Status: RO
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Mon, 09 Nov 1998 12:25:21 -0700 (MST) From: John Hernlund
Subject: Hindu-Kush/Pamirs region seismicity movie on the web X-Sender:
hernlund@stats2.asu.edu
To: Ramon Arrowsmith Cc: gao@earth.la.asu.edu,
GEORGE EARL HILLEY ,
ERIN M YOUNG , ganymede@earth.la.asu.edu,
ganymede@extremezone.com
MIME-version: 1.0
I have just finished making an animated gif movie of the Pamirs and Hindu
Kush region (the region where Pakistan, Afghanistan, Tibet, and Russia all
meet). Each frame is a cross section done at intervals of 5 degrees. The
sense of rotation is right-handed (hold your right hand up to the screen
and the events are rotating in the direction of your fingers). There are
71 frames in all (I missed one some where!). The first frame starts with a
North-South cross-section with North to the right and South to the left.
This view of the earthquake locations is providing me with a better grasp
of the geometry of anomalous deep earthquakes occuring in this region. A
number of people have suggested that continental lithosphere is being
subducted both from the south underneath the Hindu Kush and from the north
under the Pamirs. This movie illustrates two planer features: one dipping
almost at 90 degrees (slab 1, I'll call it) and which has most of the
seismicity and another which dips at an angle of around 60 degrees (slab
2, I'll call it) and which intersects the former along the area where it
has a large amount of seismicity, but throughout has much less activity.
This suggests that a collision is occuring between the two planar features
along their intersection. Slab 1 has a strike of around 40 degrees east of
north, and slab 2 has a strike of approximately 30 degrees west of north.
Also, slab 2 is not a perfect plane, but is slightly concave up.
The question remains however, whether slab 1 and slab 2 are actual slabs
of subducted lithosphere, but I tend to think this is the case. My current
idea is that this area of collision represents a location where the corner
of the Indian Plate bit hard into the Eurasian Plate, and only the corners
have been subducted. Several factors would allow for this: A) Along a
corner, the width of the crust is less, which will lend less elastic
support to any kind of bending stress imposed upon it. B) Over time as the
main portion of the plate draws near, the width of the subducting slab
becomes wider, and therefore more resistant to bending. C) Eventually the
large scale subduction of the slab slows to a stop, as the plates have had
a facelift along the borders where they collided in order to adapt the
same shape on both sides.
So I am thinking that this is a case of incompatible shapes along
colliding continental fronts and being smashed into the correct shape over
time. This is a width-dependent stiffness issue, which would explain why
the area between Tibet and India generally does not produce anomalous deep
seimicity; i.e. their edges were more compatable. If the width is greater,
less bowing or bending will occur and perhaps they will overlap one
another and result in double thick crust.
Oh yeah, the address is:
http://www.public.asu.edu/~hernlund/seismology/hindu.gif
I hope you enjoy this as much as I have!
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John Hernlund
E-mail: hernlund@asu.edu
WWW: http://www.mc.maricopa.edu/academic/phy_sci/Geology/hernlund/
http://www.public.asu.edu/~hernlund/
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