The transition from diagenesis to very low-grade metamorphism has been
characterized by conodont
colour alteration index (CAI), illite crystallinity (IC), clay-mineral assemblages
and rock fabric in a
transect (Cape Peñas–Cape Torres) located in the Cantabrian Zone
(northwest Spain). IC values show a
northwest to southeast change from epizonal to diagenetic values. CAI values
show a general congruence
for low- and high-grade areas. This correlation indicates that CAI values for
the epizone are >5.5, the
anchizone 5.5≥CAI>4, and the diagenetic zone ≤4. Cleavage develops in
incompetent rocks with CAI
values greater than 3, mainly by pressure solution. Temperature increases
towards the inner parts of the orogen
(towards the northwest) and is considered to be related to the tectonic
superposition caused by thrusting
and folding. A gradient of 35°C/km is inferred for the transect. In the
southeastern sector of the section there
are several thermal anomalies that are attributed to the presence of
epithermal fluids.