I block stream del Massiccio Peridotitico di Lanzo (Alpi Nord-Occidentali)

Abstract

G. Fioraso & G. Spagnolo, The block streams of the Lanzo Peridotite Massif (North-Western Alps). (IT ISSN 0394-3356,
2009).
The Lanzo Peridotite Massif (LPM) is one of the largest outcrops of ophiolitic peridotites in the world and it is well known because of
the freshness of its mantle rocks; furthermore, the LPM represents a peculiarity also for the extraordinary and unusual presence of a
large number of block streams (157), consisting of streams of angular blocks resting over a thick succession of fine sediments.
The LPM extends for about 150 km2 along the internal margin of the Western Alps, 35 km North-West of Turin, between the lower Susa
Valley and the lower Viù Valley; it is commonly divided into a southern body (55 km2), a central body (90 km2) and a northern body (5
km2), which are separated by two broad mylonitic and serpentinized shear zones with a NW-SE direction. Other tectonic shear zones
are localized in the marginal and more serpentinized parts of the LPM, corresponding to the contacts with the adjacent tectonic units
of the alpine chain (i.e. Piemontese Zone and Sesia-Lanzo Zone). The LPM is mainly composed of massive weakly serpentinized plagioclase peridotites with minor spinel peridotites and granular harzburgites. Plagioclase peridotites are well exposed in the core of the three bodies, whereas in the peripherical areas peridotites gradually become more serpentinized till turning into massive serpentinites.
Spinel lherzolites, spinel dunites and harzburgites are subordinated; all the peridotites are frequently intruded by various gabbroic veins and dykes and sometimes by basalts.
Block streams mainly extend on lherzolite outcropping areas, covering on the whole 14.67 km2
, equivalent to 9.8% of the LPM extent.
Block streams are located chiefly on the northern and central body of the LPM, along the slopes of the Mount Colombano - Mount
Corno and Mount Colombano - Mount Lera ridges; another notable group of block streams is localized between Col del Lys and Colle
Portia, whereas in the southern areas of the LPM such features are rather scattered. Block streams range in elevation between 1645
and 470 m a.s.l., with a maximum relief of 870 m; they take up an area ranging from 0.3 to 91.6 ha, with a maximum length of 2.23 km
and a mean slope of 13÷44°; nevertheless 76% of block streams have a mean slope of 23÷32°.
Block streams show an upper layer made of interlocked coarse angular to subangular blocks with no fine matrix, resulting in an openwork framework. Blocky cover has a relatively uniform thickness ranging between 1 and 7 m, commonly with a coarsening-upward texture. Detrital elements range between 0.2 and 1.8 m in diameter and derive exclusively from rocks belonging to the LPM. The upper layer rests on a matrix-supported unstratified diamicton, consisting of an heterogeneous mixture of silt, sand, clay and gravel and containing scattered subangular to subrounded boulders; with an ascertained thickness ranging between 0.5 and 22 m at least, diamicton in turn rest above the bedrock. Below open-work mantle a notable and perennial subdetrital runoff develops with discharge fluctuation parallel to the local pluviometric and climatic trends.
Block streams commonly exhibit a complex variety of morphological features such as longitudinal ridges and furrows, transversal ridges, narrow steep lobes and hollows: these features are caused by downslope differential movements of the blocky mantle or by the
quick removal of the underlying matrix by the subdetrital water circulation.
In the lower Viù Valley (i.e. Germagnano) the fronts of few block streams crosscut fluvio-glacial and fluvial units (ranging between middle Pleistocene to Holocene in age) reaching the actual flood plain of the Stura di Lanzo Stream. Moreover collapse features locally
affecting anthropic structures show that such phenomena in few cases are still in a steady slow evolution.
According to spatial and altimetric distribution, sedimentologic characters and morphologic features, block streams of LPM may be
regarded as creep-dominated phenomena involving sandy-silty and clayey-silty matrix. The development of such blocky-covered features, preferably regarding lherzolitic substratum, reflect the particular weathering process affecting peridotitic rocks and the absence of glacial modelling in this sector of the alpine chain: this allowed the uninterrupted long-term chemical-fisical breakdown of parent rocks, accompained by reworking of pedogenetic products by size sorting and concentration processes operating along slopes and in valley bottoms.