Identification of parameters controlling the accretive and tectonically erosive mass transfer mode at the south-central and north Chilean forearc using scaled 2D sandbox experiments [Elektronische Ressource] / Geoforschungszentrum Potsdam. Jo Lohrmann

Informations

• Publié par: freie_universitat_berlin
• Publié le: 01 janvier 2002
• Nombre de lectures: 49
• Langue: English
• Poids de l'ouvrage: 40 Mo

Extrait

ISSN 1610-0956 Identification of Parameters Controlling the Accretive and
Tectonically Erosive Mass-Transfer Mode at the South-Central
and North Chilean Forearc Using Scaled 2D Sandbox
Experiments
Jo Lohrmann
Dissertation zur Erlangung des Doktorgrades
im Fachbereich Geowissenschaften an der Freien Universitat¨ Berlin
begutachtet durch
Prof. Dr. Onno Oncken
Prof. Dr. Hans-Jurgen Gotze¨ ¨
2002Der Versuchung widerstehen durch Vermehrung ihrer Varianten.
Resist temptation by multiplying its varieties.
Dill Bitterfit, 19922
Abstract 5
1 Introduction and Theoretical Background 7
1.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Mass-transfer modes at convergent margins . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Mechanical Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4 Analogue simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.5 Scaled 2D sandbox experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 Method and Basic Studies of Material Properties 21
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2 Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3 Material properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3.1 Material characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.4 Sandbox experiments: Basic parameter studies . . . . . . . . . . . . . . . . . . . . . . . . 30
2.4.1 The sandbox apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.4.2 Experimental setup of the basic parameter studies . . . . . . . . . . . . . . . . . . 32
st2.4.3 1 Series: Variation of internal wedge properties . . . . . . . . . . . . . . . . . . . 33
nd2.4.4 2 Series: Variation of basal material properties . . . . . . . . . . . . . . . . . . . 49
2.5 Application to nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3 The Accretive Forearc of Southern Chile 57
3.1 The South Chilean Forearc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.1.1 Database obtained from nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.1.2 Mass-transfer concept derived from field data . . . . . . . . . . . . . . . . . . . . . 61
3.2 Experimental setup of the two-level experiments . . . . . . . . . . . . . . . . . . . . . . . 61
3.3 Experimental results of the two-level experiments . . . . . . . . . . . . . . . . . . . . . . . 63
3.4 Interpretation of the results of the two-level experiments . . . . . . . . . . . . . . . . . . . 73
3.4.1 Changes in structural and geometrical wedge evolution . . . . . . . . . . . . . . . . 73
3.4.2 State of stress of the wedge segments . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.5 Comparison with the South Chilean Forearc . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.5.1 Mass balance of basally- accreted sediments at the South Chilean Forearc . . . . . 79
3.5.2 Kinematics of experiments and the South Chilean Forearc . . . . . . . . . . . . . . 81
3.5.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4 The Tectonically-Erosive Forearc of Northern Chile 87
4.1 The North Chilean Forearc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.1.1 Database obtained from nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.1.2 Previously-published concepts of tectonic erosion . . . . . . . . . . . . . . . . . . . 91
4.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.2.1 Experimental concept to simulate steady-state tectonic erosion . . . . . . . . . . . 94
st4.2.2 Setup of the 1 series (mass-transfer processes) . . . . . . . . . . . . . . . . . . . . 95
nd4.2.3 Setup of the 2 series (mechanics) . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.3 Results of experiments simulating steady-state tectonic erosion . . . . . . . . . . . . . . . 97
st4.3.1 Experimental results of the 1 series (mass transfer) . . . . . . . . . . . . . . . . . 97
st4.3.2 Experimental results of the 2 series (mechanics) . . . . . . . . . . . . . . . . . . . 110
st nd4.3.3 Summary of experimental results of 1 and 2 series . . . . . . . . . . . . . . . . 112
4.4 Interpretation of experimental results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.4.1 Mass transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.4.2 Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.4.3 Summary of the basally erosive process . . . . . . . . . . . . . . . . . . . . . . . . 1183
4.5 Comparison with the North Chilean Forearc . . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.6 Discussion of published concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
4.7ion of inconsistencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
4.7.1 Geometric and kinematic inconsistencies? . . . . . . . . . . . . . . . . . . . . . . . 121
4.7.2 Inconsistencies of the erosion ratios (basal erosion versus frontal erosion) . . . . . . 123
4.7.3 Mechanical inconsistencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
4.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5 Final Discussion 127
5.1 Hierarchical order of parameters controlling the mass transfer at convergent forearcs . . . 127
5.2 Restrictions of quantitative critical-taper analysis of convergent wedges in nature . . . . . 134
Appendices 143
A Experiments directly used in this study 143
B Experiments indirectly used in this study 189
C Zusammenfassung 229
D Lebenslauf 2335
Abstract
Thisstudyattemptstoidentifyandquantifytheparametersthatcontrolmass-transfermodesinbrittle
tectonically erosive and accretive forearc settings. Scaled analogue simulations, which are specifically
designed for this task, are compared with the convergent Chilean Margin that demonstrates both of
these mass-transfer modes. Analogue simulation of geodynamic processes requires granular materials
(e.g. sand) that deforms similarly to typical crustal rocks. Accordingly, a parameter study is performed,
which yields general insight in the basic mechanics of highly-idealised convergent sand wedges.
Static and dynamic shear tests are employed to obtain the frictional strength of different sand types.
Theanalysedsandtypesarecharacterisedbyanelasticfrictional-plasticbehaviourwithatransientstrain-
hardening and strain-softening phase prior to the transition to stable sliding. This complex material
behaviour is comparable to that of natural rocks. However, it is in conflict with the assumption of
an ideal cohesionless Coulomb Material with constant frictional properties, which is commonly used in
mechanical interpretations of convergent forearcs, fold-and-thrust belts, and orogens. The influence of
these transient material properties on the kinematics, growth mechanisms, and internal deformation of
convergent sand wedges results in wedge segments with different characteristics, i.e. frontal-deformation
zone at the wedge tip, frontal-imbrication in the centre and internal-accumulation zone at the rear of
the wedge. This wedge segmentation varies — depending on material compaction — from well defined
segments with straight slopes to wedges with continuous convex topographic profiles. A new strategy
of critical-taper analysis is developed, which is restricted to individual wedge segments and considers
this complex material behaviour. The analysis shows that for most materials, only one wedge segment
(frontal-imbrication zone) is critically-tapered during material addition to the front of the wedge. Taper
and bulk strength of the wedge segments are controlled by the frictional strength of active faults. Wedge
segmentation is caused by a bulk wedge-strength increase toward the rear of the wedge. This is due to
rotation of faults into mechanically less-favourable orientations and plastic material hardening.
On this basis, the two mass-transfer modes of the North and South Chilean Forearc are investigated,
steady-state tectonic erosion and coeval frontal and basal accretion, respectively. Several scenarios of
these mass-transfer modes are simulated by systematic variation of parameters. In these experimental
series, the parameters varied are: amount of material supplied, presence of mechanically weak layers as
potential detachments, frictional strength and surface roughness of the subduction interface, as well as
transportcapacityofthe‘subductionchannel’. Thelatterisdeterminedbytheinl