Continental crustal evolution observations - Wiley Online Library

Eos, Vol. 72, No. 48, November 26, 1991

Eos,

T R A N S A C T I O N S ,

A M E R I C A N

G E O P H Y S I C A L

U N I O N

VOLUME 72, NUMBER 48 NOVEMBER 26, 1991

PAGES 537-538

Continental Crustal Evolution Observations PAGES 537, 540-541

Walter D. Mooney and Rolf Meissner How has the continental crust evolved? What are the primary processes responsible for its composition, structure, and mode of deformation? What role do fluids play in deep crustal processes? In the last dozen years, geophysicists have obtained images of the deep continental crust that can be used to examine these questions and refine geo logic models of crustal evolution. In this re port w e summarize recent progress in geo physical studies of the deep continental crust and highlight some of the more impor tant implications of deep crustal processes.

Seismic Refraction/Wide-Angle Reflection Studies Crustal seismology relies on the results of two principle techniques: refraction/wideangle-incidence reflection seismology and near-vertical-incidence seismology [Meiss ner, 1986; Mooney, 1989]. Refraction seis mology provides excellent control on crustal compressional ( V ) and shear ( V ) wave ve locities, and the relationship between seis mic velocity and rock composition has been established by numerous laboratory mea surements. However, this relationship is not unique, and metamorphic grade, in particu lar, must be considered. The V / V ratio, when available, permits a more reliable in terpretation of subsurface lithologic compo sition from crustal velocity models [Fountain and Christensen, 1989; Christensen, 1979, 1989]. p

land arcs have highly variable crustal struc ture but all are thinner than continental arcs. Young orogenic belts are typified by thick crust (as great as 60-70 km in the Himala yas) but have anomalously low seismic ve locity crust in comparison with shields and arcs (Figure 1). Continental crust for which the last tectonic event was extensional, such as the Variscan and Caledonian crust of western Europe (Figure 1) and the Basin and Range Province of the United States, have about 30-km-thick crust with little high-veloc ity (greater than 6.8 km/s) crustal material. Continental rifts have average crustal thick ness of about 35 km, but are highly variable in seismic velocity structure (Figure 1). Crustal velocity structure therefore varies sys tematically with tectonic setting, and seismic refraction measurements have found wide spread application because they provide a robust measure of crustal structure. At least three key distinguishing charac teristics can be extracted from the seismic

velocity columns shown in Figure 1: The av erage crustal velocity, crustal thickness, and the thickness of the high-velocity, lowercrustal layer. Each of these characteristics provides a significant constraint on the pro cesses that have formed and modified the crust. For example, there appear to be im portant processes that control the third char acteristic, thickness of the lower-crustal layer, including lower-crustal differentiation (an igneous process) to form a new Moho and a thinner crust with a lower average crustal velocity than the original thicker crust [Meissner, 1986], and magmatic inflation to thicken the crust with mafic material [e.g., McCarthy and Thompson, 1988]. These infer ences, based on seismic refraction studies, are complemented by reflection studies that image the fine structure of the crust.

Seismic Reflection Studies Distinct patterns of crustal reflectivity have emerged from numerous and wide spread seismic profiles [e.g., Brown et al, 1983; Matthews, 1986; McCarthy and Thomp son, 1988]. For example, in young exten sional and/or warm crust, the lower crust often contains numerous subhorizontal seis mic laminae. These laminae occur as multi ple, dense sets of reflections, often with a sharp termination at the top of the seismically transparent mantle (Figure 2 ) . This ter mination of reflectivity is termed the reflec-

s

p

IA

20--

30--

-

M M

1*0

Fig. 1. Seismic veloc ities (V ) of various crustal provinces. P

5 0

M

V

• S

= Shields a n d p l a t f o r m s

1 A = Island/continental arcs Walter D. Mooney, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025; and Rolf Meissner, Institute for Geophysics, Univer sity of Kiel, Oishausen Strasse 4 0 - 6 0 , D-2300, Kiel 1, Germany

R

V+C

10

s

The seismic structure of the crust varies widely with tectonic setting (Figure 1). Shields and platforms commonly have a 40km-thick crust, including a 5- to 10-km-thick, high-velocity (7.1-7.8 km/s) lower crust. In comparison with shields, continental magmatic arcs, such as the Oregon Cascades, have a somewhat thicker crust and a much thicker lower-crustal, high-velocity layer. Is-

0

0

= Orogenic b e l t s ( r e c e n t )

V+C = V a r i s c a n a n d C a l e d o n i a n d o m i n a t e d t e r r a n e s R

= Continental rifts

•

m

n

p

(km/s)

< 5.7 5.7-6.4 6.4-6.3 7.1 - 7 8 > 7.8

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Eos, Vol. 72, No. 48, November 26, 1991 tion Moho and agrees in depth with the refraction Moho to the precision of the depth determinations [Mooney and Brocher, 1987]. In most areas with a well-laminated lower crust the upper crust is relatively transparent, or at least much less reflective than the lower crust. Nonreflective upper crust may consist of crystalline rocks that have been fractured by brittle deformation or intruded with many near-vertical plutons that do not create coherent reflections. Compared to the relatively transparent upper crust and well-laminated lower crust found in young extensional areas, seismic reflection surveys across young and old compressional orogenic belts show more complex reflectivity patterns (Figure 3 ) . Among the most prominent features are dip ping bands of reflections in the form of seis mic duplexes, ramp and flat structures, and open wedges (sometimes referred to as "seismic crocodiles"). Reflection data dem onstrate that young compressional belts, such as the Alps, have deep crustal roots, while the older belts, such as those between the Variscan and Caledonian terranes, or the still older Grenvillian belts, no longer have roots. The original lower crustal roots appar ently have been eliminated by lateral ductile flow in an isostatic adjustment to the moun tain belt's former high topography.

features. Seismic reflection studies of Precambrian crust show pronounced structural features in the upper and middle crust that appear to date from Precambrian times [Gibbs, 1986], an observation that indicates long-term thermal and tectonic stability of Precambrian crust. A major inference based on the geometry of these structures is that they are qualitatively consistent with modern tectonic processes operating in the Precam brian. Whereas most early (pre-1985) reflection profiles of Precambrian crust indicated weak lower-crustal reflectivity and few Moho re flections, recently collected profiles for which conventional mechanical vibrators have been replaced with more powerful sources (tuned airgun arrays and dynamite shots) show a different picture. These pro files are characterized by strong lowercrustal reflectivity and Moho reflections that can be traced in a piecewise continuous fashion across all records [e.g., Behrendt et al., 1988]. Among the most convincing data are marine reflection data from the early Proterozoic Baltic shield that show a highly re flective lower crust with reflector geometries that are very similar to Phanerozoic collision zones, thereby suggesting uniformitarian tec tonic models for the past 1.9 Ga [BABEL Working Group, 1990]. Despite these widespread observations of lower-crustal reflectivity over a variety of geo-

logical settings, the ultimate source(s) of crustal laminae has still to be established. If it were principally due to extension, a recent profile across the Atlantic margin should have, but did not, show increasing reflectiv ity with increasing crustal extension [BIRPS and ECORS, 1986]. If it were principally due to magmatic intrusions or underplating, then a profile through the Eifel volcanic area in Germany should have shown increasing lam inae. The evidence, in fact, indicates that seismic lamellae are almost certainly multigenetic and arise from such causes as lithologic, metamorphic, and porosity layering, igneous intrusions, anisotropy within shear zones, or (rarely) melt zones. Regardless of the ultimate source of seismic laminae, their subhorizontal geometry is probably en hanced by a regional or global stress-strain system [Zoback et ai, 1989], which causes crustal flow and horizontal layering in the low-viscosity lower crust.

Magneto-Telluric ( M T ) Studies The electrical structure of the crust is closely correlated with tectonic setting and provides a measurement of lower crustal properties that complements seismic mea surements [Jones, 1987]. Regions of high heat flow, such as sites of crustal extension or active continental magmatism, are charac terized by high conductivity (1.0-0.05 S/m) at shallow depths (10-20 km). In a few cases,

Reflection profiling has revealed repeated and stacked upper crustal sections in several orogenic belts. Many nappes in the Alps show ramp and flat structures to mid-crustal w levels (Figure 3 ) . The Appalachians' detach ment and the North Variscan deformation front can be clearly followed as a low-angle feature to a depth of 15 km for an alongstrike distance of more than 1000 km. The ' stacking of colliding continents is not, how ever, a uniform or one-sided process. Seis mic wedges and duplexes observed in colli~ 10 CO sional belts indicate substantial interwedging and interfingering of crustal units during the shortening and thickening process (Figure 3; NEVADA U - BASIN & RANGE Price [1986]). Crustal strength-depth curves suggest that interfingering is the natural con sequence of juxtaposing crustal units with WSW (Hauser et ol. 1987b) ENE different rheologies [Strehlau and Meissner, 1987]. Thin-skin tectonics in the upper crust and indentation and interfingering in the middle crust take place along thrust zones that are strongly lubricated, and high fluid pressures make possible the transport of nappes over more than 100 km. Seismic re flection data over active thrust faults some , 10km , URACH 1 times show strong negative reflection coeffi (Bartelsen et al, 1982) cients, as would be expected for low V values within the fault zone due to high pore NNW pressures. In addition, dipping bands of re flections and interfingering patterns in the middle crust (Figure 3) are indicative of weak shear zones within a more brittle crust. Meteoric water is known to penetrate to 10-12 km [Kozlovsky, 1984], and fluids from some metamorphic reactions are likely at even deeper levels [Fyfe et al., 1978]. An (Matthews. 1986) (Luschen et al, 1987) important consequence of deep crustal flu ids is an increase in the ductility of the Fig. 2. Examples for seismic reflection profiles (line drawings) in warm and/or extensional rocks. areas. Seismic sources: oibroseis for Black Forest and Basin and Range, explosives for Urach, Important insights from reflection seis airgun for Irish Sea. Laminated lower crust and comparatively transparent upper crust. mology are not restricted to young geologic 5

p



NFP-20

EASTERN

TRAVERSE

(Schweizenscne Arbeitsgruppe ,1988) 60 80

( DEKORP Res Group .1990)

SE

NW

these mid-crustal conductive layers may re sult from partial melting of wet granitic rocks. Alternatively, they may be caused by fluids produced by dehydration during highgrade metamorphic reactions [Fyfe et al., 1978; Stanley et al., 1990a]. In colder, stable continental crust the average conductivity is generally lower (that is, higher crustal resistivity; 1000 Ohm-m and higher) as compared to technically active crust. Exceptions are suture zones that con tain significant amounts of sulfide minerals or carbon films. Examples of young suture zones with conductivity anomalies include the Carpathian Mountains of eastern Europe [Adam, 1980], the Cascades Range of Ore gon and Washington [Stanley et al., 1990a], and the Alaska Range of central Alaska [Stanley et al., 1990b]. In all these areas, dipping high-conductivity zones within the upper-to-middle crust can be correlated with substantial flysch and/or black shale basins that have been crushed and overthrust dur ing accretion and/or ocean-continent colli sion. Where coincident seismic reflection and MT profiles are available, the conductiv ity zones commonly correlate with dipping reflections that can be followed to a depth of 20 km or more. An example of this correla tion is the active subduction zone west of British Columbia, Canada, where the depth to the top of the actively subducting Juan de Fuca plate is defined by seismic reflection data [Clowes et al., 1987] and is also indi cated by MT model studies [Kurtz et al., 1986]. The electrical properties of the crust are generalized in Figure 4, which illustrates typical conductivity-depth profiles for various tectonic settings.

Conclusions

tls)

GLIMPCE LINE J

(Berendt et ol ,1988V

F/g. 3.

Line drawings from seismic reflection profiles in young and old orogenic belts.

B conductivity (Siemens/M)

conduc.

conduc.

Recent geophysical studies of the conti nental crust have provided better seismic and geo-electrical constraints on the struc ture, composition, and physical properties of the deep crust than have ever before been available. Some of the most important obser vations, summarized above, have provided important new insights into the processes of mountain building, crustal extension and rifting, and the evolution and stabilization of cratons. In a forthcoming companion paper, we discuss some of these insights and spec ulate on the key processes of crustal evolu tion.

Acknowledgments Comments and suggestions by Larry Braile, Ray Durrheim, Leonard E. Johnson, Thorne Lay, and Justin Revenaugh are greatly appreciated. Our work has been sup ported by the German Research Foundation (DFG), the U.S. Geological Survey Deep Con tinental Studies Program, and the U.S. Na tional Science Foundation, Continental Dy namics Program. 20

References

CO

z(km)

Fig. 4.

Conductivity structure of three crustal types, A, B, and C.


Adam, A . , The change of electrical structure be tween an orogenic and an ancient area (Car pathians and Russian Platform), / Geomagn. Geoelectr.,32, 1, 1980.


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western United States, Geol. Soc. Am. Bull., 100, 1361, 1988. Meissner, R., The Continental Crust: A Geophysical Approach, 426 pp., Academic Press, London, 1986. Mooney, W . D., Seismic methods for determining earthquake source parameters and lithospheric structure, in Geophysical Framework of the Con tinental United States, Geol. Soc. Am. Mem., vol. 172, edited by L. C. Pakiser, and W . D. Mooney, pp. 11-34, 1989. Mooney, W . D., and T. M. Brocher, Coincident seismic reflection and refraction measurements of the continental lithosphere: a global review, Rev. Geophys., 25, 723, 1987. Price, R. A . , t h e southeastern Canadian Cordillera: thrust faulting tectonic wedging and delamination of the lithosphere, J. Struct. Geol., 8, 239, 1986. Schweizerische Arbeitsgruppe fur Reflexionsseismik, Vorlaufige Ergebnisse der Alpentraversen des NFP-20, Geologische Tiefenstruktur

de Schweiz, Bull. Ver. Schweiz. Pet. Geol. Lug., 1-30, 1988. Stanley, W . D., W . D. Mooney, and G. S. Fuis, Deep crustal structure of the Cascade Range and surrounding regions from seismic refraction and magnetotelluric data, J. Geophys. Res., 95, 19 419 1990a. Stanley, W . D., V. F. Labson, W . J. Nokleberg, B. Csejtey, and M. A . Fisher, The Denali fault sys tem and Alaska Range of Alaska: Evidence for underplated Mesozoic flysch from magnetotellu ric surveys, Geol. Soc. Am. Bull., 103, 160, 1990b. Strehlau, J., and R. Meissner, Estimation of crustal viscosities [0090] and their role in geodynamic processes, in The Composition, Structure and Dynamics of the Lithosphere-Asthenosphere Sys tem, Geodyn. Ser., vol. 16, edited by K. Fuchs, and C. Froidevaux, pp. 69-87, AGU, Washington, D. C , 1987. Zoback, M. L., et al., Global patterns of tectonic stress, Nature, 341, 291, 1989.

Galileo Captures Asteroid Image PAGES 537-539 On October 29 the Galileo spacecraft took the first close-up image ever of an as teroid, from about 16,200 kilometers away. Galileo's encounter with Gaspra, an interme diate-sized asteroid, on the spacecraft's way to Jupiter, gave the mission's scientists a glimpse of Gaspra's "violent" geological his tory. According to Galileo project scientist Torrence Johnson, the encounter aimed to get a first look at the asteroid, assess its composi tion, and survey its environment. Michael

Belton, imaging team leader, said that the navigators used "entirely unprecedented" precision to point the camera at Gaspra. The team unveiled the image, taken through green filters, at a November 14 news briefing at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The camera operated ex actly to specification, producing multicolor coverage of the asteroid. It will be "a num ber of weeks," however, before a color im age is reconstructed from the data now in hand from four different color filters.

i

Portrait of Gaspra taken by the Galileo spacecraft is the first close-up picture of an (Diagram shows spin axis of Gaspra and direction to the Sun.)


asteroid.