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P-wave anisotropy in the lower lithosphere
58
Earth and Planetary Science Letters, 99 (1990) 58-65
Elsevier Science Publishers B.V., Amsterdam
[FB]
P-wave anisotropy in the lower lithosphere C.J. Bean * and A.W.B. Jacob Dublin Institute for Advanced Studies, School of Cosmic Physics, 5 Merrion Sq., Dublin 2 (Ireland)
Received November 23, 1989; revised version accepted March 28, 1990 ABSTRACT Long-range controlled source seismic experiments yield detailed information about the velocity depth structure of the lower lithosphere. Between Ireland and Northern Britain three such profiles sample almost the same portion of the subcrustal lithosphere. Three zones with steep velocity gradients have been detected between 30 km and 90 km depth. Both the pattern and velocity of the arrivals are incompatible with isotropy. Preferential alignment of olivine crystals, with an azimuth ca. N25 °E for the fast axis, could explain the observations, with the more highly aligned zones occurring in bands, or layers, separated vertically by zones in which the degree of alignment is slight or absent. We suggest that a shear heating mechanism may have played a part in producing these patterns. This deformation is most likely to be of Mesozoic or early Cenozoic age. It is argued that the upper mantle is not necessarily a strain marker for the last major orogenic episode, as recent findings have suggested [1], since it may undergo deformation which decouples from the brittle upper crust and hence is not "transmitted" to the Earth's surface.
1. Introduction A s m o r e d e t a i l e d studies of the E a r t h ' s seismic structure have b e e n made, it has b e c o m e clear that seismic a n i s o t r o p y is w i d e s p r e a d . T h e r e are m a n y i n d i c a t i o n s that the lithospheric m a n t l e d i s p l a y s seismic anisotropy. Pn velocity a n i s o t r o p y has been o b s e r v e d b o t h b e n e a t h the oceans [2-4] a n d the continents [5]. P, n o r m a l l y describes the velocity d i s t r i b u t i o n i m m e d i a t e l y b e l o w the M o h o . F u r thermore, C r a m p i n [6] o b s e r v e d higher m o d e surface wave a n o m a l i e s for c o n t i n e n t a l p a t h s a n d s u b s e q u e n t l y he d e s c r i b e d how this c o u l d b e used as a diagnostic of a n i s o t r o p i c layering [7]. Surface wave studies have also i n d i c a t e d the p r e s e n c e of a n i s o t r o p y in the m a n t l e b e n e a t h the oceans [ 8 10]. F u c h s [11] has shown t h a t results f r o m long range profiles in s o u t h e r n G e r m a n y are c o n s i s t e n t with P-wave a z i m u t h a l a n i s o t r o p y , d e e p in the lithosphere. Also, K i n d et al. [12] a n d Silver a n d C h a n [1] have o b s e r v e d S-wave splitting on the S K S phase a n d the evidence i n d i c a t e d that it arises in the u p p e r mantle. It c a n thus b e said that P-wave, S-wave a n d surface wave d a t a all p o i n t
* Present address: Petroleum Geology Unit, Geology Department, University College, Belfield, Dublin 4, Ireland. 0012-821X/90/$03.50
t o w a r d s the p r e s e n c e of seismic a n i s o t r o p y in the lower lithosphere. However, the p a u c i t y of d a t a f r o m a n y o n e region has m e a n t that these m e t h o d s have n o t b e e n able to resolve d e t a i l e d a n i s o t r o p i c structure at d e p t h . O n e reason for thinking that the lower lithosphere might be w o r t h y of s t u d y is a simple one. T h e a p p a r e n t velocities on m a n y long-range seismic profiles i n d i c a t e high P-wave velocities which are h a r d to e x p l a i n with a n i s o t r o p i c model. These velocities are a p p a r e n t b e c a u s e the profiles are n o t usually reversed b u t the high velocities are w i d e s p r e a d e n o u g h to e n c o u r a g e further study. N o t o n l y are high velocities f o u n d in the c o n t i n e n tal lower l i t h o s p h e r e in m a n y regions, there are low velocities too. In fact, the c o n t i n e n t a l lower l i t h o s p h e r e has b e e n f o u n d to possess a r a t h e r d e t a i l e d v e l o c i t y - d e p t h structure with, in a n u m b e r o f cases, a l t e r n a t i n g high- a n d low-velocity layers [13]. T h e v e l o c i t y - d e p t h structures in our Fig. 2 are j u s t two examples. F u r t h e r m o r e , the " l a y e r s " are n o t s h a r p l y defined a n d the transition b e t w e e n t h e m m a y be gradual. This overall p i c t u r e of the lower c o n t i n e n t a l lithosphere e n c o u r a g e d the i d e a that n o t o n l y m i g h t there be sub-horizontally stratified zones of crystalline a l i g n m e n t b u t that these zones might be studied in some detail in the region shown in Fig. 1.
© 1990 - Elsevier Science Publishers B.V.
P-WAVE ANISOTROPY IN THE LOWER LITHOSPHERE
59
58 °
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Fig. 1. Location m a p for the three profiles. The thick lines denote zones of wide angle reflections in the lower lithosphere i.e. the
zones in which the lower lithosphere is being sampled. In this work we have studied the data from three overlapping long-range refraction profiles. All these profiles sample the continental lower
CSSP
LISPB
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2. Data and interpretation
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lithosphere between Ireland and Northern Britain (Fig. 1) and each samples it at a different azimuth. Three azimuths is the minimum requirement for control in a P-wave study of seismic anisotropy. This degree of control has not, to our knowledge, been previously available for the continental lower lithosphere.
5
6 7 8 VELOCITY (KX/S}
9
Fig. 2. (a) CSSP velocity-depth ( V - Z ) function for the lower lithosphere. (b) LISPB velocity-depth ( V - Z ) function for the lower lithosphere [17,18].
Figure 1 shows the locations of three refraction/wide angle reflection profiles: (1) The Lithospheric Seismic Profile in Britain (LISPS) [14]; (2) the Caledonian Suture Seismic Project (CSSP), both in Ireland [15] and Britain [16]; and (3) a more recent experiment BB87, in Autumn 1987. In the LISPB experiment, shots were on average over 100 km apart and the average station spacing was about 3 km. Both crustal and detailed upper mantle interpretations of these data have already been
60
C.J. B E A N A N D A.W.B. J A C O B
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Fig. 3. (a) A CSSP seismic section. (b) Isotropic synthetic seismic section for model in (a). (c) Anisotropic synthetic seismic section for a 40 o view of layered preferentiallyaligned olivinein the lower lithosphere(see text).
published [14,17,18]. During CSSP, thirty shots in the North Sea (at ca. 4 km intervals) were recorded on twenty eight stations in Ireland producing a series of overlapping seismic sections which provide good reversed control on part of the subcrustal lithosphere between Ireland and Britain. This degree of control is important as it gives real and not apparent velocities. Irish crustal structure has already been derived [15] and crustal structures for Britain have also been determined [16]. One long range seismic section from CSSP is given in Fig. 3a. Three principal phases (labeled Pa, P2 and P3) were identified in the data in Fig. 3a. Through standard (isotropic) travel-time and amplitude
modelling [19,20] a velocity-depth structure was derived for these data. This velocity-depth (V-Z) function is shown in Fig. 2a and compared with the V - Z function of Faber [17] and Faber and Bamford [18] in Fig. 2b. Figure 3b shows the standard (isotropic) synthetic section for the V - Z function in Fig. 2(a). The similarity between the two V - Z functions in Fig. 2 is striking. The profiles are almost at right angles to each other (100o/80 o ), yet both show three zones in which similar velocities are attained at similar depth i.e. ca. 45, 70 and 85 km depth (corresponding to phases P1, P2 and P3 respectively). Both the nature and depth of the P1 zone is known to vary to the north [18] and west (unpubl. CSSP data). How-
P-WAVE ANISOTROPY
IN THE LOWER
61
LITHOSPHERE
ever, zones P2 and P3 are more laterally continuous both in nature and depth. The similarity between these two profiles (CSSP and LISPB) is such that, were it not for the very high velocities observed, it would not seem necessary to invoke an anisotropic solution. However, when the BB87 data and the CSSP data are considered together, giving us additional azimuthal control, a different picture emerges. While recognising the poor quality of the BB87 data, a genuine dissimilarity between the BB87 and CSSP data seems to be present.
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Fig. 4. (a) BB87 seismic section. (b) Anisotropic synthetic seismic section for a 15 o view of layered preferentially aligned olivine in the lower lithosphere (see text). (c) Anisotropic synthetic seismic section for a 75 ° view of layered preferentially aligned olivine in the lower lithosphere (see text).
(a) The strong "pulse like" arrivals in the CSSP data beyond 500 km distance (Fig. 3a), are not seen in BB87 (Fig. 4a). (b) Arrivals between 500-600 km in CSSP are confined to a narrow time window across the section. In BB87 they are distributed throughout the section. (c) Numerous en-echelon branches, some with very high apparent velocities, seen in the BB87 data are not evident in CSSP. These observations indicate the possible presence of azimuthal anisotropy. Furthermore, the modelled velocities and velocity gradients, (Fig. 2), are also at odds with an isotropic mantle lithosphere. For example, 8.6 k m / s at 85 km depth is well in excess of the predicted isotropic velocity of 7.85 k m / s (taking 80% olivine, 20% pyroxene and a calculated geotherm for the region). Density differences needed to produce the observed velocity variations would most probably lead to material instability and subsequent flow [21]. An anisotropic solution is possible if either angle between CSSP and LISPB, i.e. (100 ° / 8 0 o ), is bisected by an axis of crystallographic symmetry. Both profiles show very similar V - Z functions and therefore must be "viewing" a similar degree of anisotropy. This represents two possibilities, either the fast direction bisects the 100 ° or the 80 o angle. A search for an anisotropic solution was undertaken based on this hypothesis. Xenolith data from in and around the Midland Valley in Scotland, indicates, to a first approximation, an upper mantle composition of 80% olivine and 20% pyroxene [22]. Assuming these values and adopting laboratory determined elastic parameters for olivine [23], a structure was constructed in which anisotropic layers corresponded to the zones of observed velocity perturbations (i.e. ca. 45, 70, and 85 km depth). The degree of olivine anisotropy was allowed to vary within each zone and orthopyroxene was randomly distributed. The olivine [100] and [001] axes were in the horizontal plane. The [010] axis was vertical. Zero anisotropy (isotropy) was assumed at the extrema of a given zone, with maximum anisotropy at the zone's centre. After each iteration the structure was viewed at both an azimuth of 40 ° to the [100] (fast) axis (80 ° angle bisected) and 50 ° (100 ° angle bisected). A suitable fit, for zone three, could not be achieved by a 50 ° view for any
62
c.J. BEANAND A.W.B.JACOB
percentage of preferred olivine alignment. After several iterations, olivine anisotropies of 10, 40, and 80% were adopted for the centres of zones 1, 2, and 3 respectively. A synthetic section, for anisotropic media [24-26], for this structure viewed at 40 o, is given in Fig. 3c. A comparison of Fig. 3c with Fig. 3b shows that the principal features of the isotropically generated section have been reproduced by the anisotropic model. Such a fit could not be realised with a view at 50 o for any combination of anisotropies in zone 1, 2, and 3. This suggests a N E - S W as opposed to a S E - N W fast direction. The BB87 line is at an intermediate angle to CSSP and LISPB. It lies 15 o off a possible N E - S W fast direction and 75 o off a possible S E - N W fast direction (Fig. 5). A 15 ° view of the previous anisotropic structure shows a complex pattern of multiple arrivals in the synthetic data (Fig. 4b), there are several en-echelon branches with high apparent velocities in the synthetic seismogram. The pattern is similar to that indicated in the real data in Fig. 4a. Looking through the structure at 75 ° (i.e. assuming a S E - N W fast direction) resuits in a mismatch between the real and synthetic data Fig. 4c. The arrivals are concentrated in a narrow time window and there are no en-echelon branches in Fig. 4c. Therefore a N E - S W (ca. N25 °E) fast propagation direction best explains the CSSP, LISPB and BB87 data sets. Because the relatively large BB87 source (2 tonnes) gave a disappointingly weak signal this result is not con-
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clusive, but it is consistent with our deductions from the other two lines. The two synthetic sections in Fig. 4 illustrate an important point. If our diagnosis is correct, and the velocity variations with depth are due to anisotropy resulting from crystalline alignment, then the whole appearance of a seismic section should change as the azimuth changes. This is why, even though the BB87 section is a disappointing one, we consider it is good enough to discriminate between the two synthetic sections in Fig. 4. 3. S o m e tectonic implications
The model in section 2 consists of three horizontal anisotropic "layers" embedded in an otherwise isotropic lithospheric mantle consisting of 80% olivine and 20% orthopyroxene. The centres of the "layers" lie at 45, 70, and 85 km depth. The percentage of aligned olivine is 10, 40, and 80% respectively and the zones have half widths of 4, 4, and 1.5 km depth. The maximum P-wave azimuthal anisotropy is ca. 6%. In the anisotropic zones, the olivine a-axis (fast axis) is horizontal and points in approximately a N25 ° E direction. The orientation of the fast direction, ca. N25 o E, is probably the direction of most recent flow in the subcrustal lithosphere in this region as the olivine a-axis almost certainly aligns parallel to the flow plane and points in the flow direction during steady-state flow [27]. Two questions should be addressed: (a) When did these features form? (b) Why has deformation occurred in a banded fashion? Zoback and Fuchs [28] have found a strong positive correlation between present day maximum horizontal stress directions and present day plate motion directions. So, if crystalline alignment are present day features, they should correlate with measured present day maximum horizontal compressive stress directions. For "large" strains, the fast olivine [100] axis approaches the extension direction [1]. That is, the fast axis should lie perpendicular to the direction of maximum horizontal compressive stress. The maximum horizontal compressive stress direction in this region is N26 ° W [29]. There is no apparent correlation between this value and the orient.ation of the fast olivine axis. The crystalline alignments are thus likely to be relic, "frozen in" features.
P-WAVE ANISOTROPY
63
IN THE LOWER LITHOSPHERE
A maximum possible age for the most recent lower lithospheric deformations can be inferred through dating surface deformations which have a deep origin (e.g. sedimentary basins). Such deformations must have been "transmitted through" the lower lithosphere and hence will have deformed it also. However, as the lithosphere has a vertically inhomogeneous rheology, deformation might occur in the mantle lithosphere and not express itself at the surface. Hence, a minimum age cannot be estimated in this way. Although the structural trend in Ireland and Northern Britain is predominantly Caledonian in age, the region is surrounded by numerous early Mesozoic sedimentary basins which indicates that the Mesozoic may represent a maximum age for the proposed mantle fabrics. Therefore, as there is no apparent correlation with surface features, a Mesozoic to early Cenozoic age may be inferred. This interpretation contrasts with the recent findings of Silver and Chan [1]. Using long period S-wave data, (which has a different resolution to long-range profiling) they suggest an Upper Paleozoic (Carboniferous) age for upper mantle deformation in central Europe, based on surface geology observations. However, if deformations in the lower lithosphere are younger than surface structures, the E - W extension periods in either the late Jurassic or mid-Tertiary could also account for their observations. The mode of lower lithospheric deformation may have important consequences for the deformation of the lithosphere as a whole (including the crust). Are deep seated horizontal stresses transmitted "efficiently" Velocity
through the lithosphere during tectonically active periods or are they "attenuated" through a combination of brittle and ductile deformations in the mantle lithosphere and lower crust?. 4. Possible mode of deformation
A surprising result of long-range refraction/ wide angle reflection profiling has been the discovery of alternate layers of high and low velocities below the Moho. If anisotropy in the lower lithosphere has been caused by a constant flow episode in the asthenosphere and subsequent lithospheric growth, then a single region of anisotropic material would be expected. This has not been observed, instead, deformation is concentrated in a series of lower lithospheric layers. Large-scale shearing may be responsible for these patterns. Such shearing would produce substantial amounts of heat [30]. A relative velocity of 2 c m / y r between sliding blocks results in a temperature rise of ca. 250 ° C at 45 km depth. This heat is then free to leak into the surrounding material, increasing material ductility and rendering an envelope surrounding the shear zone more susceptible to preferred crystalline alignment by the ambient stress field (Fig. 6). The existence of upper mantle shear zones, which are not necessarily horizontal, is supported by deep vertical reflection data. Bright laterally continuous thin reflecting horizons have been detected, at appropriate depths in the mantle around Britain and Ireland, which are best interpreted as shear zones [31]. It is possible that the vertical reflection data detects % alignment
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/////////?F////////A t~
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.
Temperature
Fig. 6. Schematic representation of selective preferential alignment of olivine crystals in a zone of elevated temperature above and below a shear zone. The temperature anomalies associated with lithospheric scale shear zones can be ten times wider than the shear zone. This envelope of anomalous temperature may reduce material viscosity in the vicinity of a shear zone sufficient to allow the ambient stress field to preferentially align olivine crystals in a bell shaped region around the shear horizon.
64
the thin zone which has suffered severe m e c h a n i cal d e f o r m a t i o n (and hence chemical alteration), whereas the wide aperture data picks out a b r o a d zone which has u n d e r g o n e a more " p a s s i v e " creep-like d e f o r m a t i o n i n the regions of elevated temperature. A m e c h a n i s m of this kind would i n h i b i t the t r a n s m i s s i o n of horizontal stress through the lithosphere a n d lead to a varying strain d e p t h function. T h e u p p e r crust could c o n s e q u e n t l y be isolated from these deformations, preserving the scars of earlier tectonic episodes.
C.J. B E A N A N D A . W . B . J A C O B
4
5 6 7
8
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5. Conclusions We conclude that preferred a l i g n m e n t of olivine crystals is a n i m p o r t a n t feature of the subcrustal lithosphere in N o r t h w e s t e r n Europe. C u r r e n t data indicates a ca. N25 ° E fast direction for the anisotropic layers. This d e f o r m a t i o n is most p r o b a b l y of Mesozoic or early Cenozoic age. A l i g n m e n t occurs in a b a n d e d fashion which may, i n part, be due to a reduced viscosity associated with a shear heating mechanism. W e suggest that u p p e r m a n t l e fabrics may, i n general, be y o u n g e r t h a n the large scale d e f o r m a t i o n trends i n the overlying brittle crust. F u r t h e r controlled source l o n g - r a n g e seismic data with good azimuthal control is needed. I n particular, acquisition of long range data (including S-wave data) above k n o w n good reflectors of vertical i n c i d e n t energy is very desirable.
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Acknowledgements 16
We acknowledge the k i n d assistance of the University of Karlsruhe, F . R . G . I n p a r t i c u l a r we t h a n k C. Prodehl, A. R u t h a r d t , R Stangl a n d B. Nolte. The British Geological Survey a n d D u r h a m University, U.K. are t h a n k e d for seismic sources which, speculatively recorded i n Ireland, provided unexpectedly good signals at long range.
17
References
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1 P.G. Silver and Chan, Implications for continental structure and evolution from seismic anisotropy, Nature 355, 34-39, 1988. 2 H.H. Hess, Seismic anisotropy of the uppermost mantle under oceans. Nature, 203, 629-631, 1964. 3 G.B. Morris, R.W. Raitt and G.G. Shor, Velocity ani-
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sotropy and delay-time maps of the mantle near Hawaii, J. Geophys. Res. 74, 4300-4316, 1969. R.W. Raitt, G.G. Shor, G.B. Morris and H.K. Kirk, Mantle anisotropy in the pacific ocean, Tectonophysics 12, 173186, 1971. D. Bamford, Pn Velocity anisotropy in a continental upper. Geophys. J. R. Astron. Soc. 49, 29-48, 1977. S. Crampin, Coupled Raleigh-Love second modes. Geophys. J. R. Astron. Soc 12, 229-235, 1967. S. Crampin, Distinctive particle motion of surface waves as a diagnostic of anisotropic layering. Geophys. J. R. Astron. Soc. 40, 177-186, 1975. D.W. Forsyth, The early structural evolution and anisotropy of the oceanic upper mantle, Geophys. J. R. Astron. Soc. 43, 103-162, 1975. S.C. Kirkwood and S. Crampin, Surface wave propagation in an ocean basin with an anisotropic upper mantle: observations of polarization anomalies, Geophys. J. R. Astron. Soc. 64, 484-497, 1981. C.E. Nishimura and D.W. Forsyth, The anisotropic structure of the upper mantle in the Pacific, Geophys. J. R. Astron. Soc. 96, 203-229, 1989. K. Fuchs, Recently formed elastic anisotropy and petrological models for the continental suberustal lithosphere in southern Germany, Phys. Earth Planet. Inter. 31, 93-118, 1983. R. Kind, G.L. Kosarev, G.L. Makeyeva and L.P. Vinnik, Observations of laterally inhomogeneous anisotropy in the continental lithosphere, Nature 318, 358-361, 1985. K.Fuchs, L.P. Vinnik and C. Prodehl, Exploring heterogeneities of the continental mantle by high resolution seismic experiments, in: Composition, Structure and Dynamics of the Lithosphere-Asthenosphere System, K.Fuchs and C. Froideaux, eds. pp. 137-154, A.G.U., Washington, D.C., 1987. D. Bamford, S. Faber, A.W.B. Jacob, W. Kaminski, K. Nunn, C. Prodehl, K. Fuchs, R. King and P. Willmore, A lithospheric seismic profile in Britain I, preliminary results, Geophys. J. R. Astron. Soc. 52, 43-66, 1978. A.W.B. Jacob, W. Kaminski, T. Murphy, W.E.A. Phillips and C. Prodehl, A crustal model for a NW-SE profile through Ireland, Tectonophysics 113, 75-103, 1985. M.P.H. Bott, R.E. Long, A.S.P. Green, A.H.J. Lewis and D.L. Stevenson, Crustal structures south of the Iapetus suture beneath Northern England, Nature 314, 724-727, 1985. S. Faber, Refraktionsseismische Untersuchung der Lithosph~e unter den Britischen Inseln, Ph.D. thesis, Karlsruhe Univ., 1978. S. Faber and D. Bamford, Lithospheric structural contrasts across the Caledonides of Northern Britain. Tectonophysics 56, 17-30, 1979. K. Fuchs and G. Mtiller, Computation of synthetic seismograms with the reflectivity method and comparison with observations, Geophys. J. R. Astron. Soc. 23, 417-433, 1971. R. Kind, The reflectivity method for different source and receiver structures and a comparison with GRF data, J. Geophys. 58, 146-152, 1985. R.O. Meissner and E.R. Fltih, Probable relations between
P-WAVEANISOTROPY IN THE LOWER LITHOSPHERE
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seismic anisotropy and the fine structure of the lithosphere, J. Geophys. 45, 349-352, 1979. R.H. Hunter, B.G.J. Upton and P. Aspen, Meta-igneous granulites and ultramafic xenoliths from basalts of the Midland Valley of Scotland: petrology and mineralogy of the lower crust and upper mantle, Trans. R. Soc. Edin., Earth Sci. 75, 75-83, 1984. M. Kumazama and O.C. Anderson, Elastic moduli, pressure derivatives and temperature derivatives of single-crystal olivine and single-crystal forstedte. J. Geophys. Res. 74, 5961-5972, 1969. D.C. Booth and S. Crampin, The anisotropic reflectivity technique: theory, Geophys. J. R. Astron. Soc. 72, 755-766, 1983. M. Griinewald, Synthetic seismograms for anisotropic gradient zones, Terra Cognita 7, 294, 1987. B. Nolte, Diploma thesis, Karlsruhe Univ., 1988.
65 27 A. Nicolas and N.I. Christenscn, Formation of anisotropy in upper mantle peridotitcs--A review, in: Composition, Structure and Dynamics of the Lithosphere-Asthenosphere System, K. Fuchs and C. Froideaux, eds. pp. 137-154, A.G.U., Washington, D.C., 1987. 28 M.L. Zoback and K. Fuchs, The world stress map project, Ann. Geophys., EGS XIV Gen. Assem. 7 (Special Issue), 1989. 29 N.R. Brereton and C.J. Evans, Report of regional geophysics research group, BGS, No. RG 87/14, 1987. 30 D.A. Yuen, L. Fleitout, C. Froideaux and G. Schubert, Shear deformation zones along major transform faults and subducting slabs, Geophys. J. R. Astron. Soc. 54, 93-119. 1978. 31 M. Warner and S. McGeary, Seismic reflection coefficients from mantle fault zones, Geophys. J. R. Astron. Soc. 89, 223-230, 1987.
Earth and Planetary Science Letters, 99 (1990) 58-65
Elsevier Science Publishers B.V., Amsterdam
[FB]
P-wave anisotropy in the lower lithosphere C.J. Bean * and A.W.B. Jacob Dublin Institute for Advanced Studies, School of Cosmic Physics, 5 Merrion Sq., Dublin 2 (Ireland)
Received November 23, 1989; revised version accepted March 28, 1990 ABSTRACT Long-range controlled source seismic experiments yield detailed information about the velocity depth structure of the lower lithosphere. Between Ireland and Northern Britain three such profiles sample almost the same portion of the subcrustal lithosphere. Three zones with steep velocity gradients have been detected between 30 km and 90 km depth. Both the pattern and velocity of the arrivals are incompatible with isotropy. Preferential alignment of olivine crystals, with an azimuth ca. N25 °E for the fast axis, could explain the observations, with the more highly aligned zones occurring in bands, or layers, separated vertically by zones in which the degree of alignment is slight or absent. We suggest that a shear heating mechanism may have played a part in producing these patterns. This deformation is most likely to be of Mesozoic or early Cenozoic age. It is argued that the upper mantle is not necessarily a strain marker for the last major orogenic episode, as recent findings have suggested [1], since it may undergo deformation which decouples from the brittle upper crust and hence is not "transmitted" to the Earth's surface.
1. Introduction A s m o r e d e t a i l e d studies of the E a r t h ' s seismic structure have b e e n made, it has b e c o m e clear that seismic a n i s o t r o p y is w i d e s p r e a d . T h e r e are m a n y i n d i c a t i o n s that the lithospheric m a n t l e d i s p l a y s seismic anisotropy. Pn velocity a n i s o t r o p y has been o b s e r v e d b o t h b e n e a t h the oceans [2-4] a n d the continents [5]. P, n o r m a l l y describes the velocity d i s t r i b u t i o n i m m e d i a t e l y b e l o w the M o h o . F u r thermore, C r a m p i n [6] o b s e r v e d higher m o d e surface wave a n o m a l i e s for c o n t i n e n t a l p a t h s a n d s u b s e q u e n t l y he d e s c r i b e d how this c o u l d b e used as a diagnostic of a n i s o t r o p i c layering [7]. Surface wave studies have also i n d i c a t e d the p r e s e n c e of a n i s o t r o p y in the m a n t l e b e n e a t h the oceans [ 8 10]. F u c h s [11] has shown t h a t results f r o m long range profiles in s o u t h e r n G e r m a n y are c o n s i s t e n t with P-wave a z i m u t h a l a n i s o t r o p y , d e e p in the lithosphere. Also, K i n d et al. [12] a n d Silver a n d C h a n [1] have o b s e r v e d S-wave splitting on the S K S phase a n d the evidence i n d i c a t e d that it arises in the u p p e r mantle. It c a n thus b e said that P-wave, S-wave a n d surface wave d a t a all p o i n t
* Present address: Petroleum Geology Unit, Geology Department, University College, Belfield, Dublin 4, Ireland. 0012-821X/90/$03.50
t o w a r d s the p r e s e n c e of seismic a n i s o t r o p y in the lower lithosphere. However, the p a u c i t y of d a t a f r o m a n y o n e region has m e a n t that these m e t h o d s have n o t b e e n able to resolve d e t a i l e d a n i s o t r o p i c structure at d e p t h . O n e reason for thinking that the lower lithosphere might be w o r t h y of s t u d y is a simple one. T h e a p p a r e n t velocities on m a n y long-range seismic profiles i n d i c a t e high P-wave velocities which are h a r d to e x p l a i n with a n i s o t r o p i c model. These velocities are a p p a r e n t b e c a u s e the profiles are n o t usually reversed b u t the high velocities are w i d e s p r e a d e n o u g h to e n c o u r a g e further study. N o t o n l y are high velocities f o u n d in the c o n t i n e n tal lower l i t h o s p h e r e in m a n y regions, there are low velocities too. In fact, the c o n t i n e n t a l lower l i t h o s p h e r e has b e e n f o u n d to possess a r a t h e r d e t a i l e d v e l o c i t y - d e p t h structure with, in a n u m b e r o f cases, a l t e r n a t i n g high- a n d low-velocity layers [13]. T h e v e l o c i t y - d e p t h structures in our Fig. 2 are j u s t two examples. F u r t h e r m o r e , the " l a y e r s " are n o t s h a r p l y defined a n d the transition b e t w e e n t h e m m a y be gradual. This overall p i c t u r e of the lower c o n t i n e n t a l lithosphere e n c o u r a g e d the i d e a that n o t o n l y m i g h t there be sub-horizontally stratified zones of crystalline a l i g n m e n t b u t that these zones might be studied in some detail in the region shown in Fig. 1.
© 1990 - Elsevier Science Publishers B.V.
P-WAVE ANISOTROPY IN THE LOWER LITHOSPHERE
59
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Fig. 1. Location m a p for the three profiles. The thick lines denote zones of wide angle reflections in the lower lithosphere i.e. the
zones in which the lower lithosphere is being sampled. In this work we have studied the data from three overlapping long-range refraction profiles. All these profiles sample the continental lower
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2. Data and interpretation
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lithosphere between Ireland and Northern Britain (Fig. 1) and each samples it at a different azimuth. Three azimuths is the minimum requirement for control in a P-wave study of seismic anisotropy. This degree of control has not, to our knowledge, been previously available for the continental lower lithosphere.
5
6 7 8 VELOCITY (KX/S}
9
Fig. 2. (a) CSSP velocity-depth ( V - Z ) function for the lower lithosphere. (b) LISPB velocity-depth ( V - Z ) function for the lower lithosphere [17,18].
Figure 1 shows the locations of three refraction/wide angle reflection profiles: (1) The Lithospheric Seismic Profile in Britain (LISPS) [14]; (2) the Caledonian Suture Seismic Project (CSSP), both in Ireland [15] and Britain [16]; and (3) a more recent experiment BB87, in Autumn 1987. In the LISPB experiment, shots were on average over 100 km apart and the average station spacing was about 3 km. Both crustal and detailed upper mantle interpretations of these data have already been
60
C.J. B E A N A N D A.W.B. J A C O B
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Fig. 3. (a) A CSSP seismic section. (b) Isotropic synthetic seismic section for model in (a). (c) Anisotropic synthetic seismic section for a 40 o view of layered preferentiallyaligned olivinein the lower lithosphere(see text).
published [14,17,18]. During CSSP, thirty shots in the North Sea (at ca. 4 km intervals) were recorded on twenty eight stations in Ireland producing a series of overlapping seismic sections which provide good reversed control on part of the subcrustal lithosphere between Ireland and Britain. This degree of control is important as it gives real and not apparent velocities. Irish crustal structure has already been derived [15] and crustal structures for Britain have also been determined [16]. One long range seismic section from CSSP is given in Fig. 3a. Three principal phases (labeled Pa, P2 and P3) were identified in the data in Fig. 3a. Through standard (isotropic) travel-time and amplitude
modelling [19,20] a velocity-depth structure was derived for these data. This velocity-depth (V-Z) function is shown in Fig. 2a and compared with the V - Z function of Faber [17] and Faber and Bamford [18] in Fig. 2b. Figure 3b shows the standard (isotropic) synthetic section for the V - Z function in Fig. 2(a). The similarity between the two V - Z functions in Fig. 2 is striking. The profiles are almost at right angles to each other (100o/80 o ), yet both show three zones in which similar velocities are attained at similar depth i.e. ca. 45, 70 and 85 km depth (corresponding to phases P1, P2 and P3 respectively). Both the nature and depth of the P1 zone is known to vary to the north [18] and west (unpubl. CSSP data). How-
P-WAVE ANISOTROPY
IN THE LOWER
61
LITHOSPHERE
ever, zones P2 and P3 are more laterally continuous both in nature and depth. The similarity between these two profiles (CSSP and LISPB) is such that, were it not for the very high velocities observed, it would not seem necessary to invoke an anisotropic solution. However, when the BB87 data and the CSSP data are considered together, giving us additional azimuthal control, a different picture emerges. While recognising the poor quality of the BB87 data, a genuine dissimilarity between the BB87 and CSSP data seems to be present.
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Fig. 4. (a) BB87 seismic section. (b) Anisotropic synthetic seismic section for a 15 o view of layered preferentially aligned olivine in the lower lithosphere (see text). (c) Anisotropic synthetic seismic section for a 75 ° view of layered preferentially aligned olivine in the lower lithosphere (see text).
(a) The strong "pulse like" arrivals in the CSSP data beyond 500 km distance (Fig. 3a), are not seen in BB87 (Fig. 4a). (b) Arrivals between 500-600 km in CSSP are confined to a narrow time window across the section. In BB87 they are distributed throughout the section. (c) Numerous en-echelon branches, some with very high apparent velocities, seen in the BB87 data are not evident in CSSP. These observations indicate the possible presence of azimuthal anisotropy. Furthermore, the modelled velocities and velocity gradients, (Fig. 2), are also at odds with an isotropic mantle lithosphere. For example, 8.6 k m / s at 85 km depth is well in excess of the predicted isotropic velocity of 7.85 k m / s (taking 80% olivine, 20% pyroxene and a calculated geotherm for the region). Density differences needed to produce the observed velocity variations would most probably lead to material instability and subsequent flow [21]. An anisotropic solution is possible if either angle between CSSP and LISPB, i.e. (100 ° / 8 0 o ), is bisected by an axis of crystallographic symmetry. Both profiles show very similar V - Z functions and therefore must be "viewing" a similar degree of anisotropy. This represents two possibilities, either the fast direction bisects the 100 ° or the 80 o angle. A search for an anisotropic solution was undertaken based on this hypothesis. Xenolith data from in and around the Midland Valley in Scotland, indicates, to a first approximation, an upper mantle composition of 80% olivine and 20% pyroxene [22]. Assuming these values and adopting laboratory determined elastic parameters for olivine [23], a structure was constructed in which anisotropic layers corresponded to the zones of observed velocity perturbations (i.e. ca. 45, 70, and 85 km depth). The degree of olivine anisotropy was allowed to vary within each zone and orthopyroxene was randomly distributed. The olivine [100] and [001] axes were in the horizontal plane. The [010] axis was vertical. Zero anisotropy (isotropy) was assumed at the extrema of a given zone, with maximum anisotropy at the zone's centre. After each iteration the structure was viewed at both an azimuth of 40 ° to the [100] (fast) axis (80 ° angle bisected) and 50 ° (100 ° angle bisected). A suitable fit, for zone three, could not be achieved by a 50 ° view for any
62
c.J. BEANAND A.W.B.JACOB
percentage of preferred olivine alignment. After several iterations, olivine anisotropies of 10, 40, and 80% were adopted for the centres of zones 1, 2, and 3 respectively. A synthetic section, for anisotropic media [24-26], for this structure viewed at 40 o, is given in Fig. 3c. A comparison of Fig. 3c with Fig. 3b shows that the principal features of the isotropically generated section have been reproduced by the anisotropic model. Such a fit could not be realised with a view at 50 o for any combination of anisotropies in zone 1, 2, and 3. This suggests a N E - S W as opposed to a S E - N W fast direction. The BB87 line is at an intermediate angle to CSSP and LISPB. It lies 15 o off a possible N E - S W fast direction and 75 o off a possible S E - N W fast direction (Fig. 5). A 15 ° view of the previous anisotropic structure shows a complex pattern of multiple arrivals in the synthetic data (Fig. 4b), there are several en-echelon branches with high apparent velocities in the synthetic seismogram. The pattern is similar to that indicated in the real data in Fig. 4a. Looking through the structure at 75 ° (i.e. assuming a S E - N W fast direction) resuits in a mismatch between the real and synthetic data Fig. 4c. The arrivals are concentrated in a narrow time window and there are no en-echelon branches in Fig. 4c. Therefore a N E - S W (ca. N25 °E) fast propagation direction best explains the CSSP, LISPB and BB87 data sets. Because the relatively large BB87 source (2 tonnes) gave a disappointingly weak signal this result is not con-
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clusive, but it is consistent with our deductions from the other two lines. The two synthetic sections in Fig. 4 illustrate an important point. If our diagnosis is correct, and the velocity variations with depth are due to anisotropy resulting from crystalline alignment, then the whole appearance of a seismic section should change as the azimuth changes. This is why, even though the BB87 section is a disappointing one, we consider it is good enough to discriminate between the two synthetic sections in Fig. 4. 3. S o m e tectonic implications
The model in section 2 consists of three horizontal anisotropic "layers" embedded in an otherwise isotropic lithospheric mantle consisting of 80% olivine and 20% orthopyroxene. The centres of the "layers" lie at 45, 70, and 85 km depth. The percentage of aligned olivine is 10, 40, and 80% respectively and the zones have half widths of 4, 4, and 1.5 km depth. The maximum P-wave azimuthal anisotropy is ca. 6%. In the anisotropic zones, the olivine a-axis (fast axis) is horizontal and points in approximately a N25 ° E direction. The orientation of the fast direction, ca. N25 o E, is probably the direction of most recent flow in the subcrustal lithosphere in this region as the olivine a-axis almost certainly aligns parallel to the flow plane and points in the flow direction during steady-state flow [27]. Two questions should be addressed: (a) When did these features form? (b) Why has deformation occurred in a banded fashion? Zoback and Fuchs [28] have found a strong positive correlation between present day maximum horizontal stress directions and present day plate motion directions. So, if crystalline alignment are present day features, they should correlate with measured present day maximum horizontal compressive stress directions. For "large" strains, the fast olivine [100] axis approaches the extension direction [1]. That is, the fast axis should lie perpendicular to the direction of maximum horizontal compressive stress. The maximum horizontal compressive stress direction in this region is N26 ° W [29]. There is no apparent correlation between this value and the orient.ation of the fast olivine axis. The crystalline alignments are thus likely to be relic, "frozen in" features.
P-WAVE ANISOTROPY
63
IN THE LOWER LITHOSPHERE
A maximum possible age for the most recent lower lithospheric deformations can be inferred through dating surface deformations which have a deep origin (e.g. sedimentary basins). Such deformations must have been "transmitted through" the lower lithosphere and hence will have deformed it also. However, as the lithosphere has a vertically inhomogeneous rheology, deformation might occur in the mantle lithosphere and not express itself at the surface. Hence, a minimum age cannot be estimated in this way. Although the structural trend in Ireland and Northern Britain is predominantly Caledonian in age, the region is surrounded by numerous early Mesozoic sedimentary basins which indicates that the Mesozoic may represent a maximum age for the proposed mantle fabrics. Therefore, as there is no apparent correlation with surface features, a Mesozoic to early Cenozoic age may be inferred. This interpretation contrasts with the recent findings of Silver and Chan [1]. Using long period S-wave data, (which has a different resolution to long-range profiling) they suggest an Upper Paleozoic (Carboniferous) age for upper mantle deformation in central Europe, based on surface geology observations. However, if deformations in the lower lithosphere are younger than surface structures, the E - W extension periods in either the late Jurassic or mid-Tertiary could also account for their observations. The mode of lower lithospheric deformation may have important consequences for the deformation of the lithosphere as a whole (including the crust). Are deep seated horizontal stresses transmitted "efficiently" Velocity
through the lithosphere during tectonically active periods or are they "attenuated" through a combination of brittle and ductile deformations in the mantle lithosphere and lower crust?. 4. Possible mode of deformation
A surprising result of long-range refraction/ wide angle reflection profiling has been the discovery of alternate layers of high and low velocities below the Moho. If anisotropy in the lower lithosphere has been caused by a constant flow episode in the asthenosphere and subsequent lithospheric growth, then a single region of anisotropic material would be expected. This has not been observed, instead, deformation is concentrated in a series of lower lithospheric layers. Large-scale shearing may be responsible for these patterns. Such shearing would produce substantial amounts of heat [30]. A relative velocity of 2 c m / y r between sliding blocks results in a temperature rise of ca. 250 ° C at 45 km depth. This heat is then free to leak into the surrounding material, increasing material ductility and rendering an envelope surrounding the shear zone more susceptible to preferred crystalline alignment by the ambient stress field (Fig. 6). The existence of upper mantle shear zones, which are not necessarily horizontal, is supported by deep vertical reflection data. Bright laterally continuous thin reflecting horizons have been detected, at appropriate depths in the mantle around Britain and Ireland, which are best interpreted as shear zones [31]. It is possible that the vertical reflection data detects % alignment
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.
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Fig. 6. Schematic representation of selective preferential alignment of olivine crystals in a zone of elevated temperature above and below a shear zone. The temperature anomalies associated with lithospheric scale shear zones can be ten times wider than the shear zone. This envelope of anomalous temperature may reduce material viscosity in the vicinity of a shear zone sufficient to allow the ambient stress field to preferentially align olivine crystals in a bell shaped region around the shear horizon.
64
the thin zone which has suffered severe m e c h a n i cal d e f o r m a t i o n (and hence chemical alteration), whereas the wide aperture data picks out a b r o a d zone which has u n d e r g o n e a more " p a s s i v e " creep-like d e f o r m a t i o n i n the regions of elevated temperature. A m e c h a n i s m of this kind would i n h i b i t the t r a n s m i s s i o n of horizontal stress through the lithosphere a n d lead to a varying strain d e p t h function. T h e u p p e r crust could c o n s e q u e n t l y be isolated from these deformations, preserving the scars of earlier tectonic episodes.
C.J. B E A N A N D A . W . B . J A C O B
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5. Conclusions We conclude that preferred a l i g n m e n t of olivine crystals is a n i m p o r t a n t feature of the subcrustal lithosphere in N o r t h w e s t e r n Europe. C u r r e n t data indicates a ca. N25 ° E fast direction for the anisotropic layers. This d e f o r m a t i o n is most p r o b a b l y of Mesozoic or early Cenozoic age. A l i g n m e n t occurs in a b a n d e d fashion which may, i n part, be due to a reduced viscosity associated with a shear heating mechanism. W e suggest that u p p e r m a n t l e fabrics may, i n general, be y o u n g e r t h a n the large scale d e f o r m a t i o n trends i n the overlying brittle crust. F u r t h e r controlled source l o n g - r a n g e seismic data with good azimuthal control is needed. I n particular, acquisition of long range data (including S-wave data) above k n o w n good reflectors of vertical i n c i d e n t energy is very desirable.
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Acknowledgements 16
We acknowledge the k i n d assistance of the University of Karlsruhe, F . R . G . I n p a r t i c u l a r we t h a n k C. Prodehl, A. R u t h a r d t , R Stangl a n d B. Nolte. The British Geological Survey a n d D u r h a m University, U.K. are t h a n k e d for seismic sources which, speculatively recorded i n Ireland, provided unexpectedly good signals at long range.
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References
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