Sesia

Abstract: [1] The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65–80 Ma) subduction to different depths at high-pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0–1.2, 1.0–1.5, and 1.5–2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal-scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E-W trending mylonitic shear zone under retrograde blueschist- to greenschist-facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15–20 km occurred in the footwall of a greenschist-facies, top-SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb-Sr mineral ages constrain D2 to have occurred at about 60–65 Ma and D3 at about 45–55 Ma. Thus top-SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro-Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra-HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo-Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature-dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high-stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low-stress subduction of the Liguro-Piemont oceanic lithosphere.

Abstract: The Sesia Zone, Western Alps, is a slice of Austroalpine continental crust that was once metamorphosed at T = 500°-560°C and P > 14-16 kbar at ~130-100 Ma B.P. Pervasive blueschist/eclogite facies assemblages record this event, and a later greenschist facies overprint is only well developed in the NW part of the zone. On the basis of thermal calculations, it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift/erosion. An alternative model is proposed in which the Sesia Zone was emplaced by underthrusting of the Austroalpine continental margin prior to subduction of oceanic lithosphere of the Piemonte basin. Subduction of relatively cool oceanic lithosphere below the underthrust Sesia Zone then continued for a period of time (e.g., 20 Ma), thus suppressing thermal recovery of this latter unit. Exhumation of Sesia Zone rocks began during this subduction episode by uplift on the Insubric Line, erosion, and by underplating of oceanic ...

Abstract: The sou thern Sesia zone in the western Italian Alps consists of fou r tectonic su b-units separated by maj or thru sts. These su b-units inclu de : "gneiss minu ti" (GM) , blu eschist-facies "polymetamorphie paraschists" (PP) similar to the "11 zona diorito-k inzigitica" , "eclogitic micaschists" (E M) and blu eschist-facies "Rocca Canavese thru st sheets" (RCT) . Du ring Alpine metamorphism these su b-units u nderw ent a polyphase early­ Alpine history at high-P, follow ed by a late-Alpine low-P event. E vidence for an early-Alpine prograde path has been only fou nd in the E M and GM. The early-Alpine metamorphic peak s occu rred at T = 500-550 °C and P = 13-20 k bar in the E M, T = 400-450°C and P � 11-12 k bar in the GM and at T = 300-400°C and P � 8-10 k bar in the PP and RCT. Decompression du ring the early-Alpine event occu rred at decreasing T in the E M (trom T = 500-550 °C to T < 450-500 0C) and at more or less constant T in the other sub-u nits. In the sou thern E M, the early-Alpine post-eclogitic recrystallization produ ced law sonite porphyroblasts w hich grew in equ ilibriu m w ith quartz ± j adeite. Formation of law sonite occurred at low XC0 2 ; it w as favou red by a temperatu re decrease after the HP climax and probably the syn-metamorphic infiltration of H20 -rich flu ids along preferential channels. Law sonite also occu rs in the other Sesia sub-u nits, w here it crystallized du ring the metamorphic climax . The post-eclogitic T decrease inferred here for the sou thern E M is less evident in more northerly parts of this su b-u nit ; it may be related to a process of condu ctive-cooling du ring early-Alpine exhu mation and the stack ing at deep cru stal levels w ith the relatively cooler blu eschist-facies su b-u nits. After stack ing and recrystallization u nder fairly comparable blu eschist-facies conditions, the different Sesia su b-units u nderw ent a low -P event of probable late-Alpine age. Su ch a low -P event cau sed a metamorphic zonation, w ith low er T in the stru ctu rally high er and eastern RCT. Key-words : law sonite, blu eschist, eclogite, Sesia zone, western Alps. List of the abbreviations u sed in the tex t, tables and figu res : ab = albite, ac = acmite, act = actinolite, alm = almandine, ar = aragonite, au g = au gite, bi = biotite, cc = calcite, ch = chlorite, cpx = c\inopyroxene, ct = chloritoid, cz = clinozoisite, gf = graphite, gl glau cophane or ferroglau cophane, gr = grossu lar, gt = garnet, hbl = hornblende, Kf = K-feldspar, k y = k yanite, j d = j adeite, ilm = ilmenite, Iw = law sonite, ms = mu scovite, 01 = olivine, om = omphacite, opx = orthopyrox ene, pa = paragonite, ph = phengite, pi = plagioc\ase, pu = pu mpellyite, py = pyrope, q z = qu artz, ru = ru tile, sill = sillimanite, sp = spessartine, sph = sphene, spn = spinei, ss = serpentine, st = stau rolite, stp = stilpnomelane, tr = tremolite, zo = zoisite, a = activity, P = pressu re, Ps = lithostatic pressu re, T = temperatu re, X = molar fraction, J.L = chemical potential. * This w ork is part of a C.N .R. (Centro di Stu dio su i Problemi dell'O rogeno delle Alpi O ccidentali, Torino) programme. 001:10.1127/ejm/01/1/0089 0935-1221/89/0001-0089 $ 4. 00 <01989 E. Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart 1