Following Ben Harte's clarification, as one very interested in the
mutual constraints between tectonic evidence and metamorphic
petrologic evidence, I'd like to add my two bits again:
Piezothermal array - an attempt at further clarity.
Piezothermal array: Each metamorphic rock, as observed at present in
its position in the earth's crust, has traversed a particular
pressure-temperature-time (P,T,t) path. A characteristic point along
that path is defined by the P-T-t coordinates at the thermal maximum.
Let that point be projected onto the P-T plane. The three-dimensional
array of P-T-t points, as separately interpreted and specified, in
the three spatial dimensions (x,y,z) of a portion of the earth in its
present state constitutes the piezothermal array. In other words, the
array exists in points having positional coordinates within the
earth, each such point being defined in P and T as above.
The values used in constructing a piezothermal array are those
obtained by interpretation of the various kinds of evidence preserved
in rock specimens, by theoretical modelling, or, it is to be hoped,
by the interaction of the two. Ideally, of course, it would be most
useful if the time coordinates could be added (i.e. the above
projection did not have to be made) as, in general, the points of a
piezothermal array are not isochronous. Thus it would be desirable to
have a chronopiezothermal array! Some progress, of course, has been
made in this direction; and it is here where geophysics and
metamorphic petrology meet most productively as was recognized in
papers by Charles Babbage, friend of Lyell, Murchison, Herschel, and
Darwin, and Lucasian Professor of Mathematics at Cambridge [the chair
held by Isaac Newton], in 1834 and later. (See: v. 4, p. 142-146; p.
166-217 and v. 9, p. 84-107, (The Ninth Bridgewater Treatise, 1837),
in The works of Charles Babbage (1989), edited by Martin
Campbell-Kelly: London, William Pickering, v. 9 of 11 volumes.
Especially insightful more recently is: Sleep, N. H., 1979, A
thermal constraint on the duration of folding with reference to
Acadian geology, New England (USA): Journal of Geology, v. 87, p.
583-589. Both Babbage and Sleep concerned themselves with the manner
in which isothermal surfaces varied their positions with time as
displacements and loading affected rock strata in the earth. The
Ninth Bridgewater Treatise makes fascinating reading by showing how
Babbage and Herschel came up with the same thoughts independently and
at almost exactly the same time - apparently both had been stimulated
by Lyell to concern themselves with things geological, and Babbage
was stimulated further into thinking along these lines by his
geological investigations of the rate of flooding of the Temple of
Serapis at Pozzuoli near Naples.
It is important to note that the P-T-t coordinates of the thermal
maximum are emphasized
John L. Rosenfeld
Department of Earth & Space Sciences
University of California, Los Angeles
Los Angeles, California 90095-1567
e-mail: [log in to unmask]