Scientific contribution - FORSTER, M.A. & LISTER, G.S.:

THE NATURE OF OROGENESIS AND THE EXHUMATION OF ULTRA-HIGH PRESSURE METAMORPHIC TERRANES IN THE OVER-RIDING PLATE ABOVE RETREATING SUBDUCTION ZONES

 

Current theory divides the mountain-building process into a single “constructional” phase followed by a collapse or “destructional” phase. Rather, we suggest, the evolution of an orogenic belt involves repeated inversion cycles with tectonic mode switches.

 

Mountain belts are traditionally considered to be the result of slow convergence of lithospheric plates, and the result of overall shortening of the crust and lithosphere. Our current theories for mountain-building processes do not acknowledge a significant role for extensional tectonism, except during the late stage collapse or destruction of mountain belts. Extension during the early stages of orogenesis is thought to be restricted to local buoyancy or channel flow driven phenomena. Yet field observations show that it is major extensional shear zones that draw once deeply buried rocks from the crustal roots of the mountain belt back towards the surface.

 

This implies that mountain belts do not collapse.  Rather they are torn apart by lithosphere scale extension.  This can be explained by “roll-back” of subducting slabs adjacent to the orogenic belt.  Renewed roll-back of subducting oceanic lithosphere in front of an orogenic zone after an accretion event has the capacity to exhume very deep levels of the Earth’s crust or lithosphere,

because the crust and lithosphere can be severely extended in such environments (Fig. 1).  

 

Figure 1. Tectonic model of the formation and exhumation of  HP and UHP metamorphic rocks at Lago di Cignana, NW Italy. (a) accretion of the Briançonnais ribbon leads to thrusting, and initiation of a new subduction zone; (b) roll-back causes lithosphere-scale extension, tectonic inversion and exhumation.

 

We have researched the role of lithospheric dislocations in this process, and how lithospheric dislocations that reverse their shear sense can be recognized, using the ultra-high-pressure slice at Lago di Cignana, Valtournenche, NW Italy as a case study (Fig. 2). The UHP sliver occurs immediately beneath a major extensional shear zone (the Matterhorn Detachment), interleaved between the Zermatt Saas Unit and the Combin Shear Zone.

 

  

 

Figure 2. Cross-section through the “shuffle zone” at the ultra-high pressure locality at Lago di Cignana (parallel to the dominant NW-SE trending stretching lineation). Lenses of Briançonnais affinity metasediments, serpentinite, prasinite and UHP rocks are located at the lower boundary of the Combin Shear Zone (shuffle zone).

 

The maximum metamorphic grade recorded by the Zermatt-Saas Unit is high-pressure (HP) eclogite facies, whereas rocks in the now dominantly greenschist facies Combin Shear Zone locally contains medium-pressure (MP) blueschist facies relicts. We suggest that the UHP unit is preserved as a thin tectonic slice because the locus of later extensional structures has not precisely followed the trajectory of older thrusts. The UHP slice is located in the lower boundary of a kilometre-scale shear zone that has sheared and retrogressed rocks from the Combin Zone.

 

 

  Figure 3.Metre thick layers of marble and dolomite within the Combin Shear Zone. These layers retain evidence of earlier events of folding and extension that have been overprinted by late-stage extension. Fabrics defined by white mica have been dated using Argon geochronology and give ages of 42-40 Ma, defining the age of operation of the Combin Shear Zone.

 

Fabrics and mineralogy reveal that the Combin shear zone has operated through blueschist facies conditions until greenschist facies metamorphic conditions were reached (Fig. 3). Intense fabrics developed, commensurate with large shear strains associated with significant horizontal relative displacement. Structural geology suggests a history of large-scale overthrusting during which the UHP rocks were emplaced over the HP rocks of the Zermatt-Saas Unit. Tectonic inversion subsequent to the period of HP metamorphism led to the Alpine orogen being subject to large-scale (roughly NW-SE directed) horizontal stretching. This led to the formation of orogen-scale extensional shear zones, of which the Matterhorn Detachment may be one of the most important manifestations.