AMBITO Y ARTICULOS INCUMPLIDOS

The regional evolution of the area has been described in C hapters 2 and 3. The geological controls of the Bay of Biscay continental m argins are, at their m ost rudim entary, the d en u d atio n of a Palaeozoic orogenic belt, form ed at the end of the Carboniferous during the suturing of Pangaea, and its fragm entation d u rin g the Mesozoic breakup of the supercontinent. A t present, there are negligible traces of Mesozoic and Cenozoic sedim ents exposed overlying the A rm orican and H esperian M assifs, alth o u g h it is p ro b ab le th at they once existed and have su b seq u en tly b een ero d ed . However, it is these eras that are represented by the fission track ages.

A t the en d of the V ariscan O rogeny, the A rm o rican an d H esp erian M assifs w ere deeply d en u d ed . This took place d u rin g the Perm ian, the erosive surface possibly rep resen tin g one sim ilar to th at revealed today. There are Perm ian terrestrial sedim ents onlapping u p o n the Variscan Basement around the H esperian Massif and in the north-eastern p a rt of the A rm orican Massif. The ap p aren t rem oval of these sedim ents from o th er regions of the m assif suggests th at fu rth er erosion of the basem ent has taken place since the Permian.

Such rap id d en u d atio n of m o u n tain belts is n o t uncom m on. Regions of high elevation en d u re erosion often at rates >1000 m m ka“l (from solid and solute transport loads of the principal rivers draining from the H im alayas; M illim an and M eade, 1983). Erosion rates >5000 m m k a 'l have been recorded from the highest areas of the Southern A lps of N ew Z ealand w here local crustal uplift rates are ~7000 m m ka"^ (Selby, 1982). Erosion is a continually active process influenced prim arily by elevation. C onsequently m ountain chains will suffer erosion d u rin g orogenic activity, and this w ill continue after the cessation of activity, un til elevation is no lo n g er sufficient to p ro v id e the p o ten tial for h ig h d e n u d a tio n rates. E ngland an d R ichardson (1980) believe th at d e n u d a tio n can rem ove thicknesses of 60 km from m ountain belts, m ost of this occurring d u rin g orogenic activity. Brown (1993) suggests from data published elsew here (e.g. Jones and Brown, 1990; Dallm eyer and Brown, 1992) th at there w as rapid cooling/tectonic exhum ation of Southern Brittany at rates of approxim ately 850 m M a 'l m ain tain ed for 10 M yr to w ard s the end of the V ariscan Orogeny.

The oldest apatite fission track ages (230 -270 Ma) in this thesis are possibly representative of the initial erosion of the V ariscan M ountains, seem ingly rep resen tin g m onotonie co o lin g /u n ro o fin g of the basem ent. This is supported by the track length data where the distributions are of the

un d istu rb ed basem ent-type (Gleadow et al., 1986b). These sam ples are all from the eastern parts of the Arm orican Massif bordering the Bay of Biscay.

Tow ards the end of the Perm ian and d u rin g the low er Triassic, the region w as rifted, locally in the Bay of Biscay an d in the W estern A pproaches-E nglish C hannel region, and regionally in the w hole of the A tlantic region. This w ould have induced enhanced heat flow in the rift zones from crustal thinning and associated volcanism , alth o u g h therm al effects decay rapidly aw ay from the rift axes (Gallagher et al., 1993). This effect w as m inim al in the Bay of Biscay region, as show n by the subsequent form ation of non-volcanic passive m argins (Bott, 1992) and also by the paucity of early Triassic (230 -250 Ma) apatite fission track ages along these rifted m argins.

These p erio d s of rifting from the Triassic to the Jurassic and beginning of seafloor spreading in the N o rth A tlantic region d u rin g the m iddle Cretaceous w ere associated w ith great thicknesses of sedim entation in the N o rth A tlantic region, especially in the rift basins. H ow ever, the Biscay and Galicia m argins were starved and contain little sedim ent w hen co m p ared w ith regions such as the W estern A pproaches Basins (e.g. C h a p m a n , 1989). The P aris, A q u itain e, E n g lish C h a n n el, W estern A p proaches, A stu rian and D uero sed im en tary b asin s o n lap onto the Variscan Massifs, which have allegedly rem ained as u p stan d in g blocks for m uch of their Mesozoic and Cenozoic history (Cogné, 1974). H ow ever, this does n o t necessarily m ean th at they have escaped sed im en tary cover. T h ro u g h o u t E urope, the m ajority of the V ariscan b elt is to d ay u n d e r sedim entary cover w ith only isolated blocks u p stan d in g . V ariscan rocks occur at the surface today either reactivated as p art of the A lpine O rogeny (Alps and Pyrenees) or in the M assif Central, w hich has suffered copious Q uaternary volcanism, on the flanks of the Rhine G raben (the Vosges and Schw arzw ald), and on the w estern m argins of Europe, as the A rm orican, H esperian and C ornubian Massifs. The sedim entary h isto ry of the w est European Variscan Massifs and their im portance as source regions for the basins is outlined in section 2.4.

In the Bay of Biscay region, all the apatite fission track d ata are derived from surface outcropping samples, situated on the rift flanks of the Bay. Therefore they are unlikely to have been influenced by the therm al effects of rifting and consequently the Mesozoic ages observed are m ost probably a record of burial and subsequent exhum ation. The denudational processes w ere probably aided by an increase in elevation as a result of rift- related dom ing in the U pper Jurassic and Lower Cretaceous (Ziegler, 1990). The observation of clastic sedim ent thickness in the surro u n d in g basins and

their provenance cannot be ignored. In the m ajority, the fission track ages represent a Jurassic to Cretaceous evolution w hich is presum ably linked to these processes.

The post-C retaceous evolution of the A rm orican M assif is less w ell docum ented, although relicts of Tertiary transgressions are exposed in p a rts of the massif. G entle deform ation w as in curred as a resu lt of the A lpine orogenic phases. The surrounding offshore basins, particularly the W estern A pproaches and the English C hannel u n d erw en t m ajor inversion as a result of tectonism related to the early A lpine O rogenic phases and N o rth A tlantic O pening. These events w ere unlikely to have been lim ited only to the basinal regions and p robably affected the adjacent blocks, including the A rm orican, C ornubian and H esperian M assifs (see d ata of Lewis et al., 1992a). The Iberian Plate w as tectonically active at this time, culm inating in the Pyrenean collision. G uim erà (1984) reports tw o m ain co m p ressiv e p h ases affecting the in te rio r of the Ib erian p late (Late C retaceous to Lower Eocene and Oligocene), follow ed by a post orogenic end-O ligocene m inor extensional regime. This tectonism is reflected in the Eocene ages in the A sturian Arc.

The possibility of hot fluid system s as a resetting m echanism is as ever, an un k n o w n quantity. The area is structurally com plex and highly m ineralised, w ith n u m ero u s regional-scale th ru sts an d faults, m any of w hich are deep seated, and all are capable of acting as conduits for fluids.

R ecognition of resettin g in fission track sy stem s a n d the determ ination of the resetting time has been facilitated by use of the 'banana curve' (Fig. 5.8). w here apparent age is plotted against m ean track length. G reen (1986) first used this w ith a range of apatite ages betw een 270 and 60 Ma on rock sam ples from N orthern England. At that tim e the younger (-80 Ma) apparent apatite ages could not be ascribed to a know n therm otectonic event. G reen (1986) therefore proposed th at the track length p o p u latio n form ed broadly as tw o separate populations, an older and a younger, w ith different distributions related to the different apatite ages. This graph show s the variation of m ean track length and standard deviation as plotted against apatite fission track age, and the banana' shaped curve is characteristic of g rad u a l resetting of apatites and replacem ent of the 'old' p o p u latio n of tracks w ith a 'new' population.

The data presented in this thesis are p resen ted in this fashion, enhanced w ith the overlay of data fields (Fig 5.9). Those sam ples w ith the oldest apparent ages and longer m ean track lengths have seen no significant therm al perturbations since their formation, w hereas those at the young age end of the graph have been affected by m ore recent activity. The m iddle

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