Provenance and Paleozoic tectonic evolution of Ganderia and its relationships with Avalonia and Megumia in the Appalachian-Caledonide orogen

At most mid-ocean ridges, a wide region of decompression melting must be reconciled with a narrow neovolcanic zone and the establishment of full oceanic crustal thickness close to the rift axis. Two competing paradigms have been proposed to explain melt focusing: narrow mantle upwelling due to dynamic effects related to in situ melt or wide mantle upwelling with lateral melt transport in inclined channels. Measurements of seismic attenuation provide a tool for identifying and characterizing the presence of melt and thermal heterogeneity in the upper mantle. We use a unique data set of teleseismic body waves recorded on the Cascadia Initiative's Amphibious Array to simultaneously measure seismic attenuation and velocity across an entire oceanic microplate. We observe maximal differential attenuation and the largest delays ([Formula: see text] s and δTS ~ 2 s) in a narrow zone <50 km from the Juan de Fuca and Gorda ridge axes, with values that are not consistent with laboratory estimates of temperature or water effects. The implied seismic quality factor (Qs ≤ 25) is among the lowest observed worldwide. Models harnessing experimentally derived anelastic scaling relationships require a 150-km-deep subridge region containing up to 2% in situ melt. The low viscosity and low density associated with this deep, narrow melt column provide the conditions for dynamic mantle upwelling, explaining a suite of geophysical observations at ridges, including electrical conductivity and shear velocity anomalies.

High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt

SUMMARY Head waves are extremely important in determining the structure of the predominantly lay- ered Earth. While several recent studies have shown the diffractive nature and the 3-D Frechet kernels of finite-frequency turning waves, analogues of head waves in a continuous velocity structure, the finite-frequency effects and sensitivity kernels of head waves are yet to be care- fully examined. We present the results of a numerical study focusing on the finite-frequency effects of head waves. Our model has a low-velocity layer over a high-velocity half-space and a cylindrical-shaped velocity perturbation placed beneath the interface at different locations. A 3-D finite-difference method is used to calculate synthetic waveforms. Traveltime and ampli- tude anomalies are measured by the cross-correlation of synthetic seismograms from models with and without the velocity perturbation and are compared to the 3-D sensitivity kernels constructed from full waveform simulations. The results show that the head wave arrival-time and amplitude are influenced by the velocity structure surrounding the ray path in a pattern that is consistent with the Fresnel zones. Unlike the 'banana-doughnut' traveltime sensitivity kernels of turning waves, the traveltime sensitivity of the head wave along the ray path below the interface is weak, but non-zero. Below the ray path, the traveltime sensitivity reaches the maximum (absolute value) at a depth that depends on the wavelength and propagation dis- tance. The sensitivity kernels vary with the vertical velocity gradient in the lower layer, but the variation is relatively small at short propagation distances when the vertical velocity gradient is within the range of the commonly accepted values. Finally, the depression or shoaling of the interface results in increased or decreased sensitivities, respectively, beneath the interface topography.

Finite-Frequency Sensitivity Kernels for Head Waves

Rise and fall of the Acadian altiplano: Evidence for a Paleozoic orogenic plateau in New England

Seismic characteristics of the eastern North American crust with Ps converted waves: terrane accretion and modification of continental crust

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JB017387

Crustal and Upper Mantle Structure of the Tien Shan Orogenic Belt From Full-Wave Ambient Noise Tomography

Teleseismic receiver functions were calculated to image the Moho geometry in the northern Appalachian Mountains in order to explore crustal thickness variations and possible linkages with tectonic units. Waveforms from 1995 to 2016 were analyzed for a total of ~ 200 broadband seismic stations, yielding a well-distributed data set with high lateral resolution. Consistent P-to-S phases converted at the Moho can be clearly observed. The Moho depth increases from the coastal plain northwestward to the Appalachian Plateau, with a sharp east-west gradient in southern New England. A distinct subrectangular downward deflection of the Moho is imaged in northern New Hampshire and western Maine. There is a spatial correlation observed between Moho depth variations and the NE-SW trending Appalachian orogenic strike. Variations of crustal thickness along and across the orogenic strike provide insights into the formation and modification of the crust during and after the major Appalachian orogenic events. Plain Language Summary The Wilson tectonic cycle is one of the most fundamental processes leading to growth and modification of continental crust on Earth. Eastern North America provides a complete record of the eastward growth of the continent and has been used as a type section and model of plate tectonics for decades. Although the surface boundaries between former continental fragments are increasingly well known, a question persists about the timing and processes involved with the growth and stabilization of the continental crust. In this study, we provide high-resolution crustal thickness variations in New York and New England, in order to identify the boundaries between tectonic units within the crust. The abundance of broadband seismic stations has significantly increased in the last few years, including the EarthScope Transportable Array and many long-running stations. The crust thickness demonstrates a much sharper east-west gradient in southern New England than in northern New England. A distinct subrectangular downward deflection of the crust is imaged in northern New Hampshire and western Maine. Variations of the crustal thickness along and across the orogenic strike provide insights into the formation and modification of the crust during and after the major Appalachian orogenic events.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018GL078777

Crustal Thickness Variation in the Northern Appalachian Mountains: Implications for the Geometry of 3‐D Tectonic Boundaries Within the Crust

A Comprehensive Quality Analysis of Empirical Green’s Functions at Ocean‐Bottom Seismometers in Cascadia

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