Download COCORP: new perspectives on the deep crust

Geophys. J . R. astr. SOC. (1987), 89,47-54
COCORP: new perspectives on the deep
crust
L. Brown, D. Wille, L. Zheng, B. DeVoogd, J. Mayer,
T. Hearn, W. Sanford, C. Caruso, T.-F. Zhu,
D. Nelson, C. Potter, E. Hauser, S. Klemperer,
s. Kaufman and J. Oliver Instituefor the study ofthe Continents
(INSTOC), Cornell University, Ithaca, N.Y.14853
Summary. Relict sutures from colliding continents, regions characterized by
a "young" Moho, layering and faulting throughout the crust, mid-crustal
magma traps, and seismic "bright spots" which suggest deep crustal fluids are
among recent COCORP findings. In addition, new studies of signal
penetration, noise mitigation, recording geometry, and coherency filtering
have yielded better understanding of, and substantial improvements in, data
quality. Amplitude anomalies, or "bright spots", in the Basin and Range may
be due to magma at mid-crustal levels; in one case, a normal fault appears to
link the deep magma with young surface volcanics. Another bright spot, 15
km deep in southeastern Georgia, has a flat geometry that suggests a
gadliquid interface, perhaps within fluids underthrust along an Appalachian
suture. The Moho continues to appear relatively undisturbed in many regions
of past deformation, suggesting that its formation post-dates these major
tectonic episodes. The diversity of reflection patterns from the U.S. Cordillera
casts further doubt on the generality of the common model of a reflective,
layered lower crust underlying a transparent upper crust.
1. Introduction.
COCORP deep seismic profiling in the US. now exceeds 8000 km (Fig. 1). During the
past three years this work has focussed on four major regional traverses of 1) the Basin and
Range extensional province, 2) the southeastern Appalachian orogen (expanding on
COCORPs earlier transect in this area), 3) the Northwest Cordillera and adjacent craton, and
4) the Colorado Plateau and its southwest margin. In addition, continuing investigations of
technique, including new field and processing experiments, have led to better data
representations and more confident interpretations. This report briefly reviews some of the
progress on the broader issues of deep seismic profiling'as reflected in recent COCORP
activities. Details of specific traverses are covered by Nelson et al. (this issue), and Potter et
al. (this issue).
2. The Moho
The premier issue in deep reflection profiling is still the nature and geometry of the Moho.
Moho reflections are now a sufficiently common observation that their absence sometimes
48
L. Brown et al.
DEEP
SEISMIC
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JUNE 1986
Figure I. COCORP field operations. Surveys completed to date.
takes on special significance. For example, the lack of distinct Moho reflections beneath the
Proterozoic craton, in spite of adequate signal penetration probably reflects a difference in the
nature of the cratonic versus the adjacent Phanerozoic terranes (e.g. Brown et al. 1986;
Brown 1986). Although sub-Moho reflectors are known from other areas (e.g. McGeary and
Warner; Warner et al., this issue), the mantle immediately beneath the crust in most areas of
the U.S. appears to be non-reflective (e.g. Mayer & Brown 1986; Klemperer et al. 1986).
The most striking aspect of the Moho reflections on several COCORP transects is their
relative continuity and flat orientation beneath crustal blocks which have undergone major
orogenic deformations. Dipping reflections in the lower crust are often discordant with flatter
Moho events, merging into or being truncated by the Moho (e.g. Fig. 2; Potter et al., this
issue). Sanford et al. (1986) show for the NW Cordillera that this discordance is not an
artifact of "sideswipe", and that the juncture between the two can be quite abrupt.
Beneath the Basin and Range Province, the contrast between surficial deformations and
Moho continuity is equally impressive (Fig. 3). Klemperer et al. (1986) conclude that the
reflection Moho beneath the Basin and Range corresponds (for the most part) with the
refraction Moho, that the Moho is relatively flat (in depth, not necessarily travel time)
beneath the entire Basin and Range, that the Moho is marked by distinct reflections (as
opposed merely to the cessation of lower crustal reflections) and that in places the Moho
exhibits a distinct doublet character. The Moho beneath the hinterland (Piedmont/Coastal
Plain) of the SE Appalachians also lies at virtually constant depth, even beneath an inferred
continent-continent suture zone (Nelson et al. 1985). Since large deformations of the crust
must have occurred in all of these areas, the present Moho must post-date compressional
orogenesis and, at least in the case of the Basin and Range, be relatively young.
The relatively undisturbed nature of the Moho beneath many areas is not to imply that
there are no significant variations in crustal thickness; such variations are as evident from
reflection results as they are from refraction surveys. For example, reflections beneath the
Colorado Plateau extend at least 5 s deeper than those beneath the adjacent Basin and Range,
in rough agrement with limited available refraction data (e.g. Allmendinger et al. 1986;
Farmer et al. 1986). Furthermore, although there are numerous examples where upper crustal
COCORP: new perspectives on the deep crust
COCORP
7
49
Idaho Line 1 - Lower Crust
8
u)
U
9
c
0
0
CD
v, 10
11
17
I
5 km
I
V:H=l4 @ 6 km/s
Figure 2. Lower crustal reflections from COCORP Idaho Line 1. Note the general discordance between the
dipping lower crustal events and the Moho. In this case the crustal reflections appear to merge into the Moho
events. (Potter et al. 1986).
faults fail to penetrate the Moho (e.g. Fig. 3), there are also instances where the Moho
appears to be offset (eg. Peddy et al. 1984; McGeary, this issue).
3. Magma
One of COCORPs earliest results was a set of unusually strong reflections at mid-crustal
depths in the Rio Grande Rift, which for a variety of reasons were interpreted as magma (e.g.
Brown et af. 1980). New surveys suggest that the New Mexico body is not unique (Fig. 4).
50
L. Brown et al.
800
I
400
NORTHERN SHAKE RAHOE
1
SPRINO VY
I
000
1
200
CONFUIlON RANOE
SNAKE V Y
400
tw
1
Figure 3. COCORP Nevada Line 5 . Line drawing illustrating layered reflections in lower crust, deeply penetrating
normal fault and relatively undisturbed Moho reflections. The reflections beneath the major normal fault are
unusually strong, perhaps indicative of magma trapped in the footwall of this fault. (Hauser et al., 1986).
Unusually strong mid-crustal reflections from both Death Valley, S . Cal., and eastern
Nevada may be from magma (e.g. DeVoogd et al. 1986a; Mayer 1986; Serpa and DeVoogd,
this issue). Although the cause of these new "bright spots" is more equivocal than in the
Rio Grande Rift case, it is certainly plausible that they too represent trapped magma; both
lie within the Basin and Range, an area noted for its Cenozoic extension and volcanism. In
Death Valley, a transverse normal fault appears to connect the bright spot with a young
volcanic structure at the surface (DeVoogd et al. 1986a). The Nevada (Snake Range) bright
spots appear to lie within the footwall of a deeply penetrating normal fault (Fig. 3 ; Hauser et
al. 1986).
Ensemble, the most prominent characteristic of these bright spots is their similarity in
depth (Fig. 5). All occur at about 18 - 20 km, leading to the suspicion that density or
rheology is responsible for entrapping these magmas as they migrate through the crust (e.g.
DeVoogd 1986).
4. Bright spots and deep gas
The Surrency Bright Spot (SBS), recorded in southeastern Georgia, is not only a strong
reflection, it is extremely flat over part of its length (Fig. 6; Wille et al. 1986). Both
COCORP:new perspectives on the deep crust
DV
e
51
8
10
Death Valley
. -.
11
..I
W
E
Abo P a s s 1
Socorro 1A
NW
Socorro 2A
0
Abo Pass 2
20 km
Figure 4. True amplitude seismic sections from New Mexico and Death Valley, showing midcrustal bright spots
that may represent magma. (de Voogd 1986).
characteristics are similar to those sometimes observed in association with gas/liquid
interfaces in shallow sedimentary reservoirs. Yet the Surrency Bright Spot lies 15 km deep
within metamorphic basement. At these pressures and.temperatures, candidate gases would
exist in a supercritical state (Wille et al. 1986). Thus the SBS is more properly considered in
terms of two (or more) immiscible fluids in contact. The nature and origin of such fluids is
highly speculative. However, the SBS lies within a group of reflections which can be
interpreted as remnants of the suture that marks the culminating collision of the Appalachian
orogeny (Nelson et al. 1985; see also this issue). Surface fluids (water? methane? oil?) may
well have been partially subducted and subsequently trapped along crustal thrust faults in t h ~ s
zone.
5. Related research and future directions
In addition to their relevance to bright spots, amplitudes have also been a concern in studies
of noise mitigation (Klemperer 1986), source coupling (DeVoogd el al. 1986b) and signal
52
L. Brown et al.
Figure 5. Amplitude decay curves for various COCORP bright spots (after Mayer 1986). Note that these
anomalies occur at very similar depths, suggesting a common rheological/density control for entrapment.
penetration (Mayer & Brown 1986). Improvements in data processing, particularly from
the use of pre-stack deconvolution and FK filtering, are now evident in routine processing of
current recordings and in reprocessing of older data sets (e.g. Zhu & Brown 1986a; DeVoogd
et a/. 1986; McBride & Brown 1986). New techniques for velocity inversion.(e.g. Zhu &
Brown 1986b) and coherency filtering (Li & Brown 1986) have also found important
application. Acquisition has been more flexible in the use of non-standard spreads (e.g.
asymmetric split) and hardware modifications now allow routine use of broader signal
bandwidth and higher frequencies (e.g. 12 - 48 Hz vs 8 - 32 Hz). Special experiments to
study noise reduction techniques (Klemperer, 1986) have led to effective use of alternative
noise rejection schemes (e.g. mantissa summing), and special recording to look for mantle
reflections (e.g. 57 s full sweep recording) have helped define the limits of convincing
profiling. Particularly promising are efforts to use Cornell's new Production Supercomputer
Facility for seismic processing (Brown & H e m 1986). Though COCORS's efforts have
been considerable, much of the continental U.S. remains largely unexplored by seismic
means, especially the Precambrian interior. A prime goal of future COCORP profiling is the
completion of a transcontinental traverse, with new segments across the Proterozoic orogenic
belts buried beneath the Palaeozoic cover of the interior craton.
Acknowledgements
COCORP is supported by NSF Grants No. EAR 83-13378, EAR 85-18400 and EAR
84-18157. INSTOC Contribution No. 68.
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