Geophys. J. Int. (2002) 150, 91–108
Receiver function analysis of the North American crust
and upper mantle
D. S. Ramesh,1 R. Kind2,3 and X. Yuan2
1 National
Geophysical Research Institute, PB 724, Hyderabad 500 007, India
Telegrafenberg, 14473 Potsdam, Germany. E-mail: [email protected]
3 Freie Universität Berlin, Germany
2 GeoForschungsZentrum,
Accepted 2002 January 26. Received 2001 October 25; in original form 2001 April 18
SUMMARY
Data from 27 permanent broad-band stations spread over North America and sampling diverse
tectonic regimes of varying ages from the Archaean to the Phanerozoic, were analysed using
the receiver function approach. Receiver functions, a few hundred at most of the locations,
provide valuable constraints on the composition of the crust shaped by diverse processes
characteristic of their time of formation. The chemically inert and stable nature of the Archaean
domains of Canada is well manifested in their low Poisson ratio (0.26, felsic, lower than global
averages for Precambrian age crust) accompanied by a thicker than global average (>35 km)
Archaean crust. Data from the Canadian Cordillera and Innuitian orogens of Phanerozoic
times indicate the possible operation of two distinctly different crustal processes resulting in
thin crusts (<35 km) with dominantly felsic and more mafic compositions, respectively. These
processes are restricted to the region above 410 km without disturbing the integrity of the
mantle transition zone (MTZ), which is evident from the remarkably homogeneous nature of the
410 and 660 km discontinuities beneath the Canadian landmass. Contrastingly, the thermally
complex Phanerozoic southern California region suggests the presence of a heterogeneous
mantle transition zone possibly owing to localized 660 topography with a relatively uniform 410
overlain by a crust heterogeneous in composition and perhaps maturing to that of a continent.
Key words: Poisson ratio, receiver functions, upper-mantle discontinuities.
1 I N T RO D U C T I O N
Knowledge of the nature of the crust and its thickness beneath vastly
different geological provinces, in age and tectonics, is recognized as
an important step in differentiating diverse models of crustal evolution. The complexity of the continental crust largely stems from its
development over a large time span in response to varied tectonothermal stresses over its geological history. The Mohorovicic discontinuity (Moho), is marked by a significant change in seismic
velocities or chemical compositions with not necessarily coinciding depths (Griffin & O’Reilly 1987). Most of the information built
into the popular models of crustal evolution (Durrheim & Mooney
1991; Nelson 1991) is largely abstracted from seismic refraction or
reflection experiments that provide accurate estimates of the depth
to the Moho and compressional wave velocities (v p ) alone. Though
attractive, these models suffer from a lack of constraints on shear
velocities (vs ) in the crust. Measurement of vs becomes particularly important while discriminating the rock types with similar v p ,
as the ratio v p /vs is sensitive to the composition and hence is a
better discriminant (Christensen 1996). It is important to recognize
that interpretation of v p /vs , however, is not straightforward enough
to reduce the ambiguity regarding crustal composition. Laboratory
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2002 RAS
experiments (Christensen 1996) reveal that the Poisson ratio (σ )
shows little variation with temperature, while the effects of crack
density, pore fluid pressure become negligible at pressures larger
than 150 MPa. However, presence of complex structures (anisotropy,
scatterers or partial melt) bias the σ values (Zandt & Ammon
1995; Krishna & Ramesh 2000). The mineral composition of rocks
plays an important role in determining the σ values of the crust
(Christensen 1996), suggesting that v p /vs measurements indeed assume prominence in the investigation of the composition of the
crust. We therefore attempt to interpret the measured σ values
mainly in terms of compositional differences constrained by other
geological factors relevant for a region. It therefore, seems crucial to determine accurately the Moho depth (H ), σ (related to
v p /vs ) and the disposition of the 410 and 660 km discontinuities
to understand the tectonic evolution of a region through geological
times.
Some topics of current research interest that largely revolve
around issues such as the reported dichotomy between the Archaean
and Proterozoic crustal forming processes (Durrheim & Mooney
1991) or the Poisson ratio being a function of crustal age (Zandt &
Ammon 1995) come into sharper focus with analysis of data from
the prolific deployment of broad-band stations in regions of vast
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D. S. Ramesh, R. Kind and X. Yuan
Table 1. Location of stations with codes used in this study, grouped by the geological age of the basement
rocks; Measured Ps times from the Moho, the 410 and 660 km discontinuities with measured ‘660–410’
differential times; Estimated crustal parameters (H = Moho depth, σ = Poisson ratio, v p /vs ) and quality
of the observations.
Station code
Ps times
Moho (s)
Archaean–early Proterozoic
YKW3
4.4
FFC
4.7
FRB
5.3
SCHQ
5.8
Late Proterozoic
INK
4.2
DRLN
4.1
ANMO
4.5
CCM
5.8
Phanerozoic
ALE
3.3
MBC
3.8
RES
5.5
ADK
4.6
COLA
3.5
WHY
4.7
PMB
4.2
LLLB
4.1
HRV
3.3
HKT
3.6
COR
6.1
California (Mesozoic)
CMB
4.9
BAR
4.8
DGR
4.3
PFO
3.5
GSC
3.6
PAS
3.5
SVD
5.0
ISA
4.7
SUM
—
Moho depth
H (km)
σ
v p /vs
Quality
410 (s)
660 (s)
660–410 (s)
43.3
43.5
42.7
43.7
67.3
67.4
66.5
66.5
24.0
23.9
23.8
22.8
35
37
42
44
0.261
0.240
0.265
0.273
1.76
1.71
1.77
1.79
A
A
A
B
42.9
42.8
44.8
43.7
66.7
66.7
69.7
67.4
23.8
23.9
24.9
23.7
35
33
39
40
—
0.257
0.240
0.305
—
1.75
1.71
1.89
C
B
A
B
43.7
44.3
44.4
—
41.4
46.8
—
—
44.4
—
—
67.3
67.4
67.7
—
65.1
70.3
—
—
68.0
—
69.7
23.6
23.1
23.3
—
23.7
23.5
—
—
23.6
—
—
22
31
42
36
29
37
35
31
30
22
40
0.276
0.280
—
0.269
0.253
0.265
0.249
0.280
0.220
—
0.308
1.80
1.81
—
1.78
1.74
1.77
1.73
1.81
1.67
—
1.90
A
B
C
B
A
A
A
B
A
C
A
46.0
46.2
46.4
45.5
46.0
46.4
—
—
46.4
—
69.4
71.2
70.7
69.6
69.2
67.4
—
69.4
—
23.2
24.8
25.2
23.6
22.8
—
—
23.0
35
33
32
29
29
25
34
39
—
0.293
0.290
0.280
0.257
0.265
0.290
0.308
—
—
1.85
1.84
1.81
1.75
1.77
1.84
1.90
—
—
B
B
A
A
B
B
B
C
—
Note.—Station qualities: A = clear Ps phase and its related multiples; B = Ps is clear, its multiples are
detectable and with clear maxima in the H–v p /vs domain; C = combination of clear Ps phase with poor
multiples and not so clear Ps phase with poor multiples. Note that the Moho depth estimates at quality C
stations are based on Ps times of 3 s, low-pass filtered sum traces. Also note that entry SUM refers to
values corresponding to the sum trace of all California stations. The values of the ‘410’, ‘660’ and
‘660–410’ for the IASP91 model are 44.1, 68.1 and 24.0 s, respectively (at 67◦ distance).
geological diversity such as North America, India, Australia and
Africa. It is in this context that we present this receiver function
analysis (Langston 1979; Owens et al. 1984; Kind et al. 1995; Yuan
et al. 1997) of the crust–mantle beneath 27 stations spread over the
continent of North America (Table 1), sampling crustal regimes from
the Archaean to the Phanerozoic covering Canada and mainly the
California region in the USA. Of these, stations FRB, FFC, YKW3
and SCHQ lie on rocks of Archaean to early Proterozoic age;
ANMO, CCM, INK, DRLN on mid to late Proterozoic formations.
The remaining essentially sample regions of Phanerozoic orogen
with station HRV on the Appalachians and stations MBC, RES,
ALE on Innuitian. While stations WHY, COLA, PMB, LLLB, COR
lie on the Cordilleras, stations BAR, CMB, DGR, PFO, GSC, PAS,
SVD and ISA in California are underlain by Mesozoic Basement.
Such tectonic variety and diversity in age of the crust is expected to
have a bearing on the architecture of the crust and the underlying
mantle transition zone.
Numerous seismic refraction and reflection transects, under the
aegis of LITHOPROBE, covered many geological provinces of interest, mainly in Canada. Previous studies (Cassidy 1995; Bostock
1996) using Ps conversions, recent results from focused regions
(Bank et al. 2000; Rondenay et al. 2000) and seismic transects
(Hammer et al. 2000) provide stimulating insights into the makings of the diverse geotectonic provinces of Canada. It is relevant
to mention that estimates of the Moho Ps, P410s and the P660s
times from the two previous Ps conversion studies (Cassidy 1995;
Bostock 1996) at the same locations with far less data match reasonably well with our measurements mainly either owing to the
fortuitously simple nature of the crust or the presence of a largely
homogeneous MTZ beneath Canada. Surprisingly, no direct estimates of the Poisson ratio are available at these locations. We provide the first estimates of this diagnostic for composition beneath the
Canadian shield using substantially more data than before. With regard to the stations sited in the USA, we obtain signatures of the
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2002 RAS, GJI, 150, 91–108
North American crust and upper mantle
410 and 660 discontinuities beneath them, perhaps the first estimates
from receiver functions at several locations, and draw compositional
inferences of the crust from v p /vs estimates. Finally, we glean this
information to attempt geological age correlations and discuss their
implications.
2 D AT A A N D M E T H O D
Data of teleseismic P waveforms up to 2000 April, from 27
IRIS broad-band stations located on a variety of tectonic settings
(Goodwin 1996; Hoffman 1989) in North America (Table 1) are
used in this study. An initial data set of 12 000 seismograms is first
quality checked, to extract 8000 records with signal-to-noise ratio
(SNR) > 3.0, which are later subjected to visual inspection. Such a
rigorous quality check still resulted in few hundred receiver functions at most of the stations.
The mechanics of the receiver function method is summarized as
below. Owing to the large velocity contrast across a discontinuity
(e.g. Moho, 410 or 660 km), part of the steeply incident teleseismic
P-wave energy becomes converted to an SV wave and forms part
of the P-wave coda. Besides the direct conversion Ps, there are also
many multiple reflections and conversions that occur between the
surface and the interface (e.g. Moho). Whilst the P wave and its
multiples dominate the vertical component, Ps conversion and its
multiples are prominently registered on the horizontal component
(SV ). Therefore, an appropriate component rotation isolates the Ps
energy from that of P. The effects related to source, mantle propagation and instrument response, are suppressed by deconvolving
the P waveform from the SV component, to obtain what are called
the receiver functions. In the flat layered case only these contain
Ps converted waves with related multiples, and hence are sensitive to the shear wave structural impulse response. Also, drawing
analogy from reflection seismology, a display of data in the time
domain using distance moveout corrections (Yuan et al. 1997) or
in the depth domain using migration (Kosarev et al. 1999) is applied to the receiver functions. Here, we essentially use the methods
described in Kind et al. (1995) to construct and treat the receiver
functions.
The crustal multiples (Ppps and Ppss) together with Ps, contain a
wealth of information concerning the average crustal properties such
as the Moho thickness and σ in a well-constrained manner. Zhu &
Kanamori (2000) proposed a stacking technique that sums receiver
function amplitudes at the predicted arrival times of these crustal
phases for a combination of crustal thicknesses (H ) and v p /vs ratios. For the correct pair of H, v p /vs values, all three phases stack
coherently and the sum reaches a maximum value. This method is
advantageous as it is not so sensitive to the average crustal P velocity. Since the need to pick arrival times for the various converted
phases is dispensed with, large amounts of data can be processed
quite conveniently.
Methodological developments in probing the nature of the 410
and 660 km discontinuities using conversions of primary phases
P, S; ScS reflections; precursors to standard seismological phases
such as the PP, SS, P P , etc. are summarized in Helffrich (2000).
The P–S conversions, P410s and P660s, from these discontinuities,
unlike those from crustal interfaces, show significant move-out with
epicentral distance. After a Ps move-out correction is applied to the
traces, Ps conversions are parallel to P and multiples are inclined
if plotted as a function of distance. This important refinement to
individual receiver functions in the form of a dynamic move-out
correction for a reference epicentral distance of 67◦ is applied to
our data following Yuan et al. (1997).
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3 S A M P L E S O F T H E N O RT H
A M E R I C A N C RU S T — R E S U L T S
AND DISCUSSIONS
3.1 The Phanerozoic crust
The Phanerozoic regions of North America lie to the north, southeast, east and west of the Canadian shield. The region to the west is
a result of Cordillera orogeny and hosts stations INK, PMB, LLLB,
WHY and COLA. The other region lying to the west is California,
which is treated separately. The east-southeast segment represented
by the Appalachian Orogen is sparsely sampled by HRV and DRLN.
The Innuitian orogen in the north is sampled by stations ALE, MBC
and RES.
3.1.1 The Appalachian orogeny crust
HRV. The station HRV in the southern Appalachian province belongs to the complex eastern terranes. This region has Paleozoic
granitic intrusions and gneisses. The stark simplicity of the receiver
crust (Fig. 1) in such a geologically complex terrain is quite surprising. An outstanding Moho Ps phase at 3.3 s with strong multiples testifies to its simple character. Receiver functions numbering
about 280 also reveal no azimuthal variations of the Moho Ps phase
(Fig. 2). Among all the stations modelled in this study, we obtain
the thinnest crust (30 km) and the least v p /vs value (1.67) (Fig. 3,
Table 1) for HRV. The dominantly felsic nature of the thin crust
beneath HRV is also revealed from the seismic refraction profile
O-NYNEX results (Musacchio et al. 1997) and from the laboratorymeasured v p /vs ratios on granitic gneiss rock samples along this
profile close to HRV (Musacchio et al. 1997).
DRLN. Station DRLN, although classified as being on Proterozoic
geology, is located in a region affected by the Appalachian orogen. It yields a relatively simple receiver function stack, with a
prominent Moho Ps arriving at 4.1 s and devoid of any subcrustal
or intracrustal complexities (Fig. 1). The Ppps and Ppss multiple
phases are identifiable. A relatively thin crust of 33 km with a lowσ value of 0.26 commensurate with the felsic composition of the
crust, and in general agreement with the station HRV also located
on the Appalachians, is measured. These values seem to follow the
trend indicated in the seismic refraction profile O-NYNEX results
(Musacchio et al. 1997).
3.1.2 California region crustal domain
The southern California subregion, primarily consists of the Transverse Ranges (TR), the Mesozoic Peninsular Ranges (PR) batholith
and the actively rifting Salton Trough (ST) together with the San
Andreas fault (SAF) system. The complex nature of this subregion
is brought out as the presence of a high-velocity uppermost mantle
with low heat flow beneath the TR to PR; a well-imaged slower mantle under the ST with high heat flow values (Humphreys & Dueker
1994; Lachenbruch et al. 1985) and a compositionally distinctive
western (mafic and older) and eastern (siliceous and younger) portions (Silver & Chappell 1988; Ichinose et al. 1996) of the PR
batholith. These two sections of PR are believed to have formed
in the oceanic and continental-type lithospheres, respectively. The
TR, southern Sierra Nevada and PR are interestingly underlain by
low = Pn velocities, which appear realistic (Hearn & Clayton 1986).
Therefore, structurally, these lower-Pn velocities more probably reflect the conditions when this part of North America formed, while
the crustal thicknesses are a consequence of later tectonics in the
region.
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D. S. Ramesh, R. Kind and X. Yuan
Figure 1. Moho Ps conversions and its multiples designated as Ppps and Ppss on sum traces at individual stations (three- or four-letter codes). The letters
that appear as superscripts after each station code denote the geological age of the crust underlying them. Ar, Archaean–early Proterozoic geological age of the
crust; Pt, mid to late Proterozoic age crust; Ph, Phanerozoic age crust. The stations are displayed in increasing order of Moho depth (crustal thickness) from top
to bottom. The figure in the right-hand panel shows a low-pass filtered version of the sum traces of the broad-band data presented in the left-hand panel. Note
the longer time window of the right-hand panel needed to cover the time range of arrival of multiples Ppps and Ppss. Note the coherent nature of the Moho Ps
and the corresponding multiples.
CMB. The northernmost station in California (CMB) far removed
from the stations of southern California is perhaps comparable
in tectonics to station HRV to the east. Analysis of 405 receiver
functions reveals a complex crust constrained by an azimuthally
consistent Moho Ps signal at 4.9 s, with weak multiples. It is
for this reason that we refrain from interpreting data from this
station.
ISA. For station ISA, sited in the southernmost Sierra Nevada, a
Moho depth of 39 km derived from 235 Receiver Functions (RFs),
generally agrees with that of Zhu & Kanamori (2000) within the error limits. A prominent Moho conversion appears at 4.7 s with almost
no multiples to follow (Fig. 1). The estimated Moho depth agrees
reasonably well with the results from PmP arrivals (36 km, RichardsDinger & Shearer 1997) and Pn (Jones et al. 1994) data (33–35 km).
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North American crust and upper mantle
95
Figure 2. Broad-band receiver functions binned and sorted for backazimuth at individual stations representing a variety of crustal regimes in North America. YKW3 (Archaean); FRB (early Proterozoic); ANMO (late Proterozoic); HRV (Phanerozoic, Appalachian Province); DGR (Phanerozoic, west of the
compositional boundary, southern California); PFO (Phanerozoic, east of the compositional boundary, southern California).
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2002 RAS, GJI, 150, 91–108
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D. S. Ramesh, R. Kind and X. Yuan
Figure 3. Average crustal v p /vs ratio versus Moho depth at a few sample stations as in Fig. 2.
The absence of clear multiples could, however, be a result of the presence of a several kilometre thick intracrustal layer with velocities in
the range 7.3–7.5 km s−1 (Jones et al. 1994).
PAS. Station PAS, in the Los Angeles basin, modelled with 348 receiver functions shows a strong Moho converted phase at 3.5 s with
a sharp Ppps multiple but a relatively broad Ppss multiple (Fig. 1).
The complex nature of the second multiple is clearly evident from
its ‘double-peak’ nature of near equal amplitude between 15 and
20 s (Fig. 4). Earlier estimates (Zhu & Kanamori 2000) at this
location yield a crustal thickness of 28 km and a lower v p /vs of
1.73 compared with our estimates of 25 km and 1.84, respectively.
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North American crust and upper mantle
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Figure 4. Receiver functions binned and sorted for distance at a few individual stations. All time domain receiver functions in this study and differential times
are Ps moveout corrected for a reference distance of 67◦ . FFC (Archaean); PAS (Phanerozoic, southern California); WHY (Phanerozoic, Cordillera orogen);
ALE (Phanerozoic, Innuitian orogen). Traces are low-pass filtered and show clear arrivals of the Ps conversions from the 410 and 660 km discontinuities
designated as P410s and P660s, respectively. Also Moho Ps conversion and its multiples can be seen clearly. Thick vertical lines mark the IASP91 arrival times
of 410 and 660 km Ps conversions.
We suspect that this discrepancy stems from the complicated behaviour of the Ppss multiple. However, the Moho depths obtained
from both the studies do conform with that from the 3-D P-velocity
models (Magistrale et al. 1992) in the region, which also infer a
more mafic oceanic crust in the adjacent Ventura basin.
GSC, SVD, DGR, BAR, PFO. Near identical crustal parameter estimates to those of Zhu & Kanamori (2000) are obtained for sta
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tion GSC (29 km, 1.77) located in the Mojava block, whilst SVD,
situated in the eastern Transverse Ranges, yields a Moho depth
(34 km) remarkably close to that from the PmP studies (33 km,
Richards-Dinger & Shearer 1997). In the Peninsular Ranges (PR),
stations DGR and BAR lie to the west of the compositional boundary with PFO to the east. Excellent quality receiver functions
(Figs 1 and 2) yield thinner (29–33 km) Moho depth values than
those inferred from previous studies (36–41 km, Ichinose et al.
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D. S. Ramesh, R. Kind and X. Yuan
1996) but closer to re-estimations (30–36 km) by Zhu & Kanamori
(2000) bringing out the previously observed east–west difference in
crustal thickness. Our v p /vs measurements (Fig. 3) suggest a dominant mafic composition (1.81–1.84) of the crust in the PR region
with the exception of PFO (1.75). Incidentally, station GSC that also
lies to the east of the compositional boundary yields similar crustal
parameters (29 km, 1.77) as PFO. A detailed modelling of receiver
functions at PFO (Baker et al. 1996) constrains the Moho depth to
be around 30 km but reveals a complicated crust. Though stations
PFO, GSC suggest a significantly different crustal composition to
the east of PR compared with the well-documented mafic nature of
the western PR, it is prudent to wait for more data to accumulate to
further discuss this division.
In summary, the Moho thickness and v p /vs estimates from this
as well as earlier studies (Zhu & Kanamori 2000) show the presence of a thinner crust (around 30 km) with an abnormally high
v p /vs , hovering around 1.8, in the general region of southern
California. These measurements are clearly in sharp contrast to
those from other tectonic terrains of North America (Fig. 5, see
Table 1). Such an anomalous nature of the southern California crust
is indeed clearly reflected in Fig. 5. The measured high v p /vs values together with a thinner crust (30–32 km) in southern California
requires the presence of a highly mafic crust or partial melt in the
crust. The measured high v p /vs , in the background of generally low
heat flow values, lower-Pn velocities and a subduction environment
(Humphreys & Dueker 1994), can be explained by a small degree
of serpentinization in the crust. However, this would result in abnormal lowering of crustal velocities, which is not supported by any
model in this region. Therefore, the most plausible candidate seems
to be a crust dominant in gabbroic rock composition, typical of a
subduction environment, with possible transition to eclogite facies
assemblages in deeper domains. Hence we may infer that the crust
in the southern California region is still maturing from the oceanic
to continental in composition. Stations PFO and GSC in the eastern section of the PR with lower-v p /vs ratios (around 1.75) indeed
lend support to this observation by grading towards a more felsic
composition crust (Fig. 5).
3.1.3 The Cordilleran orogeny crust
WHY, COLA. In the northern part of the Cordillera, stations WHY
and COLA yield relatively simple receiver functions (Fig. 1), mainly
because of their location slightly away from the coastal belt but still
close to the active margin. Excellent quality data sampling the total azimuth range with clear Moho Ps and corresponding multiple
phases at these stations yield well-constrained crustal parameters
(Table 1). Near-zero time positive arrival is related to the basement layer. Additionally, for WHY, a prominent velocity inversion
is suggested at sub-Moho depths by the negatively polarized arrival,
immediately after the Moho (Fig. 4). At WHY and COLA, wide
apart from each other, we obtain a Moho thickness of 37 and 29 km
and the Poisson ratio of 0.27 and 0.25, respectively (Fig. 5).
INK. Station INK, though it falls in the general region of the
Proterozoic domain is in close proximity to younger orogens. INK
with 250 receiver functions located on the boundary between the
shield and the Cordilleran deformation front, is, however, treated as
a Phanerozoic station. As is true for a complicated crust, the generation of crustal multiples is generally hindered in such situations.
Nevertheless, some general observations can be abstracted from the
receiver functions at this station mainly owing to the quantum of
data inspite of its complicated structure. A double-peaked Moho
Ps at 4.2 s immediately followed by a broader negatively polarized arrival at 5.3 s essentially describes the receiver function at
INK. There is no discrepancy with the Moho Ps values, between
35 and 36 km, from previous studies at these locations (Cassidy
1995). The absence of multiples at INK does not permit us to give
a well-constrained Poisson ratio.
PMB, LLLB. Stations PMB and LLLB are located in the general
area of the coastal belt of southern Cordillera with LLLB sited to
the east of PMB. Both the stations show some degree of complexity.
The Moho Ps time at PMB (4.2 s) matches well with that already
reported (Bostock 1996). The presence of clear multiples (Fig. 1) at
both the stations ensured well-constrained H and σ values (Table 1).
Station LLLB perhaps predominantly samples the Cascade terrane
to the east. This nature of LLLB is reflected in its v p /vs value (1.81),
which differs dramatically from the value of 1.73 at PMB (Fig. 5)
on the coastal belt and from a value of 1.90 measured at COR on
the Cascadia subduction zone to the south. The broader range of
31–37 km crustal thickness in the southern coastal belt reported
from earlier refraction studies in this region (Clowes et al. 1995) are
in conformity with our values. Station PMB is characterized by an
azimuthally consistent arrival around 1.5 s from a shallow interface
corresponding to about 8–10 km. Though station LLLB shares this
feature, the Moho is sharp compared with PMB.
COR. In our analysis, station COR located in the forearc region of
the Cascadia subduction zone presents an interesting setting. Data
consisting of 330 excellent receiver functions are marked by a simplicity devoid of any sharp intracrustal interface. Apart from a signal
close to zero time, related to a near-surface interface, two Ps converted phases of negative polarities between 4 and 5 s and the Moho
Ps at 6 s are prominently registered. The Moho-related multiples
are clear. We obtain a crustal thickness of 40 km with a large v p /vs
value of 1.90 for this station. The presence of a large σ of 0.31 in
the subducted slab environment is easily explained by ascribing a
gabbroic composition to the entire crust, which seems reasonable in
this setting. A more detailed analysis of COR data from an earlier
study (Li 1996) agrees very well with our measurements, and additionally quantifies the easterly dip of the plate to be around 10◦ –15◦ .
Our interpretation of a thick oceanic crust made up of highly mafic
material was already suggested (Trehu et al. 1994).
In the Cordillera on the whole, the average crustal thickness is
around 33 km and the σ values show a tendency to be around 0.26
(Fig. 5). These average estimates do not vary even with the inclusion
of stations DRLN and INK, which seem to be more influenced by the
Phanerozoic orogens. The presence of a thin crust (<35 km) with an
average crustal Poisson ratio of 0.26 in the Cordillera is a surprise
given the convergent nature of the margin in the region. This thin
crust could be a result of post-orogenic tectonics. Recent results from
wide-angle studies in NW British Columbia (Hammer et al. 2000)
and earlier studies in southern Cordillera (Clowes et al. 1995) show
that the crustal structures in coastal belts compare favourably with
global extended crust regimes while the interior regions show values
to lie between the orogenic and extended crust. So, our inference
of a thin crust receives support from the latest results also from
this region. Also, receiver function analyses from previous studies
and this study document clearly the presence of a thin crust, with
an average thickness of between 30 and 32 km in the Phanerzoic
southern California region. The main difference between the results
of Canadian Cordillera and southern California is the consistently
high Poisson ratio (average 0.28) of the latter. Seismic refraction and
reflection traveltime and amplitude modelling with constraints from
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2002 RAS, GJI, 150, 91–108
Figure 5. The left-hand panel displays the colour-coded Moho thickness estimates at stations sited on diverse geology as indicated with appropriate symbols; Archaean–early Proterozoic (circles), mid to late
Proterozoic (diamonds) and Phanerozoic (triangles). The colour codes are as mentioned in the figure. The right-hand panel shows the average v p /vs values broadly divided into two groups, felsic (blue) and mafic
(red) recognizing 1.79 as a global average value. For detailed discussion on each station, please refer to the text. Note that the Moho depths for all Archaean stations are consistently >35 km (see Table 1) and the
Proterozoic stations also compare in range with these.
North American crust and upper mantle
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2002 RAS, GJI, 150, 91–108
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D. S. Ramesh, R. Kind and X. Yuan
xenoliths suggest the presence of a thin (30–34 km), felsic composition crust beneath Sierra Batholith in the southern Sierra Nevada
thinning further west (Ruppert et al. 1998). The batholiths present
along the western margin of North America in the Cordillera orogen
could indeed be analogous to the Sierra batholith sharing its thin and
felsic nature. Possible mechanisms capable of thinning a crust as observed in the Phanerozoic Cordillera range from delamination and
lateral flow (Zandt & Carrigan 1993) to simple tectonism-related
crustal heating and differentiation (Meissner 1986). Most models
explain crustal thinning successfully but shift the bulk composition
towards being more mafic. In this context, one possible model that
thins the crust preserving its felsic composition, as desired in the
Cordillera, is to produce crustal melting by a thermal event where
the light melt migrates to the upper crust. The residue is then incorporated into the mantle, resulting in a thinner than original crust
with a more felsic component (Meissner 1986). The presence of
a slow upper mantle and inferred high upper-mantle temperatures
throughout the western portion of the Cordillera together with the
limited measured high heat flow values in the region (Hyndman &
Lewis 1999) lend support to such a mechanism.
3.1.4 The Innuitian orogeny crust
MBC, RES, ALE. The Innuitian orogen-related areas in north
Canada are quite far-flung from the main landmass. Stations MBC,
RES and ALE yield Moho Ps times in the range 3.3–5.5 s, with
ALE and MBC showing a narrower range. These measured values
compare well with the previous estimates at these locations (Bostock
1996). The station RES with 205 receiver functions reveals by far
the most complicated receiver structure in this study. Owing to this
complicated nature, no multiple phases were observed. At station
MBC, besides the Moho Ps at 3.8 s, a large-amplitude, azimuthally
consistent Ps conversion at 1 s related to an upper-crust interface
and an oppositely polarized phase at 5 s corresponding to sub-Moho
depth are conspicuous (Fig. 1). A Moho depth of 31 km and a σ
of 0.28 for the crust are obtained. Station ALE shows a simple thin
crust of 22 km with an average σ value of 0.28. Stations MBC and
ALE differ enormously in their crustal thickness values but show
almost similar σ values (Fig. 5) around 0.28. These two large σ
values can easily be explained by invoking the simple model of orogenic tectonic thickening of the crust and subsequent erosion until
isostatic equilibrium is achieved (Chevrot & van der Hilst 2000).
This is quite an efficient mechanism, widespread in Phanerozoic
times, that yields a thin crust with the major felsic component getting eroded, resulting in a mafic end product. For this model to be
viable in the Innuitian orogen, the rate of erosion might need to
be quite different to account for the large variations in thickness
of the crust at MBC and ALE. Alternately, extensional tectonics
can be invoked to explain crustal thinning and the consequent magmatic intrusion material results in a more mafic composition of the
crust leaving behind a refractory upper mantle. The upper-mantle
residuals computed by Bostock (1996) in this general region indicate (the northern parts) the presence of a slightly faster upper
mantle.
In summary, in the Phanerozoic provinces of Canada, a dichotomy
in models of evolution of the thinner felsic crust is related to
Cordilleran orogeny and a mafic thin crust for Innuitian orogeny
(Fig. 5) are suggested. However, more data from these two orogens
are much needed to discriminate between competing models. Also,
the modes of Phanerozoic crust formation in California and within
Canada can indeed be diverse and it remains to be seen whether such
processes were in vogue during Precambrian times.
3.2 Mid to late Proterozoic formation response
ANMO. The receiver functions at ANMO (numbering 370), on a
mid to late Proterozoic basement, reveal a simple crust with a sharp
Moho Ps signal at 4.5 s and a transparent subcrust. The presence of a clear intracrustal discontinuity corresponding to 1.2 s followed by a reasonably strong phase of negative polarity particularly
from the western azimuth at 2.2 s describes the receiver structure
(Figs 1 and 2). The excellent quality of the data is brought out by
the well-constrained maxima in the H−v p /vs plots (Fig. 3).
CCM. Station CCM, though located in the continental interior as
ANMO, is in close proximity to the Ozark uplift composed of
Precambrain crystalline rocks. The receiver functions show a simpler crust and reveal two azimuthally consistent primary conversions
at times 7.2 s and about 8.8 s. The latter has an opposing polarity to
the Moho Ps phase observed at 5.8 s. The Ps conversion around 7 s
and the following phase at 8.8 s are indeed recognized by Langston
(1994) from fewer records. Though the Moho depth estimates at
CCM (40 km) and ANMO (39 km) are near identical, the v p /vs
value at the former is 1.89 compared with 1.71 at ANMO (Fig. 5,
Table 1). This suggests that the compositon of the crust perhaps
changes from purely felsic at ANMO to rock types of high σ values
beneath CCM. Langston (1994) explained the high-velocity mantle
layer (7.2 s signal) as representing the residuum of a magma chamber from a Precambrian magmatic event. In such a case, it would be
reasonable to expect some associated fluids to be trapped in the crust,
resulting in high σ values at CCM. Alternately, given the presence
of crystalline rocks beneath the station, abundance of plagioclase
feldspar in the igneous rocks at CCM would also explain the measured high v p /vs ratios. The values at ANMO, are on expected felsic
compositional lines and comparable to those of PFO and GSC with
a similar nature (Fig. 5) but opposed to other Californian stations.
3.3 Nature of the Archaean crust
YKW3, FRB, FFC, SCHQ. The Archaean–early Proterozoic crust
of Canada is sampled by the shield stations YKW3, FRB, FFC and
SCHQ. The number of events analysed varied between 300–354 for
the first three stations and 140 for SCHQ. This older crustal entity
shows relatively less complexity, except at SCHQ, as demonstrated
by the presence of a clear Moho Ps between 4.4 and 5.3 s and wellidentified multiples yielding σ in the range 0.24–0.26 (Figs 1–3,
Table 1). The location of the station SCHQ at the boundary between
the Superior and Churchill provinces would have resulted in the
observed complex nature of its receiver functions. Incidentally our
Moho Ps estimate is identical to the previous estimate at this location. An interesting sidelight of this study is the gradual increase in
crustal thickness from west YKW3 (35 km), to east SCHQ (44 km).
The results from this oldest block of North America are rather uniform in both crustal thickness and v p /vs estimates (Fig. 5).
The average crustal thickness and the Poisson ratio in the
Archaean–early Proterozoic domains of Canada come to 40 km and
0.26, respectively. Receiver function modelling of the five Archaean
stations of the SKIPPY experiment in Australia (Chevrot & van der
Hilst 2000) reveals crustal thickness values in the range 30–41 km
and σ values of between 0.25 and 0.28. Their average Moho depth
of 35 km agrees with the global average Archaean crust of 35 km
(Durrheim & Mooney 1991) and their average σ of 0.27 is less
than the global average by Zandt & Ammon (1995) of 0.29 ± 0.02
for shield regions that include both the Proterozoic and Archaean
crust. This value of 0.27 in western Australia is considered as
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2002 RAS, GJI, 150, 91–108
North American crust and upper mantle
indicative of a mafic-composition Archaean crust in this region. Our
measurements of H and σ are directly comparable with the results
from the Tanzanian Archaean crust, which show a crustal thickness in the range 36–42 km and a Poisson ratio of between 0.24
and 0.26 (Last et al. 1997). Archaean crust with an average thickness >35 km together with a low (0.25) average Poisson ratio is
also reported recently from the disparate Archaean provinces of the
Indian shield (Ravi Kumar et al. 2001) based on receiver function
modelling. Their values of crustal thickness vary between 33 and
39 km, whilst σ is largely centred around 0.25. Laboratory measurements of rock samples document the rapid decrease in Poisson
ratio from 0.30 to 0.23 with increase in silica content of the sample from 55 per cent to about 75 per cent. This dominant effect
on the Poisson ratio with abundance of silica content is well constrained for common igneous rocks (Christensen 1996) that make
up most of the Archaean crust. Therefore, the measured low average σ values in the Canadian Archaean crustal regimes seem to
suggest a crust that is more felsic in composition following the inferences from the Tanzania craton based on a similar low Poisson ratio.
Such a felsic continental crust composition is also inferred for the
Archaean crust beneath western and eastern Dharwar cratons in the
south Indian shield based on extensive modelling of mine tremors
and explosions (Krishna & Ramesh 2000), which is indeed confirmed by the latest study (Ravi Kumar et al. 2001) using direct Ps
conversions in the region. These results assume significance for two
reasons. First, the lower than expected global average Poisson ratio
values from these Archaean nuclei suggest that σ may not increase
with age of the crust, and the presence of a mafic lower crust at
least beneath the Archaean core terrains becomes doubtful. These
observations deviate from the earlier inferences made by Zandt &
Ammon (1995) based on 76 measurements around the globe. Secondly, the presence of a consistently thicker than Archaean global
average crust in the Archaean domains discussed above together
with the absence of an Archaean–Proterozoic dichotomy in crustal
thickness in the Tanzania craton serve as counter examples to the
(Durrheim & Mooney 1991) model. The low average Poisson ratio
values from these Archaean terrains seem to be more consistent with
the average continental crust composition estimates of Rudnick &
Fountain (1995) showing 59 per cent silica content.
3.4 Accuracy of crustal data
Moho conversion times of all the Canadian stations used in our
study were given earlier by Bostock (1996). However, only about
50 traces per station were available at that time, whereas several
hundred traces per station are used here. In most cases our Moho
conversion times match with his observations to within 0.1 s, which
indicates an excellent accuracy. Only station FFC yields significant
difference, with a reported time of 3.5 s against 4.7 s obtained by
us. The data in Fig. 1 show (especially the longer period data in the
right-hand panel), that there is a large Moho signal at 4.7 s. However, the weaker signal at 3.5 s (short-period data in the left-hand
panel of Fig. 1), seems to originate from the lower crust. Bostock
(1996), however, did not convert these conversion times into Moho
depths because of insufficient velocity information. This time-todepth transformation is conducted using crustal multiples (Zhu &
Kanamori 2000) while determining the required average v p /vs values taking advantage of its weak dependence on v p . This method
is probably now the most effective and accurate technique to determine the crustal thickness, as it also provides as a by-product, very
valuable information concerning the Poisson ratio in the crust. This
is an extremely important step because the Ps conversion time of
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2002 RAS, GJI, 150, 91–108
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the Moho alone could be very misleading, if converted into depth
without an accurate v p /vs ratio. On comparison, for example the
stations HRV and ALE, we see that their Moho conversion times
are identical but the crustal multiples of HRV arrive much later than
those of ALE. Consequently, this leads to very different v p /vs ratios and Moho depths at both stations (see Table 1). The crust at
HRV is 8 km thicker than at ALE (although their Moho conversion
times are identical) and the v p /vs ratio is 0.13 smaller. In southern
California, Zhu & Kanamori (2000) have given Moho depths and
v p /vs ratios for the same stations as in our study, also using several
hundred traces per station. Our results agree with their results within
their formal error limits at most stations. Only at the stations PAS
and SVD did we obtain about 4 km shallower depths and 0.1 largerv p /vs ratios. The waveforms of the direct conversions and multiples
at SVD and BAR (see Fig. 1) are very similar. This indicates that
the structures at both sites are also similar. At BAR our results and
those of Zhu & Kanamori (2000) agree. We conclude from this that
our results at SVD are probably more accurate. Comparing PAS and
PFO, we find that the Moho conversions have the same time, but
the PFO multiples are later than the PAS multiples (Fig. 1). This
is similar to the comparison between HRV and ALE. We, therefore
conclude that PAS must have a shallower crust and a larger-v p /vs
ratio than PFO, in agreement with our original results. For the other
stations (besides the Canadian and southern Californian stations)
we found no published receiver function data from the crust.
4 THE MANTLE TRANSITION
Z O N E — R E S U LT S A N D D I S C U S S I O N S
4.1 California region
In the study area of Phanerozoic southern California, the observed depression of the 410 km discontinuity by about 2 s (Figs 6
and 7), is in conformity with the seismically slow upper mantle in the
western US. The P660s–P410s differential times (Fig. 7) show variations within the region, suggesting the heterogeneous nature of the
MTZ that can be attributed mainly to slight topography at 660 km
(Table 1). That such subtle variations documenting the heteregeneous nature (Fig. 7) of the MTZ in a small region can be obscured
in an average picture of the region becomes clear from the sum trace
values presented for southern California in Table 1. The differential
times beneath PAS (22.8 s) and PFO (25.2 s) form the end-members.
In view of the reported presence of multiple discontinuities in the
MTZ near 660 km (Simmons & Gurrola 2000), the receiver functions were subjected to a special scheme. Receiver functions from
this region provide dense sampling of the MTZ (Fig. 8). About 1800
receiver functions with piercing points on the 660 km discontinuity
between 30 and 39◦ N and 114–122◦ W are selected and summed
in non-overlapping intervals of 0.10 by latitude and longitude. In
most of the intervals, the number of receiver functions varied between 30 and 60. These Ps moveout-corrected broad-band binned
traces sorted in longitude (west to east) and latitude (north to south)
are presented in Figs 9 and 10, respectively. The main features of
these receiver functions and the sum trace are the presence of a
large coherent phase corresponding to the 660 km discontinuity at
about 70 s followed by a broad weak arrival around 73 s (arrows
in Fig. 9). The coherent 70 s arrival, is deeper than the predicted
660 km one shown by a thick line (Figs 9 and 10) and can be followed throughout the section and comes closest to the predicted 660
between 117 and 116◦ W. We, however, do not observe any coherent
signature shallower than the predicted 660 km. Our analysis, on the
whole, provides marginal support to the reported possible presence
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D. S. Ramesh, R. Kind and X. Yuan
Figure 6. (a) Ps moveout-corrected receiver function sum traces showing Ps conversions at the 410 and 660 km discontinuities for individual stations. The
thick vertical lines represent the predicted 410 and 660 km arrival times corresponding to IASP91 model. Also, note that the P660s–P410s differential times
across varied age provinces show marginal variations (within 1 s, 10 km), indicating a laterally homogeneous MTZ thickness in North America. The stations in
California and in Cordillera show generally delayed 410 and 660 arrivals. (b) Ps moveout-corrected sum traces as in (a) but for stations that have clear P410s
and P660s times. Note that the traces are aligned to IASP91 predicted P660s times. Superscripts following the station names indicate the broad geological ages
as in Fig. 1. The traces are placed in increasing order of P660s–P410s differential times from top to bottom.
of an additional discontinuity around 720 km (the weak phase at
73 s) associated with the probable garnet–perovskite (gt–pv) transition (Simmons & Gurrola 2000) but no evidence for the precursory gt–il (garnet–ilmenite) transformation at depths shallower than
660 km nor the anticorrelated nature of the spinel (sp) and gt–pv
discontinuities as suggested by Simmons & Gurrola (2000) in this
region. Two possible reasons could result in this discrepancy. Our
data points are slightly fewer in number compared with those in
the other study (a total of 1800 high-quality receiver functions of a
possible 2021 as against 2609), which might have resulted in less
dense spatial sampling of critical subregions and the other could be
caused by the different data processing techniques used in the two
studies.
4.2 Mid-continent craton
The cratonic stations ANMO, CCM and HRV, though give widely
separated yield values (410, 660 and 660–410 times) closer to the
global averages (Fig. 7, Table 1). Our observations at CCM and
HRV, that are incidentally the end stations on either side of the east–
west MOMA array, are also reflected in the analysis of data from this
array (Li et al. 1998) at the same locations. The 410, 660 km and the
MTZ at CCM and HRV conform to global averages to within 5 km,
whilst the intervening stations in the MOMA array show a relatively
flat 410 but with significant local topography on the 660 km one.
Station ANMO has been studied previously in detail and the presence of significant local topography at 410 km is inferred. This resulted in a bi-modal distribution of the MTZ thickness with a 20 km
difference based on the stacking velocity spectrum method (Gurrola
& Minster 2000). The thicker MTZ to the east (253 km) typifies a
normal cratonic value, while the thinner MTZ to the west (232 km)
is argued to be similar to the active southern California value represented by PFO (236 km). Our analysis (Fig. 7, Table 1) shows a
MTZ in slight excess of 250 km for both PFO and ANMO. Conceding that the reported azimuthal bi-modal observation at ANMO is
real, our PFO value (25.2 s) still compares better with the reported
ANMO east MTZ value (253 km) rather than ANMO west. The data
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2002 RAS, GJI, 150, 91–108
Figure 7. In this figure, yellow indicates values in accord with global averages. Red indicates delayed arrivals (thin MTZ) and blue, faster arrivals (thicker MTZ values). The left-hand panel shows the P410s
times in the study region. The slower nature of the upper mantle beneath California is clearly reflected in the delayed P410s times. The remaining study region is characterized by a normal to faster upper mantle
shown in yellow and blue. The right-hand panel shows the P660s–P410s differential times related to the thickness of the MTZ. The MTZ thickness is relatively uniform over most of the North American continent.
Contrastingly, in a smaller region such as California, the MTZ appears to be more heterogeneous compared with a large region such as Canada.
North American crust and upper mantle
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D. S. Ramesh, R. Kind and X. Yuan
Figure 8. Location of Ps conversions at the 660 km discontinuity (piercing points at 660 km) for the southern California stations used in this study. The
piercing points share the same symbol as the station to enable easy correlation. ISA (square), GSC (inverted triangle), SVD (star), PAS (triangle), DGR (circle),
PFO (diamond) and BAR (plus).
at ANMO especially from westerly directions have a very ringing
character, which could easily be misinterpreted.
4.3 Canadian shield
The best documented study of the Canadian MTZ by Bostock
(1996), though with fewer data, is quite illustrative of this predominantly Precambrian region. The Ps times of the MTZ at stations
PMB, LLLB and ADK are not presented as they are contaminated
either by scattered energy or owing to the presence of coherent noise.
We note that because of such complications earlier studies either do
not provide estimates of the MTZ discontinuities at some locations
(ALE, FFC and PMB) or present estimates from partial data subsets with limited azimuthal coverage (BBB, PGC, PNT, WHY) or
mention that their estimates are doubtful owing to inadequate data
(DLBC, SADO).
4.3.1 The Precambrian transition zone
Most of the stations on the older crust in the region (YKW3, FFC,
FRB, SCHQ, INK and DRLN) behave along expected lines with
both the P410s and P660s arrivals being early by similar amounts
mainly owing to an overlying faster upper mantle, typical of shield
regions (Fig. 7). However, SCHQ shows a clear deviation from
other locations with a smaller than normal MTZ thickness (Fig.
7). Its P410s and P660s delay times show opposing signs. With
the observed P410s time closer to the IASP91 model predicted arrival time, the presence of a local velocity heterogeneity within
the MTZ could explain such a behaviour. Alternately, localized
topography on the 660 km discontinuity beneath SCHQ can also
result in the measured thinning. Besides the anomalous nature of
SCHQ, Bostock (1996) also mentions that stations FCC and DRLN
are anomalous. We, however, note that DRLN and INK are normal and similar to other shield stations with regard to their MTZ
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North American crust and upper mantle
105
Figure 9. Ps moveout-corrected receiver functions summed in narrow intervals of 660 km pierce point longitude as described in text. The thick vertical lines
show the IASP91 model predicted 410 km (P410s) and 660 km (P660s) Ps converted arrival times. Note the relatively simple nature at 410 km and slightly
complicated nature at 660 km both in the sum trace and individual summed receiver functions between 118 and 115◦ W longitude. In the total sum trace
displayed in the top box, note a weaker later phase at 73 s following the large-amplitude phase around 70 s corresponding to the 660 km discontinuity that can
followed in the section with the help of arrows. This weaker arrival could indicate the inferred gt–pv transition discontinuity of Simmons & Gurrola (2000).
(Fig. 7). The P410s and P660s delay times, in general, show good
correlation with the upper-mantle structure overlying the 410 km
discontinuity.
4.3.2 Cordillera and Innuitian transition zone
Amongst the Cordillera stations presented in Figs 6 and 7, COLA
shows the presence of a faster upper mantle as beneath other shield
stations with a normal MTZ thickness value. Station WHY in response to the slow upper mantle underlying the Cordillera, yields
delayed P410s and P660s but maintains its normal MTZ thickness as
others. The stations on the Innuitian orogen (ALE, MBC and RES)
largely follow the IASP91 model with ALE showing a marginal
tendency for shield like MTZ thickness. This orogen is underlain
by a uniform but slightly thinner MTZ than the Cordillera stations
(Fig. 7, Table 1). Our results are in general agreement with the ear
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lier inferences (Bostock 1996) that the Canadian landmass has quite
a uniform MTZ in accord with the global averages and shows no
strong lateral variations.
Figs 6 and 7 succinctly summarize our results on the 410 and
660 km discontinuities in the Canadian shield. Fig. 4 shows some
examples of data binned in 2◦ distance intervals, from the shield
(FFC), Cordillera (WHY) and Innuitian orogen (ALE) as a function
of distance. Both the discontinuities can be followed throughout
the distance range irrespective of the geological terrain indicating
their well-developed nature. The sum traces plot (Fig. 6) and the
P660s–P410s differential times (Table 1) show a very narrow range
(within 1 s, with the exception of SCHQ), strongly suggesting the
presence of a uniform MTZ thickness (Fig. 7) beneath this part of
North America. This supports the similar inference from an earlier
study cited above. Most of the P660s–P410s differential times in the
Canadian landmass are indeed very close to the global average of
24 s (Fig. 7).
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D. S. Ramesh, R. Kind and X. Yuan
Figure 10. Ps moveout-corrected receiver functions summed in narrow intervals of 660 km pierce point latitude as described in the text. The thick vertical
lines show the IASP91 model predicted 410 km (P410s) and 660 km (P660s) Ps converted arrival times. Note that the P410s arrivals can be traced up to 38◦ N
latitude and the P660s phase up to 36◦ N latitude and become poorly coherent beyond that.
4.4 Overview of the North America
mantle transition zone
The heterogeneous character of the MTZ beneath Phanerozoic
southern California, in particular, and its relatively homogeneous
nature under the predominantly Precambrian Canadian landmass
becomes evident from this study. The broad correlation of P660s
with P410s for the older terrains and for stations sited on younger
orogens (Table 1) suggests the influence of overlying upper-mantle
structure (<400 km) on these times, correlatable to surface tectonics. Whilst the Canadian region shows a more uniform MTZ
overlain by a normal upper mantle in accord with the IASP91 model,
a smaller region such as California hosts contrasting MTZs within
it. Stations PFO and DGR that are close by and underlain by a slow
upper mantle show the colder (thick) nature of their MTZ as the
cratonic station ANMO lying further east. Though stations GSC,
BAR and PAS share the same slow (hot) upper mantle as other
California stations, the MTZ beneath them is contrastingly warm
(thin). Therefore, the thermally complex nature of southern
California is indeed clear and is reflected in its heterogeneous
MTZ in contrast to the relatively uniform normal MTZ beneath the
Canadian Shield with the exception of SCHQ.
4.5 Accuracy of transition zone data
The reading accuracy of our data from the 410 and 660 km discontinuities is about 0.5 s (see Fig. 6). Comparing our readings from
the Canadian stations with those by Bostock (1996), we notice that
most data agree within this limit. We note that Bostock (1996) used
a reference distance of 68◦ , whereas we used 67◦ (the difference is
0.1–0.2 s, less than the reading accuracy). There are some differences at a few stations, which could mainly be a result of the different number of traces used in both studies (we used several hundred
traces, whereas Bostock 1996 had only about 50 available). Bostock
(1996) obtained 67.6 s at DRLN for 660 km, which is clearly larger
than our value of 66.7 s. Fig. 6 shows that INK and DRLN are
very similar and at INK our results agree with the earlier study. For
this reason we think that our value for 660 km at DRLN is more
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North American crust and upper mantle
realistic. There is another small discrepancy at 410 km MBC. We
obtain 44.3 s and Bostock (1996) reports 43.3 s. Fig. 6 shows that at
410 km this station is close to the IASP91 value of 44.2 s, and not
smaller. At WHY the reported values are more than 1 s smaller than
ours at both discontinuities. Fig. 6 shows that at least at 410 km WHY
has a very clear signal and the published value of 45.5 s is clearly too
early for our data. We have already compared our southern California
data with those from Simmons & Gurrola (2000) and qualified the
results. As already discussed, there exists only marginal support for
an additional discontinuity at 73 s and none for a third discontinuity
just above 660 km. The character of the receiver functions from the
MTZ at ANMO is strongly varying in a complicated manner with
backazimuth (Gurrola & Minster 2000). More data in the neighbourhood of ANMO would be required to resolve this structure reliably.
The lateral variations in MTZ across the MOMA array are close to
the normal values at both ends of the array where the stations HRV
and CCM are located. At these two stations we see no significant
variation of the MTZ with backazimuth (no figure).
5 C O N C LU S I O N S
The Moho Ps and its multiples Ppps and Ppss together with Ps
conversions from the 410 and 660 km mantle discontinuities in a
variety of crustal regimes of North America succinctly bring out
the underlying similarities and dissimilarities in their composition
and formation. The most unifying feature that emerged from the
P410s and P660s study is the generally uniform mantle transition
zone thickness close to the global average with local excursions in
southern California. Such an observation permits us to regard the
large continental region of North America as being largely isolated
from convective processes in the upper mantle.
The presence of a thick (>35 km), felsic (σ = 0.26) Archaean
crust in Canada, India and Tanzania together with presence of both
mafic (σ = 0.28) and felsic crusts (σ = 0.26) beneath three different
Phanerozoic terrains (thickness <35 km) suggests that the Poisson
ratio may not depend as much on crustal age as on the mode of its
formation and the nature of the underlying mantle.
The more than average (35 km) Archaean crustal thickness in
Canada and its similarity in crustal thickness to the clearly Proterozoic stations of the mid-continent craton (39 and 40 km) along with
the results from Tanzania present a counter example to the Durrheim
& Mooney (1991) model. While the thick Proterozoic crust is well
established globally, more Archaean terrains remain to be sampled
in this regard.
ACKNOWLEDGMENTS
DSR gratefully acknowledges financial support from the Alexander
von Humboldt Foundation. DSR thanks the GFZ, Potsdam authorities for permission to work with excellent facilities at the institute.
The staff of the IRIS Data Management Center are thanked profusely for providing the data in an efficient manner. J. Saul, G. Bock
and S. Sobolev are thanked for their interest and a critical reading of
the manuscript and Ravi Kumar for help in revising it. Colleagues
from the Global Seismology Project at GFZ are thanked for their
support at various stages of the work and J. Kummerow in particular. DSR is grateful to the Director, NGRI, India, and DG CSIR,
New Delhi, India, for granting him leave for this research work.
Most of the figures were generated using GMT freeware developed
by Paul Wessel and Walter H. F. Smith.
C
2002 RAS, GJI, 150, 91–108
107
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