OJS 631 Monegato et al proofcopy 6.pub Available online http://amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 35 (2), 2022, 1-16 1. INTRODUCTION Glacial dispersal trains (Shilts, 1982) - i.e. trails of deposits and clasts eroded from a bedrock source, transported downglacier and deposited in the strati- graphic record (Cummings & Russell, 2018) - represent an important tool for understanding the flow of glaciers and the delivery of glacigenic sediments, including the distribution of particular types of rocks or minerals in the glacial till (e.g., Shilts, 1993; McClenaghan et al., 2001). Empirical tests in tills of boreal ice sheets (e.g., Larson & Mooers, 2004) highlight the key role that distance from the source areas plays in the downflow distribution of the indicator lithotypes in glacigenic deposits. In glaci- ated mountains the dispersal train largely follows the major valleys towards the piedmont plains. Gravitational processes (e.g., rock fall, slope failures) occurring at the interface of the glacier and the ice-free valley slopes promote debris accumulation and its consequent em- bedding into the ice (e.g., Evans, 2003; Goodsells et al., 2005). Supraglacial delivery - including large (erratic) boulders - represents a peculiar characteristic of moun- tain glaciers. Erratic boulders evidence past glacier ex- tents, even where no till can be found. They are key to inferring the pathways and transfluences within the net- work of valley glaciers that developed in the Alps during Pleistocene glaciations (e.g. van Husen, 2004; Kelly et al., 2004; Bini et al., 2009). Understanding the dynamics of large mountain glaciers during Pleistocene glaciations is intriguing be- cause present-day analogs are scarce and located at different latitudes (i.e. Alaskan Range or Patagonian Andes) or at different elevations (Himalaya). Modern glaciers with piedmont lobes are very rare and limited to the Alaskan Range, Kunlun Mountains and Iceland. Consequently, a multidisciplinary approach is needed to reconstruct the functioning of past Alpine ice-stream networks. For the European Alps, the excellent knowledge on Last Glacial Maximum (LGM) glacier ex- tent has fostered numerous glacier modelling studies and related ELA interpretations (Becker et al., 2016; Jouvet et al., 2017; Cohen et al., 2018; Seguinot et al., 2018; Imhof et al., 2019; Višnjevi� et al., 2020; Seguinot & Delaney, 2021; Del Gobbo et al., 2022). Modelling results additionally provide information on paleoglacier velocity, sliding, thickness and flow path. Even if the simulations show certain discrepancies with the sedi- mentological and geomorphological data with regard to glacier extent and ice thickness, they raise important questions about ice development during major glacia- tions and the building of interconnections among the valley glaciers (Bini et al., 2009; Seguinot et al., 2018). https://doi.org/10.26382/AMQ.2022.07 THE TICINO-TOCE ICE CONVEYOR BELTS DURING THE LAST GLACIAL MAXIMUM. Giovanni Monegato 1, Sarah Kamleitner 2, Franco Gianotti 3, Silvana Martin 4, Cristian Scapozza 5, Susan Ivy-Ochs 2 1 Institute of Geosciences and Earth Resources, National Research Council, Padova, Italy. 2 Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland. 3 Dipartimento di Scienze della Terra, Università di Torino, Torino, Italy. 4 Dipartimento di Geoscienze, Università di Padova, Padova, Italy. 5 Istituto scienze della Terra, Scuola Universitaria Professionale della Svizzera Italiana SUPSI, Mendrisio, Switzerland. Corresponding author: G. Monegato ABSTRACT: The provenance and distribution of erratic boulders of the Ticino-Toce glacier network yields key information for determining glaciers’ paleoflow and highlights the interaction between two major Alpine glacier systems during the Last Glacial Maximum (LGM). Boul- ders in the central and western parts of the Verbano, as well as in the smaller Orta end moraine systems, originate from the Toce catch- ment. Erratics pertaining to the Ticino mountain basin characterize the eastern flank of the Verbano amphitheatre and the glacial deposits in the Ceresio system. The wide distribution of Toce lithologies in the Verbano end moraine system can, despite its smaller overall size, be ascribed to the hypsometry and valley course of the Toce catchment. Areas with highest-elevation (>4000 m a.s.l.) and a short flow path (<100 km), favored the early spread of the Toce glacier. In the first phase of the LGM, the preceding advance of the Toce glacier may have suppressed the larger, possibly inert, Ticino glacier towards the east forcing its diffluence into the prealpine area of Ceresio, which had no local glaciers and was likely impacted by a western branch of the Adda glacier. The dynamics of the Ticino-Toce glacier network during the LGM highlight the role of the topography and location of the accumulation areas in driving differential development of glaciers originating from high catchments that can force nearby glaciers with greater inertia towards a different path. Keywords: European Alps, Last Glacial Maximum, palaeoglacier, erratic boulder, provenance. . 2 Monegato G. et al. Reconstructions based on geomorphological/geological data become more difficult moving from the outer sec- tors of the chain, where piedmont lobes spread onto the plain, to the axial sector, where the accumulation areas were located. In this inner portion of the cathement the landforms related to the glacial maxima are scarce, while glacigenic sediments are present but hardly useful for paleoglacier reconstruction. A key tool for under- standing the flow of paleoglaciers and their possible interconnections through transfluences is the prove- nance of glacial debris and particularly that of erratic boulders carrying the lithological signature of the source catchment (e.g., Coutterand et al., 2009; Coutterand, 2010; Jouvet et al., 2017; Braakhekke et al., 2020). Large erratic boulders are more suited than smaller clasts for such a task as they are large enough to barely suffer from the possible bias of reworking, as their da- tings to the LGM have shown (see section Materials and Methods). While not every type of rock can produce big boulders, those boulders transported on the ice surface would not suffer from mixing and would generally re- main on the side of the glacier that they fell onto. An exception in this assumption is related to large land- slides, which may scatter the debris across the whole width of the flowing glacier (e.g., Menzies et al., 2017; Frasca et al., 2020). Boulder roundness can additionally inform about the transport position (supraglacial or sub- glacial). The potential of using erratic boulders to model glacier flow was shown by several studies (Florineth & Schlüchter, 1998; Jouvet et al., 2017; Cohen et al., 2018), yet the coupling with chronology is critical. Through field studies and different absolute dating tech- niques, the chronology of the Alpine LGM has been refined over the last decades, allowing to depict the LGM ice margin in several sectors of the mountain range (Monegato et al., 2007, 2017; Reber et al., 2014; Scapozza et al., 2014; Gianotti et al., 2015; Bernoulli et al., 2018; Ivy-Ochs et al., 2018; Braakhekke et al., 2020; Kamleitner et al., 2022a; Kamleitner et al., 2022b). The present study shows the reconstruction of the paleo -ice streams of the Ticino-Toce glacier system (Fig. 1), one of the largest paleoglaciers in the central southern Fig. 1 - Extent of the LGM Ticino-Toce glacier system (modified after Bini et al. 2009, Braakhekke et al., 2020 (Orta lobe), Kamleitner et al., 2022a (Verbano lobe)). Red dots represent erratic boulders included in different studies (Bernoulli et al., 2018; Braakhekke et al., 2020; Kamleitner et al., 2022a). 3 The Ticino-Toce Ice conveyor belts during the Last Glacial Maximum Alps, and possibly interconnections with neighboring catchments of Rhone, Reuss, Rhine and Adda. The LGM chronology of the Ticino-Toce glacier system was recently assessed by the means of exposure dating of erratic boulders and geomorphological considerations (Braakhekke et al., 2020; Kamleitner et al., 2022a). This allowed to reconstruct the shape of the Orta and Ver- bano piedmont lobes in detail (Fig. 1). Evaluating the boulders' source areas may allow to infer the flow lines of the Ticino-Toce ice streams and test existing glacier models (Seguinot et al., 2018), which suggest asyn- chrony in the development between the Ticino and the Toce valley glaciers. 2. STUDY AREA The Ticino-Toce catchment is about 6700 km2 in size and located in the Western Alps, south of the main Alpine divide. The drainage basin is characterized by an unequal distribution of elevation (Fig. 1, Fig. 2). The highest peaks are located at the western boundary along Monte Rosa-Fletschhorn line with elevations up to above 4600 m a.s.l. Whereas, the northern and espe- cially the eastern sectors have accumulation areas be- low 3500 m a.s.l. The southern parts of the catchment are characterized by several valley reaches, some filled by lakes. The longitudinal profiles of the valleys illustrate the asymmetry between the Toce and Ticino catch- ments, with the former being characterized by high gra- dients and the latter by longer reaches (Fig. 3). The Ticino and Toce valleys are overdeepened (Preusser et al., 2010), with the base of bedrock reported at 600 m b.s.l. at the Magadino plain, progressively decreasing to maximum basin depth of 800 m b.s.l. halfway between Luino and Laveno (Cazzini et al., 2020). The overdeep- ening of these valleys, similarly to other valleys on the southern flanks of the Alps, is assumed to have formed during the Messinian Salinity Crisis (Bini et al., 1978, Finckh, 1978, Hantke, 1983). Debris flow, fluvial and fluvio-deltaic sediments of Messinian age (Miocene), as well as marine sediments of Zanclean age (Pliocene), were indeed observed in the valleys between Mendrisio (southern Switzerland) and Varese (northern Italy) (Bernoulli et al., 2018). Nevertheless, this interpretation is debated and a glacial origin of the upstream sections of southern Alpine valleys is currently proposed (Winterberg et al., 2020). The lake bottom at the outlet of the Ossola Valley does not show a fluvial incision and, during the Messinian, the Toce River was inferred to have flowed into the Orta Valley (Cazzini et al., 2020), Fig. 2 - Geological Map of the Ticino-Toce catchment. Data synthesized from swisstopo (2005). tinental crust, including the mantle peridotite series, which was tilted vertically and brought to the surface during Alpine orogenesis (Rutter et al., 2007). A thick sequence of high-grade metapelitic schists referred to as the “Kinzigite” formation, intruded by gabbro and diorite plutons at the mantle-crust boundary. These "Kinzigite" schists contain mainly garnet, biotite, plagio- clase, quartz, sillimanite and possible muscovite (Rutter et al., 2007). Early Permian granites (named Graniti dei Laghi, GLA) intruded the LAM (i.e. plutons of Mottarone and Montorfano). These plutons are unaffected by Al- pine metamorphism (Boriani et al., 1990b). The Prealps east of Lake Maggiore are largely characterized by sedi- mentary covers of the southalpine domain (Bertotti et al., 1993). These are Permo-Mesozoic (PMC) to Tertiary (TC) shelf to basin marine formations. Lugano Volcan- ites (LV) outcropping on the left side of the Valcuvia Valley are of Permian age (Hunziker & Zingg, 1980). The Insubric Line is crosscut by the Toce Valley and the Ticino Valley in the W-E reach of Lake Maggiore (Fig. 2). Here the narrow (about 2 km wide) Canavese Zone (ZC; Southalpine domain) is in tectonic contact with the Ivrea-Verbano and Sesia-Lanzo zones to the south and north, respectively. The Canavese Zone in- cludes many types of rocks, such as gneisses, schists, Permian diorite, serpentinites and sedimentary car- bonate and siliceous rocks, that are locally strongly de- formed, with a low-grade Alpine metamorphism (Ferrando et al., 2004). West of Bellinzona the tail of the Bergell pluton (BG) is characterized by tonalite (Schmid et al., 1996). The Sesia-Lanzo Zone (SL) is the first unit showing Alpine metamorphism (Compagnoni et al., 1977). It is composed of rocks derived from the Southalpine domain and forming the distal edge of the Adria microplate be- fore the Alpine collision (Austroalpine domain). SL was subdivided by previous authors into: “Gneiss Minuti”, “Micascisti eclogitici” and “II Zona Diorito-kinzigitica” where the bedrock crops out underneath the frontal mo- raines of the Orta end moraine system (Braakhekke et al., 2020). 2.1. Geological setting The bedrock of the Ticino-Toce mountain basin (Fig. 2) can be split into two major domains (Dal Piaz, 2010): (a) the Southalpine domain with a Paleozoic basement and the Permo-Mesozoic sedimentary covers, both lacking Alpine metamorphism; (b) the second do- main was affected by Alpine metamorphic phases that include: the Sesia-Lanzo Zone (SL), the Penninic and the Helvetic nappes. The second domain is character- ized by a stack of basement and cover nappes with high -pressure Alpine metamorphism of Late Cretaceous - Eocene age and a Late Eocene-Early Oligocene greenschist to amphibolite facies overprinting (Lepontine Dome: Frey et al., 1999). The two domains are separat- ed by the Periadriatic/Insubric lineament, a 1000 km long post-collisional transpressive fault with post-Late Oligocene to Miocene activity. The valleys of Lake Orta and Lake Maggiore are situated in the Southalpine domain (Fig. 2) dominated by Palaeozoic metamorphic rocks called Massicio dei Laghi (MdL), by late Variscan volcanic suites and Permo- Mesozoic sedimentary covers at the outlets and east of Lake Maggiore (see Geological Map of Switzerland, 1:500,000; Federal Office of Topography swisstopo 2005; Piana et al., 2017). The Alpine metamorphism is interpreted to have had only minimal effects on the MdL (Rutter et al., 2007), whose main metamorphism is of Caledonian (Late Cambrian-Early Devonian) to Variscan (Late Devonian-Early Permian) age (Boriani et al., 1990a; Pinarelli et al., 1993). The MdL is divided into the Ivrea-Verbano Zone (IVK) and the Serie dei Laghi (LAM) separated by the Cossato-Mergozzo-Brissago (CMB) tectonic line (Fig. 2). The Ivrea-Verbano Zone (IVK) is one of the most spectacular sections through lower con- 4 Fig. 3 - Longitudinal profiles of the major valleys; those related to the Toce catchment are in solid lines, those of the Ticino catchment in dashed lines; in dotted line the Vedeggio Valley related to the transfluence towards the Ceresio morainic amphitheatre. Blue arrows show the elevation of the major transfluences from the respective glaciers. Longitudinal distance from the outern LGM moraines of the Verbano according to Kamleitner et al. (2022a). Monegato G. et al. 5 bordered by a southern marginal shear belt. “Gneiss Minuti” are albite-white mica gneisses and schists, with a local porphyroclastic texture. Locally, "Gneiss Minuti" complex contains coarse-grained metagranitoids. “Micascisti eclogitici” are made of high-pressure (eclogite to blueschist facies) micaschists with jadeitic pyroxene-garnet + glaucophane + chloritoid with interca- lated eclogites, glaucophane metabasites and marbles. The term "kinzigite" is used in the Alpine literature for indicating high-grade (amphibolite facies) sillimanite- Fig. 4 - Photos of glacial landforms and erratic boulders (see Table 2) from the Ticino-Toce systems: A) boulder Puffer 11 made of IVK gneiss resting on a rocky relief (right slope of Lake Orta); B) impressive group of seven right lateral moraines of the Verbano glacier lobe on the watershed between the Lake Maggiore and Lake Orta basins, supporting many granite boulders (Alpe Canà); C) sub-rounded boulder VR30 made of GLA white granite, which provided a pre-LGM 10Be age (Alpe Canà moraines; Kamleitner et al., 2022a); D) small angular boulder VR37 made of LAM1 garnet paragneiss (S. Michele, eastern slope of Lake Maggiore, NW of Laveno); E) big polished sub-angular boulder VR22 made of AON serpentinite (Alpe Pala at Miazzina on the western slope of Lake Maggiore); F) Velmaio Megalith Invorio Brec- cia (PMC Breccia). Other photos of erratic boulders from the Orta basin (Bugnate 17-18, Grassona 25, Briallo 33, Carcegna 50, Colma 20 and Armeno 39) are found in Braakhekke et al. (2020). Photos of boulders VR16, VR31, VR45-46 and VR51-52 from the Verbano system are found in Kamleitner et al. (2022a). The Ticino-Toce Ice conveyor belts during the Last Glacial Maximum 6 Fig. 5 - Plate of thin section samples (see also Table 2): A) VR23, green biotite orthogneiss (LAM2); B) VR14, granite (GLA); C) VR02, serpentinite (AON); D) VR39, paragneiss (LAM1); E) VR37, garnet paragneiss (LAM1, Fig. 4D), F) VR24, green biotite orthogneiss with myrmekite (LAM2); G) VR20, granulite (IVK); H) VR27, garnet orthogneiss (MR1). Monegato G. et al. 7 garnet-biotite gneisses of the IVK and of the “II Zona Diorito-kinzigitica”. The SL crops out in the lower Ossola Valley for about 4 km at the Anzasca-Toce confluence, while it pinches out in the Ticino Valley near Ascona (Fig. 2). In the north-western sector, the topmost Penninic nappe is the Zermatt-Saas Fee Zone (ZSFZ), which forms a very thin and discontinuous oceanic crust slice in the Toce basin (Fig. 2). It is predominantly composed of serpentinized ultramafites, associated to mafic rocks (metagabbros, metavolcanites and metasediments) crystallized under high-pressure conditions. Towards the east, the ZSFZ pinches out at the northern tip of Lake Maggiore. The Monte Rosa (MR) is an Upper Penninic conti- nental unit characterized by a large SW-vergent recum- bent antiform (Berger et al., 2011) with a core com- posed of high-pressure micaschists and “gneiss minuti” derived from high-grade Paleozoic paragneiss (MR1, Piana et al., 2017) cropping out along the Anzasca Val- ley bottom (Fig. 2) and a shell composed of dominant metagranitoid (MR3, Frey et al., 1976; Piana et al., 2017). These are characterized by K-feldspar meg- acrystals up to ten centimeters in size, preserved within the Alpine schistosity. Monte Rosa “gneiss minuti” are characterized by albite, fine-grained quartz, white mica, chlorite and garnet. In the micaschists, high-pressure assemblages include chloritoid, phengite, garnet with kyanite, talc, Mg-chlorite and glaucophane. The Alpine high-pressure assemblages are overprinted by a greenschist Tertiary metamorphism (Keller et al., 2005) as shown by crystallization of biotite from white mica, albite blastesis (poikiloblasts) and quartz recrystalliza- tion to form homogeneous coarse-grained aggregates. The Antrona metaophiolite (AON) is a nappe of oceanic crust interposed between the overlying Monte Rosa and the underlying Camughera and Moncucco nappes (Bigioggero et al., 1981). It crops out on the left side of Anzasca Valley and more extensively in the up- per Antrona Valley (Fig. 2). The AON suite includes serpentinized ultramafites, metagabbros and metabasites (Turco & Tartarotti, 2006). Eclogite metabasite with garnet, amphibole and relict omphacitic clinopyroxene are described by Colombi & Pfeifer (1986) in the Antrona and Anzasca valleys. Middle Penninic unit to the west (Toce catchment) is the Gran San Bernardo (SB). In the central eastern to the eastern sectors of the Ticino-Toce mountain basin the Middle Penninic units are characterized by greenschist facies paragneiss and orthogneiss with blueschist relics (Bigi et al., 1983). They were grouped (MO-CAM-BEL) in the geological map of Figure 2. The Moncucco-Orselina (MO) and Camughera (CAM) are folded together with AON, MR and ZS units. They show a medium-grade metamorphic overprinting by the Le- pontine metamorphism (Keller et al., 2005). CAM is made of micaschists derived from mid- to high-grade Paleozoic gneisses (paragneiss, orthogneiss, amphibo- lites) and dominant metagranitoids similar to those of MR3, with Alpine high-pressure overprint. MO is com- posed of a Paleozoic basement covered by Permo- Carboniferous sequences including graphitic schists, basic metavolcanites and Permian sub-volcanic bodies, suggesting affinity with the Gran San Bernardo nappe (Bigioggero et al., 1981). Tambo, Bosco and Bellinzona nappes are made of Paleozoic basement and slices of Mesozoic covers and Bundner schist (Baudin et al., 1993). Their lithological setting is similar to that of the Monte Leone nappe (Maxelon & Mancktelow, 2005). Tambo, Bosco, Bellinzzona and Monte Leone nappes are recrystallized under amphibolite facies (biotite, stau- rolite, garnet). The deep Penninic nappes (Monte Leone, Lebendun, Simano, Lucomagno, Leventina, Antigorio and Verampio, LPN) show a strong Lepontine amphibo- lite facies metamorphic overprint. It is characterized by crystallization of new minerals over the early Alpine met- amorphism (Bigioggero et al., 1981; Berger et al., 2011). The LPN are composed of a typical Paleozoic basement (paragneisses, orthogneisses, amphibolites), with late Paleozoic intrusions and Permo-Carboniferous to Meso- zoic sequences similar to the other Penninic nappes. These units characterize the innermost sectors of Ticino and Toce catchments (Fig. 2). The Ultra-Helvetic Mesozoic cover sequences and the Helvetic Gotthard basement (HN), including the Per- mian granitoids (Sergeev & Steiger, 1995), make up the far northern part of the Ticino mountain basin (Fig. 2). 2.2. The LGM Ticino-Toce glacier system The Ticino-Toce accumulation area is located south of the major Rhone and Rhine (or Vorderrhein) ice domes (Fig. 1; Kelly et al., 2004; Bini et al., 2009). In the inner Alpine accumulation areas, additional input of ice could have been added to the Ticino basin (4900 km2) by overflowing of the Rhone ice dome to the East along Bedretto Valley via Nufenen Pass (Fig. 2). In Leventina Valley, the Ticino glacier received large ice masses built up in the high-mountain cirques of Gotthard Pass. Downstream of Leventina Valley, Ticino trunk glacier was joined by ice draining Blenio and Moesa valleys. Both tributaries were influenced by likely transfluences from the Vorderrhein ice dome via Lukmanier, Cristallina and Greina passes, as well as from the Hinterrhein ac- cumulation areas via San Bernardino Pass (Fig. 2). Right tributary glaciers of Verzasca and Maggia valleys fed the Ticino glacier together with Toce ice overflowing through Centovalli Valley. On the left side of the Ticino Valley, another diffluence existed over Monte Ceneri Pass, draining ice directly to the south and towards Lu- gano. In parts, these ice masses united with ice outflow- ing the Adda glacier systems and terminated in the small Ceresio lobe south of Lake Lugano (Fig. 1; Bini et al., 2009). Through an overspill located near Ponte Tresa, some of the drained ice also re-entered the Ticino catch- ment at Valcuvia Valley (Bini et al., 2009) and merged with the united Ticino-Toce glacier at the western slope of the Campo dei Fiori (Fig. 1, see Fig. 6). Advancing southwards, the large Ticino valley glacier filled the overdeepened basin of Lake Maggiore (Preusser et al., 2010) and merged with Toce glacier coming in from the northwest. In its upper parts, the smaller Toce catchment (~1800 km2) was dominated by ice draining through Divedro- and Antigorio valleys, two tributary glaciers with additional linkage to the Rhone glacier system adjacent The Ticino-Toce Ice conveyor belts during the Last Glacial Maximum 8 in the north (Fig.1, Fig. 2). The Toce glacier system received overflowing ice from the Rhone valley glacier via Simplon Pass and through a direct link to the Rhone ice dome (Florineth & Schlüchter, 1998; Kelly et al., 2004). An ice diffluence existed in the middle of the Ossola Valley, where ice of the Toce catchment did overflow into the Ticino catch- ment along the western Vigezzo Valley (a tributary of Toce Valley) and the Centovalli Valley (a tributary of Ticino Valley). Right tribu- tary glaciers of Antrona and Anzasca valleys joined the Toce trunk glacier downstream. Originating in the eastern slope of the Monte Rosa, ice outflowing Anzasca Valley repre- sented the major tributary glacier. Downstream from the Anzasca confluence, one part of the Toce glacier branched off to the south into the Orta Valley, building up the correspondent Orta glacier lobe (~85 km2 in area) and a small morainic amphitheater (Novarese, 1927; Braakhekke et al., 2020). The main branch of the Toce glacier, however, continued to the southeast to merge with the Ticino glacier. Downstream of the confluence at the Gulf of Borromeo, between Stresa and Verbania, the merged Ticino-Toce glacier filled the basin of Lake Maggiore and advanced several kilo- meters (about 28 km) towards the SSE (Fig. 1, Fig. 2). The topographic control was reduced and the LGM Ticino-Toce piedmont lobe termi- nated on the plain. The bulbous terminus (known as Verbano lobe) spread to cover ~380 km2 (Kamleitner et al., 2022a). Pre-LGM and LGM advances of the Ticino-Toce glacier built up a multi-ridged, closely stacked end moraine system, interlaced and succeeded by glaciofluvial plains (Bini et al., 2014; Piana et al., 2017; Bernoulli et al., 2017; Kamleitner et al., 2022a). The LGM maximum advances of both, Orta and Verbano lobes, were recently dated to ca. 25 ka and are interpreted to have been followed by a several thousand-year long period of glacier fluctuations near the maxi- mum position (Braakhekke et al., 2020; Kam- leitner et al., 2022a). Late LGM readvances (~20-19 ka) were likely followed by rapid ice decay and retreat from the foreland (Braakhekke et al., 2020; Kamleitner et al., 2022a). The occurrence of large erratic boulders within the Orta and Verbano morainic amphi- theatres was stated by several authors since the 19th century (Omboni, 1861; Gentilli, 1866; Salmojraghi, 1882; Sacco, 1892; CAI, 1914). The biggest erratics were quarried for building purposes already in prehistory (De Marinis, _______________________________ >>>>>> Tab. 1 - Details of erratic boulders of the Orta- Verbano and Ceresio systems lithologically analysed in this study. Monegato G. et al. 9 2012) or used as holy sites (e.g. Prea Guzza). Within the Orta and Verbano morainic amphitheaters, big boul- ders are not limited to the last glaciation, but were also found external to the LGM frontal moraines (Braakhekke et al., 2020; Kamleitner et al., 2022a). 3. MATERIALS AND METHODS In the present work, provenance analysis was ap- plied to erratic boulders found in the Orta, Verbano, and Ceresio end moraine systems (Fig. 1). The respective boulders were located during field campaigns related to different mapping and dating projects (Bernoulli et al., 2017, 2018; Braakhekke et al., 2020; Kamleitner et al., 2022a). In total more than 500 erratic boulders were considered, out of which 105 boulders were chosen for provenance studies as their source rocks were well rec- ognized (Tab. 1). Erratic boulders were grouped by their source rock on the base of the simplified geological map of the catchments, which derives from Bigi et al. (1983), swisstopo (2005) and Piana et al. (2017). More than 260 erratic boulders were located in the lower Ticino and Orta valleys, geographically positioned in a GIS database and described in size and roundness type (Tab. 1). Most of the erratics are related to frontal or lateral moraine ridges but boulders situated on bed- rock close to the trimline are also included (Fig. 1). The size of the boulders varies from 1 m3 minimum to >300 m3 max (Tab. 1, Fig. 4). The exposure age of 54 erratic boulders was defined with cosmogenic nuclide exposure dating (Braakhekke et al., 2020; Kamleitner et al., 2022a). Out of these, 19 boulders are related to the maximum advances and 31 located in the withdrawal moraines or in the valley floor. Four boulders from the Verbano end moraine system were dated to pre-LGM ice advance(s) (VR19, VR42, VR30 and VR45). Four more boulders (GUDO01, GUER01, GUER02, PARU01) come from the Lateglacial deposits in Ticino Valley. These eight boulders are included in Table 1 but not considered in the discussion. A description of the boul- der petrography was provided in the field. For 85 boul- ders (including exposure dated and other selected errat- ics), thin section analysis was performed in order to recognize the metamorphic facies of gneiss and micas- chist lithotypes (Fig. 5). An overall 289 erratic boulders related to the Cere- sio morainic amphitheatre were mapped and inventoried in a GIS geodatabase during Quaternary geological cartography for the Sheet 152 Mendrisio-Como of the Swiss Geological Atlas 1:25,000 (see Bernoulli et al., 2017, 2018). 101 erratic boulders were considered as deposited during pre-LGM glacial advances, whereas 188 boulders are located within limits of the LGM expan- sion in the geological map (Bernoulli et al., 2017). The eleven boulders included in the present work were litho- logically recognized (Tabs. 1 and 2) and, even if outside the mapped LGM (Bini et al., 2014; Bernoulli et al., 2017), they yield information about the flowline of this branch of the glacier. 3.1. Provenance of the boulders The provenance of more than hundred erratic boul- ders related to the LGM of the Orta, Verbano, and Cere- Tab. 2 - Overview of the geological units of the Ticino-Toce catchment and the provenance of the studied erratic boulders. The Ticino-Toce Ice conveyor belts during the Last Glacial Maximum sio glacier lobes, was identified using thin section analy- sis (85 samples) and visual interpretation in the field (30). The petrographic characteristics of the boulders are reported in Table 2 and illustrated in Fig. 6. Boulders show petrographic provenance clustering in the wide area covered by the Ticino and Toce glacier snouts. An overall 48 samples are related to bedrock units located south of the Insubric Line, the sector closest to the glaci- er termini. The most frequently encountered lithologies are those related to the Southalpine metamorphic base- ment (IVK and LAM, with three boulders in the Orta and 19 in the Verbano system, respectively), the Permian granites (GLA, with three boulders in Orta, ten in the Verbano system, and one erratic in the Ceresio end moraines, respectively) and the Lugano Volcanites (LV, two erratics in the Verbano end moraine system). The occurrence of GLA erratics was also documented in till north of Sesto Calende (Omboni, 1861; Salmojraghi, 1882). Erratic boulders from the Sesia-Lanzo Zone (SL) build a second important petrographic group, particularly in the Orta end moraine system (seven boulders in the Orta Valley and three in the Verbano system, respec- tively). MR and MO-CAM-BEL units are well represented in the Orta system (eight samples), while one boulder (VR27, Fig. 5H) was collected in the Verbano area. Oph- iolitic boulders (Antrona Zone, AON) are present in both Orta and Verbano amphitheatres with two and seven samples, respectively. In the Verbano end moraine sys- tem, erratics sourced from the Antrona Zone are widely scattered. AON erratics were found associated with lateral moraine deposits north of Verbania (Fig. 4E) and along the western margin of the former Verbano lobe. Three boulders sampled from Verbano frontal moraines show AON signature. One of them (ER192) is located 10 Fig. 6 - Distribution and lithology of the studied erratic boulders in the Orta-Verbano and Ceresio morainic amphitheaters. For color codes see legend of Fig. 2. Red crosses indicate areas where Baveno granite boulders (GLA unit) were recognized during railway constructions in the 19th century (Omboni, 1861; Gentilli, 1866; Salmojraghi, 1882). Mapped moraine ridges (Bini et al., 2014; Braakhekke et al., 2020; Kam- leitner et al., 2022a) are shown as red lines. Distribution of glaciofluvial and glacial sediments was modified from the geological maps of Piedmont (1:250,000; Piana et al., 2017) and Lombardy (1:250,000; Montrasio et al., 1990) and the Swiss Geological Atlas 1:25,000 (Bernoulli et al., 2017, 2018). Monegato G. et al. just south of Lake Varese. Lower Penninic boulders (LPN) are lacking in the Orta amphitheatre while 15 and four LPN samples were collected in the Verbano and Ceresio end moraine systems, respectively. Finally, one boulder of the Helvetic Gotthard Massif was collected from Monte Pian Nave (VR36; Fig. 6), while one gran- odioritic boulder, coming from the Bergell batholith (BG), was found south of Lake Varese. In the Ceresio sector most of the boulders belong to the Southalpine sedi- mentary covers (Tab. 1, Fig. 4F); because no local val- ley glaciers could have developed in the low-elevation sector, these boulders were transported by glacier difflu- ence. Boulder lithologies indicate that both the Ticino and the Adda branches could have been responsible for erratic transport. As expected from the scarce mixing of the suprag- lacial fluxes, the distribution of analyzed erratic boulders shows an inhomogeneous lithological distribution. The lithotypes from bedrock of the Ossola Valley (AON, MR, IVK, SL) are largely distributed in the Orta lobe and on the western side of the Verbano lobe. Nevertheless, erratics sourced from Ossola Valley were also found in the central part of the end moraine system, south of Lake Varese. On the other hand, lithotypes of BG, ADMG and HN, originating from the Ticino Valley net- work as well as Lugano Volcanites (LV) were found only in the very left sector of the Verbano end moraine sys- tem. In contrast, the Lower Penninic bedrock (LPN) characterizes the upper catchments of Ticino and Toce glaciers. Occurrence of Penninic erratic boulders is com- mon across the Verbano lateral-frontal moraine com- plex. Roundness of erratic boulders varies from angular to sub-rounded (Tab. 1). Most of the angular boulders are located on the outermost frontal and lateral mo- raines, while those located on internal moraines are mostly sub-angular to sub-rounded. An exception is represented by GLA boulders that, despite their position, are mostly sub-rounded (Fig.4C). This can be ascribed to the particular surficial exfoliation (Ollier, 1967) of granite bedrock on the slopes of the Mottarone (Fig. 6) and the subsequent shaping during the transportation, even if from short distance. 11 Fig. 7 - Flowlines of the Ticino-Toce glacier system (extent modified after Bini et al. 2009, Braakhekke et al., 2020, Kamleitner et al., 2022a) during the LGM inferred from erratic boulder lithologies. Dashed trajectories in the Ceresio lobe are suggested by boulders located outside the pre-LGM limit (Bini et al., 2014; Bernoulli et al., 2017; no dates available). For symbols see legend of Figure 6. The Ticino-Toce Ice conveyor belts during the Last Glacial Maximum 4. DISCUSSION 4.1. Inferred ice flow paths in the Ticino-Toce sys- tem The distribution of index erratics within the Orta, Verbano, and Ceresio end moraine systems found in this study can be coupled with observations made by early authors. The lithological provenance of granite and gneiss erratic boulders that have been subsequently removed by anthropogenic quarrying activities is report- ed by CAI (1914). It is remarkable that Omboni (1861) already noticed that the moraines west of Lake Maggio- re are dominated by boulders of the Toce catchment, while those close to the Campo dei Fiori slope (opposite, eastern side) are characterized by boulders of the inner Ticino basement. Baveno granite boulders (GLA) are common in the lateral moraines (Fig. 4B) of the western part of the Verbano end moraine system deposited by the Toce glacial branch at this point weld- ed with the Ticino glacier. However, Baveno granite boulders, as well as the presence of this type of rock in lodgment till, were recognized in early excavations for the construction of a railway line between Sesto Calen- de and Lake Varese in the 19th Century (i.e. in the east- ern frontal sector of the Verbano system; Fig. 6; Om- boni, 1861; Gentilli, 1866; Salmojraghi, 1882). Petrological investigations undertaken in this study and from the literature (Omboni, 1861; Gentilli, 1866; Salmojraghi, 1882; CAI, 1914) combined with exposure dating of selected erratic boulders and geomorphologi- cal analysis of earlier work (Braakhekke et al., 2020; Kamleitner et al., 2022a) yield crucial insights into glaci- er flow paths of the Ticino-Toce glacier during the LGM (Fig. 7). Boulder lithologies originating from the Ossola Valley indicate that the LGM Orta morainic amphitheatre and the western section of the LGM Verbano end mo- raine system correspond to the flow of the Toce glacier. Most of the boulders belong to the southalpine base- ment (IVK and LAM), the Permian batholith (GLA), the SL and MR Alpine nappes and the ophiolites (AON). It is remarkable that the inner sector of the Toce catchment is scarcely represented in the Orta moraines (Braakhekke et al. 2020), whereas boulders of the LPN are common in the western frontal moraines of Verbano (Fig. 6), indicating a significant persistence of the dis- persal train. Boulders transported by the Toce glacier are widely found on LGM lateral and frontal moraines of the Verbano amphitheatre, including the sector south of Lake Varese. This suggests a dominance of the Toce glacier over the Ticino glacier. Ticino glacier may have hence been confined to the eastern flank of the system (Fig. 7). The Toce system is characterized by a shorter stream course while at the same time being notably steep with accumulation areas reaching above 4000 m a.s.l. In comparison, the Ticino catchment is more than two-and-a-half times the size of the Toce basin, yet mountain peaks are lower, the slopes are gentler, and the thalweg is longer (Fig. 3). An important divergence of the Toce glacier was located along the Vigezzo Val- ley draining ice towards Ticino Valley and into high Canobbina Valley, as testified by glacigenic deposits (Boriani & Burlini, 1995); again this indicates the pre- dominance of the Toce glacier. These thoughts agree well with the palaeoglacier model by Seguinot et al. (2018), which shows that the Ticino glacier likely reached the mountain front slightly later, where it faced the physical obstacle of the spreading Toce piedmont glacial lobe. Likely due to ice congestion, flow of the Ticino glacier was forced to diverge to the east- southeast across Monte Ceneri Pass and to occupy the Vedeggio and Cassarate valleys (Fig. 2). In the Ceresio sector, it merged with the branch of the Adda glacier system, coming through Porlezza Valley (Bini, 1997; Castelletti et al., 2014). The abundance of sedimentary rock boulders indicates that the Ticino glacier eroded the sedimentary covers of the area and then formed the main part of the western Ceresio lobe ending at Ar- cisate. Probably it made also a part of the eastern Cere- sio lobe, ending at Stabio where it merged with the west- ern part of the Adda glacier system (Bini, 1997; Scapoz- za et al., 2014; Bernoulli et al., 2018). In the lower reaches of Ticino Valley, a second divergence at Luino may have promoted ice exchange between Valcuvia Valley and Tresa Valley. According to the distribution of erratic boulder li- thologies in the Orta, Verbano, and Ceresio end moraine systems, the major LGM ice streams of Ticino and Toce glaciers are suggested to have been as follows (Fig. 7): - the LGM Orta glacier lobe was mainly fed by the An- zasca tributary glacier (Braakhekke et al., 2020) with a contribution of ice from the Ossola/Toce valley glacier in the eastern parts of the lobe. - ice from the Toce valley glacier built up the western sector of the LGM Verbano glacier lobe and possibly extended far towards the southeast, at least during the early phase of the LGM. - the Ticino glacier dominated the eastern flank of the Verbano system, in the Lake Varese and Campo dei Fiori sectors. - through a diffluence across Monte Ceneri Pass, Ticino glacier fed western parts of the Ceresio lobe between Porto Ceresio and Arcisate. 4.2. Implications for Ticino-Toce glacier dynamics The complex dynamics of the Ticino-Toce glacier network were strongly influenced by the asymmetry of the mountain catchments. A key role was played by the highest sector of the Toce accumulation area represent- ed by the high Monte Rosa - Fletschorn ridgeline, run- ning over 3000 m a.s.l. with peaks over 4000 m a.s.l. The rapid spread of the Toce catchment glaciers, taking into account the short travel distances (~100 km of the Toce against ~150 km max of the Ticino Leventina, Fig. 3), promoted early arrival of the Toce glacier in the pied- mont area. The possibly rapid response of the Toce catchment to climate changes of MIS 3 and the onset of the LGM supports the marked spread of Toce ice over a large sector of the Verbano piedmont lobe. Presumably, the larger but slower Ticino glacier was pushed to the east and forced to spread in the prealpine area of the Lake Lugano region. These results yield interesting im- plications for the reconstruction of the evolution and interconnections of large glaciated networks in mountain ranges. The fastest glaciers might have played a role in blocking the flow of glaciers with greater inertia and forced transfluences/diffluences if topography allowed 12 Monegato G. et al. for it. During the LGM, the Alps were covered by a net- work of interconnected valley glaciers (Ivy-Ochs et al., 2022). Hence, many glacier systems were affected by merging trunk glaciers. The presented data from Toce and Ticino catchments underlines how the reaction time of neighboring glacier systems may vary due to inherent differences in catchment geometry, especially when the accumulation areas show a strong asymmetry. A similar behavior of ice overflowing into previously non-glaciated valleys forced due to blockage by another glacier was reported for trunk glaciers in the lower Inn and Salzach valleys (van Husen, 2004; Reitner et al., 2010), but can be conceivable in other sectors of the Alps and in other mountain ranges as well. In addition, palaeoclimatic models suggest that precipitation was irregularly distributed in time and space across the Alps contributing to anomalous ice accumulation throughout the LGM time bracket (Kuhlemann et al., 2008; Luetscher et al., 2015; Del Gobbo et al., 2022). Moreover, the different distribution of major ice-flows can be related to how the accumula- tion areas, and specifically the major ice-domes, reacted to the climate change promoting the spread of the glaci- ers. According to modelling studies on the Isere, Rhone, and Rhine paleoglaciers (Coutterrand, 2010; Jouvet et al., 2017; Cohen et al., 2018), the contribution from dif- ferent sectors of an accumulation area can be traced from the highest parts to the outlet. Glacier simulations further suggest increased basal erosion underneath confluencing ice streams (Cohen et al., 2018; Jouvet et al., 2021). Some valley reaches of the Rhine catchment were characterized by very slow advance (Cohen et al., 2018), and this characteristic may be inferred for the advance of the Ticino glacier through the valley section south of Bellinzona, which is up to 4.5 km wide with a very low gradient. The model of Seguinot et al. (2018) shows that the Toce glacier extended to Gravellona Toce several times already in the MIS 4 and MIS 3 cold phases and may have molded the valley outlet. On the other hand, the Ticino glacier arrived in the Lake Maggi- ore area only during the LGM (Seguinot et al., 2018), when the paleoclimatic conditions (Del Gobbo et al., 2022) allowed the spread of a large glacier and its per- sistence until the final collapse at about 18 ka (Wirsig et al., 2016; Ivy-Ochs et al., 2022). Moreover, the shallow elevation in the lower Toce Valley bottom and in the Gulf of Borromeo is in contrast to the deep trough of Lake Maggiore (Cazzini et al., 2020) and may indicate a more effective subglacial carving by the Ticino glacier. This may suggest that the Toce glacier prevailed at the LGM onset and the Ticino glacier at the LGM climax. 5. CONCLUSIONS The study of the provenance of erratic boulders of the Ticino-Toce glacier system provides interesting in- sights into the development of a large LGM glacial net- work and its outflow in the prealpine and piedmont are- as. The distribution of erratic boulders shows a domi- nance of the Toce glacier in the Orta lobe and in the western-central sector of the Verbano lobe. The LGM Orta lobe was predominantly fed by ice from Anzasca Valley, a right tributary glacier of the Toce valley glacier. Despite the much larger catchment, lithologies indicative of the Ticino glacier are largely limited to the eastern sector of the LGM Verbano lobe and the western parts of the Ceresio system, where Ticino ice merged with the westernmost branch of the Adda glacier system. The topographic differences in the overall accumulation are- as, with maximum elevations (>4000 m a.s.l.) and steep slopes in the Toce catchment, promoted the sudden and rapid spread of the Toce glacier, whose path was short- er (<100 km) in comparison to the Ticino glacier (around 150 km). The early arrival of Toce glacier possibly re- sulted in the damming of the Ticino glacier and fostered the diffluence of Ticino ice into the prealpine area to the east, merging with the western branch of the Adda glaci- er, where elevations were too low for hosting independ- ent valley glaciers during the LGM. The dynamics of the two studied LGM glaciers highlight the importance of topography as a driver for ice build-up and development of valley glaciers. Faster glac- iers may force slower trunk glaciers to a different path. Considering the topographic setting of the Alpine area this could have been a common phenomenon also in past glaciations. The provenance of boulders can thus provide an important tool for determining glacier flow trajectories that could have been affected by unequal glacier evolution. ACKNOWLEDGMENTS Funding by the Swiss National Science Foundation is gratefully acknowledged [SNF grant number 175794, 2017]. 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