3D dynamics of crustal deformation along the western Andean margin Investigations on strain ...

139 Pages·2017·2.83 MB·English

by  Jorina Schütt

3D dynamics of crustal deformation
along the western Andean margin
Investigations on strain partitioning above
an obliquely convergent oceanic
subduction zone using a 3D numerical
geodynamical model

JORINA MARLENA SCHÜTT

ACADEMIC DISSERTATION
To be presented, with the permission of the Faculty of Science of the University of Helsinki,
for public examination in Auditorium A111, Exactum (Gustaf Hällströmin katu 2b, Helsinki)
on 23 February 2018, at 12 o’clock noon.

Helsinki 2018

DEPARTMENT OF GEOSCIENCES AND GEOGRAPHY A63 / HELSINKI 2018
Author: Jorina Marlena Schütt
Department of Geoscience and Geography
P.O. Box 68
00014 University of Helsinki
Finland

Supervisor: Associate Professor David Whipp
Department of Geoscience and Geography
University of Helsinki

Reviewers: Professor Ulrich Riller
Department of Earth Sciences
University of Hamburg

Professor Hemin Koyi
Department of Earth Sciences
Uppsala University

Opponent: Professor Claire Currie
Department of Physics
University of Alberta

ISSN 1798-7911
ISBN 978-951-51-3978-8 (pbk)
ISBN 978-951-51-3979-5 (PDF)

Unigrafia
Helsinki 2018
Schütt J., 2018. 3D dynamics of crustal deformation along the western Andean margin.
Unigrafia. Helsinki. 139 pages, 69 figures and 3 tables.

Abstract

Subduction zones play an important role in the dynamics of the Earth. At plate margins,
where one tectonic plate subsides underneath a second one, mountain ranges are built,
earthquakes experienced and volcanic activity observed. The subduction zone of South
America, where the Nazca Plate slides underneath the South American Plate, extends for
roughly 7000 km along the plate margin of the continent. Ongoing since at least the
Cretaceous this subduction zone is a laboratory for tectonic systems. This study focuses on
how the geometry and relative motion of the two convergent plates affects dynamics in the
overriding plate utilizing 3D numerical mechanical and thermo-mechanical models.
Along the South American subduction zone the obliquity angle, subduction dip angle and
strength of the continental crust vary. All these factors influence the dynamics and
deformation of the continental crust and affect – especially in oblique subduction systems –
how oblique convergence is partitioned onto various fault systems in the overriding plate.
The partitioning of strain into margin-normal slip on the plate-bounding fault and horizontal
shearing on a strike-slip system parallel to the margin is mainly controlled by the margin-
parallel shear forces acting on the plate interface and the strength of the continental crust.
While the plate interface forces are influenced by the dip angle of the subducting plate (i.e.,
the length of plate interface in the frictional domain) and the obliquity angle between the
normal to the plate margin and the plate convergence vector, the strength of the continental
crust is strongly affected by the presence or absence of weak zones such as regions of arc
volcanism, pre-existing fault systems, or boundaries of stronger crustal blocks. In order to
investigate which of these factors are most important in controlling how the overriding
continental plate deforms, this study compares results of lithospheric-scale 3D numerical
geodynamic experiments from two regions in the north-central Andes: the Northern Volcanic
Zone (NVZ; 5°N - 3°S) and adjacent Peruvian Flat Slab Segment (PFSS; 3°S -14°S).
The NVZ is characterized by a ~35° subduction dip angle, an obliquity angle of about 40°,
extensive volcanism and significant strain partitioning in the continental crust. In contrast,
the PFSS is characterized by flat subduction (the slab flattens beneath the continent at around
100 km depth for several hundred kilometers), an obliquity angle of about 20°, no volcanism
and minimal strain partitioning. The geometry and strength characteristics of these regions
are incorporated in 3D (1600 x 1600 x 160 km) numerical experiments of oceanic subduction
beneath a continent, focusing on the conditions under which strain partitioning occurs in the
continental plate. In addition to different slab geometries and obliquity angles, the effect of a
continental crust of uniform strength (friction angle ) versus one including a weak
zone in the continental crust ( ) that runs parallel t°o the margin is compared in
(cid:596)=15
°
(cid:596)=2
iii
systematic purely mechanical experiments. Finally, reference models representing the NVZ
and PFSS are revisited with fully coupled thermo-mechanical experiments.
Results of the experiments show that the obliquity angle has, as expected, the largest effect
on initiation of strain partitioning, but development of sliver movement parallel to the margin
is clearly enhanced by the presence of a continental weakness. While velocities in the
continental sliver in the NVZ reference model are similar to the values observed in nature,
the model without a continental weakness develops slivers with velocities half as fast. In
addition, a shallower subduction angle results in formation of a wider continental sliver.
Based upon the results, the lack of strain partitioning observed in the PFSS results from both
a low convergence obliquity and lack of a weak zone in the continent, even though the
shallow subduction should make strain partitioning more favorable. Purely mechanical
models are despite their simplicity a good suited tool for investigations of subduction zone
dynamics and in very good agreement with observations in nature in the particular study
areas of this work as well as global subduction zones. The sliver movement observed in the
thermo-mechanical models agrees well with purely mechanical reference experiments
representing the NVZ and the PFSS. While this is the case, the thermo-mechanical
experiments do expose the boundary between successful (NVZ) and partly failing (PFSS)
incorporation of temperature dependence into mechanical models. With this they provide a
good starting point for further refinement and development of temperature dependent
experiments.

iv
Acknowledgments

I would like to thank my supervisor David Whipp for introducing me to the world of
subduction zone modeling and for his support and encouragement during the project. Also, I
would like to thank Lars Kaislaniemi for his help and advice, Emilia Koivisto and Ilmo
Kukkonen as well as Seija Kultti and Tapani Rämö for their feedback, ideas and support.
Thank you also to Annakaisa Korja and the people of the Institute of Seismology for
accommodating me.

I wish to thank my parents and friends for being there – near and far.

This project was financially supported by a Three-Year Research Grant from the University
of Helsinki.

Helsinki, February 2018

You are braver than you believe,
stronger than you seem,
and smarter than you think.
Winnie the Pooh
A. A. Milne
v
Contents

Abstract iii
Acknowledgements v
List of symbols and abbreviations viii

1 Introduction and motivation 1

2 Tectonic and geological setting in the northern and northern central
Andes (5°N and 14°S) 8

2.1 The South American Plate 8

2.2 The Northern Volcanic Zone (NVZ, 5°N - 3°S) 8

2.3 The Peruvian Flat Slab Segment (PFSS, 3°S - 14°S) 14

3 Theory and numerical modelling software 18

3.1 Governing geodynamic equations 18
3.1.1 Continuity equation (conservation of mass) 19
3.1.2 Stokes flow (conservation of momentum) 21
3.1.3 Heat transfer (conservation of energy) 22
3.1.4 Nonlinear viscosity 23
3.1.5 Frictional plasticity 24

3.2 Numerical modelling software 27
3.2.1 General overview of the software 27
3.2.2 Implementation of geodynamic equations 31

3.3 Strain partitioning at obliquely convergent margins 32

Case Study I - Mechanical modelling of oblique subduction in the Northern
Volcanic Zone (5°N- 3°S) 39

1.1 Model design 39
1.2 Results and discussion 44
1.2.1 The NVZ – Model 1 (reference model) 45
1.2.2 Effect of a weak zone in the continental crust – Model 2 51
1 2 3 Effect of changes in the subduction angle 55
1.2.4 Effect of changes in the obliquity angle 58

Case Study II – Oblique subduction and continental deformation in the
Peruvian Flat Slab Segment of the Andes (3°S - 14°S) 67

2.1 Model design 67
vi
2.2 Results and discussion 72
2.2.1 The PFSS – Model 1 (reference model) 73
2.2.2 Effect of changes in the obliquity angle 77

Case Study III – Effects of temperature-dependent rock properties on the
oblique subduction and continental deformation, Northern Volcanic Zone (5°N-
3°S) and Peruvian Flat Slab Segment (3°S - 14°S) of the Andes 81

3.1 Model design 81
3.2 Results and discussion 89
3.2.1 The NVZ – Model 1 89
3.2.2 The PFSS – Model 2 97

Discussion 106

Conclusions 119

References 121

vii
List of symbols and abbreviations

symbol or unit description
abbreviation

° subduction dip angle
A proportionality coefficient; arbitrary point
(cid:2009) area of basal thrust in a subduction zone
(cid:2870) area of vertical rear shear zone of a subduction zone
(cid:1827)A(cid:3029)LE (cid:1865) Arbitrary Lagrangian-Eulerian
(cid:2870)
(cid:1827)c (cid:3045) (cid:1865)Pa cohesion
CCPP Chingual-Cosanga-Pallatunga-Puná (Shear Zone)
J/K heat capacity
CFL the Courant–Friedrichs–Lewy (condition)
(cid:1829)(cid:3043) Pa· s viscosity
Eulerian time derivate

(cid:2015)
(cid:3105) Lagrangian time derivate
(cid:3105)(cid:3047)
d(cid:3005)iv divergence; defined as a scalar function over a three
(cid:3005)(cid:3047) (cid:3105)(cid:3007)(cid:3299) (cid:3105)(cid:3007)(cid:3300) (cid:3105)(cid:3007)(cid:3301) dimensional vector field F
DGM (cid:3105)(cid:3051) + (cid:3105)(cid:3052) +(cid:3105)(cid:3053) Dolores Guayaquil Mega (Shear Zone)
strain
1/s strain rate
(cid:2035)FEM Finite Element Method
(cid:2035)(cid:4662) yield function for the Mohr-Coulomb yield criterion
Function F of x
(cid:1832)(cid:3014) (cid:3004) kg/ms2 inter plate frictional force
(cid:1832)( (cid:1876) ) kg/ms2 margin parallel force component
(cid:2028)(cid:3033) kg/ms2 resisting force
(cid:1832)g(cid:3043) m/s2 gravitational acceleration (g = 9.81 m/s2)
(cid:1832)g(cid:3019)rad gradient; defined as a differential operator over a three

dimensional scalar field
(cid:3105)(cid:3020) (cid:3105)(cid:3020) (cid:3105)(cid:3020)
H (cid:4672)J (cid:3105)(cid:3051),(cid:3105)(cid:3052),(cid:3105)(cid:3053)(cid:4673) volumetric heat production
i natural number; index nu(cid:1845)mber
ICF incompressible fluid
j natural number; index number
W/ (m·K) thermal conductivity
Pa·s penalty factor
(cid:1863) ° obliquity angle
(cid:2019)M kg mass
(cid:2011)MA material acceleration
moment magnitude (earthquake scale)
N natural number
(cid:1839) (cid:3024) set of all natural numbers
n natural number; power law exponent
(cid:1331)NAB North Andean Block
NAS North Andean Sliver
NVZ Northern Volcanic Zone
viii
set of normal vectors in
P kg · m/s momentum (cid:2871)
(p(cid:1866) (cid:2869),(cid:1866)(cid:2870),(cid:1866)(cid:2871)) Pa pressure (cid:1337)
friction angle
i component of shape function
(cid:2030)PFSS Peruvian Flat Slab Segment
(cid:560)Q (cid:3036) kJ/mol activation energy
W/m2 heat flux
m2kg/s2K the Boltzmann gas constant (R = 1.3·10-23 m2kg/s2K)
(cid:1869) real coordinate space in three dimensions
(cid:1844)(cid:2871) kg/ms2 slab bending resistance force
(cid:1337) kg/m3 density
(cid:1844)(cid:3003) stress tensor
(cid:2025) Pa differential stress
(cid:2026)(cid:3036)(cid:3037) Pa normal stress
(cid:2026)(cid:3031) Pa mean stress
(cid:2026)(cid:3041) Pa yield stress
(cid:2026)(cid:3040) Pa maximum principle stresses
(cid:2026)(cid:3052) Pa minimum principle stresses
(cid:2026)t (cid:2869) s time
(cid:2026)T(cid:2871) °C temperature
Pa shear strength
Pa subduction shear stress
(cid:594) Pa maximum shear strength
(cid:2028)(cid:3033) Pa margin – normal component of subduction shear stress
(cid:594)(cid:2923) Pa margin – parallel component of subduction shear stress
(cid:2028)(cid:3041) Pa margin – vertical component of subduction shear stress
(cid:2028)U(cid:3043) J energy
(cid:2028)(cid:3049) velocity vector
m/s i-th component of velocity field
(cid:1874)(cid:1318) cm/a advancing subduction velocity
(cid:1874)(cid:3036) cm/a total relative convergence velocity
(cid:1874)(cid:3028) (cid:3031) (cid:3049) cm/a subduction velocity
(cid:1874)(cid:3030)(cid:3042)(cid:3041)(cid:3049) cm/a margin parallel velocity component of a fully partitioned
(cid:1874)(cid:3046) system
(cid:1874)(cid:3043)(cid:2869)(cid:2868)(cid:2868) cm/a northward pointing velocity component of a fully
partitioned system
(cid:1874)x (cid:3041)(cid:2869)(cid:2868)(cid:2868) m x-coordinate in a Cartesian coordinate system
coordinates in a Cartesian coordinate system
set of body forces on a three dimensional body in
((x(cid:1876),(cid:2869)y,)(cid:1876) (cid:2870),(cid:1876)(cid:2871)) pair of coordinates in (cid:2871)
((x(cid:1850),(cid:2869)y,,z(cid:1850))(cid:2870) ,(cid:1850)(cid:2871)) set of coordinates on (cid:2870) (cid:1337)
y m y-coordinate in a Carte(cid:1337)sian coordinate system
(cid:2871)
z m z-coordinate in a Cart(cid:1337)esian coordinate system
m depth range of faults

(cid:1878)(cid:3033)

ix
x