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\title{Mantle strain from Paleozoic orogenesis beneath the New
England Appalachians}


\author{Vadim Levin, Jeffrey Park, Mark T. Brandon \\ 
Department of Geology and Geophysics, Yale University \\ 
P.O. Box 208109, New Haven,
Connecticut 06520-8109, USA \\ \\
William Menke \\
Lamont-Doherty Earth Observatory \\
Palisades, New York 10964, USA 
}
\date{ manuscript for {\em Geology}, revised \today}
%\date{ {\Huge PREPRINT}\\ \today}
\maketitle

{ABSTRACT}

Across the Appalachian orogen of New England, the splitting of
core-refracted shear waves from a wide range of arrival directions
indicates the presence of two nearly uniform horizontal layers of
anisotropic upper mantle.  
The anisotropy in the lower layer has a fast axis nearly parallel to the
motion  of the North American plate in an absolute reference frame  and
thus is attributed to basal shear as the plate plows through
asthenospheric mantle.
The anisotropy of the upper layer is inferred to be a fossil fabric,
residing in lithospheric mantle.  
The finite extension direction of the
upper fabric is subhorizontal and oriented normal to the local trend of
the Appalachian orogen.
The upper fabric is consistent over a broad region beneath
and west of the New England Appalachians, which indicates that it
formed after Devonian closure of the Iapetus Ocean, probably during the
late Paleozoic Acadian and Allegheny orogenies.
Tectonic scenarios for synconvergent extension predict rapid surface
uplift and increased heat flow due to lithospheric thinning, consistent
with coeval late-orogenic mantle-derived magmatism in both the
northern Appalachians and Morocco.


{INTRODUCTION}

There is a controversy about what happens to the excess mantle
lithosphere in continent-continent collision zones.
Does the continental mantle lithosphere subduct like oceanic
lithosphere or does it deform with the rest of the orogen, forming a
combined mantle and crust root?
If the mantle lithosphere is subducted, slab ``rollback'' can cause
widespread horizontal extension, in both the asthenosphere and mantle
lithosphere (Willett and Beaumont, 1994).
If mantle lithosphere does not subduct, modeling studies suggest that,
after 10--50 m.y., it may founder, detach, and sink through the
asthenosphere (Molnar et al. 1993; Houseman and Molnar, 1997).
Detachment might occur via ductile necking, or by
delamination (Bird, 1979; Schott and Schmeling, 1998).
All scenarios predict late-orogenic or postorogenic lithospheric
thinning, uplift, near-surface extension, and substantial mantle strain
(Fig. 1).

Continental delamination has been proposed to explain the regional
geology of the New England Appalachians (Robinson, 1993), but the
potential impact of upper-mantle tectonics has not been evaluated as
fully as for many coeval locales in Canada (Lynch and Giles, 1995;
Murphy et al. 1999) and in Europe (Wenzel et al. 1997).
More than 100 My of plutonism and tectonic activity follows 
the final closure of the Iapetus ocean.
During this interval, Appalachian orogenesis continued as an
intra-plate process within the supercontinent Pangaea, with a muted
connection to global-scale plate movements.

In the lithosphere beneath old orogenic belts, fossil strain should be
detectable in the form of seismic anisotropy, caused by lattice-preferred
orientation (LPO) of olivine (Zhang and Karato, 1995), whose mineral
fast axes tend to align with the axis of maximum extension (Ribe,
1992).
Anisotropy estimates for orogenic zones from the birefringence, or
``splitting,'' of seismic shear waves have often, but not
universally, indicated an olivine fast axis parallel to the orogen,
consistent with collisional shortening of the mantle ``root'' (Silver,
1996).
Post-orogenic mantle extension scenarios, as diagrammed in Fig. 1,
would develop a fast axis perpendicular to the orogen.


{SEISMIC EVIDENCE FOR COHERENT PALEOZOIC STRAIN}
{IN THE NEW ENGLAND LITHOSPHERE}

Initial shear-wave splitting observations from the northeastern United
States were related to both present-day strain associated with
absolute plate motion (Fischer et al. 1996) and fossil strain acquired
during the Appalachian orogenies (Barruol et al.  1997).
When ``split'' shear waves from a larger set of earthquakes are
analysed, the apparent fast axes of individual shear waves vary with
their arrival back azimuth in a pattern diagnostic of two layers of
anisotropic rock (Levin et al. 1999a; Fouch et al., 1999; see Fig. 2).
We combined data from two long-running permanent seismic observatories
with observations from a temporary seismic network that collected data
in spring and summer 1995 (Fig. 3), to study the lateral variation of
this anisotropy.
The pattern of variation in apparent fast-axis strike $\phi$
at all stations is consistent with the two-layer anisotropic model
derived by Levin et al (1999a) for station HRV alone  (Fig. 4).
Estimates of splitting delay times $\tau$ from individual seismograms
are less robust than estimates of $\phi$, because the former depend on
the choice of frequency bandpass and pulse duration.

Aside from distinguishing ``upper'' from ``lower'' anisotropic layers,
splitting observations offer no direct constraint on layer thickness
and placement within the crust-mantle seismic profile.
Examination of P-to-S converted waves from the crust-mantle Moho
discontinuity at HRV precludes the top layer of anisotropy from
residing in the crust (Levin et al. 1999a).
We model the anisotropic mantle as 30\% orthorhombic olivine and 70\%
isotropic olivine, a mixture that is $\sim$6\% anisotropic.
For this choice, the upper and lower layers are 60 and 90 km thick,
respectively.
The array data exhibits a regional variation: an overlap of two
data ``branches'' near back azimuth 290$^{\circ}$.
This can be modeled successfully with small ($\sim$10 km) variations in
layer thicknesses:  a thinner upper layer under the Adirondacks and
Grenville areas, and a thicker upper layer under other inland
stations.
Small regional trends in layer-anisotropies ($\sim$1\%), could also
cause these variations in splitting pattern.

Our anisotropy model bears an interesting relationship to the 
structure and dynamics of the lithosphere in this region.
The inferred fast-axis directions are horizontal in both layers.
In the lower layer the fast axis azimuth (S53$^{\circ}$W) is
sub-parallel to the absolute plate motion of North America
($\sim$S65$^{\circ}$W) as reported by Gripp and Gordon (1990).
In the upper layer the fast axis is nearly perpendicular to the local
trend of the Appalachian orogen.
If axial symmetry, simpler than orthorhombic, is prescribed, the fast
axis in the lower layer must dip 40$^{\circ}$ below the horizontal to
fit the observations (Levin et al. 1999a).

We interpret the lower layer to lie within an actively deforming
asthenosphere, with anisotropic rock fabric maintained by contemporary
plate motion.
We interpret the upper layer to reside in the stable part of the
continental lithosphere, with a frozen fabric that formed during a past
deformation event.
It is unlikely that, alternatively, both anisotropic layers reflect
contemporary dynamics at the base of the North American plate, e.g., a
corkscrew shear in the underlying asthenosphere.
It also seems unlikely that one or both of the layers reside in
the deep mantle, e.g.,  the D'' region, as a coherent anisotropic
layer near the core-mantle boundary would need to span a
$\sim$30$^{\circ}$ arc.

Near-uniform anisotropic properties beneath the northeastern United
States contrast with regional variations of geology within the
Appalachian orogenic belt, as well as with significant short-scale
variations in isotropic seismic velocity (Levin et al 1999b).
The contrast between ``rough'' velocity variations, indicative of rock
composition, and smooth, weak anisotropy variations, indicative of
mantle strain, argues that the anisotropy developed after the Paleozoic
accretion of the Avalon terrane to the Proterozoic Grenville
province.

We propose that the upper layer of anisotropy beneath the
northeastern United States is associated with convergent, late-orogenic
tectonism in the late Paleozoic, not with divergent, syn-rift
tectonism in the early Mesozoic.
Mantle fast axes along mid-ocean ridges are both predicted
and observed to align perpendicular to the rift axis (Ribe, 1992; Wolfe
and Solomon, 1998), and our upper-layer fast axis is perpendicular to
the general due-north trend of the failed Connecticut Valley rift.
However, splitting fast axes in the Arabian shield are not normal to
the Red Sea rift (Wolfe et al. 1999).
Moreover, shear wave splitting studies within active continental rift
zones typically indicate fast polarization aligned with the axis of the
rift, not normal to it (Sandvol et al. 1992; Gao et al. 1997).
The disruption of continental lithosphere during rifting differs from
the steady-state growth of oceanic lithosphere, typically involving a
narrow upwelling rather than a broad mantle flow.

POST-OROGENIC MANTLE EXTENSION SCENARIOS

Petrologists often attribute ``late-orogenic'' magmatism
to the thinning of cold lithospheric mantle, and the bouyant
rise of asthenosphere.
The fingerprints of such magmatism identify hot mantle in situations
where one might expect collisional thickening of the lithosphere to
thermally insulate the crust.
These include trace-element enrichment patterns that indicate partial
melting of reheated continental lithosphere, and paired mafic and
felsic melts that indicate ponded asthenosphere-derived gabbroic
intrusions at the base of the crust (Huppert and Sparks, 1988; McKenna
and Walker, 1990; Arnaud et al. 1992; Turner et al. 1992).
Possible causes include slab rollback or an initial partial removal of
an orogenic mantle root, coupled with convective thinning and/or
ablation of the remaining continental lithosphere.

Although these different tectonic scenarios vary in detail, all would
develop anisotropy in the uppermost mantle, formed by either horizontal
extension of the residual mantle lithosphere or the asthenospheric flow
that replaces the mantle root (Fig. 1).
The orogen-perpendicular fabric that developed during the extension of
the Appalachian mantle differs from orogen-parallel fast axes observed
in Tibet (McNamara et al. 1994; Hirn et al. 1998) and the
Alps and Carpathians (Dricker et al. 1999), but both
those regions are thought to be influenced by orogen-parallel motion of
``escaping'' blocks (Meissner and Mooney, 1998).
     
Several lines of evidence suggest that stretching and uplift of the
Appalachian mantle accompanied the Acadian and Allegheny orogenies,
which span the time between the final closure of Iapetus (Devonian,
390--410 Ma, depending on the reconstruction) to the cessation of
thrusting, magmatism and thermal metamorphic activity in southern New
England (Permian, 275 Ma).
This evidence includes removal of 15 km or more of upper crust by
erosion (Carmichael, 1978), which implies high topography during the
late Paleozoic; granitic magmatism and thermal metamorphism, indicating
high heat flow (Zartman, 1988; Zartman et al. 1988; Lux and Guidotti,
1985; Sevigny and Hanson, 1993); and crustal extension
(Getty and Gromet, 1992).
Mantle-derived magmatism and mafic underplating have been
observed or inferred for various late Paleozoic New
England igneous rocks (Hannula et al. 1998; Wiebe et al. 1997). 
Some large Acadian plutons show little evidence of mantle influence
(Lathrop et al. 1996), but note that two of the largest, the
Bethlehem Gneiss and the Kinsman Quartz Monzonite, are dated at
$\sim$410 Ma, and thus are not late-orogenic.

In the Canadian Appalachians, where crustal erosion is less deep,
Devonian continental tholeiites, inferred to be a partial melt of the
underlying lithosphere, are reported in the Magdalen basin, as well as
later mixed mafic and felsic magmas, indicative of gabbroic
underplating of the crust (Pe-Piper and Piper, 1998).
On the African side of the orogen, Hercynian volcanics have been
identified in Morocco, and include both calc-alkaline (shoshonitic)
sequences (Ajaji et al. 1998) and mixed mafic and felsic magmas
indicative of mafic underplating (Gasquet et al. 1992; Chalot-Prat,
1995).

Seismic studies (Hughes and Luetgert, 1991; Hennet et al. 1991; Zhu and
Ebel, 1994) show the lower crust of northern New England to be
reflective and to lack large-scale structures, e.g., throughgoing fault
zones.
The compressional velocity of the lower crust ($\sim$6.8 km/sec) is
consistent with mafic underplating after collision and
the removal of an upper mantle root.
With mantle surface waves, Van der Lee and Nolet (1997) imaged a broad
shallow ($\sim$100 km) deficit in shear velocity beneath New England,
which they attributed to volatiles released by a long-departed slab
during the closure of Iapetus.
However, high-pressure mineral elasticity  experiments suggest that the
major-element compositional differences that distinguish ``undepleted''
from ``depleted'' mantle peridotite could also explain a large portion
of this anomaly (Chai et al. 1997; J. Michael Brown, personal
communication).
Therefore, the low velocities could indicate the post-collisional
replacement of depleted lithospheric mantle with undepleted
``asthenospheric'' rock.

{CONCLUSION}

Observations of shear-wave birefringence (splitting) on a network of
permanent and temporary seismic stations are consistent with nearly
uniform upper-mantle strain across a broad region of the northeastern
United States.
This contrasts with significant variations in basement geology,  and in
the traveltime delays of seismic body waves.
The near-uniformity in anisotropy suggests strongly that any fossil
strain must postdate the assembly of the Appalachian orogen.
Vertical layering in the anisotropy indicates that both modern and
fossil deformation contribute to the signal.
The upper layer of deformation, which we interpret as fossil, implies
maximum extension nearly perpendicular to the Appalachian orogen.
Combined with the nature and timing of late Paleozoic plutonism, this
suggests that hot asthenosphere replaced cooler 
lithosphere in the northern Appalachians during the late Paleozoic, and
later cooled to form new lithosphere.
This process, similar to spackle applied to repair damaged
plaster, may have an important role in continental dynamics.

{ACKNOWLEDGMENTS}

This work was supported by National Science
Foundation grants EAR-9707189 and EAR-9805206.  Jay Ague adviced us
regarding New England geology.  J. Michael Brown allowed us to quote
his results before publication.  Karen Fischer and Steve Roecker
provided seismic data prior to its public release.  We used GMT
software (Wessel and Smith, 1991) to prepare figures.


{REFERENCES CITED}

{\baselineskip=15pt

\reff
Ajaji, T., Weis, D., Giret, A., and Bouabdellah, M., 1998, Coeval
potassic and sodic calc-alkaline series in the post-collisional
Hercynian Tanncherfi intrusive complex, northeastern Morocco:
Geochemical, isotopic and geochronological evidence: Lithos, v.  45, p.
371--393.

\reff
Arnaud, N. O., Vidal, Ph., Tapponnier, P., Matte, Ph.,  and Deng, W.
M., 1992,  The high K$_2$O volcanism of northwestern Tibet:
Geochemistry and tectonic implications: Earth and Planetary Science
Letters, v.  111, p. 351--367.

%\reff
%Babuska, V., J.  Plomerova, and J. Sileny, 1993. Models of seismic anisotropy
%in the deep continental lithosphere, \PEPI v. 78, p. 167--191.

\reff
Barruol, G., Silver, P. G., and Vauchez, A., 1997, Seismic anisotropy
in the Eastern United States; deep structure of a complex continental
plate:  Journal of Geophysical Research, v.  102, p. 8329--8348.

\reff
Bird, P., 1979, Continental delamination and the Colorado Plateau:
Journal of Geophysical Research, v.  84, p. 7561--7571.

%\reff
%Bonin, B.,  A. Azzouni-Sekkal, F. Bussy, and S. Ferrag, 1998.
%Alkali-calcic and alkaline post-orogenic (PO) granite magmatism:
%petrologic constraints and geodynamic settings: Lithos, v.  45, p.
%45--70.

%\reff
%Brechner, S., K. Klinge, F. Kruger, and T. Plenefisch, 1998. Backazimuthal
%variations of splitting parameters of teleseismic SKS phases observed
%at the broadband stations in Germany: Pure and Applied Geophysics, v. 
%151, p. 305--331.

\reff
Carmichael, D. M., 1978, Metamorphic bathozones and bathograds: A
measure of the depth of post-metamophic uplift and erosion on the
regional scale:  American Journal of Science, v.  278, p. 769--797.

\reff
Chai, M., Brown, M. J., and Slutsky, L. J., 1997,  The elastic
constants of an aluminous orthopyroxene to 12.5 GPa:  Journal of
Geophysical Research, v.  102, p. 14779--14785.

\reff
Chalot-Prat, F., 1995, Genesis of rhyolitic ignimbrites and lavas from
distinct sources at a deep crustal level: Field, petrographic, chemical
and isotopic (Sr, Nd) constraints in the Tazekka volcanic complex
(Eastern Morocco): Lithos, v.  36, p. 29--49.

%\reff
%Conrad, C. P., and P. Molnar, 1997. The growth of Rayleigh-Taylor-type
%instabilities in the lithosphere for various rheological and density
%structures: Geophys. J. Int., v.  129, p. 95--112.

\reff
Dricker, I., Vinnik, L., Roeker, S., and Makeyeva, L., 1999,
Upper-mantle flow in Eastern Europe:  Geophysical Research Letters, v.
26, p. 1219--1222.

\reff
Fischer, K. M., Wysession, M. E., Clarke, T. J., Fouch, M. J.,
Al-Eqabi, G. I., Shore, P. J., Valenzuela, R. W., Li, A., and Zaslow,
J. M., 1996, The 1995-1996 Missouri to Massachusetts broadband
seismometer deployment, IRIS Newsletter, v. 2, p. 6-9.
    
\reff
Fouch, M. J., Fischer,  K. M., Parmentier, E. M., Wysession, M.  E.,
and Clarke, T. J., 1999, Shear wave splitting, continental keels, and
patterns of mantle flow: Journal of Geophysical Research, in press.

\reff
Gao, S. S., Davis, P. M., Liu, H., Slack, P. D., Rigor, A. W., Zorin,
Y. A., Mordvinova, V.  V., Kozhevnikov, V. M., and Logatchev, N. A.,
1997, SKS splitting beneath continental rift zones: Journal of
Geophysical Research, v.  102, p. 22781--22797.

%\reff
%Gao, S. S., P. M. Davis, H. Liu, P. D. Slack, A. W. Rigor, Y. A. Zorin,
%V. V.  Mordvinova, V. M.  Kozhevnikov, and N. A. Logatchev, 1999. Reply: J.
%Geophys. Res., v.  104, p. 10791--10794.

\reff
Gasquet, D., Leterrier,  J., Mrini, Z., and Vidal, P., 1992,
Petrogenesis of the Hercynian Tichka plutonic complex (Western High
Atlas, Morocco): Trace element and Rb-Sr and Sm-Nd isotopic
constraints: Earth and Planetary Science  Letters, v.  108, p. 29--44.

\reff
Getty, S. R., and Gromet, L. P., 1992, Geochronological constraints on
ductile deformation, crustal extension and doming about a
basement-cover boundary, New England Appalachians: American Journal of
Science, v.  292, 359--397.

%\reff
%Getty, S. R., and L. P. Gromet, 1992b. Evidence for extension at the
%Willimantic Dome, Connecticut: Implications for the late Paleozoic
%tectonic evolution of the New England Appalachians: Amer. J.
%Sci., v.  292, p. 398--420.

\reff
Gripp, A. E., and Gordon, R. G., 1990, Current plate velocities
relative to the hotspots incorporating the NUVEL-1 global plate motion
model:  Geophysical Research Letters, v.  17, p. 1109--1112.

\reff
Hannula, K. A., Coish, R. A., and Applegate, G. S., 1998, Geochemistry
and Ar cooling ages of the syn-Acadian Victory Pluton, northeastern
Vermont, USA (abstract): Geological Society of America Abstracts with
Programs, v.  30, p. 240.

\reff
Hennet, C. G., Luetgert, J. H., and Phinney, R. A., 1991, The crustal
structure in central Maine from coherency processed refraction data:
Journal of Geophysical Research, v.  96, p. 12023--12037.

\reff 
Hirn, A., Diaz, J., Sapin, M., and Veinante, J.-L., 1998, Variation of
shear-wave residuals and splitting parameters from array observations
in southern Tibet: Pure and Applied Geophysics, v.  151, p. 407--431.

%\reff
%Houseman, G. A., D. P. McKenzie, and P. Molnar, 1981. Convective instability
%of a thickened boundary layer and its relevance for the thermal
%evolution of continental convergent belts: Journal of Geophysical Research, v. 
%86, p. 6115--6132.

\reff
Houseman, G. A., and Molnar, P., 1997, Gravitational (Rayleigh-Taylor)
instability of a layer with non-linear viscosity and convective
thinning of continental lithosphere: Geophysical Journal International,
v.  128, p. 125--150.

\reff
Hughes, S., and Luetgert, J. H., 1991, The 1988 New England-New York
seismic refraction profile: Crustal structure of the northern
Appalachian and Adirondack mountains: Journal of Geophysical Research,
v.  96, p. 16471--16494.

\reff
Huppert, H. E., and Sparks, R. S. J., 1988, The generation of granitic
magmas by intrusion of basalt into continental crust: Journal of
Petrology, v.  29, p. 599--628.

%\reff
%Keppie, J. D., 1993. Synthesis of Paleozoic deformational events and
%terrane accretion in the Canadian Appalachians: Geologische Rundschau, v.
%82, p. 381--431.

\reff
Lathrop, A. S., Blum, J. D., and Chamberlain, C. P., 1996. Nd, Sr, and
O isotopic study of the petrogenesis of two syntectonic members of the
New Hampshire Plutonic series: Contributions to Mineralogy and
Petrology, v.  124, p. 126--138.

%\reff
%Levin, V., Lerner-Lam,  A., and Menke, W., 1995,  Anomalous mantle
%structure at the Proterozoic-Paleozoic boundary: Geophysical Research
%Letters, v.  22, p. 121--124.

%\reff 
%Levin, V., W.-Y. Kim, and W. Menke, 1995b. Seismic velocities in the
%shallow crust of western New England and northern New York: Bull.
%Seism. Soc. Amer., p. 85, 207--219.

\reff
Levin, V., Menke, W., and Park, J., 1999a, Shear wave splitting in
Appalachians and Urals: a case for multilayered anisotropy: Journal of
Geophysical Research, v. 104, p. 17975--17994.

\reff
Levin, V., Menke, W., and Park, J., 1999b, Dynamic or relict anisotropy
in the northeast US: Why not both? (abstract): EOS Transaction AGU, v.
80, p. S216.

%\reff
%Levin, V., W. Menke, and J. Park, 1999.  A failed search for regional
%anisotropic domains in eastern North America: J. Geophys. Res.,
%{\it submitted}.

%\reff
%Liegeois, J.-P., J. Navez, J. Hertogen, and R. Black, 1998. Contrasting
%origin of post-collisional high-K calc-alkaline and shoshonic versus
%alkaline and peralkaline granitoids: The use of sliding normalization:
%Lithos, v.  45, p. 1--28.

\reff
Lux, D. R., and Guidotti, C. V., 1985, Evidence for extensive Hercynian
metamorphism in western Maine: Geology, v.  13, p. 696--700.

%\reff
%Lynch, G., and C.  Tremblay, 1994. Late Devonian-Carboniferous detachment
%faulting and extensional tectonics in western Cape Breton Island, Nova
%Scotia, Canada: Tectonophysics, v.  238, p. 55--69.

\reff
Lynch, G., and Giles, P. S., 1995, The Ainslie detachment, a regional
plat-lying extensional fault in the Carboniferous evaporitic Maritimes
Basin of Nova Scotia, Canada: Canadian Journal of Earth Science, v.
33, p.  169--181.

\reff
Lyons, J. B., Boudette, E. L., and Aleinikoff, J. N., 1982, The
Avalonian and Gander zones in central eastern New England, in {\em
Major Structural Zones and Faults of the Northern Appalachians}, P.
St.-Julien and J.  Beland: Geological Association of Canada Special
Paper, v.  24, p.  43--66.

\reff
McKenna, L. W., and Walker, J. D., 1990. Geochemistry of crustally
derived leucogranitic igneous rocks from the Ulugh Muztagh Area,
northern Tibet and the implications for the formation of the Tibetan
plateau: Journal of Geophysical Research, v.  95, p. 21483--21502.

\reff
McNamara, D. E., Owens, T. J., Silver, P. G., and Wu, F. T., 1994,
Shear wave anisotropy beneath the Tibetan Plateau: Journal of
Geophysical Research, v.  99, p. 13655--13665.

\reff
Meissner, R., and Mooney, W., 1998, Weakness of the lower continental
crust: A condition for delamination, uplift and escape: Tectonophysics,
v. 296, p. 47--60.

%\reff
%Moecher, D. P., M. A. Cosca, and G. N. Hanson, 1997. Petrologic and
%$^{40}$Ar/$^{39}$Ar geochronological constraints on the middle to late
%Paleozoic thermotectonic history of the southern Connecticut Valley
%zone, New England Appalachians: Geological Society of America
%Bulletin, v.  109, p. 164--175.

\reff
Molnar, P., England, P., and Martinod, J., 1993,  Mantle dynamics,
uplift of the Tibetan Plateau, and the Indian monsoon: Reviews of
Geophysics, v.  31, p.  357--396.

\reff
Murphy, J. B., van Staal, C. R., and Keppie, J. D., 1999, Middle to
late Paleozoic Acadian orogeny in the northern Appalachians: A
Laramide-style plume-modified orogeny?: Geology, v.  27, p.  653--656.

\reff
Pe-Piper, G., and Piper, D. J. W., 1998, Geochemical evolution of
Devonian-Carboniferous igneous rocks of the Magdalen Basin, Eastern
Canada: Pb- and Nd-isotopic evidence for mantle and lower crustal
sources: Canadian Journal of Earth Science, v.  35, p. 201--221.

\reff
Ribe, N. M., 1992, On the relation between seismic anisotropy and finite
strain: Journal of Geophysical Research, v.  97, p. 8737--8747.

\reff
Robinson, P., 1993, Acadian magmatism and metamorphism in New England: A
product of mantle-lithosphere delamination in front of an east-dipping
subduction zone? (abstract): Geological Society of America Abstracts
with Programs, v.  25, p. 179.

\reff
Sandvol, E., Ni,  J., Ozalaybey, S., and Schlue, J., 1992, Shear-wave
splitting in the Rio Grande Rift:  Geophysical Research Letters, v.
19, p. 2337--2340.

\reff
Schott, B., and Schmeling, H., 1998, Delamination and detachment of a
lithospheric root: Tectonophysics, v.  296, p. 225--247.

\reff
Sevigny, J. H., and Hanson, G. N., 1993, Orogenic evolution of the New
England Appalachians of southwestern Connecticut: Geological Society of
America Bulletin, v.  105, p.  1591--1605.

%\reff
%Shalev, E., J. Park, and A. Lerner-Lam, 1991. Crustal velocity and Moho
%topography in central New Hampshire: J. Geophys. Res., v.  96, p.
%16415--16427.

%\reff
%Silver, P. G., and M. K. Savage, 1994. The interpretation of shear-wave
%splitting parameters in the presence of two anisotropic layers:
%Geophys. J. Int., v.  119, p. 949--963.

\reff
Silver, P. G., 1996,  Seismic anisotropy beneath the continents:
Probing the depths of geology: Annual Reviews of Earth and Planetary
Science, v.  24, p.  385--432.

%\reff
%Sylvester, P. J., 1998. Post-collisional strongly peraluminous granites:
%Lithos, v.  45, p. 29--44.

\reff
Turner, S., Sandiford, M., and Foden, J., 1992, Some geodynamic and
compositional constraints on ``postorogenic'' magmatism: Geology, v.
20, p. 931--934.

%\reff
%Turner, S., N. Arnaud, J. Liu, N. Rogers, C. Hawkesworth, N. Harris,
%S.  Kelley, P. Van Calsteren, and W. Deng, 1996. Post-collision, shoshonic
%volcanism on the Tibetan plateau: Implications for convective thinning
%of the lithosphere and the source of ocean island basalts: J.
%Petrology, v.  37, p. 45--71.

\reff
van der Lee, S. and Nolet, G., 1997, Upper mantle S velocity structure
of North America: Journal of Geophysical Research, v.  102, p.
22815--22838.

%\reff
%Vauchez, A., Barruol, G., and Nicolas, A., 1999, Comment on ``SKS
%splitting beneath continental rift zones'' by Gao et al: Journal of
%Geophysical Research, v. 104, p. 10787--10789.

%\reff
%Vinnik, L. P., G. L. Kosarev,  and L. I. Makeyeva, 1984. Anizotrpiya
%litosfery po nablyudeniyam voln SKS i SKKS {\it in Russian}: Docl.
%Akad. Nauk SSSR, v.  278, p. 1335--1339.

\reff
Wenzel, Th., Mertz, D. F., Obberhansli, R., and Becker, T., 1997, Age,
geodynamic setting and mantle enrichment processes of a K-rich
intrusion from the Meissen massif (northern Bohemian massif) and
implications for related occurrences from the mid-European Hercynian:
Geologische Rundschau, v.  86, p. 556--570.

\reff
Wessel, P., and Smith, W. H. F., 1991, Free software helps map and
display data: Eos, Transactions of the American Geophysical Union, v.
72, p. 441.

\reff
Wiebe, R. A., Smith, D., Sturm, M., King, E. M., and Seckler, M. S.,
1997, Enclaves in the Cadillac Mountain granite (coastal Maine):
Samples of hybrid magma from the base of the chamber: Journal of
Petrology, v.  38, p. 393--423.

\reff
Willett, S. D., and Beaumont, C., 1994, Subduction of Asian
lithospheric mantle beneath Tibet inferred from models of continental
collision:  Nature, v.  369, p. 642--645.

\reff
Wolfe, C. J., and Solomon, S. C., 1998, Shear-wave splitting and
implications for mantle flow beneath the MELT region of the East
Pacific Rise:  Science, v.  280, p. 1230--1232.

\reff
Wolfe, C. J., Vernon III, F. L., and Al-Amri, A., 1999, Shear-wave
splitting across western Saudi Arabia: The pattern of upper-mantle
anisotropy at a Proterozoic shield:  Geophysical Research Letters, v.
26, p. 779--782.

\reff
Zartman, R. E., Hermes, O. D., and Pease, M. H., 1988, Zircon
crystallization ages, and subsequent isotopic disturbance events, in
gneissic rocks of eastern Connecticut and western Rhode Island:
American Journal of Science, v.  288, p. 376--402.

\reff
Zartman, R. E., 1988, Three decades of geochronological studies in the
New England Appalachians: Geological Society of America Bulletin, v.
100, p. 1168--1180.

\reff
Zhang, S.,  and Karato, S., 1995,  Lattice preferred orientation of
olivine aggregates deformed in simple shear: Nature, v.  375, p.
774--777.

\reff
Zhu, H., and Ebel, J. E., 1994, Tomographic inversion for the seismic
velocity structure beneath northern New England using seismic
refraction data:  Journal of Geophysical Research, v.  99, p.
15331--15357.


}


\vspace{20pt}
\centerline{\bf\large Figure Captions}

{\bf Figure 1}. 
End-member models for extension of the upper mantle beneath a
convergent orogen.
Thin lines in the upper mantle represent the
deformation of an originally horizontal set of material lines.
Convergence of the lines indicates a component of extension parallel to
the lines.
Panel a): Rollback: after initial collision,
lithospheric mantle continues to subduct due to its greater density.
Rollback or retreat of the mantle slab away from the orogen induces
horizontal extension in the asthenosphere as it flows to fill the gap.
Panel b): Detachment: subduction halts following
collision, and continental lithospheric mantle forms a negatively bouyant orogenic root.
The root detaches from the
lithosphere via thermal softening and viscous flow.
Panel c) shows this detachment as a ductile necking instability, but
delamination near the surface, e.g., within the lower crust, is also
possible.
Both residual mantle lithosphere and the inflowing asthenosphere
develop a deformation fabric that would be characterized, at the
regional scale, by a nearly horizontal extension direction.

{\bf Figure 2.}
Observed (solid) and predicted (shaded) shear wave splitting data for
HRV. $S$ phases from South American earthquakes with hypocenters deeper
than 500 km are included to provide coverage from the south. Splitting
values are shown as bars centered on the nominal back azimuth and
apparent velocity (from IASPEI91 velocity model). 
The bar orientation parallels the azimuth $\phi$ of the fast direction.
Bar length is proportional to the time delay $\tau$ between fast and slow polarizations.
Near-zero splitting delays are plotted with open circles.

{\bf Figure 3.} Map of station network.  Major tectonic 
boundaries for the Northern Appalachian orogen are indicated.
The dashed boundary indicates the inferred extent of Avalonian basement
in southern New England (Lyons et al. 1982).
Horizontal projections of our best-fit anisotropic fast-axis directions are
indicated with double-headed arrows within the cluster of station
locations (triangles).  The light arrow represents the top layer.  The
shaded arrow represents the bottom layer. The dashed ellipse within
stable North America indicates the Adirondack Mountains.



{\bf Figure 4.}
Observed and predicted variation of the apparent fast direction.  
Some of the visual mismatch between the observed and predicted patterns
stems from the variety of incidence angles for the incoming
S-waves; the data-fitting algorithm compensates for this.
{\it Left:} data for station HRV, covering 1990-1997.  Observations are
shown by triangles with error bars. A subset of "robust" data points
(circled) is indicated  for which $\sigma_\tau < \tau / 3 $. Clearly, robust
and "poor" data points follow the same pattern. Crosses show values of
the fast-axis azimuth $\phi$ reported for HRV by Barruol et al. (1997). 
{\it Right:} data for all stations (see Fig. 1) observed during the
spring and summer of 1995.  Crosses show all measurements, circles
identify measurements with $\sigma_\tau < \tau / 3 $.
All stations in the region follow the same general pattern. 
Solid lines on both plots show a pattern of fast direction values
predicted by our model of seismic anisotropy (Table 1), for one value
of phase velocity.


\end{document}


{\bf Figure 4.}
Schematics of ``late-orogenic'' tectonics.
In scenario a), subduction of the stalled slab leads to ``rollback''
extensional stresses
in the former convergent zone.  Extension and thermal erosion of this zone
leads to buoyant uplift.
In scenario b), convergence leads to thickened crust and a stable
orogenic mantle root. 
The thickened crust fosters bouyant uplift, but anatectic melts form
without mantle involvement.
In scenario c), the orogenic root founders, and is replaced by hot
bouyant asthenosphere.  Extension and uplift follow as in scenario a).
Our anisotropy model for the northeastern United States is consistent with scenarios
a) and c), because they can lead to widespread orogen-perpendicular
stretches.
Our model does not favor scenario b), in which maximum extension in the
orogenic root would either be orogen-parallel, or else dip steeply.




%As a result, highly-strained mantle rocks are expected to have a bulk
%elastic anisotropy with approximate axial symmetry, with maximum
%seismic velocity parallel to the maximum extension direction (Ribe, 1992).
Anisotropic media ``split'' a propagating shear wave into two orthogonal
time-shifted pulses, with particle motion aligned with ``fast'' and
``slow'' polarizations in the medium (Vinnik et al. 1984).
Path-integrated seismic anisotropy can be estimated most readily from
shear waves that enter an anisotropic region with a known polarization,
e.g., core-refracted SKS.


Dextral transcurrent motion along faults within the Meguma terrane in
Nova Scotia has been dated to occur in several episodes that span
270--370 Ma, suggesting some orogen-parallel motion of crustal blocks
within an evolving collision zone (Keppie, 1993), but these structures lie
northeast of our seismic stations.


On a single seismic record, the fast and slow polarizations of a
``split'' shear wave can be reconstructed into two pulses of identical
shape, shifted by time delay $\tau$, with a coordinate rotation to an
apparent fast-polarization direction $\phi$.
If, at a particular seismic station, $\phi$ and $\tau$ for incoming
shear waves are invariant over a range of back azimuths
and incidence angles, a single layer of anisotropy with a horizontal
fast axis can be inferred beneath the station.
A tilted fast axis or vertical layering of anisotropy would give rise
to systematic (and distinct) variations in apparent $\phi$ and $\tau$,
dependent on the direction of the incoming wave (Babuska et al. 1993;
Silver and Savage, 1994).

Lateral variation in shear-wave traveltime delays (not shown) also
suggest significant ($\sim$3\%), short-scale ($\sim$100 km)
shear-velocity anomalies in either the lower crust or upper mantle.


The accretion of the Avalonian terrane, whose basement rocks underlie
our easternmost stations HRV and STME, is estimated variously to
coincide  with the Acadian orogeny in New England ($\sim$410 Ma - see
Lyons et al. (1982)), or with an earlier orogeny recognized in
Newfoundland (late Silurian, starting 430 Ma - see Keppie (1993)).

\begin{table}
\caption{
{\em Anisotropic Structure (Orthorhombic Symmetry) Consistent With Shear
Wave Splitting Observations at station HRV.}  Velocity values in anisotropic
layers are the isotropic averages of respective elastic tensors. 
The anisotropic medium is modeled as 30\% orthorhombic
olivine and 70\% isotropic olivine, a mixture that is $\sim$6\%
anisotropic.
The angles $\theta$ and $\phi$ define the tilt (from vertical) and
azimuth (CW from north) of the symmetry axes ($f$=fast, $i$=intermediate, $s$=slow)
within each anisotropic layer.}

\begin{tabular}
{cccccccccc}
\hline 
Depth, km & $\rm V_p$, km/s & $\rm V_s$, km/s & $\rho$, $\mbox{g/cm}^3$ &  $\theta_f$, deg & $\phi_f$, deg & $\theta_i$, deg  & $\phi_i$, deg & $\theta_s$, deg  & $\phi_s$, deg \\
\hline
40 & 6.8 & 3.9 & 2.85 & - & - & - & - & - & \\
100 & 8.3 & 4.8 & 3.3 & 89 & 115 & 80 & 25 & 10 & 210 \\
190 & 8.3 & 4.8 & 3.3 & 90 & 53 & 67 & 323 & 23 & 143 \\
$\infty$ & 8.3 & 4.8 & 3.3 & - & - & - & - & - & \\

\hline  
\end{tabular}
\end{table}

