Supplementary Material

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1 GSA Data Repository item Brardinoni, F., Picotti, V., Maraio, S., Paolo Bruno, P., Cucato, M., Morelli, C., and Mair, V., 2018, Postglacial evolution of a formerly glaciated valley: Reconstructing sediment supply, fan building and confluence effects at the millennial time scale in Vinschgau/Val Venosta, Eastern Italian Alps: GSA Bulletin, Supplementary Material DESCRIPTION OF THE SEISMIC STRATIGRAPHIC UNITS Seismic Unit I The upper boundary of this unit, which is found at the beginning of seismic profile Lasa 1 at about 50 m from the topographic surface (Fig. 4, m), deepens gradually moving southward until it reaches at least 250 m in correspondence of 1750 m on profile Lasa 2. From this point, this structure rises at progressively shallower depths toward the southern valley margin, until the end of the profile, where, after describing an abrupt step at 2000 m, becomes almost exposed. Unit I is also is also visible along the tie line Lasa 3, where it shows a clear east-dipping geometry (Fig. DR4). Overall, the definition of this unit s boundary is more uncertain on profile Lasa 2, i.e., beneath the current location of the Adige River bed. Unit I is characterized by very weak internal reflectivity, if any (Table 3). This behavior is typically associated with massive homogeneous formations. Some coherent reflectivity, characterized by moderate amplitude and low frequency, is found in the deeper portion of the seismic profile toward the valley axis (Fig. 4), where, according to available geological information, an Alpine tectonic contact between two nappes should occur (Schmid and Haas, 1989; Pomella et al., 2016). Overall, the top of this unit depicts a rather asymmetric valley cross-section, with the deepest part of river incision focused toward the southern valley side. Seismic Unit II Unit II is found in profile Lasa 2 between 1400 and 2050 m, where it reaches a maximum thickness of 65 m, and in a smaller patch on the southern valley side, where it is only 20-m thick. The topping unconformity of this unit (orange line in Figures 4 and DR3a) consists of a continuousto-semicontinuous reflector, with mid-to-high amplitude and low frequency. Seismic Unit III This unit is the most reflective one within the seismic line (Figs. 4 and DR3a). Its lower boundary is represented by the erosional unconformity (orange) separating Unit II from Unit I. It is both covered and flanked by surfaces characterized by complex geometries associated with distinctive facies changes of Unit IV. We further subdivide this unit into subunits IIIa, IIIb and IIIc, which are characterized by different reflectors amplitude, continuity and geometry. Seismic Subunit IIIa Seismic Subunit IIIa (Figs. 4 and DR3a) is located between 720 and 780 m asl. The transition with the overlaying Subunit IIIb is marked by a gradual increase in continuity and amplitude, whereas to the south this subunit becomes Subunit IIIc, due to an abrupt change in the geometry of the reflectors (Fig. DR3a). Subunit IIIa has been sampled by core S5 for 21 m (from Page 1 of 6

2 740 to 761 m a.s.l., i.e., 152 m to 173 m), and is characterized by fine-grained deposits, mostly consisting of mud and sand (Fig. 2). Seismic Subunit IIIb Seismic Subunit IIIb (light blue in Figures 4b and DR3a) is visible on seismic profile Lasa 2 above Subunit IIIa, between 1350 and 2000 m, at a depth range between 780 and 840 m a.s.l. To the south, reflectors are adjacent to Subunit IIIc at around 2000 m, where they change abruptly in continuity, amplitude, geometry and frequency (Fig. DR3a). At the transition with the overlying Unit IV, the reflectors are truncated with a downcutting toward north. Toward the transition with Subunit IIIc, some discontinuous, widely concave and higher amplitude reflectors are visible (Fig. DR3a). Seismic Subunit IIIc This subunit is recognized on seismic profile Lasa 2 (i.e., between ~2000 and ~2300 m), above seismic Unit IIIa, and is adjacent to Unit IIIb (Fig. DR3a). Toward south, this subunit is adjacent to Unit IV, whose reflectors are characterized by a northward clinoform pattern. Seismic Unit IV This is the uppermost unit of the study area and is associated with the sediment delivered by the Gadria-Strimm and Lasa Creeks. Along profile Lasa 3, we observe west dipping internal reflections (Fig. DR4) that describe downlap relationships with Unit I and show the lateral shifting of the Gadria fan progradation to the west. Another important seismic morphological characteristic, visible only within the Gadria fan, is the widespread hummocky-to-mounded facies (see Figures 4 and DR3). Unit IV shows clear evidences of internal deformation in the northern valley side (profile Lasa 1 between 400 and 600 m, and between 900 and 1200 m), where the reflectors are folded more or less gently. The internal reflectors, as well as the boundary reflector toward the basement (Unit I) are also faulted between 300 and 400 m and at ~1000 m. Seismic Subunit IVa Description: Toward the margins of the cross-valley profile (Fig. 4), we have delineated a Subunit IVa to describe steep clinoform reflectors (see Figure 4c with line drawings). To the north (profile Lasa 1 in Fig. 4), the clinoforms are around 300 m long and 50 m deep. They are grading to a toplap without clear unconformity. To the south (profile Lasa 2 in Fig. 4), the clinoforms are shorter ( m long) and steeper (60 70 m deep). They are buried by a clear toplap unconformity (Fig. DR3b). REFERENCES CITED Maraio, S., 2016, Geomorphic Features Revealed by the Acquisition, Processing and Interpretation of High-Resolution Seismic Reflection Profiles across a Large Debris-Flow Fan (Vinschgau/Val Venosta, Italian Alps): Amsdottorato, p. 113, Pomella, H., Flöss, D., Speckbacher, R., Tropper, P., and Fügenschuh, B., 2016, The Western end of the Eoalpine High Pressure Belt (Texel unit, South Tyrol/Italy): Terra Nova, v. 28, p , doi: Schmid, S.M., and Haas, R., 1989, Transition from near surface thrusting to intrabasement decollement, Schlinig Thrust, Eastern Alps: Tectonics, v. 8, p , doi: Page 2 of 6

3 Figure DR1. Map showing the location of the study fans (filled polygons) and the relevant source basins (empty polygons). Note in Lasa Creek basin the presence of sediment stores such as debrisflow fans and sedimentary valley fills. Page 3 of 6

4 Figure DR2. Post-stack depth migrated section from CMP stacking along Lasa_1 and Lasa_2 profiles, overlaid with the results of refraction tomography. No vertical exaggeration. Modified after Maraio (2016). Page 4 of 6

5 Figure DR3. Close-up views of the central (a) and northernmost (b) portions of the interpreted depth converted seismic imaging of profiles Lasa 1 and Lasa 2. Page 5 of 6

6 Figure DR4. (a) Depth converted seismic imaging of profile Lasa 3 transversal to the Gadria fan; and (b) interpretation of the main seismic units described in the text. Note the prominent E-dipping reflector interpreted as the top bedrock (Unit I), interrupted by a structure to the east. The Unit IV shows internal clinoform reflectors downlapping to the west. Red dashed line indicates the shear plane of a deep-seated gravitational slope deformation. No vertical exaggeration. Modified after Maraio (2016). Page 6 of 6