deJong-Zeck_Very High Cooling & Uplift Rates (Betics, Spain)_Geology 1992

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Comment and Reply on "Very high rates of cooling and uplift in the Alpine belt of the BeticCordilleras, southern Spain"

COMMENT

Koende Jong

Institut de Geodynamitjue, Universite de Nice-Sophia Antipolis, Pare

Valrose, 06108 Nice Cedex 2, France

Zeck et al. (1992) discussed very high cooling and uplift rates in theAlpujarride complex (Betic Zone, southern Spain) in the context of

extension-induced mantle diapirism due to lithospheric slab detachment. I

agree with them on the very high cooling rates of these (very) high-grade

metamorphic rocks (de long, 1990) and on the proposed extension mech

anism (de Jong, 1991), but I do not believe that extension is the prime

cooling mechanism. Geological and geopbysical observations and

pressure-temperature-time modeling imply that the extensional structure

is, in fact, inverted; thrusting of mantle rocks and of strongly attenuated

and reheated crustal rocks of the Alpujarride complex over a less extended

and thus cooler lithosphere provides an alternative explanation for theirrapid cooling, as pointed out below.

The importance of extension during the late Oligocene to early Mio

cene tectonic evolution of the Betic Zone has been envisaged by manyauthors from different points of view (e.g., Tubia and Cuevas, 1986; Platt,1987; Bakker et ai., 1989; Platt and Vissers, 1989; de long, 1990, 1991;

Van Wees et al., 1992; Van der Beek and Cloetingh, 1992). Reheating of

the rocks of the Betic Zone has been assigned to extension-induced mantleupwarping (Bakker et ai., 1989; de long, 1991; Van Wees et aI., 1992).

The sharp downward increase in metamorphic grade in the AJpujamde

complex (Los Reales unit; Westerhof, 1977; Tubia and Cuevas, 1986)

toward the underlying ultramafic rocks may also be attributed to this

mechanism, as well as the near-isothermal decompression (Westerhof,

'1977) undergone by these mantle rocks. It s tempting to relate the present

day outcrop of ultramafic rocks in the western Betics directly to extension.

Interpretation of seismic refraction profiles over the Ronda ultramafic

massif, however, implies that the peridotites do not root in the mantle, as

would be expected in a disposition due to extension, but instead representa thrust sheet (Barranco et al., 1990). Field relations show that the ultra

mafic rocks rest on very high grade metamorphic crustal rocks of the

Alpujarride complex (Westerhof, 1977). This implies that the extensional

mantle uplift has been decapitated and transported over the crustal rocks

of the Alpujarride complex (Tubia and Cuevas, 1986), which in turn

overthrust crustal rocks of the Nevado-Filabride complex, characterized

by less extension-related reheating. Thus, the extensional structure has

been inverted (de long, 1991; Van Wees et aI., 1992). Thrusting of the hotultramafic rocks bwesulted in an upward increase in metamorphic grade

in the underlying rocks of the Alpujarride complex (Blanca unit-

Westernof, 1977; Ojen nappe-Tubia and Cuevas, 1986), culminating inlocalized fusion. Deformation structures in the mylonite zone separatingthe crustal rocks from the overlying peridotites indicate that ductile over

thrusting occurred during a change from high-temperature to low-

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temperature conditions, accompanied by retrograde reactions (Westerhof,

1977; Tubia and Cuevas, 1986). The temperature field for blocking of theisotope systems used by Zeck et a1. (1992) was, therefore, entered during

overthrusting. K-Ar biotite cooling ages obtained by Priem et al. (1979),cited by Zeck et aI. (1992, Table 1, Sierra Alpujata), from high-grade

metamorphic rocks below the peridotites (ALM 67, 76, and 79; Blanca

unit or Ojen nappe) and above them (ALM 75 and 77; Los Reales unit)are concordant, pointing to a similar cooling history. Two-dimensional

pressure-temperature-time modeling (Van Wees et aI., 1992), in combina

tion with 4OAr/39Ar data (de long, 1991; de long et al., 1992), implies

that cooling of the Alpujarride complex by about 200°C is possible within

a time span of 4 m.y., owing to thrusting of these rocks over coolerlithosphere. .

Thus,. tectono-metamorphic characteristics and the concordance ofcooling ages obtained from crustal rocks both above and below the perido

tites imply that very rapid cooling of high-grade metamorphic rocks of the

Alpujamde complex was achieved by their being thrust over less extended,

cooler lithosphere during inversion of the late Oligocene to early Mioceneextensional structure in the Betic Zone and not during progressive exten

sion, as envisaged by Zeck et aI. (1992).

REFERENCES CITEDBakker, H.E., de Jong, K., Helmers, H., and Biermann, C., 1989, The geodynamic

evolution of the Internal Zone of the Betic Cordilleras (SE Spain): A modelbased on structural analysis and geothermobarometry: Journal of Metamorphic Geology, v. 7, p. 359-381.

Barranco, L.M., Ansorge, J., and Banda, E., 1990, Seismic refraction constraints onthe geometry of the Ronda peridotite massif (Betic Cordillera, Spain): TeetonophysiCl, v. 184, p. 379-392.

de Jong, K., 1990, Alpine tectonics and rotation pole evolution of Iberia: Tectonophysics, v. 184, p. 279-296.

de Jong, K., 1991, Tectono-metamorphic studies and radiometric dating in the BeticCordilleras (SE Spain)-With implications for the dynamics ofextension andcompression in the western Mediterranean area [Ph.D. thesis]: Amsterdam,Vrije Universiteit, 204 p.

de Jong, K., Wijbrans, J.R., and Feraud, G., 1992, Repeated thermal resetting ofphen/tites in the Mulhacen Complex (Betic Zone, southeastern Spain) shownby 4iJAr/38Ar step heating and single grain laser probe dating: Earth andPlanetary Science Letters, v. 110 (in press).

Platt, J.P., 1987, The uplift of high-pressure-Iow-temperature metamorphic rocks:Royal Society of London Philosophical Transactions, ser. A, v. 321,p.87-103.

PIatt, J.P., and Vissers, R.L.M., 1989, Extensional collapse of thickened continentallithosphere: A working hypothesis for the Alboran Sea and Gibraltar arc:Geology, v. 17, p. 540-543.

Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Den, I.S., Verdurmen, E.A.Th.,and Verschure, R.H., 1979, Isotopic dating of the emplacement of the ultramafic masses in the Serrania de Ronda, southern Spain: Contributions toMineralogy and Petrology, v. 70, p. 103-109.

TubIa, J.M., and Cuevas, J., 1986, High temperature emplacement of the LosReales peridotite nappe (Betic Cordilleras, Spain): Journal ofStructural Geology, v. 8, p. 473-482.

Van der Beek, P.A., and C loetingh, S., 1992, Lithospheric flexure and the tectonic

GEOLOGY, November 1992



evolution of the Betic Cordilleras (SE Spain): Tectonophysics, v. 203,p.325-344.

Van Wees, J.D., de Jong, K., and Cloetingh, S., 1992, Two-dimensional P-T-tmodelling and the dynamics of extension and inversion in the Betic zone (SESpain): Tectonophysics, v. 203, p. 305-324.

Westerhof, A.B., 1977, On the contact relations of high temperature peridotites inthe Serrania de Ronda, southern Spain: Tectonophysics, v. 39, p. 579-592.

Zeek, H.P., Monii:, P., Villa, I.M., and Hansen, B.T., 1992, Very high rates ofcooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain:Geology, v. 10, p. 79-83.

REPLY

H. P. Zeek

Geological Institute, Copenhagen University, 1350K Copenhagen,

Denmark

P. MonieGeological Institute, Montpellier University, Montpellier, France

I. M. Villa

Isotope Geological Unit, Bern University, Bern, Switzerland

A major point in our paper (Zeck et aI., 1992), namely the paleonto

logical age that terminates the time-temperature calibration ofour cooling

uplift-exhumation sequence apparently was not quite clear to de Jong.

Unlike most other cooling-uplift studies, our reconstructed cooling histo

ries for three areas within the Alpujarride nappe complex end under

ambient surface conditions. This implies active removal of cover material

after passing through the isotopic closure temperature windows to produce

a transgression surface. Thrusting of Alpujarride nappe complexes over

cooler Nevado-Fihibride basement, claimed by de Jong as an alternative

cooling model, is clearly not enough. The situation is best illustrated in

Figure 1, which reproduces one of de Jong's figures (Van Wees et aI.,

NW<===:J E X T E N S IO N r::::=::::)

SE

MANTLE

Figure 1. Schematic northwest-southeast sections illustrating suggested cooling thrust phase (de Jong's Comment) inverting a suggested earlier extension. N =Nevado-Filabride nappe complex; LA =

Lower Alpujarride nappe complex; HA = Higher Alpujarride nappecomplex.


--- ~ ~ - - - ~

1992). The section indicates that after the hypothesized cooling thrust

phase the cover stilI bas to be removed; if anything, the thrusting will

thicken the cover. The quintessence of the data we presented (Zeck et ai.,

1992) is that the time span indicated between the biotite and muscovite

isotopic closure ages and the paleontological age of the transgressive sedi

mentary rocks is within a few million years. On this basis, we expressed

doubt whether erosion alone could have been the operational process and

instead suggested an extensional tectonic setting, tectonic unroofing, uplift,

and exhumation as the cause of the rapid cooling (Zeck et aI., 1992).

De long makes a lot out of the presumed concordance ofKIAr ages

for rocks overlying and underlying "the peridotites" and claims it supports

his cooling-thrust theory. For the Sierra Alpujata slab the concordance is

indicated by present data, but there are no data to support a regional

generalization. Furthermore, the conclusive power of such concordance is

doubtful. It is in agreement with, or does not deny, several other models,

including our suggestion of tectonic unroofing. The cited apparent age

concordance (De Jong's Comment) merely indicates that the cooling

through the closure temperature of the K-Ar system took place at approx

imately the same time below and above the peridotite slab in the Sierra

Alpujata.

De long states that we discussed the very high cooling and uplift rates

within the context of extension-induced mantle diapirism due to litho

sphere slab detachment, and he claims that we believe that extension is the

prime cooling mechanism and that we envisage the very rapid cooling to

take place during progressive extension. These references and several oth

ers made to our paper (Zeck et aI., 1992) are not quite correct and draw amisleading picture of our suggestions. The background for our suggestion

of tectonic unroofing has been explained above; it should be noted that we

specifically kept open the possibility of body-force-controlled extensional

tectonics in the crustal section within an overall compressive regime. Apart

from assigning this tectonic event to the late stage of the orogeny, we

pointedly did not attempt to incorporate it into a regional orogenic scena

rio. Space limitations did not allow it, and our data do not restrain and are

not dependent on, e.g., direction of movement, correlation with existing

schemes ofdeformation phases, or the occurrence and importance ofshearbeating. In the last paragraph of the paper (Zeek et al., 1992) we drew

attention to a recent tomography analysis by Blanco and Spakman (1992)

which, following upon earlier seismic work by Chung and Kanamori

(1976), Udias et aI. (1976), and Buforn et a\. (1988), indicates the presenceof a northeast-striking, roughly vertical, detached lithospheric fragment at

a depth of 200-700 km in the mantle of the Betic- Alboran region. We

regarded this as very important evidence and suggested that the detached

slab might represent a remnant of an earlier active, northwestward-dipping

subduction zone. Its location and orientation would agree with the sug

gested southeastward drift of the Iberian block in connection with the

opening of the North Atlantic and the resulting involvement of the western

realm of the Tethys-i.e., the Alpine orogeny. We made the carefully

phrased suggestions that (1) the detachment of the lithospheric slab might

have been a contributive factor to the very high rates of cooling, uplift, and

exhumation which our data seem to imply for the Alpujarride nappe

complex within the remaining, nonsubducted lithosphere, and (2) the

sinking of the slab possibly induced diapirism in the surrounding mantle.

We conclude that the detailed and decisively formulated geodynamicscenario presented by de Jong is not supported by the data we presented(Zeck et aI., 1992). The phase of late-stage tectonic unroofing we sug

gested is accommodated much better by other orogenic models(e.g., Tubia et aI., 1992).

REFERENCES CITEDBlanco, M.J., and Spakman, W., 1992, The P-wave velocity structure of the mantle

below the Iberian peninSUla: Evidence for subducted lithosphere below southern Spain: Tectonophysics (in press):

Buforn, E., Udlas, A., and Mezcua, J., 1988, Seismicity and focal mechanisms insouth Spain: Seismological Society of America Bulletin, v. 78, p. 1372-1382.

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Chung, W.-Y., and Kanamori, H., 1976, Source process and tectonic implicationsof the Spanish deep-focus earthquake of March 29,1954: Physics ofthe Earthand Planetary Interiors, v. 13, p. 85-96.

Tubia, J.M., Cuevas, J., Navarro-Vila, F., and Alvarez, F., 1992, Tectonic evolutionof the Alpujarride Complex (Betic Cordillera, southern Spain): Journal ofStructural Geology, v. 14, p. 193-203.

Udias, A., L6pez Arroyo, A., and Mezcua, J., 1976, Seismo-tectonics of the AzoresAlboran region: Tectonophysics, v. 31, p. 259-289.

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Van Wees, J.D., de Jong, K., and Cloetingh, S., 1992, Two-dimensional P-T-tmodelling and the dynamics of extension and inversion in the Betic Zone (SE

Spain): Tectonophysics, v. 203, p. 305-324.Zeck, H.P., M o n j { ~ , P., Villa, I.M., and Hansen, B.T., 1992, Very high rates of

cooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain:Geology, v. 20, p. 79-82.


deJong-Zeck_Very High Cooling & Uplift Rates (Betics, Spain)_Geology 1992

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