Numerical modeling of tectonically driven river dynamics and deposition in an upland incised valley

    Numerical modeling of tectonically driven river dynamics and deposition in an upland incised valley Valeria Bianchi, Tristan Salles, Massimiliano Ghinassi, Paolo Billi, Edoardo Dallanave, Guillaume Duclaux PII: DOI: Reference:

S0169-555X(15)00204-4 doi: 10.1016/j.geomorph.2015.04.007 GEOMOR 5173

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

23 September 2014 6 April 2015 8 April 2015

Please cite this article as: Bianchi, Valeria, Salles, Tristan, Ghinassi, Massimiliano, Billi, Paolo, Dallanave, Edoardo, Duclaux, Guillaume, Numerical modeling of tectonically driven river dynamics and deposition in an upland incised valley, Geomorphology (2015), doi: 10.1016/j.geomorph.2015.04.007

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ACCEPTED MANUSCRIPT NUMERICAL MODELING OF TECTONICALLY DRIVEN RIVER DYNAMICS AND

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DEPOSITION IN AN UPLAND INCISED VALLEY

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VALERIA BIANCHI1, TRISTAN SALLES2, MASSIMILIANO GHINASSI3, PAOLO BILLI4, EDOARDO DALLANAVE5, GUILLAUME DUCLAUX6

School of Earth Science, University of Queensland, Brisbane, Australia (corresponding

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1

author: [email protected], [email protected]) School of Geosciences, The University of Sydney, Sydney Australia

3

Dept. of Geoscience, University of Padova, Italy

4

Dept. of Physics and Earth Sciences, University of Ferrara, Italy

5

Department of Earth and Environmental Science, Ludwig-Maximilians University,

Dept. of Earth Science, University of Bergen, Norway

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6

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Munich, Germany

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2

Abstract

Within upstream reaches of incised valleys, fluvial sedimentation occurs where it is controlled by interaction between climate and tectonics. This study focuses on a Plio– Pleistocene fluvial paleovalley, which drained the northeastern margin of Siena basin (northern Apennines, Italy). Valley filling resulted from the interaction between river drainage and active normal faults striking perpendicular and parallel to the main valley. Through numerical modeling, this study aims to refine temporal and spatial mesoscale deposit variations, which highlight the upset of fluvial architectures derived from the interplay between the river system and uplift. Geomorphological and hydrodynamic 1

ACCEPTED MANUSCRIPT parameter calibration was performed integrating field studies with paleohydraulic and paleomagnetic data. The numerical model simulates the evolution of valley formation

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with the development of (i) a pre-tectonic steady state system, followed by (ii) a

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syntectonic aggradation and avulsion phase, and (iii) a post-tectonic relaxation phase. The syntectonic phase shows fine sediment back-filling upstream of the uplifted area and coarse sediment down-filling downstream of the upwarping. The recorded aggradations

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are asynchronous with upstream deposition preceding downstream deposition.

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Keywords: alluvial-valley fill; numerical modeling; syndepositional tectonics; Tuscany

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1. Introduction

Over the past three decades, numerical modeling methods have been developed

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to address geomorphic landscape evolution at catchment to orogeny scale and over

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temporal dimensions from 104 to 106 years (e.g., Koons, 1989, 1994; Kooi and Beaumont, 1996; Dietrich et al., 2003; Braun and van der Beek, 2004; Willgoose, 2005; Paola et al.,

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2009; Tucker and Hancock, 2010). Unlike the methods involved in physical analogue modeling, which preclude meticulous spatial and temporal scaling of processes (Peakall and Warburton, 1996; Peakall et al., 1996; Lague et al., 2003; Hickson et al., 2005), this kind of reconstruction can detect the impact of extrinsic and intrinsic processes of the geomorphic evolution and the stratigraphy. According to Beerbower (1964), extrinsic factors are defined by allogenic processes, such as variation in climate, tectonics, and relative sea level. By contrast, intrinsic factors consist of autogenic processes such as the redistribution of energy within the depositional system. In a valley system, the sediment transfer is commonly controlled by allogenic forcing (Zaitlin et al., 1994; Blum and Tornqvist, 2000; Boyd et al., 2006). In the valley 2

ACCEPTED MANUSCRIPT headwaters, tectonic and climate are considered to be the main controls on fluvial dynamics (Shanley and McCabe, 1991; Currie, 1997; Blum and Tornqvist, 2000; Holbrook,

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2001), whereas in downstream sectors, relative sea-level changes are dominant (Zaitlin et

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al., 1994; Boyd et al., 2006). Among the allogenic forcings, tectonic control on fluvial dynamics has been widely described by several authors (Burnett and Schumm, 1983; Ouchi, 1985; Schumm, 1986; Holbrook and Schumm, 1999; Blum and Tornqvist, 2000;

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Blum et al., 2013). River response to tectonic activity includes (i) changes in fluvial styles

modification

of

the

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(Twidale, 1971; Ouchi, 1985; Blum and Tornqvist, 2000; Gibling et al., 2011); (ii) equilibrium

profile

(Bridge,

2003);

(iii)

triggering

of

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aggradation/degradation processes (Holbrook and Schumm, 1999; Dalrymple, 2006; Holbrook et al., 2006) and (iv) development of different patterns of channel avulsion

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(Bryant et al., 1995). The effects of tectonic subsidence on rivers has been the goal of

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several studies (among others Burnett and Schumm, 1983; Leeder, 1978; Hickson et al., 2005), providing significant insights on river channel dynamics and related sedimentary

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products. This topic was also investigated through detailed LAB (acronym for Leeder, Allen and Bridge, the first to define a numerical approach to alluvial landscape evolution) models (Allen, 1978; Leeder, 1978; Bridge and Leeder, 1979; Alexander and Leeder, 1987; Bryant et al., 1995; Mackey and Bridge, 1995), which provided significant results concerning the interaction between river systems and subsidence at basin scales. On the contrary, less information is available on the effects of tectonic uplift, which can cause rapid and marked changes in alluvial dynamics, especially if rivers are confined within valleys (Schumm, 1986; Guiseppe and Heller, 1998). In the case of localized uplift, rivers tend to regain an equilibrium state through channel incision (Whipple and Tucker, 1999; Montgomery and Grant, 2001; Whipple, 2001, 2004; Willet 3

ACCEPTED MANUSCRIPT and Brandon, 2002; Whittaker et al., 2010; Whittaker, 2012). During this adjustment phase, the uplifted zone may introduce a marked element of discontinuity in the profile

that

acts

as

a

local

base

level

causing

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longitudinal

changes

in

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aggradation/degradation processes, grain-size distribution, and avulsion patterns (Ouchi 1985; Heller and Paola, 1992; Holbrook and Schumm 1999; Blum and Tornqvist, 2000; Hickson et al., 2005; Blum et al., 2013).

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The present study uses a numerical modeling approach to investigate the effects

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of tectonic uplift on alluvial system dynamics, with particular emphasis on time and mode of channel aggradation and degradation processes. The numerical model presented here

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is developed on the data derived from the stratigraphic record of a Plio–Pleistocene fluvial infill of the Ambra Valley in the northern Apennines, Italy. This study presents the

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physical modeling and the calibration techniques used to simulate valley-fill

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accumulation. The results of this approach are then compared with field data and

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discussed in terms of river response to tectonic disturbance of the longitudinal profile.

2. Geological setting 2.1. Overview

The study area is located in central Italy (Fig. 1A), in the headwaters of the Ambra and Ombrone rivers. The Ambra River belongs to the Valdarno drainage, wheareas the Ombrone River belongs to the Siena basin drainage (Fig. 1B). The Ambra River is a tributary of the Arno River. Its source is in Mt. Luco (Siena province); and after a short reach oriented to the south, the river turns N-NE in correspondence with Castello di Montalto to join the Arno River near the Bucine area (Fig. 1C). The Ombrone River is a 4

ACCEPTED MANUSCRIPT major river, the source of which is in the same area of the Ambra River The Ombrone River flows to the south and reaches the Tyrrhenian Sea near Grosseto (Fig. 1B).

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The valley-fill succession considered for this study is located along the northern

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margin of the Siena basin, one of the Neogene–Quaternary depressions developed on the northern Apennines as a superficial response to lithospheric scale extension (Carmignani et al., 1994, 1995, 2001; Brogi, 2008; Barchi, 2010) (Fig. 1B). These structural basins, up to

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200 km long and 25 km wide, are filled with continental to marine deposits (Martini and

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Sagri, 1993), NW- to NNW-trending, and segmented into minor depressions by NNE-SSW– oriented tectonic transverse lineaments (Liotta, 1991; Martini and Sagri, 1993; Bonini and

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Sani, 2002; Pascucci et al., 2007). These lineaments exert a marked control on the modern river network (Fig. 1C; Bartolini and Pranzini, 1981).

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The northern margin of the Siena basin is made of Cretaceous to Oligocene

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sedimentary rocks (Abbate and Sagri, 1982) mainly consisting of sandstone with subordinate marls and shale. Sedimentation in the Siena basin started in the late Miocene

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with fluviolacustrine deposits, followed by marine sediments from the early Zanclean to the late Piacentian (Bambini et al., 2010; Martini et al., 2011, 2013; Arragoni et al., 2012; Martini and Sandrelli, 2015) when a regional tectonic doming induced a widespread marine–forced regression and establishment of continental sedimentation (Martini et al., 2001).

2.2. The study area: geomorphological and structural features The study valley was incised following the late Piacentian regression and then filled with fluvial deposits (Aldinucci et al., 2007; Bianchi et al., 2014) sourced from the north (i.e., Chianti ridge; Fig. 2). During the late Calabrian, the headwaters reach of the 5

ACCEPTED MANUSCRIPT main, southward-draining watercourse was deflected to N-NE (i.e., toward the Upper Valdarno basin; Fig. 1A) as a consequence of a piracy, which promoted the setting of the

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modern drainage pattern (Bartolini and Pranzini, 1981). This piracy is documented by the

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abrupt turn to N-NE of the modern Ambra River (Fig. 1C) and by the occurrence of a dry, hanging valley in the Monastero d’Ombrone area (Fig. 1C).

This study focuses on the southward-draining paleovalley, which is characterized

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by a complex structural setting (Brogi et al., 2013) with NW-SE and NE-SW trending fault

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systems including two main normal faults, named here Ambra River (Minissale, 2004; Baldi et al., 2006) and Terre Rosse faults (Fig. 2). The former cuts the pre-Neogene

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bedrock and dips toward the NE (i.e., perpendicular to the valley axis) and is still affected by intense CO2 emissions (Brogi et al., 2014). The latter dips toward the SW (i.e., parallel

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to the valley axis) and displaced Pliocene marine deposits (Bianchi et al., 2013).

3. Depositional history

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Regional observations suggest that deposition in the study valley occurred without any influence of relative sea level (Aldinucci et al., 2007). Bianchi et al. (2013, 2014) divided the valley-fill succession into two main sedimentary units (V1 and V2) and summarized their tectono-sedimentary evolution as follows (Fig. 3A): 1) Tectonic uplift of the Chianti ridge provided a pulse of sediment that caused accumulation of about 60-70 m of gravelly sand deposits within the whole valley (V1 unit; Fig. 3B). 2) Activation of the Ambra River fault triggered subsidence upstream of the fault, with an overall decrease of fluvial transport capacity, the consequent accumulation of mud-rich deposits (V2fla; Fig. 3C) and lateral coeval alluvial-fan gravels (V2af). In the 6

ACCEPTED MANUSCRIPT uplifted area, the fluvial incision shifted progressively toward the eastern flank of the valley in response to the progressive increase of displacement SE of the Ambra River

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fault. Downstream of the uplifted zone, the activation of the Terre Rosse fault caused an

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eastward shift of the fluvial system, which started to accumulate gravel and sand (V2flb) sourced from the uplifted area (Fig. 3D).

3) The stream capture of the Ambra River to the north gave way to the present-

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day drainage network.

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The Ambra River and the Terre Rosse faults substantially controlled river dynamics

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and depositional history of the study area (stage 2 of Fig. 3A).

4. Simulated processes and governing equations

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Until recently, geomorphic and stratigraphic codes provided general results in

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separated simulations. Increasing demand for paired assessment of geomorphic changes and sedimentary record tracking led to the generation of new codes (e.g., CHILD:

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Gasparini et al., 2004; sedsim; golem). LECODE is one such code (Salles and Duclaux, 2014). It performs three-dimensional parallel geomorphic and stratigraphic modeling, and it is able to solve overland and open-channel flows. This modeling code addresses largescale landscape evolution and the associated stratigraphic record at valley or basin scales (Salles and Duclaux, 2014), emphasizing the interaction between external forces and resulting sedimentary architecture (Alexander et al., 1994; Crave et al., 2000; Bonnet and Crave, 2003; Bishop, 2007; Jerolmack, 2011). The geomorphic and stratigraphic modeling method used in this study, is based on a Lagrangian particle-in-cell finite difference scheme (Tetzlaff and Harbaugh, 1989; Griffiths et al., 2001). The method has the 7

ACCEPTED MANUSCRIPT advantage of allowing the flow to follow the topography in a natural way, and it also offers an alternative to a purely diffusive approach often used in stratigraphic models

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(Heimsath et al., 2005).

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LECODE solves the flow evolution using the marker-and-cell approach. It uses Lagrangian fluid elements (flow walkers) as depth-averaged fluid properties. With their acceleration or deceleration, these flow walkers are able to erode or deposit (Harlow,

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1964; Hockney and Eastwood, 1981). The flow-walkers path recognized by the model

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simulates long-term channelized flow pathways. The flow walker dynamic is governed by a set of two equations: (i) the momentum conservation and (ii) the mass conservation.

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Interaction with topography and sediment erosion/transport and deposition are simulated using a nonuniform total load equation that takes into consideration surficial

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sediment mixture defined as a combination of multiple grain-size classes (Zhang, 1989;

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Wu et al., 2000; Wu, 2007). The bottom friction is expressed by Manning’s n coefficient (Arcement and Schneider, 1984; McCuen, 1998). More details about the equations used

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are given in Salles and Duclaux’s (2014) paper.

5. Parameters calibration The following sections shows how paleohydraulic and timescale parameters required to constrain and initialize LECODE simulations have been derived from field data.

5.1. Paleohydrology Paleohydrological studies focused on the gravelly deposits from unit V1, which is exposed in the Arcidosso area, where a specific cross section (Fig. 4) allowed measurement of grain-size distribution of bed material and the hydraulic geometry (e.g., 8

ACCEPTED MANUSCRIPT bankfull area, wetted perimeter, maximum and mean depths). These gravels were deposited by single, relatively sinuous channels (Bianchi et al., 2014), labelled as

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wandering (sensu Rice et al., 2009) or pseudo-meandering (sensu Bartholdy and Billi,

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2002).

Bed material grain-size distribution of channel lag deposits was measured in the field by the frequency by number method stretching a tape ruler across the outcrop cliff

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and collecting the particle tangent to the tape every 50 cm. A minimum of 100 particles

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were collected and a frequency curve, based on a ½ φ scale, was obtained. Channel lag deposits were selected because they are the most effective in determining the boundary

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flow resistance conditions required to calculate the hydraulic parameters. Characteristic diameters of bed material such as D50, D84, and D90, i.e., the grain size for which 50%, 84%,

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and 90% of the distribution is finer, are shown in Table 1.

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The thickness of large-scale inclined bedsets accounts for a paleochannel depth of about 4.5 m (Fig. 4; Table 1). Channel-fill deposits are not entirely exposed, but their lateral

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extent suggests that the channel was at least a few tens of meters wide. The width of a 4.5-m-deep, single channel forming gravelly lateral bars can be estimated at about 70 m (Wb in Table 1) according to Leeder (1978) and its bankfull cross section at about 200 m2 (A in Table 1) (McGowen and Garner, 1970). Mean flow velocity (v) was calculated using the Chezy uniform flow equation v = C (RS)0.5

[1]

where R is the hydraulic radius, S the streambed gradient, and C the roughness coefficient. C = (8g)0.5/f 0.5

[2] 9

ACCEPTED MANUSCRIPT where g is gravity and 1/f 0.5 is the Darcy-Weisbach roughness coefficient.

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To calculate 1/f 0.5, the following equations were used: 1/f 0.5 = 1 + 2*log (h/D84) (Leopold and Wolman, 1957)

[3]

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where h is mean depth, and D84 is the grain size for which 84% of the distribution is finer; [4]

1/f 0.5 = 0.82*ln (4.35*R/D84) (Knighton, 1996)

[5]

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1/f 0.5 = 1.16 + 2*log (R/D84) (Limerinos, 1970)

These equations were selected since they were developed for gravel-bed streams with

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grain size similar to those of the studied cross section.

Because tectonic tilting affected the study deposits, the streambed gradient that

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could be inferred from field exposures is not representative of the actual one.

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In order to assess the streambed gradient of the studied paleochannel, the

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following equations were used:

S = 0.002*Wb-0.06 H-0.91 (Williams, 1984)

[5]

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where Wb is bankfull width, and H is maximum depth; S = 0.0173 *(H / D84) 0.785 (Dingman and Sharma, 1997)

[6]

Equations [5] and [6] provided streambed gradients of 0.0004 and 0.0007529, respectively (Table 1). Combining C and v values, we obtained the v values reported in Table 1. For each mean flow velocity (v) values, bankfull discharge Qb was calculated Qb = v / A

[7]

The bankfull stage corresponds to the discharge at which the channel has power to maintain moving sediment, and so moving bars (Dunne and Leopold, 1978). The bankfull discharges calculated with different criteria are very close with an average value 10

ACCEPTED MANUSCRIPT of 266 m3s-1 (Table 1), comparable with an alluvial system in similar conditions (e.g., the Arno

River

upstream

of

the

confluence

with

the

Ambra

River

-

URL:

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http://www.sir.toscana.it/index.php?IDS=8andIDSS=38, latest access 5th February 2015).

5.2. Paleomagnetism

In order to provide time constraints for the valley-fill successions we integrated

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regional geological evidence with paleomagnetic analyses, which were carried out on 56

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oriented samples collected for unit V1 in the Arcidosso (n = 30) area and V2 in Pian di Bari (n = 26) area (Figs. 2 and 5A). The details about the rock, paleomagnetic data, and

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analyses are described in the supplementary material available online. The analyses revealed that the Arcidosso section is characterized by reverse magnetic polarity from the

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base up to 26.3 m and by normal magnetic polarity upsection, whereas the whole Pian di

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Bari section is characterized by normal magnetic polarity (Figs. 5B and C). The termination of marine sedimentation in the Siena basin (i.e., latest Piacentian;

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Bambini et al., 2010; Martini et al., 2011, 2013; Arragoni et al., 2012) and the valley abandonment from the capture of the Ambra River (i.e., latest Calabrian; Bartolini and Pranzini, 1981; Fidolini et al., 2013a,b; Ghinassi et al., 2013) allow us to confine accumulation of units V1 and V2 within the Gelasian to Calabrian time span (Fig. 5D). Two distinct scenarios stem from these paleomagnetic analyses. In the first case (Fig. 5D), most of the studied successions accumulated during Subchron C1r.1n (Jaramillo), with an average sedimentation rate close to 1.1 mm/y. In the second case, it was mainly deposited with an average sedimentation rate of about 0.5 mm/y during Subchron C2n (Olduvai; Fig. 5D). The sedimentation rate for the first scenario seems to be unrealistic considering the scarce elevation and uplift rate of the Chianti ridge during the 11

ACCEPTED MANUSCRIPT Pliocene–Pleistocene (Thomson et al., 2010). Considering the duration of Olduvai

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models, considering the deposition of V1 and V2 units.

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Subchron (~200 ky), a time of deposition of 300 ky will be therefore used for numerical

6. Ambra Valley simulation

This sedimentary succession documents alluvial dynamics in an upstream valley

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reach, where aggradation is primarily driven by an upstream-dipping normal fault. Recent

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studies have provided detailed field-based data sets (Aldinucci et al., 2007; Bianchi et al., 2013, 2014) that are necessary to produce meaningful numerical results. The initial

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setting of the Ambra Valley numerical simulation requires the definition of sedimentary composition and thicknesses of the pre-Gelasian buried geological layers, the

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reconstruction of paleotopography, and the assumption that tectonics was the main

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factor controlling sedimentation (Bianchi et al., 2013, 2014). As attested by field evidence, tectonics plays the dominant role on sedimentation because the facies heterogeneity

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distribution is recorded just across the localized uplift, though a minor climate influence on sedimentation cannot be ruled out a priori (cf. Holbrook and Schumm, 1999; Schumm et al., 2000).

6.1. Initial geological layers The geological units were characterized by two different sedimentary layers, both of which define the substratum geology, and the paleotopography (Fig. 6). The topography reconstruction was obtained by geomorphic and geological interpretation of the early Gelasian paletopography, which is consistent with field data. The reconstruction started with a digital elevation model (DEM) of the modern Ambra 12

ACCEPTED MANUSCRIPT valley

(http://www.regione.toscana.it/-/cartografia-tecnica-regionale-e-scarico-dati-

geografici, latest access 5 February 2015). This modern topography was modified with

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GRASS GIS software (Neteler and Mitasova, 2008) using a suite of methods to (i) tilt the

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whole region toward the south, (ii) uplift flattening in correspondence to Castello di Montalto (see Bianchi et al., 2013), and (iii) carve the whole valley channel. The DEM was tilted 300 m toward the south, which was necessary to simulate the Gelasian channel

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slope, based on calculated paleohydrological values (S = 0.06%). The uplifted portion was

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flattened to guarantee a continuum with the altitude of the northern portion and to provide a uniform valley floor, consistent with the calculated slope. The valley carving was

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performed by removing 100 m of initial valley-fill deposits through a localized subsidence along the actual valley axis.

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The substratum lithology was simulated using two initial underlying sedimentary

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layers with specific hardness and grain-size distribution (Fig. 6), based on the lithological features of the valley documented in the local geological map (Bianchi et al., 2013). Two

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types of substratum were considered: (i) one representing the sandstone of Macigno Fm., simulated as hard rock (h = 2, d = 0.25 mm, h is the hardness, and d the grain-size diameter), and (ii) the second corresponding to the Pliocene marine deposits belonging to the Siena basin infill, simulated as unconsolidated sand and clay (h = 1, d = 0.20, 0.01 mm).

6.2. River parameterization River input is represented in LECODE by flow walkers holding information such as discharge, velocity, flow height, sediment concentration, and sediment-type percentages.

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ACCEPTED MANUSCRIPT According to paleohydrological evidence, the considered initial discharge has been set to 200 m3/s. The initial velocity of the source was directed toward the south and was

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set to 2 m/s at the source point, in agreement with paleocurrent distribution (Bianchi et

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al., 2013, 2014). Flow height defines the flow walker depth at the source point; the maximum height defined in the input file is 10 m. For sediment concentration, a value of 3.5 kg/m3 was fixed and corresponds to suspended and bedload sediments.

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Owing to the use of nonuniform sediment mixture for sediment transport

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calculation, it is convenient to divide the mixture in several grain-size classes (Zhang, 1989; Wu et al., 2000), which are characterized by individual densities and angles of

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repose. Relative to the river within the valley, six sediment classes were imposed: four for gravel, one for sand, and one for silt-clay. Table 2 shows the physical properties of each

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sediment class.

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The choice of four gravelly classes was used to match the sediment distribution outcropping in the Arcidosso section (Fig. 2) and corresponding to the V1 deposits

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(Bianchi et al., 2013). In order to finalize the river parameterization, each grain-size class that composes the source has to be allocated based on their respective percentages. After analyzing the sedimentary logs, it was chosen to define the following grain-size distribution from coarsest to finest grain sizes: 1%, 1.5%, 1.25%, 1.25%, 37%, and 58%.

6.3. Allogenic forcing Two factors control the sedimentation in the model: tectonics and climate (Fig. 7). Since the tectonics is the main controlling factor, we hold the climatic conditions steady throughout the simulation. Rainfall was held at a steady rainfall rate of 0.7 m/y, which

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ACCEPTED MANUSCRIPT corresponds

to

the

modern

average

precipitation

in

the

area

(http://www.sir.toscana.it/supports/download/report, latest access 5 February 2015).

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In order to simulate the Ambra River fault dynamics (Bianchi et al., 2014), the

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movement of the footwall block was replicated. The uplifting portion was displayed with a hill shape bounded to the north by a steep NW-SE trending slope mimicking the Ambra River fault plane. The maximum displacement, centered on the main valley axis, was

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about 100 m and occurred over 50 ky. Although Thomson et al. (2010) calculated the

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average uplift rate of the northern Apennines as 1 mm/y, with peaks of 1.4 mm/y (Mt. Falterona, NE Tuscany) and 0.5 mm/y (Valdarno, central Tuscany), we simulated a

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tectonic event for a limited uplifted portion and for a brief period, thus imposing an increased uplift rate of 2 mm/y. Furthermore, we impose the presence of a localized

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subsidence in the eastern valley flank of the southern sector to simulate the Terre Rosse

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fault creating accommodation space for the input of lateral tributaries. This lineament has the same trend of the fault mentioned above. Both tectonic movements, uplift and

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subsidence, have been simulated in two pulses which correspond to two running steps. This assumption has been considered after some tests, starting from the uplift rate stated in the literature (England and Molnar, 1990; Thomson et al., 2010). The two events of fault reactivation promoted landscape modifications and valley accumulation. The effects of the tectonic pulses are described in section 6.5.2.

6.4. Running phases Three phases simulate valley evolution. The first, pre-tectonic phase, is aimed at reaching the steady state of the system. The second, syntectonic phase, focused on the monitoring of the sedimentary dynamics. The third, post-tectonic phase, involved the 15

ACCEPTED MANUSCRIPT adjustment of the system to reach a new steady state. This approach highlighted the signature of autogenic factors on sedimentation while reaching the pre-tectonic steady

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state while avoiding their disturbance during syntectonic sedimentation (Whipple, 2004).

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The model duration is strictly linked with the bankfull discharge from paleohydraulic analyses. Although the bankfull discharge is associated with a specific return time (Dalrymple, 1960; Chow, 1964; Eagleson, 1972; Fielding et al., 1999), it

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provides the most dynamic phase for channel construction (Dunne and Leopold, 1978)

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and is therefore simulated here in a continuous virtual time. We assumed a bankfull discharge frequency of 35 h/y. In this framework, assuming that the Ambra valley

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succession accumulated over about 300 ky, the time arising from the flood frequency analysis could be approximated to 1100 years. This time period was confirmed by several

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tests that defined the suitability to simulate the former three stages. The first simulation

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phase (i.e., reaching of the steady state) was run for 700 years, the second phase (i.e., fluvial response to tectonic disturbance) for 200 years, and the third phase (i.e., reaching

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of the second steady state) for another 200 years. Every 100 years the model generates one sedimentary layer and every 50 years new flow walkers are released. During the syntectonic phase, the tectonic perturbation does not create a decreasing valley slope, which was, however, highlighted by a field study (Bianchi et al., 2014). In order to mimic the reduction of valley slope, we decrease the velocity and height of the source (V = 2 > 1.5 m/s; H = 2.25 > 1.5 m) only in the syntectonic phase (from 700 to 900 virtual years) to simulate the effects of river potential energy loss in the upstream part of the system (Blum and Tornqvist, 2000).

6.5. Results 16

ACCEPTED MANUSCRIPT The interaction between climatic–tectonic forcing and sediment supply on initial geological layers led to aggradation/degradation patterns, with the maximum

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aggradation within the valley flanks and degradation over the surrounding hills (Fig. 8).

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The incision pattern underlines the distribution of areas with differential erosion: hard bedrock promotes narrow, deep, and elongated incision; unconsolidated substratum endorses short, wide, and shallow erosional valley forms (Fig. 8).

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The valley evolution is here summarized in three phases: pre-, syn-, and post-

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tectonic phases (Fig. 9), associated with virtual time spans measured in virtual years (vy).

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6.5.1. Pre-tectonic phase (0–700 virtual years)

In planview (Fig. 8), the system passes from a river in a confined valley (100–300

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vy) to a wider valley (400–700 vy), associated with the infilling of the valley and ascribed

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to a well-drained riverine landscape. In the pre-tectonic phase, the valley fills up (Fig. 10A) and reaches its steady state (Fig. 10B). The overall grain-size distribution (Fig. 10A)

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shows a downstream fining sequence, whereas thickness of infill deposits varies as a consequence of irregularities of the valley floor. From 0 to 500 vy, thick layers up to 15 m each are deposited, showing a sandy-gravelly coarsening-upward tendency (Fig. 10A) and the system displays a fluctuating sedimentation rate (Fig. 10B). From 500 to 700 vy the system is defined by a virtual sedimentation rate that is constant and lower than previous values (0.3 cm/vy; Fig. 10B). The grain size is mainly sandy-gravelly, with a fining-upward trend. Deposits younger than 500 vy present sharp bases that incised the older deposits with almost no depositional features (Fig. 10A), suggesting the development of a bypass zone (Whipple, 2004). When the bypass zone develops, the sedimentation rate reaches the steady state at about 500 virtual years. 17

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6.5.2. Syntectonic phase (700–900 virtual years)

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The geomorphic features from 800 to 900 vy (Fig. 9) appear different in the two

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sectors: upstream of the fault zone a poorly defined riverine landscape develops, whereas downstream of the fault zone a distributive system developed.

During this phase the virtual fault displacement reaches 100 m, which is equally

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distributed in two distinct tectonic events (Fig. 11). After the first pulse (700–800 vy)

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aggradation occurred upstream of the fault (Fig. 11A), whereas only after the second pulse (800–900 vy) did river shift eastward and then aggradation of coarse sediments in

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the downstream part started (Fig. 11B). This river shifting forms a second valley taking advantage of the eastern tip fault depression (Fig. 11C).

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As the collection of cross sections in Fig. 12 shows, in the upstream sector the

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valley is narrow (about 1.7 km) and has a deep floor up to 60 m (Fig. 12A). About 30 m of muddy sediments and 10 m of sand accumulated during the first (800 vy) and second

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tectonic pulses (900 vy), respectively (Fig. 12A). Both these intervals tend to thin upstream. In this area sedimentation occurred with a rate of about 10 cm/vy at 800 vy, then decreasing to 3 cm/vy at 900 vy (Fig. 13). In the sector downstream of the fault zone, cross sections show a wider (about 3.2 km) and shallower (about 35 m deep) valley. Deposits form a fan-shaped lithosome confined within the newly formed valley (Fig. 11B) and are mainly gravelly, up to 10 m thick (Fig. 12B). The presence of the secondary Terre Rosse fault is laterally recorded by architectures showing gently rollover layers (Fig. 12B). Along-valley sections show that the layers pinch southward. Aggradation occurred during the second tectonic pulse (900 vy) with a rate of about 3 cm/vy (Fig. 13). 18

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6.5.3. Post-tectonic phase (900–1100 virtual years)

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During the post-tectonic phase, the river system maintains its new pathway (Fig.

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9; 900–1100 vy) and only a reduced thickness of sediments is accumulated along the valley (Fig. 14). Substantial accumulations of bedrock-derived material occur at the main tributaries outflow into the main valley, upstream and downstream of the fault zone (Fig.

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14). The dominant presence of bedrock material, released in this phase at the outlet of

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the main tributaries, marks a variation of valley-fill composition, which therefore establishes a change in geomorphic processes and drainage configuration (Church, 2002).

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This phase was also characterized by a reduced aggradation rate of about 0.3 cm/vy, which remains stable even after 1100 vy, portion cut by authors for shortening reason

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(Fig. 13). The stabilization of a reduced aggradation rate indicates the achievement of a

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7. Discussion

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steady state (Fig. 13).

This study provides insights about tectonic-driven alluvial dynamics through a comparison, at mesoscale resolution, between a virtual landscape evolution and the Ambra Valley stratigraphic record (Fig. 15). All the simplifications adopted in the simulation are aimed at decreasing the complexity of the model, intending to test the depositional processes and stratigraphic successions in the easiest way (Paola, 2000). 7.1. Pre-tectonic phase During this phase of the simulation, sediments accumulate in the valley until the system reaches a steady state after 500 vy and a longitudinal equilibrium profile was dominated by sediment transfer (Whipple, 2004; Blum et al., 2013). The unscaled time 19

ACCEPTED MANUSCRIPT required by the system to reach a virtual steady state corresponds to 135 ky (real time) and can be considered as the intrinsic response time (or equilibrium time) introduced by

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Blum et al. (2013). In this context, the steady state indicates the tendency of the inland

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valley system to attain an equilibrium profile irrespective of the marine or lacustrine influence (Gibling et al., 2011). 7.2. Syntectonic phase

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During the syntectonic phase, river transport capacity limited conditions (induced

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by the local uplift) result in a consistent aggradation upstream of the deformed zone as far as the river achieves a new equilibrium profile. Virtual sediment accumulations in the

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upstream reach fit with the V2fla deposits of the Ambra succession (Bianchi et al., 2014; Fig. 15) and find their equivalent also in laboratory experiments (Ouchi, 1985) and

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modern river examples (e.g., Holbrook and Schumm, 1999). Upstream pinching of the

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fine-grained deposits suggests that the development of the new river profile started close to the fault and moved upstream, producing an upvalley-thinning sedimentary lithosome,

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which resembles the back-filling architecture (sensu Schumm, 1993) of alluvial valley fills genetically linked to relative sea-level rises (Boyd et al., 2006). The higher the block is uplifted the more upstream the aggradation propagates. The characteristic time required by the fault-induced uplift to affect the fluvial dynamics (T*) is referred as reaction time and is expressed by (Blum et al., 2013 and references therein): T*= T/Teq

[8]

where Teq is the equilibrium time for the system to reach the steady-state configuration, and T is the time scale of the tectonic forcing (Paola et al., 1992; Marr et al., 2000; Swenson, 2005; Blum et al., 2013). The reaction time of the simulated river is found to be 0.4 and corresponds to the field of ‘fast variations’ of Swenson (2005). Thickness and 20

ACCEPTED MANUSCRIPT grain size of the virtual V2fla deposits correspond to the real V2fla deposits (Fig. 15) and are consistent with the dominance of overbank sedimentation because of increase of

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flood height and velocity, as documented also in similar tectono-geomorphic settings

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(Dales, 1966; Russ, 1982; Schumm, 1986; Hengesh and Lettis, 2002; Gibling, 2006; Ethridge and Schumm, 2007; Hole, 2011).

Across the uplifted block, river incision dominated, as documented also in the

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Ambra Valley case (Bianchi et al., 2014) and in similar tectono-morphological settings

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(e.g., Rio Grande River; Holbrook and Schumm, 1999; Snyder et al., 2000, 2003). The narrow secondary valley, cut into the uplifted bedrock, reflects a condition of uplift rate

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balanced by river incision (Whipple, 2004). The model highlights also that the fault trend strongly influences the lateral shift of the valley axis across the uplifted block, following

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the mechanisms of lateral drainage adjustment described for the Carizzo Plain (California)

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by Schumm (1986). The absence of terraced surfaces on the virtual uplifted block is consistent with a constant incision rate (Lavé and Avouac, 2001). The model also shows

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that the Terre Rosse fault does not affect the eastward shift of the main watercourse; instead it controls the rollover architecture present in the downstream deposits. Aggradation of virtual coarse sediment downstream of the uplifted block is strictly linked to the amount of sediments eroded from the uplifted area as also suggested by field examples (e.g., New Madrid earthquake; Russ, 1982; Schumm, 1986). These virtual deposits find their equivalent in the gravelly V2flb deposits of the Ambra River succession (Fig. 15) and fit also with laboratory experiments (Ouchi, 1985). Aggradation was triggered by a substantial increase in sediment supply from the uplifted block and can be labeled as a downfilling process (sensu Schumm, 1993) that caused a relative rise of the river profile (Blum and Tornqvist, 2000). 21

ACCEPTED MANUSCRIPT Notwithstanding that both the numerical simulation and the Ambra valley succession document that aggradation occurred upstream and downstream of the

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uplifted area, the model shows that aggradation of the virtual V2flb unit post-dates

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accumulation of V2fla (Fig. 13). Such a diachronism, which cannot be inferred from the sedimentary record, represents the time required for accumulation of potential energy, which will allow the river to increase its stream power and cut through the uplifted block.

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At this stage, where the modeled valley is cut into the bedrock, its W/T ratio

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(channel-width and thickness ratio sensu Gibling, 2006) is about 11.6, whereas this value rises to 175 where the valley is cut into the unconsolidated substratum. These two values

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correspond respectively to the field of ‘Valley Fills on Bedrock Unconformities’ and ‘Valley Fills within Alluvial and Marine strata’ as defined by Gibling (2006) and fit with those

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observed for the V1 and V2 units of the Ambra Valley succession (Bianchi et al., 2014).

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7.3. Post-tectonic phase

During the post-tectonic phase, the dominance of bedrock material in fan-shaped

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bodies distributed along the valley flanks highlights a change in geomorphic processes (Heller and Paola, 1992; Church, 2002), which was represented by a decrease in fluvial transport along the valley trunk accompanied by accumulation of virtual alluvial fan deposits along the valley flanks. These fans find their equivalent in similar deposits (V2af) of the Ambra River succession (Bianchi et al., 2014; Fig. 15). The numerical model highlights that alluvial fans prograded during the end of the post-tectonic steady state, confirming that they represent the rejuvenation of the relief (Whittaker et al., 2010) and that they are the expression of the system relaxation phase (Fidolini et al., 2013b; Sømme et al., 2013).

Development of alluvial fans during the latest depositional stage is

confirmed in the stratigraphic record by the superposition of alluvial fan deposits over the 22

ACCEPTED MANUSCRIPT axial fluvial succession (Bianchi et al., 2014). Similarly, the monogenic composition of

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alluvial fan gravels (Bianchi et al., 2014) exhibits a reorganization of the local drainage.

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8. Conclusion

Based on previous field study (Bianchi et al., 2014), this paper investigates mesoscale sedimentary architecture evolution due to the interplay between upstream

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river system and uplift forcing. Through the numerical modeling we have assessed and

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time-constrained the tectonic control on aggradation of the studied valley fill. The physical and hydrological laws that constrain numerical modeling validated

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our tectono-sedimentary scenario, derived from field observations. The validation comes from the similarity between the modeling results and the sedimentary and geomorphic

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features of Ambra paleovalley, reproducing the two differential and characteristic

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aggradations within the valley and the landscape evolution at any phase. Temporal control, taking advantage of numerical modeling, highlighted that the

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tectonic variation felt in the ‘fast variation’ field and syntectonic aggradations occurred anachronously. The time span between the two aggradations was sufficient for achieving the required stream power, which is responsible for the uplift incision. The accumulation of alluvial fans dominantly in the post-tectonic phase marks a change in geomorphic process such as a relaxation of the system and reorganization of the drainage. The model presented here, provides valuable information for petroleum and aquifer research in architecture connectivity, where the interplay between river valley systems and tectonics is important.

ACKNOWLEDGEMENTS 23

ACCEPTED MANUSCRIPT This work is derived from PhD work of the corresponding author (V.B.), which was supervised by M.G. and co-supervised by T.S. Modeling was conducted in CSIRO (North

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Ryde, Sydney), and it was economically supported in part by the University of Padova

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(Fondi di Ateneo ex 60%, 2009 and 2010 - M. Ghinassi) and in part by CSIRO (T. Salles). M. Gazley and L. Quigley are kindly thanked for reviewing the English of the manuscript. The CE Dr. Richard A. Marston and three anonymous reviewers are here thanked for their

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helpful suggestions.

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Fig. 1. (A) Sketch of Italy and inset of Fig. 1B. (B) Simplified geological sketch of the northern Apennines showing the Neogene–Quaternary basins bounded by bedrock ridges and main tectonic features. (C) Digital elevation model of the area (location in Fig. 1B).

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Fig. 2. Detailed geological map of the studied area (location in Fig. 1C) with the main localities mentioned in the text. In this map the valley infill, formed by facies associations, overlaps different substrata, which are defined as Zanclean (?) to Piacenzian coastal to marine deposits and Pre-Neogene bedrock, classified in turn in Tuscan, Subligurian, and Ligurian units. Fig. 3. (A) Tectono-sedimentary evolution of the studied area illustrated in a three-step sketch with geological cross sections. Stage 1: sedimentation of gravelly sand deposits within the whole valley (unit V1). Stage 2: activation of the Ambra River fault triggered upstream the accumulation of mud-rich fluvial deposits (V2fla) and coeval alluvial fan gravels (V2af). Fluvial incision in the uplifted area shifting eastward. Downstream, the activation of the Terre Rosse fault recalls the fluvial system, accumulating gravel and sand (V2flb) sourced from the uplifted area. Stage 3: piracy of the Ambra River and presentday drainage configuration. (B) Sedimentary log showing depositional features of unit V1, with a detail of the first fining-upward (FU) interval. (C) Sedimentary log showing depositional features of unit V2fla. (D) Sedimentary log showing depositional features of unit V2flb; note the FU trend. Fig. 4. (A) Panoramic view of deposits belonging to the last FU interval of unit V1, picture taken in Arcidosso locality (Fig. 2); (B) line-drawing highlighting the sedimentary parameters involved in the calculation of the palaeohydraulic section.

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Fig. 5. (A) Geological cross section showing the site of palaeomagnetic sampling (location in Fig. 3A) B) From left to right declination, inclination, and virtual geomagnetic pole (VGP) latitude associated to each characteristic remnant magnetization (ChRM) direction, plotted vs. the stratigraphic position of the samples. The relative latitude of each VGP with respect to the paleomagnetic (North) pole was used to interpret the magnetic polarity: black (white) bar indicates normal (reverse) polarity. (C) Representative sedimentary log with positioning of paleomagnetic samples in the valley-fill succession. (D) Constraining temporal sketch of valley-fill deposition based on the paleomagnetic data.

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Fig. 6. Illustration of initial layers composed by substratum geology and topography. The substratum geology is represented by a bulk made of deposits with two different hardnesses. The paleotopography is represented by a modified modern DEM (see the text for details).

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Fig. 7. Collection of forcing parameters in layers. From the top: (A) representation of sediments related to the source and the source track in red overlaying the topography; (B) rain-derived drainage represented by fluid-element tracks in blue with a constant rainfall rate of 0.7 m/y; (C) vertical displacement characterized by uplift (red) and subsidence (blue).

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Fig. 8. Schematic sketch describing the parameters (stacked layers) involved in the formation of aggradation/degradation pattern. Aggradation is expressed in red high relief and degradation in blue low relief.

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Fig. 9. Set of frames showing the landscape evolution of the valley. Green line comprises frames belonging to the pre-tectonic phase. Red line borders frames of syntectonic phase. Blue line frames scenarios of post-tectonic phase. Fig. 10. Pre-tectonic phase. (A) Planview of the area with cross section of the valley fill framed at 700 vy; note the enlarged cross-section of pre-tectonic deposits, highlighting the grain size. (B) Sedimentation rate graph of the pre-tectonic phase; note the low sedimentation rate from 500 to 700 virtual years. Fig. 11. Syn-tectonic phase. (A) Plan view of grain-size distribution (left-hand) at 800 vy; detail of thickness distribution (right-hand) in plan view at 800 vy, showing the aggradation localised in the valley upstream portion. (B) Plan view of grain-size distribution (right-hand) at 900 vy; highlighted in the left-hand the deposits thickness at 900 vy, showing an aggradation framed in the downstream portion and spatially limited in the upstream portion. (C) Cross sections (position shown in B) concerning the valley axis shifting. Note the bedrock incision and formation of a second valley in the cross section at 900 vy. Fig. 12. Two collections of cross sections taken from the upstream sector (A) and from the downstream sector (B) of the uplifted portion. They are both displayed on the basis of a combination of different parameters: time of layer deposition, thickness of layers, grainsize variation in each layer, and percentage of material derived from bedrock erosion of 35

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Fig. 13. Sedimentation rate graph calculated on the basis of the instantaneous aggradation detected in the valley during the three phases; for the syn-, and post-tectonic phases the calculation of sedimentation rate is split for the northern and the southern sectors; note the high aggradation detected at 800 vy in the northern sector, and the high aggradation identified at 900 vy in the southern sector.

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Fig. 14. Post-tectonic phase. Planview of the area across the fault highlighting the proportion of bedrock material. Cross sections of the downstream area (A) and the upstream area (B) displayed on the basis of the proportion of the bedrock material.

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Fig. 15. Comparison of stratigraphy in planview and cross sections between Ambra Valley field data and numerical simulation.

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H (m) Wb (m) A (m2)

68.9 206.8

Field data

D50 (m) D84 (m) D90 (m)

0.0363 0.0830 0.1004

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S S C C C V (m/s) V (m/s) V (m/s) V (m/s) V (m/s) V (m/s) Qb (m3/s) Qb (m3/s) Qb (m3/s) Qb (m3/s) Qb (m3/s) Qb (m3/s) Qb (m3/s)

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Williams (1984) Dingman (pers. comm.) Knighton (1996) Limerinos (1970) Leopold & Wolman (1957) Knighton + Williams Knighton + Dingman Limerinos + Williams Limerinos + Dingman Leopold & Wolman + William Leopold & Wolman + Dingman Knighton + Williams Knighton + Dingman Limerinos + Williams Limerinos + Dingman Leopold & Wolman + Williams Leopold & Wolman + Dingman Qb average

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Field data

Arcidosso 4.5

0.0004 0.0007529 36.24 37.35 35.94 1.207 1.591 1.244 1.718 1.197 1.653 249.6 329.0 190.8 263.6 247.5 341.8 265.8

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Table 1. Parameters involved in the calculation of bankfull discharge.

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Gravel 1

Gravel 3

Gravel 4

Sand

Silt/Clay

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1

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Table 2. Parameters characterizing the examined grain-size classes.

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