How sharp is the sharp Archean Moho? Example from eastern Superior Province.
Vadim Levin, Jill A. VanTongeren, Andrea Servali
Department of Earth and Planetary Sciences, Rutgers University
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been through the copyediting, typesetting, pagination and proofreading process which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1002/2016GL067729
©2016 American Geophysical Union. All rights reserved.
Abstract:
The Superior Province of North America has not experienced major internal
deformation for nearly 2.8 Gyr, preserving the Archean crust in its likely original state. We
present seismological evidence for a sharp (less than 1 km) crust-mantle boundary beneath
three distinct Archean terranes, and for a more vertically extensive boundary at sites likely
affected by the 1.2 – 0.9 Ga Grenville orogeny. At all sites crustal thickness is smaller than
expected for the primary crust produced by melting under higher mantle potential
temperature conditions of Archean time. Reduced thickness and an abrupt contrast in seismic
properties at the base of the undisturbed Archean crust are consistent with density sorting and
loss of the residues through gravitational instability facilitated by higher temperatures in the
upper mantle at the time of formation. Similar sharpness of crust-mantle boundary in
disparate Archean terranes suggests that it is a universal feature of the Archean crustal
evolution.
Key Points:
1. Teleseismic receiver functions used to measure Moho thickness in eastern Superio
Province.
2. Sharp (~1 km) Moho likely is an original feature of Archean continental crust formation
3. Density sorting and delamination due to high mantle potential temperature explain sharp
Archean Moho
©2016 American Geophysical Union. All rights reserved.
Introduction:
The transition from the seismically slower material of the crust to the faster material
of the mantle, rarely accessible for direct sampling, bears the (somewhat shortened) name of
its discoverer Andrija Mohorovičić, and has been a subject of seismological studies for over a
century (Prodhel et al., 2013). A degree of ambiguity remains in associating the change in
seismic speed at the Moho with a change in lithology at the crust-mantle boundary (O’Reilly
et al., 2013). Nevertheless, Earth’s crust is most commonly delineated by seismological
means (e.g., Cook et al., 2010).
In global surveys (e.g. Christensen and Mooney, 1995) and regional studies (e.g. Yuan, 2015)
the depth to the Moho within stable continents is ~40 km, with oldest areas (cratons) having
systematically smaller values. A recent survey of diverse seismological definitions of the
Moho (Abbott et al., 2013) argues for systematic differences in its character as a function of
the continental consolidation age, with the oldest Archean-age cratonic crust having a larger
fraction of “sharp” Moho observations. Criteria used for the “sharp” designation depend on
the methods in specific studies, and include confident detection of PmP reflected waves in
controlled seismic source profiles, as well as shapes of P-to-S converted phases from distant
earthquakes.
Here we use the teleseismic receiver function technique to probe Moho structure at
six sites within the eastern Superior Province of North America. We present evidence for the
Moho being sharp (~1km) within all undisturbed Archean terranes, even those with different
formation ages and compositions, and notably more diffuse (> 4 km) near the ~1.0 Ga
Grenville Front. We also confirm that the crust is thinner than the global average. In both its
thickness, and the vertical extent (sharpness) of the Moho, the Superior Province crust differs
from the typical continental crust generated in the Phanerozoic.
©2016 American Geophysical Union. All rights reserved.
Regional Setting
The Superior Province is the largest of the Archean cratons (Card, 1990), made up of
terranes with origin dates as early as 3.3 Ga, assembled mainly during 2.72 Ga and 2.68 Ga
collisional events (Percival et al., 2007), and further modified as late as 2.51 Ga (Moser et al.,
2008). Seismic studies of the Superior province (e.g., Calvert et al., 1995; Calvert and
Ludden, 1999; Mereu, 2000; Musacchio et al., 2004; Darbyshire et al., 2007) showed its crust
to be 40 km or less, with velocities typical of felsic lithology, and with many instances of
“sharp” Moho.
We investigate the vertical extent of the crust-mantle transition in three different
terranes within the eastern part of the Superior Province (Fig. 1). Two stations (WEMQ and
NMSQ) are located within the La Grande terrane, which contains the oldest rocks of the
eastern Superior Province, with 3.3 Ga to 2.9 Ga continental basement, and juvenile
magmatic rocks emplaced between 2.75 and 2.70 Ga. One station (YOSQ) is located within
the Opinaca subprovince in late Archean paragneiss (Reed et al., 2005). Further to the south,
three stations (MATQ, LSQQ, CHGQ) are within the Abitibi granite-greenstone belt, known
for its abundance of komatiitic lavas considered by some to be indicative of plume-sourced
oceanic plateaus in the Archean (Fan and Kerrich, 1997). Station MATQ is located on a
sequence of late Archean volcanic rocks, whereas stations LSQQ and CHGQ have
experienced post-Archean events. LSQQ is located directly on top of a dike from the late
Proterozoic Abitibi dike swarm (1141±1 Ma), and CHGQ is only 30 km west of the 1250-980
Ma Grenville front (Reed et al., 2005).
Measuring Moho sharpness
We investigate the vertical extent, or sharpness, of the Moho transition using
©2016 American Geophysical Union. All rights reserved.
observations of shear (S) waves present in the coda of first-arriving compressional (P) waves
from distant earthquakes. The receiver function (RF) analysis technique (Ammon, 1991)
assumes that such S waves originate by the process of mode conversion from P waves at a
contrast in seismic properties. The constraint on the vertical extent of the boundary comes
from the consideration of the pulse shape of a P-to-S converted wave. Figures 2a&b illustrate
expected effects of the vertical profile of seismic velocities across the crust-mantle transition.
Three cases are considered: an instantaneous increase, a 4 km wide gradual increase, and a 5
km wide sequence of layers with alternating properties. At frequency 0.25 Hz (corresponding
to S wavelengths ~15 km, Figure 2b) resulting time series are virtually identical. Differences
emerge as higher frequencies (shorter wavelengths) are introduced. A vertically
instantaneous change yields a pulse with progressively diminishing width. A smooth vertical
gradient results in a pulse with a characteristic width that does not change after a certain limit
is reached. Finally, a complex crust-mantle transition zone results in a set of distinct pulses
arising from individual contrasts within it.
We adopt the wavelength of the P-to-S converted wave that departs from the simple
pulse shape as a measure of the boundary width. This approach is similar to the consideration
of pulse shapes of waves reflected from closely spaced boundaries (Widess, 1973), which
informs a widely accepted ¼ wavelength rule for the vertical resolution of seismic reflection
data. The time separation between two vertically propagating P-to-S converted waves from
horizontal boundaries separated by h (Figure 2c) will be
, where Vp,Vs are constant velocities of P and S waves between the boundaries, and
. If we set
then
, where
 is the wavelength of the P-to-S converted wave,
for =1.75.
Simulated P-to-S converted waves in Figure 2a show that a departure from the simple pulse
©2016 American Geophysical Union. All rights reserved.
shape becomes obvious at the f =1 Hz corresponding to  km, a wavelength
commensurate with the width of the velocity transition. This observation is more in line with
the h~P/2 estimate (P – P wavelength) proposed by Bostock (1999) and based on
amplitudes of mode-converted waves in zones of generalized velocity heterogeneity. In our
context this will yield h~
measure of the converting boundary width.
Data and Analysis
Continuously operating seismic observatories in the Superior Province (Figure 1)
provide data for the study. We select groups of nearby earthquake sources (see Supplement
Figure 1) from datasets containing between 90 and 140 records per site. All records chosen
for one site in the frequency domain are stacked to form a single RF time series. All sources
used to form this RF have similar ray parameters and backazimuths, eliminating possible
distortions of the converted wave pulse shape due to lateral changes in Moho properties and
variations in the incidence angle. We use a multitaper spectral correlation variant of the RF
technique that affords an exceptional resolution of higher frequency RF components (Park
and Levin, 2000). For each site, the RF time series with different frequency content (Figure
3) are constructed in order to examine the resulting pulse shapes of the P-to-S converted
waves from the Moho (designated Pms). We identify the highest frequencies at which these
pulses appear “pure” (i.e. as predicted by an abrupt change in properties, Figure 2a), evaluate
their wavelengths, and derive two estimates of the vertical distance over which seismic
properties change, an optimistic one assuming a constant velocity layer (i.e.,
a conservative one assuming a complex vertical transition (i.e.
), and
).
Results.
With the exception of the site CHGQ discussed in more detail below, all sites within
©2016 American Geophysical Union. All rights reserved.
the Superior Province show a simple Pms phase, with delay times in the 4.3 – 4.5 s range
(Figure 3). At three locations (WEMQ, YOSQ, MATQ) Pms phases display a progressive
decrease in width with increasing f. The highest usable frequency in our dataset is f~3 Hz,
and the corresponding
~1.2 km. At site NMSQ the Pms pulse does not decrease in width for
f>1.75 Hz, and at site LSQQ the highest frequency where the Pms pulse retains its pure shape
is f=1 Hz. Consequently,
~2.1 km for NMSQ, and
~3.7 km for LSQQ. Our estimates for
the maximum vertical extent over which seismic properties change at the Moho are ~0.7 km
for sites WEMQ, YOSQ and MATQ; ~1.2 km for site NMSQ; and ~2.2 km for site LSQQ.
Using a more conservative approach (
these estimates are 1.2 km, 2.1 km and 3.7 km,
respectively.
At site CHGQ there appears to be two positive pulses with properties similar to the
Pms phase at lower frequencies (e.g., 1 Hz), one at ~3.7 s and another at ~6.2 s. At higher
frequencies additional phases appear between them. Examination of the full dataset
(Supplementary Figure 1) shows that the pulse at 3.7 s is seen from most directions, and thus
is more likely to represent the P-to-S conversion at the crust-mantle boundary. This pulse
loses its pure shape for f>0.5Hz, or
~7.4 km, and thus our estimates of Moho width here
are 4.3 - 7.4 km. Presence of additional converted phases in 4 – 6 s time window points to
considerable complexity of the uppermost mantle down to depths of at least 50 km.
The PmS delay is a measure of the crustal thickness, the calculation of which is based
on an assumption of Vp =6.5 km/s (e.g. stacking velocity of Calvert and Ludden, (1999) for
the Superior Province), and Vp/Vs = 1.75 (see Supplementary Material for discussion). Using
these values we obtain crustal thickness values for the eastern Superior Province of 34.9 36.5km (Supplementary Table 1). Choosing global-average values of Vp=6.454 km/s and
Vp/Vs=1.768 (Christensen and Mooney, 1995) reduces these estimates by ~1 km, while
adopting values from controlled-source studies of the Abitibi province (Vp~6.6 km/s and
©2016 American Geophysical Union. All rights reserved.
Vp/Vs=1.72; Mereu, 2000) increases them by 1.6-1.9 km (Supplementary Table 1).
Stacking of direct and multiply scattered Ps phases in RF timeseries provides a way to
estimate Vp/Vs and crustal thickness (Zhu and Kanamori, 2000), with an assumption of
either Vp or Vs value. Applying this technique (often called H-k stacking) to our data we
obtain tight constraints on the Moho depth, while Vp/Vs ratio is variable (Supplementary
Figure 2). While a choice of Vp value strongly influences the estimate of the crustal
thickness, for Vp<6.8 km/s our data favor crustal thickness values under 40 km at all sites
(Supplementary Table 2).
Discussion
Estimates of Moho sharpness rely on the ability to resolve RFs at high frequencies
(Park and Levin, 2000). In their review of controlled source reflection studies in Canada,
Cook et al. (2010) defined the Moho as “sharp” or “diffuse” using patterns of near-vertically
reflected waves. In cases of “sharp” Moho (e.g., at the northern end of Abitibi-Opatica line
near our site NMSQ, Figure 13 in Cook et al., (2010), originally from Calvert and Ludden,
(1999)) the change from strong reflectivity (in the crust) to no reflectivity (supposedly in the
mantle) takes place over ~0.2 s of two-way time. For a range of compressional wave speeds
of 6.7 – 7.5 km/s at the bottom of the crust (Musacchio et al., 2004), this translates into ~0.7
km of vertical distance, close to our optimistic estimate for site NMSQ,
Clarity and prominence of the PmP Moho reflection is another commonly used
indicator of a "sharp" Moho in controlled seismic source studies (cf. Abbott et al., 2013). In
western Superior Province Musacchio et al. (2004) report a PmP phase being best observed in
the 2-6 Hz pass-band, with a corresponding shortest wavelength in the lower crust of ~1.1 1.2 km, calculated as
, where Vp= 6.7 - 7.5 km/s. According to the 1/4 wavelength
measure of Widess (1973), this implies a velocity transition over ~0.3 km or less. Although
©2016 American Geophysical Union. All rights reserved.
the frequency range of our current RF data is not sufficient to detect boundaries separated by
300 m, the behavior of the PmS pulse at site WEMQ (Figure 3) is suggestive – it becomes
progressively narrower with each increase of frequency. Given enough bandwidth in the
earthquake sources, RF resolution may be brought closer to that of controlled source studies.
At three locations we estimate the transition between the cratonic crust of the eastern
Superior Province and its lithospheric mantle as being ~1 km thick. Our results are very
similar for three locations with different early histories. Common to the Abitibi terrane,
which is dominantly mafic volcanics, and the La Grande and Opinaca terranes, which are
dominated by granitoids (Figure 1, and Percival et al., 2007) is the lack of tectonic activity
after their incorporation into the Superior province. Sites with considerably more diffuse (4-7
km) crust-mantle transition are LSQQ and CHGQ. Both have experienced post-Archean
crustal reworking such as dike emplacement and extensive faulting related to Grenville-age
continent-continent collision. A plausible interpretation of our findings is that the Superior
Province Moho was initially sharp, and was subsequently disturbed by tectonic events in the
Proterozoic. This disturbance did not relax over the subsequent ~1 Gyr. Thus, a sharp Moho
is likely an original feature of craton formation and stabilization in the Archean.
Many researchers suggest that mantle potential temperature in the Archean was up to
250°C higher than today (Davies, 1992; 2009; Korenaga, 2008a,b; Herzberg et al,. 2010;
Brown, 2007). Under these conditions the mantle is expected to melt to a greater extent,
thereby producing primary picritic-basaltic crust up to 40 km in thickness (Herzberg et al.,
2010; Herzberg and Rudnick, 2012). One consequence of the uniformly thick crust may have
been the inhibition of subduction sensu stricto by extreme bending stresses (e.g., Korenaga,
2006; Davies, 2009). However, preserved crust of the cratons is significantly smaller than
the 41.1 km modern continental average of Christensen and Mooney (1995), and seismic
imaging yields convincing evidence of subduction episodes during the Archean (e.g. Calvert
©2016 American Geophysical Union. All rights reserved.
et al., 1995; Chen et al., 2009). Furthermore, the predominant TTG composition of the
cratonic crust (Moyen and Martin, 2012) likely requires multiple episodes of re-melting of
the primary picritic-basaltic crust (Johnson et al., 2014).
In contrast to preserved Archean cratonic crust, juvenile continental crust developed
in modern island arcs has a very different seismic structure, with higher absolute values of
compressional velocity, and a gradual change to upper mantle values over a distance of
multiple km (e.g. Holbrook et al., 1999; Kodaira et al., 2010). The expected RF signature of a
sample profile through the juvenile arc crust is illustrated in Supplementary Figure 3. In order
to transition from juvenile island arc to modern continental crust, a significant portion of the
lower crust and the mantle lithosphere must be removed (e.g. Jagoutz et al., 2011; Jagoutz
and Schmidt, 2013).
Examples of lithospheric instability during the Phanerozoic are inferred from exposed
terranes of island arcs, such as the Kohistan and Talkeetna arc sections (Jagoutz and
Kelemen, 2015). In Talkeetna, a ~100-400 m thick section of dense garnet gabbro crust is
the only remaining density-unstable lithology present under a 40 km thick crustal section of
granites and gabbronorites (Greene et al., 2006). It is proposed that a further 10-12 km of the
original crust were delaminated from the Talkeetna arc section prior to obduction (Greene et
al., 2006; Jull and Kelemen, 2001; Kelemen et al., 2004; Behn and Kelemen, 2006). Jagoutz
and Behn (2014) show that density sorting, and removal of density-unstable material, occurs
in the modern day crust on 0.5-5 Myr time-scales, and that higher Moho temperatures (e.g.
~1000°C in Talkeetna relative to ~700-800°C for Kohistan) allow for a sharper Moho by this
density sorting process.
Thus, delamination of density-unstable volumes of primary Archean crust is a
plausible scenario to yield thinner Archean crust with an abrupt lower boundary (e.g.
Hamilton, 2013). The transition from density-stable to density-unstable crustal lithology is
©2016 American Geophysical Union. All rights reserved.
pressure and composition dependent, corresponding roughly to the appearance of garnet in
the stable phase assemblage. Johnson et al. (2014) modeled the stability of primary crust
using the appropriate compositions for high MgO picrites and basalts from the Archean, and
showed that the removal of dense mafic lower crust by Rayleigh-Taylor instability occurs
rapidly in undifferentiated crust at mantle potential temperatures greater than 1550°C. Lighter
crustal material left behind after an episode of gravitational removal of dense residues is
likely to form an abrupt contrast with the underlying mantle. This conclusion is supported by
the similarity of Moho sharpness and crustal thickness in all Archean terranes investigated
here, regardless of dominant lithology, and suggests that the process of granitization does not
disrupt or alter the Moho signature.
Conclusions
At a set of locations broadly distributed within the eastern Superior Province we use
high-quality records of P-to-S converted waves from distant earthquakes to quantify the
length-scale of the contrast in seismic properties delimiting the bottom of the crust. Our
estimates of the vertical extent of the Moho boundary range from less than 1 km in
undisturbed crust to over 7 km near major post-Archean tectonic boundaries. We find short
vertical transition (< 1km) from the crust to the mantle in all Archean terranes probed,
regardless of their formation age or crustal lithology (basaltic vs. granitic). This finding
suggests that the sharp Moho observed in Archean cratons is a first order feature, likely
related to higher mantle potential temperatures resulting in hot (1000°C) Moho temperatures
during their formation. Our data also show that regions affected by Grenville-age tectonism
have a more diffuse Moho, suggesting that disturbances to the originally sharp Moho are
unable to relax on the >1 Gyr time scale.
Our observations of sharp Moho and relatively thin crust in the Archean terranes of
©2016 American Geophysical Union. All rights reserved.
the eastern Superior Province are consistent with density sorting of the crust under the
conditions of higher mantle potential temperatures in the Archean. The formation of a sharp
Moho transition must be a primary and ubiquitous feature of Archean craton formation and
stabilization.
Acknowledgements
This work was supported by the NSF Earthscope grant EAR-1147831 and the Aresty Center
for Undergraduate Research, and made possible by the open data policy of the Canadian
National Data Centre for Earthquake Seismology and Nuclear Explosion Monitoring.
Discussions with C. Herzberg, O. Jagoutz , W. Menke and M. Bostock, as well as reviews by
Andrew Hynes and anonymous, helped us sharpen the arguments. Figures were drawn using
GMT (Wessel and Smith, 1995).
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©2016 American Geophysical Union. All rights reserved.
Figure 1. 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.
©2016 American Geophysical Union. All rights reserved.
Figure 2. (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.
©2016 American Geophysical Union. All rights reserved.
Figure 3. Observed RF arranged by frequency content for 6 locations within the Superior
Province. Pms pulses with progressively diminishing width are found at sites WEMQ, MATQ
and YOSQ. Sites NMSQ and LSQQ show pulses that have a simple shape up to f=1.75 Hz
and f=1Hz, respectively (marked by stars). Site CHGQ displays frequency dependence of the
Pms pulse suggesting complexity in the crust-mantle transition zone.
©2016 American Geophysical Union. All rights reserved.