Structural analysis and analogue modeling of the kinematics and dynamics of rockslide avalanches Thomas Shea* Benjamin van Wyk de Vries* Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, 63000, France ABSTRACT ics: hummocky, nonhummocky, dominantly dynamics, because the link between deposit and extensional, and dominantly compressional emplacement is unclear. We present a structural analysis of subaer- rockslide avalanches. The models require ial natural and analogue rockslide avalanches. a brittle core and surface that spreads and Faults in Rockslide Avalanches Such deposits often have well- developed contracts by adjustment on large numbers of faults, folds, and hummocks. These structures faults that bottom into a low-friction décol- A striking feature of well-preserved rockslide- can be used to determine the kinematics and lement layer. Spreading is accommodated by avalanche deposit surfaces is their morphology, dynamics of emplacement. Large-scale terres- normal and strike-slip faults, while on decel- which, through ridges and escarpments, shows trial rockslide avalanches show large runout eration, thrust faulting generates thickening. the presence of a complex succession of faults distances compared to their fall height. Most To be realistic, any physical predictive model and folds. One superb example of this is the attempts to explain this phenomenon invoke must take into account these fundamental Socompa rockslide avalanche (Central Andes, fl uidizing mechanisms or lubricating agents kinematic and structural aspects. Chile and Argentina; Fig. 1), where the entire to reduce forces opposed to momentum, surface displays a dense network of normal, especially at the base. However, the proper- Keywords: rockslide-avalanche deposits, struc- strike-slip, and thrust faults (Wadge et al., 1995; ties and mechanics of low friction are still tures, faults, analogue modeling, avalanche van Wyk de Vries et al., 2001, 2002; Kelfoun and poorly understood. Any model for motion transport, kinematics, hummocks. Druitt, 2005; Kelfoun et al., 2008). The interior and emplacement must integrate geometric, of this avalanche deposit, as seen in road cuts, morphologic, and structural features, all cru- INTRODUCTION shows that the fault structures incise deeply into cial in constraining kinematics and dynam- the deposit interior (e.g., Fig. 3 in van Wyk de ics. Here we fi rst examine the morphological Rockslide Avalanches Vries et al., 2001; Figs. 1B, 1C herein). Other and structural features displayed by 13 natu- examples include the avalanches of Mombacho ral rockslide avalanche deposits; we then use Large-scale rockslide avalanches are part of Volcano (Nicaragua; Shea et al., 2007), Pari- simple and well-constrained analogue mod- the Earth mass-movement processes. Subaerial nacota (Clavero et al., 2002), and Flims (Pol- els involving the slide of stratifi ed granular examples on Earth involve volumes of as much let and Schneider, 2004). In each case, road material down smooth, curved ramps. These as 100 km3, and their deposits cover areas as cuts show either original material or granular differ from previous analogue models in that large as thousands of square kilometers, reach- rockslide-avalanche breccia layers being offset we concentrate on observing the structures ing distances >100 km (Stoopes and Sheri- by numerous faults. Displacement is localized produced by brittle deformation and use a dan, 1992). They cause considerable human on narrow shear zones in the breccia, where the low-friction sliding surface. Models show that and material loss, directly or through second- simple shear of the fault zone is accommodated. variations in the sliding surface curvature, ary events such as tsunamis, river obstruction, Such a structural pattern is common in faults in lateral profi le, roughness, and modifi cations lahars, or magmatic eruptions (Siebert, 1984; granular materials, seen in sandbox analogue in material cohesion can successfully repro- Siebert et al., 1987). Although the onset of models of volcano deformation (e.g., Merle duce the majority of rockslide-avalanche fl ank collapse was photographed at Mount St. and Borgia, 1996) or in natural breccia or pyro- deposit features. After discussing the geo- Helens in the 1980 eruption (Voight, 1981), clastic successions deformed by slow tecton- metrical and dynamic similarity between rockslide-avalanche formation and motion have ics (e.g., Borgia and van Wyk de Vries, 2003). experiments and natural examples, we pro- never been clearly observed; as a result, study is These observations allow us to characterize the pose a model for structure formation and a mostly done through their deposits, or by theo- structures observed here as faults, as this is the fourfold classifi cation based on model and retical modeling. Important gaps thus arise in closest and most suitable broad defi nition avail- natural deposit morphology and dynam- understanding the emplacement kinematics and able. The data indicate that large-scale brittle *Shea, corresponding author: present address: Geology and Geophysics Department, SOEST, University of Hawaii at Manoa, Honolulu, USA; [email protected]. van Wyk de Vries: [email protected]. Geosphere; August 2008; v. 4; no. 4; p. 657–686; doi: 10.1130/GES00131.1; 15 fi gures; 2 tables; 2 supplemental tables; 1 supplemental fi le; 2 animations. For permission to copy, contact [email protected] 657 © 2008 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/4/657/3336451/i1553-040X-4-4-657.pdf by guest on 30 September 2021 Shea and van Wyk de Vries Bolivia large normal fault with back-facing normal A major strike-slip component fault at terminus 20° 3250 m isolated back thrust area, 3000 m formed like median scar before distal lobe active distal lobe has many bookshelf Socompa faults and a strike-slip boundary 30° 3100 m strike-slip and graben unit structures join distal a n i structures here t n e g r 3455 m eastward spreading A e indicated by grabens l i h C 3100 m northeast unit lineations continue 50° through median scarp 80° abrupt change in direction of NE unit large -throw normal faults 3600 m well-preserved early thrusts, cut by conjugate 3100 m strike-slips 3500 m spreading debris units longitudinal thrusts area of backflow sliding off mountain range 200 m -high debris units continue through fault scarp median scarp but are deflected buried scar Median scarp starts as trace en-echelon folds, thrusts 3580 m and strike-slips longitudinal grooves 3450 m large-throw (<300 m) Toreva faults levée and marginal strike-slips scar infilled with later W / N distal lavas and pyroclastics margin lobe strike-slip/ areas of dense graben normal faulting 6061 m 3600 m east Key Normal faults Strike-slip faults Major thrust faults al im ox Folds Grooves/striae SB RIF pr N Toreva Levees scar Lithological repetitions Later cover 5 km B F C SBa F F SBb repetition of vertical stratigraphy F F F F Transport direction RIF Figure 1 (continued on next page). (A) Volcán Socompa (Central Andes, Chile and Argentina) rockslide-avalanche deposit struc- tural map with main features (Guilbaud, 2000; van Wyk de Vries et al., 2002). The map is made from detailed aerial photographs, a digital elevation model (DEM), and fi eld observations. Only the main structures are shown. (B) Outcrop photograph of normal faults developed in breccia at Socompa (from van Wyk de Vries et al., 2001). These are proximal normal faults that are deformed by latter strike-slip faults as shown in Figure 2A. (C) Vertical lithological repetitions (man for scale) at a marginal site shown in vertical cut, normal to transport direction of the dense banding seen in F. The repetition is similar to that seen in the analogue models. The white (basal) layers are Reconstituted ignimbrite facies (RIF; dominantly fi ne materials from the remobilized ignim- britic substratum), and the gray (upper) is the Socompa Breccia (SB; blocks + matrix) (van Wyk de Vries et al., 2001). 658 Geosphere, August 2008 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/4/657/3336451/i1553-040X-4-4-657.pdf by guest on 30 September 2021 Structural analysis and analogue modeling of rockslide avalanches D RIF 1 km N scar s s T E raft N ms 1 km F RIF N 1 km Key normal fault thrust fault strike-slip fault DAD unit boundary (in E) thrust bands (in F) Figure 1 (continued). Three detailed structural maps of the Socompa deposit, made from aerial photographs after fi eld inspection (van Wyk de Vries et al., 2001, 2002). Structures are shown at a greater resolution. Note that there are numerous crosscutting relationships between faults that indicate multiple generations of fault structures. Some crosscutting relationships have displacements too small to show on the map, others have displacements of several kilometers. (D) Proximal zone including the Toreva block margin (T) and edge of collapse scar (scar). Here Toreva blocks have 10–100-m-spaced large normal faults that become denser in the direction of transport. (The Torevas transform over a short zone of ~500 m to a fully brecciated rockslide avalanche.) Normal faults (illustrated in B) are displaced and deformed by later strike-slip faults and broad zones of shear that at small scale, are made up of many smaller discrete fault planes. S indicates small salar deposits on the deposit surface. (E) Central zone of deposit cut by the major Median Scarp (ms) thrust belt (Guil- baud, 2000; Kelfoun et al., 2008). Note that the proximal structures to the southeast are cut and the deposit banding is defl ected across the median scarp. Note also that islands or rafts of normally faulted blocks are preserved in the predominantly strike-slip deformed zone to the northwest of the median scarp (raft). (F) Distal area with fi ne-scale banding, cut by thrusts and strike-slip faults. Note that the outer margin of the deposit is composed of the RIF basal facies.