ELSEVIER Earth and Planetary Science Letters 164 (1998) 569–575

Late Cenozoic mineralization, orogenic collapse and slab detachmentin the European Alpine Belt

H. de Boorder Ł, W. Spakman, S.H. White, M.J.R. Wortel

Vening Meinesz Research School of Geodynamics, Faculty of Earth Sciences, Utrecht University, 3508 TA Utrecht, Netherlands

Received 15 September 1998; revised version received 15 October 1998; accepted 15 October 1998

Abstract

It is known that mineralization in orogenic belts often occurs in extensional settings late in the collisional history, andwithout voluminous co-temporal magmatism. In search of the underlying geodynamic processes we consider the spatialand temporal distribution of Late Cenozoic hydrothermal mineralization in the European Alpine Belt and relate this tothe distribution of heat in the lithosphere as inferred from seismic tomography. Specifically, we propose a hypothesisfor mineralization in the European Alpine Belt involving increase in heat flow and fluid flow in response to tearingand detachment of lithosphere slabs and concomitant emplacement of the hot asthenosphere at lower crustal levels. Weconclude that detailed knowledge of lithosphere=mantle structure and processes can help to define potentially mineralizedzones within regions of orogenic collapse. Our approach pioneers the application of lithosphere and mantle tomography tometallogenesis and to earth resources exploration. 1998 Elsevier Science B.V. All rights reserved.

Keywords: Alps; orogenic belts; mineralization; orogeny; tomography; lithosphere; mantle; Mediterranean region

1. Introduction

Mineralization in orogenic belts often occurs inextensional settings [1,2] late in the collisional his-tory [3] and without voluminous co-temporal mag-matism [3]. The possible importance of the mantlewedge, above the subducted slab, in metallogene-sis has recently been amplified [3] and leads to agrowing consensus that mantle wedge processes inthe late stages of subduction may contribute to fluidstransporting metals to shallower levels. In particu-lar, gold mineralization in convergent belts has beenattributed to dehydration of subducted sedimentsand oceanic crust [4]. We connect this to recent

Ł Corresponding author. Tel.: C31 30 2535102; Fax: C31 302537725; E-mail: hdboordr@geo.uu.nl

modelling work on detachment of lithosphere slabsin continental collision zones and concomitant em-placement of the hot asthenosphere at lower crustallevels [5]. Our study starts from the observation inthe European Alpine Belt of a close spatial associ-ation between Late Cenozoic mineralization [6] andregions of orogenic collapse [7]. The mineralization,comprising vein and replacement deposits of Hg, Sb,Au, Zn, Pb and Ag together with porphyry coppersystems, is hosted in belts of Late Cenozoic calc-alkaline to alkaline volcanism. Precise radiometricages of the mineral deposits are scarce. Maximumages are mostly inferred from those of the host-ing volcanic complexes and are generally of middleNeogene to Quaternary age (see caption of Fig. 1).On the basis of the spatial and temporal associationsin the European Alpine Belt (Fig. 1) we propose

0012-821X/98/$ – see front matter 1998 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 2 4 7 - 7

570 H. de Boorder et al. / Earth and Planetary Science Letters 164 (1998) 569–575

Aegean Basin

Vardar Zone

Menderes Massif

Alboran Basin

Transsylvanian BasinPannonian Basin

Tyrrhenian Basin

1000 km

Fig. 1. Distribution of Hg (circles), Sb (diamonds) and Au (squares) mineralization [6] and extent of postulated regions of orogeniccollapse [7] (heavy dashed lines) in the European Alpine belt. Altitudes >1500 m are in dark red. Depths >4000 m are in dark blue. Thethree straight lines give the locations of the cross sections of Fig. 4. The age of the collapse is broadly placed as Miocene and extensionmay continue to the Present [7–9]. Deposits of Hg, Sb and Au are known around the northern Tyrrhenian–Apennine post-collisionalsystem [10–14], the Carpathian–Pannonian system [15,16] and around the northern Aegean [17]. In the Tuscany region of the northTyrrhenian–northern Apennine post-collisional system, cinnabar and stibnite are related to Quaternary geothermal systems [12]. In theCarpathian–Balkan region, an Early Miocene belt through northern Greece, eastern Macedonia and Serbia with porphyry copper depositsand a Mid Miocene to Early Pliocene Inner Carpathian volcanic arc through Slovakia, northern Hungary, western Ukraine and northernRomania with Zn–Pb and Zn–Pb–Au vein and replacement deposits were recently discussed [2]. In western Romania, epithermal Auand porphyry-type deposits are possibly related to a western segment of the Inner Carpathian Arc [2]. In the Carpathians, two distinctvolcanic trends have been reported [18] of Middle Miocene to Early Pliocene and Early Pliocene to Quaternary age, respectively.Here, adularia from hydrothermal aureoles of polymetallic deposits postdates the andesitic and dacitic volcanic hosts by up to 2.2Ma. In western Turkey, Hg and Sb districts are associated with the sedimentary–volcanic basins within and around the Menderesmetamorphic core complex [6,19]. Calc-alkaline to alkaline volcanic host complexes in the Aegean and western Turkey range from EarlyMiocene to Present [20,21]. Along the Vardar Zone, connecting the Pannonian and Aegean Basins, a close relation is inferred betweenrelatively small, late stage sedimentary–volcanic basins, Plio-Pleistocene alkaline volcanism and Hg and=or Sb mineralization [6,22]. Insoutheastern Spain, Au, Sn and polymetallic mineralization trends were recently defined in a narrow northeasterly striking zone [23].This zone also comprises four Hg deposits [6]. Here, the hosting volcanic trend [23] ranges from calc-alkaline rocks (15.5–7.9 Ma) inthe southwest to lamproitic rocks (10.8–6 Ma) in the northeast. This volcanism ended with alkaline basalt emission (4.3–2.6 Ma).

the hypothesis of an alternative, tighter causal rela-tionship between late mineralization and lithospheredetachment processes (Fig. 2).

2. Orogenic collapse and mineralization

Orogenic collapse is taken to result from detach-ment and sinking of either a thickened, gravitation-ally unstable lithospheric root [7,8,24–26] or of arelatively cold subducted slab [27–29] (Fig. 2b). Inboth cases, consequent emplacement of hot astheno-sphere at shallow levels will increase the heat fluxinto the continental lithosphere, causing generation

of felsic and mafic melts and increased fluid activ-ity in the crust which are conditions favorable formineralization (Fig. 2c). In our hypothesis slab de-tachment (or alternatively lithosphere delamination)is responsible for both orogenic collapse and theinflux at shallow levels of heat and fluids from thehot mantle wedge asthenosphere, and thus it pro-vides an internally consistent dynamical cause forthe observed spatial correlation between mineraliza-tion and regions of orogenic collapse. A prerequisitefor any test is the validity of an important aspectof this hypothesis: a spatial correlation should existbetween the above mineralization and hot regions atlithosphere levels within the European Alpine Belt

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Fig. 2. Schematic illustration of our hypothesis that orogeniccollapse due to subducted slab detachment is relevant to Alpinemineralization. (a) The thickened orogenic belt prior to slabdetachment. (b) The initiation of slab detachment. The subductedslab breaks off and starts to sink into the mantle. This leads touplift of and extension in the thickened belt. Asthenosphere and,possibly, mantle wedge materials begin to rise into the spacevacated by the sinking slab. The heat flow into the lithosphereincreases. The breakoff depth of the subducting lithosphere islargely determined by its thermal structure and rheology [5]. (c)Collapse of the orogen and ascent of fluids and magmas into thecrust to produce mineralization.

which are associated with mantle wedge and litho-sphere detachment processes. Because detachmentis a mantle process, this correlation should be mostclearly exhibited by late stage mineralization involv-ing metals with a suspected mantle affinity such asHg, Sb and possibly Au [30–36] (Fig. 1).

3. Lithosphere and mantle structure

We demonstrate the above correlation using re-cent results of lithosphere and mantle structure

obtained by seismic tomography [29,37,38]. FromFig. 3 we observe that the locations of Late Ceno-zoic deposits of Hg, Sb and Au all correspond withzones of reduced P-wave velocity in the lithosphere.A similar correspondence occurs with low S-wavevelocities in recent models derived from waveformtomography [37,38]. The P- and S-wave models areindependent because completely different data setsand methods have been used. Both P- and S-wavevelocities are significantly lower than the ambientmantle reference values. This leads to the conclusionthat, independent of compositional differences, thetemperature in these regions — or belts — is raisedabove average by possibly as much as a few hun-dred degrees [39]. Further, the existence of elevatedtemperatures is corroborated by the noted presenceof volcanic host rock and by the general correspon-dence of low velocity regions with increased surfaceheat flow [40].

A crucial element in our hypothesis is that thehot belts can be associated with lithosphere subduc-tion and detachment processes. Below the EuropeanAlpine Belt, the regions of higher P-wave velocities(only partly displayed in Fig. 3) can be interpreted asa result of lithosphere subduction during the Ceno-zoic [41]. We interpret the low seismic velocity re-gions (Fig. 3) as due to hot mantle wedges associatedwith present and past subduction. Slab detachmentwould produce additional inflow of hot material atupper lithospheric levels [5], specifically in the latestage of collision and just prior to and during oro-genic collapse. The higher seismic velocities of slabanomalies are mostly due to the relatively lower tem-perature in the subducted lithosphere. Further, thepattern of subducted material is largely in agreementwith scenarios of the tectonic evolution of the AlpineMediterranean region in terms of location and lengthof subducted slab [41]. From the interpretation [28]of the subduction-related anomalies it is proposedthat slab detachment at crust–lithosphere levels, andspecifically lateral migration of the tear in the slabalong the strike of the subduction zone, provides afeasible mechanism for slab roll-back and arc migra-tion, for orogenic collapse (and concurrent creationof large sedimentary depo-centers), and for changingchemical signatures of arc volcanism due to replace-ment of magma source regions by asthenosphericmaterial. This could qualitatively explain [28,42] the

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52 km

95 km

145 km

-1.5% +1.5%

1000 km248 km

Fig. 3. Four depth slices through the tomographic P-waveanomaly model EUR89B [29]. Colors represent the relativedeviation of P-wave velocity with respect to the regional ref-erence model PM2 [29] where red (blue) colors indicate lower(higher) velocities. The reference velocities are (from top to bot-tom) 7.853, 8.028, 8.117, and 8.176 km=s. Symbols in the toppanels give the location of mineralization (cf. Fig. 1). The spatialresolution of the displayed anomalies in most of the regionspertinent to mineralization is sufficient (50–100 km laterally, 50km in depth) for our purpose. The exceptions are the anoma-lies below southeastern Spain and the Alboran Basin, and be-low northern Africa where spatial resolution is generally poorer(100–200 km). Note that a close correspondence exists betweenthe occurrence of mineralization and lower P-wave velocities atlithospheric levels (52 km, 95 km).

evolution of the following orogen–back-arc basinsystems: the Carpathian–Pannonian, the Apennines–West-Mediterranean, and the Hellenic–Aegean sys-tems. The occurrence of slab detachment is alsoproposed below the Betic–Alboran system [43] asan alternative for the earlier hypothesis put forward[24] on the cause for gravitational collapse of theBetic orogen. Slab detachment has also been invokedas the trigger for heat supply in studies of magma-tism and exhumation of metamorphic rocks [44] andhas been related to the onset of alkaline magmatismfollowing calc-alkaline magmatism [28].

Most of the regions of mineralization and asso-ciated hot belts (Figs. 3 and 4) correspond spatiallywith the proposed locations of lateral slab rupture atdepth [28]. A notable exception is in the region ofthe Menderes Massif in western Anatolia. The south-ern segment of the Hellenic slab has most likely notdetached [28,29] (cf. Fig. 3, panel ‘145 km’) andthe lack of spatial resolution below central Anatoliaprevents a detailed interpretation of slab detachment,although the image does not contradict detachment.However, below western Anatolia (at the longitudeof the Menderes Massif) the slab anomalies belowthe Aegean and Anatolia are separated in depth by aconspicuous low velocity zone (cf. Fig. 3, panel ‘248km’, Fig. 4c). This zone is interpreted [45] as a resultof a vertical rupture in the subducted slab. This couldchannel a similar inflow of hot material, e.g. fromthe Aegean mantle wedge, at shallower levels andthus provide the heat source and material supply formineralization in and around the Menderes Massif.

In the Carpathians, the locally recorded time span(up to 2.2 Ma) between the formation of a volcanichost complex and mineralization [18] may be an in-dication of the time delay between the occurrence ofslab detachment and actual mineralization. In viewof such a time lag the apparent absence of miner-alization along the southern part of the TyrrhenianBasin corroborates the earlier suggestion [28] thatdetachment is still in progress here, or has happenedonly very recently.

4. Conclusions

We find sufficient indication that slab rupture, ei-ther laterally or vertically, can be invoked to provide

H. de Boorder et al. / Earth and Planetary Science Letters 164 (1998) 569–575 57340

02

00de

pth

(km

)

0

a

400

200

dept

h (k

m)

b

0

c

Fig. 4. Three cross sections (cf. Fig. 1), with map on top, through tomographic model EUR89B [29] illustrating the correlation betweenmineralization and hot belts. Color key as in Fig. 3. Large blue regions are images of subducted lithosphere [29,42]. Panel (b) givesa clear example of slab detachment. The low velocities in panel (c) separating the subducted lithosphere below the Aegean and belowwestern Turkey give the location of possible vertical slab rupture [45].

a plausible explanation for the existence of rela-tively hot domains at lithosphere levels associatedwith regions of orogenic collapse. The alternativeprocess of delamination of thickened lithosphere ofthe overriding plate would similarly result in flowof asthenosphere to shallower levels. However, theevidence from seismic tomography combined withpredictions for lithosphere subduction from tectonicreconstructions [41], renders lithosphere delamina-tion as a less likely process in most regions of theEuropean Alpine Belt.

We propose that the mineralization late in the his-tory of the European Alpine Belt is a function of heatand, possibly also, of fluid phases provided by up-welling asthenosphere in regions where slab ruptureprocesses occur in the late stage of collision. Ourhypothesis is based on two combined observations.First, the spatial correlation between regions of oro-genic collapse and mineralization and, second, thecorrelation between mineralization and hot regions

in the lithosphere (and presumably the lower crust).The first correlation requires a tectonic=dynamiccause since orogenic collapse is a rapid processon the geological time scale. The second needs thetransport of heat and fluids to shallow depths. A crit-ical first test of our hypothesis will be in the intrinsictime relations between mineralization and orogeniccollapse. Our study illustrates the significance of to-mographic studies for investigations of the crust andupper lithosphere, locally also allowing identificationof potentially mineralized structures within regionsof late orogenic extension.

Acknowledgements

H.d.B. gratefully acknowledges advice on carto-graphic matters by P.G.M. Mekenkamp and encour-agement by B.H. de Jong. [RV]

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