoirs, consistent with “mush zones”. Furthermore, the depth of the inferred reservoirs below several of the volcanic centers (e.g., Chiles-Cerro Negro and Tungurahua) are in broad agreement with previous geobarometry and geodetic modeling. Our results provide new observations of possible long-term magma reservoirs below other less-studied volcanic systems in the Ecuadorian arc as well, and further contributes to a mounting number of observations indicating long-term magma storage at low melt percentages in the mid-crust beneath active arc systems.

Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three‐dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (<1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (>1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period.