12 May 1997

Dear Bill,
	I have started to write the "Introduction" and "Regional Setting" 
for the proposal. I know that we are too close to June 1 to succed to 
present the proposal on that date, but I will keep working in the project. 
Today evening, I need to go to Buenos Aires and will be there for three 
days.  I will take the opportunity to look for geological information in 
the Andes region and its foreland south of 33 S.  I have the necessary 
information for the region north of that latitude.  I will try to organize 
a little more what I have written for the proposal and send to you.  It is 
only a draft of a few lines, but it may be useful that you know what I am 
thinking about.	
	The first week of July I may have the chance to be in New York. In 
that case, I could stop at Lamont for a few days and define with you all 
the aspects of the project (if we have not succeded to do that before). 
Will you be at Lamont at that time?
	Today I will be leaving the office at about 3 to 4 pm.
	Best regards,
		Enrique


IMAGING THE ANDEAN CRUST AND UPPER MANTLE OVER THE FLAT AND DIP
SUBDUCTION SEGMENTS, AND THEIR TRANSITION AT ~33 S

Introduction

The late Cenozoic Andean Orogeny have along-strike variations
in crustal deformation, uplift, and magmatism.  These
variations can be correlated with the three-dimensional shape
of Nazca plate subducted segments.  Thus, the non-collisional
convergent process in western South America appears to be
related with the Andean mountain building. Although many
studies have assumed that the Andean crustal thickening is
produced by magmatic additions [e.g., Thorpe et al., 1981], an
increasing amount of evidence shows the importance of the
crustal shortening, and the uplift due to thermal thinning of
the lithosphere particularly in the Altiplano-Puna plateau
[e.g., Isacks, 1988].  The earlier Andean orogenic episodes
began in the Jurassic.  During Middle to Late Cretaceous
experienced severe deformation, and the diverse segments
reacted in different ways, but the present-day morphotectonic
pattern is more related to deformation and segmentation that
started in the Miocene.  Mesozoic and early Terciary intrusive
are now found west of the modern Andes were probably formed in regions
of crustal thickening and uplift but are now located in areas of
relatively low elevation and relief.  It is recognized that the
Oligocene was a period of relative tectonic and magmatic quiescence and
of probable reduction of relief formed in earlier Terciary times.  The
amount of crustal thickening remaining from earlier episodes is unknown
but it is likely that a large fraction of modern relief and crustal
thickness did form during the Neogene.
This project focuses in the Andes between latitudes 30-35 S
which includes a large portion of the flat subduction segment
of the Nazca plate (~28-32 S), part of its normal dipping
segment South of ~33 S, and the transitional segment in
between.  The study area also include the Precordillera and a
few other North-South trending ranges to the East of the Andes
between latitudes 30-33 S (Fig. 1).  One of the conpiscuous
missing elements for the understanding of the Andean mountain
building is the information about crustal and lithospheric
structure.  In the Argentinian-Chilenian Andes the available
information includes a limited number of gravity data and
refraccion data.  In the section of normal dip subduction
between 22-24 S a continental transect study within the Global
Geoscience Transects Project [Omarini and Gotze edt., 1991]
have produced crustal velocity profiles from refraccion work,
and Bouguer and free-air anomaly maps from areal gravity data.
To the South of those latitudes, only gravimetric linear data
exist on transects at 30 S, 33 S,  ---, and --- (Introcaso et
al., 1992; ------).  The two PANDA seismic array experiments
[Chiu et al., 1991,----] operated in two different regions of
the Andean foreland of Argentina.  One in San Juan Province
between latitudes 31-32 S and longitudes 67-69 W, and the other
one in Jujuy Province between latitudes 23.5-24.5 S and
longitudes 64-66 W.

The southern region analysis of converted wave travel times from
intermediate-depth local earthquakes suggest a continuous east-west
deepening of the Moho from 50 to 60 km [Regnier et al., 1994].  In this
region the experiment also produced a wealth of well located  crustal
seismicity, and that of intermediate-depth related to the
sub-horizontally subducting segment.  The crustal seismicity mainly
associated with the Sierras Pampeanas block uplift of Pie de Palo, and
the eastern Precordillera showed a depth range of 15 to 30 km, and a
maximum depth of 30 to 40 km which is unussually deep for intraplate
activity and related to a strong lower crust rheology [Smalley and
Isacks, 1990; Regnier et al., 1992; Smalley et al., 1993].  The PANDA
experiment in Jujuy provide well located crustal earthquakes and
estimations of the thickness of the crust [Cahill et al., 1992;
Whitman, 1994].
The regional-scale morphology of the subduction zone (Fig. 2)
is known from the spatial distributions of earthquakes [Isacks,
1988; Cahill, 1990; Cahill and Isacks, 1992].  The amount of
shortening that accomodated plate convergence is not accurately
constrained in any single cross section through the Andes, its
forearc and foreland.  Isacks [1988] attempted estimates in
Central Andes considering the cross-sectional area of the
uplifted terrain as an effect of crustal shortening and
thickening.  In general he established estimates of about 100
km.  Almendinger et al. [1990] studied crustal-scale balanced
sections at a latitud of 30 S where the magmatic addition to
the crust has been insignificant in the last ~9 Ma [Kay et al.,
1988].  They concluded that 150-170 km of horizontal shortening
has occurred in three major belts located between the trench
and the foreland.  Other estimates in different sections vary
from 90 km to 250 km [Jordan and Allmendinger, 1986;
Allmendinger et al., 1983; Lyon-Caen et al., 1985; Sheffels et
al., 1986; Megard, 1984].  The most poorly constrained part of
all of these estimates is the lack of any direct measure of
crustal thickness.
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Regional Setting

The experiment will be performed in two different regions of the Andean
orogenic system, and in the transition region between both of these.
The northern section between latitudes 30-33 S is a region where the
subduction of the Nazca plate is suhorizontal, there is no Quaternary
volcanism and the South American plate exhibits in the foreland thin
and thick-skinned deformation.  In contrast to this, in the section
south of ~33 S the subducted plate is more steeply deeping and the
upper plate has an active volcanic arc.
In the northern segment the subducting plate descends with a
moderate eastward dip (~30 deg.) from the Chile trench to a
depth of 100 km, before flattening to a dip of less than 5 deg.
east farther eastward [Cahill, ----;  Bevis and Isacks, 1984].
Numerous intermediate-depth earthquakes in the region clearly
define the subducted plate geometry [Bevis and Isacks, 1984,
Cahill and Isacks, 1985; Smalley and Isacks, 1987].  The
flattening of the subducted plate occurs beneath of the crest
of the Andean Cordillera [Kadinsky-Cade et al., 1985], about
200 km from the trench.  In the southern segment the subducted
plate dips normally (~30 deg.), but the geometry of the plate
is less well defined by seismicity than in the northern
segment.  In coincidence with the gradual change from the
dipping to the flat segment of the Nazca plate at about 27-28
S, there is a change in the morphology of the Andes from the
Puna-Altiplano plateau in the northern segment to a high and
narrower cordilleran uplift composed by two ranges, the
Cordilleras Principal and Frontal. The Quaternary volcanism
that is present in the dipping segment to the North is absent
in the flat segment.  In the southern boundary of this segment
at 33-34 S (Fig. ---), the change of the geometry of the plate
to a normal dipping segment is more abrupt (Fig. ---) than in
the northern part. This change is in coincidence with the end
of the Frontal Cordillera, the end of the Quaternary volcanic
gap, and the begining to the south of the stationary
Jurassic-Quaternary magmatic arc developed along the axis of
the present-day Cordillera Principal which still active.  The
Andes foreland in the two sections are also very different.  In
the northern section the Precordillera thrust belt, whicch forms
the foothills of the Andes, and farther east the Sierras
Pampeanas, a distinct morphostructural province of mountain
blocks and broad valleys, are both an integral part of the
Andean deformation.  The southern ends of Precordillera and
Sierras Pampeanas are both at about 33 S.  In the southern
segment (south of 33 S), the foreland of the Andes (Cordillera
Principal) shows a magmatic activity of Miocene to Pliocene age
that resulted in emplacement of andesitic subvolcanic
intrusions and lava flows  [Groeber, 1980; Haller et al.,
1985].  Between lat. 34 and 37 S, extensive backarc plateau
basalts developed at the same time


Some of the important things to add:

-Discussion about the athenospheric wedge. It is far to the east of the
Andes in the flat subduction segment, and closer to it in the normal
dipping segment.  (See Isacks, 1988).

-The crustal seismicity of the upper plate over the flat subduction
segment (only known to exist in Precordillera and Sierras Pampeanas).
It is the region with the largest and more destructive earthquakes in
the Argentinian foreland (severals earthquakes of magnitude about
7.4).  The southern segment have lower seismicity in the foreland, but
at least one earthquake of magnitude ~ 6.

-Geology of the southern segment.  What is known about the volcanos.

-Something about the forearc (chilenian side of the experiment).

-A few words about Precordillera and Sierras Pampeanas, and the region
east of Cordillera Principal south of 33 S.

Bibliography

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and B. L. Isacks, Foreland Shortening and Crustal Balancing in the
Andes at 30 S Latitude., Tectonics, Vol. 9, No. 4, 789-809, 1990.

Bevis, M., and B. L. Isacks, Hypocentral trend surface analysis:
Probing the geometry of Benioff Zone, Journal Geophysical Res., vol.
89, N0. B7, 6153-6170, 1984.

Cahill, T., Earthquakes and tectonics of the Central Andean subduction
zone. Ph.D. Dissertation, Cornell University, 110 pp, Ithaca, 1990.

Cahill, T., Isacks, B. L., Whitman, D., Chatelain, J. L., Perez, A.,
and J. m. Chiu,  Seismicity and tectonics in Jujuy Province,
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1992.

Cahill , T., and B. L. Isacks, Shape of the subducted Nazca plate, EOS,
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