Some research results of the QMIII project

 

A summary of results obtained by Rutgers undergraduate student Michael Klaser in the course of a year-long Aresty Research Assistanship.

A summary of crustal properties (thickness and Moho width) along a 1259 km long profile (line on the map at left).

Topography is shown in the upper panel, vertical scale in meters. Estimates of crustal thickness using receiver function pulse timing (circe) and H-k stack are shown in the lower panel (scale in km). Green horizontal line shows an average crustal thickness based on our new results. Ages of last significant tectonic activity are marked above corresponding regions of the transect.

G - Grenville Front

A - Appalachian Front

N - Norumbega Fault Zone


Receiver function analysis was applied to long-term records of teleseismic P waves gathered by permanent installations in Quebec. Attempts to detect the characteristic signature of the LAB (a “negative” P-to-S converted phase) proved futile at two sites in the Superior province, while a candidate phase appears in records from the site on the Appalachian Front.

Studies of the properties of the crust-mantle boundary

initiated by Andrea Servali’s work in the course of the Aresty Research Assistanship.

One important general development produced in the course of this study is a formula for Moho sharpness/width based on observed teleseismic compressional waves (receiver functions).

Two papers resulting from this research are published in GRL and as a chapter in the GSA Special Paper.

Figure to the right.  A map of sites studied (circles) with the outlines of tectonic boundaries (from Clowes et al., 2010). The locator map (inset) shows the study area as a red box. Four-letter codes designate seismic observatories. Numbers next to sites show ranges of Moho thickness, in km.


Figure below. (a) Synthetic RFs computed in 1D layered velocity structures using a reflectivity algorithm of Levin and Park, (1997). Time series are shaded according to the model (instant step – light grey, linear gradient – grey, complex structure – solid). The shortest wavelength is computed as , where f is highest frequency, and VS = 3.7 km/s.  An inset in (b) shows the P-to-S wave pulse for f=3 Hz. (b) Values of shear wave speed for three vertical profiles at the crust-mantle boundary used to produce synthetic RFs. (c) Raypaths and waveforms of two P-to-S converted waves from closely spaced boundaries.

The Northern Appalachian Anomaly* (NAA) is an intense, laterally-localized (400 km diameter) low velocity anomaly centered in the asthenosphere beneath southern New England. Its maximum shear velocity contrast, at 200 km depth, is about 10% and its compressional-to-shear velocity perturbation ratio is about unity, values compatible with its being a modern thermal anomaly. Although centered close to the track of the Great Meteor hotspot, it is not elongated parallel to it and does not cross-cut the cratonic margin. In contrast to previous explanations, we argue that the NAA’s spatial association with the hotspot track is coincidental and that it is caused by small-scale upwelling associated with an eddy in the asthenospheric flow field at the continental margin. That the NAA is just one of several slow velocity features along the eastern margin of North America suggests this process may be globally ubiquitous.

*first imaged in 1990s using a different data set, see

Levin, V., A. Lerner-Lem & W. Menke,  (1995), Anomalous mantle structure at the Proterozoic-Paleozoic boundary, Geoph. Res. Let., vol. 22, pp. 121-124, doi: 10.1029/94GL02693


Imaging upper mantle structure beneath northern New England

(A-C) Compressional velocity perturbations at 100, 200 and 300 km depths, respectively. The poorly-resolved peripheral part of the model is masked. The Appalachian Front (AF), Grenville Front (GF) and Great Meteor Hotspot (GMHS) track are shown. (D-F). Corresponding shear velocity model.

Results of multi-event inversion for parameters of seismic anisotropy beneath selected sites of the QMIII network that recorded 10 or more core-refracted shear waves.

Symbols show fast axis orientation and % of anisotropy in a horizontal 100 km thick anisotropic layer. Red bars indicate the uncertainty in the fast orientation, and the size of circles indicates % anisotropy, ranging from <2% (at the NW end of the line) to >5% (near St. Lawrence river and the Atlantic coast).


Seismic anisotropy studies: looking for signatures of mantle deformation