Geology Lecture Outline

Convergent Plate Tectonism and Continental Evolution (Ch 14)

 

I. Lecture Content

Introduction - Makings of High Places

Origin of Mountains - The Nature of Orogeny

Tectonic Terranes - Orphaned crust

Continental Accretion - Crustal-scale Squeegee Process

Evolution of Continents - Billions of Years of Accretion and

Recycling

 

II. Introduction

A. Deformation and Mountain Building

1. Spectacular crustal upheavals are the results of:

            Same forces that power plate tectonics

            Interplay between crustal rocks and the global

and regional-scale deviatoric stresses created

by inter-plate motions

2. Deformation events are typically part of much larger

regional mountain building events called orogenies.

 

III. Origin of Mountains

A. Mountain Structures

1. Mountains defined:

              Areas of land that stand significantly taller than the

surrounding landmass

 

2 Several Types of Mountains

              Fold and Thrust belts

              Volcanic centers and chains

              Metamorphic-Plutonic Core complexes

              Horst and Graben Fault Block systems

 

B. Processes that Build Mountains

1. Orogeny --- a mountain-building episode

              Includes a variety of mountain-building activities

and events

       Regional deformation and metamorphism

 

       Magmatism

 

       Uplift and erosion

 

              Orogenic belts are regions where major mountain-

building activities have taken (or are taking) place

       Characteristic of convergent plate boundaries

 

       Compressional tectonics major driving force

 

       Accretionary tectonics prevalent

 

             Earth's Recent Orogenic Belts (see Figure )

 

2. Regional Deformational activities and events

              Compressional tectonics

       Associated mainly with convergent boundaries -

especially cont-cont collision zones

 

       Folding and thrust faulting common

 

       Mountain building accompanied by major shortening,

thickening, and uplift of crust

 

       Examples: Himalayas, Andes & Appalachians

 

              Tensional tectonics

               Associated mainly with divergent boundaries like

ocean spreading ridges & continental rifts

 

       Horst and graben normal faulting common

 

       Mountain building accompanied by major extension,

thinning, and uplift of crust

 

       Best examples are Mid-Atlantic Ridge and the

East Africa Rift Valley

 

       Regional example is the Sierra Nevada and Basin

and Range Province

 

              Accretionary tectonics

       Associated with convergent and transform plate

boundaries at continental margins

 

       The accretion of an exotic or suspect terrane onto

the edge of a continent is often the primary trigger

for many orogenic episodes

 

       A suture zone marks the boundary where a terrane

is accreted to the edge of a continental plate

 

              Regional metamorphism

       Accompanies folding and faulting activities

 

       Regional metamorphism is more prevalent in

compressional tectonic setting like continental

collision and subduction zones

 

3. Magmatism

              Building of mountains by piling up of solidified lava

forming volcanic mountains and mountain chains

 

              Formation of massive underground batholiths builds up

the crust from the inside out

 

4. Regional Uplift

              Instrumental in creating high places

 

              Causes of Regional uplift

       Sustained regional compressional forces

 

       Sustained upward-directed forces

 

       Isostatic equilibrium (rebound)

 

5. Erosion

              Instrumental in causing greater topographic relief

 

              Helps sustain buoyant mountain root uplift

C. Nature of Orogeny at Convergent Plate Boundaries

1. Oceanic-Oceanic Plate Boundary

              Paired set of parallel orogenic belts

       Juvenile volcanic-plutonic arc complex made up

mainly of new andesitic crust

 

       Uplifted subduction accretionary prism

 

       Little to no compressional deformation

 

              Illustration of this orogenic setting (see Fig. )

 

2. Oceanic-Continental Plate Boundary

              Pair set of parallel orogenic belts

       Mature volcanic-plutonic arc complex made up of

mainly new/reworked granitic crust

 

       Uplifted, subduction accretionary prism that may

include blocks of ophiolite

 

       Accreted terranes may be sutured into the paired

volcanic arc-trench deposit belts

 

       Moderate compressional deformation

 

             Illustration of this orogenic setting (see Fig. )

 

 

3. Continental-Continental Plate Boundaries

              Broad, massive, and complex orogenic belt

       Number of Fold and Thrust mountain systems

 

       Massive uplift of mountain systems and formation

of deep crustal root

 

       Numerous accreted terranes may be sutured into

the collision zone

 

       Strong compressional deformation

 

              Illustration of this orogenic setting (see Fig. )

 

IV. Terranes and Continental Accretion

A. Suspect or Exotic Terranes - Orphaned Lithosphere

 

1. Orogenic mountain systems typically include elongate

blocks of exotic lithosphere

 

2. Terranes have geologic origins far from their present

geographic location

 

              Terranes are "rafted" to a continental margin from

potentially great distances (pole to pole)

 

       Carried via the ocean crust conveyor belt to

a continental margin subduction zone

 

       Carried laterally along a continental margin

via regional-scale transform faulting

 

             Terranes can have great variation in age and geology

 

       Old slivers of exotic continental crust

 

       Island arcs from near and far

 

       Obducted oceanic crust

 

3. The terrane accretion process is an important part of the

orogenic cycle of continental mountain-building

 

              Substantial means of continental growth and evolution

 

              Western margin of North America has experienced major

continental growth in the last 200 million years due in large

part to continental accretion events

 

              Numerous terranes have been recognized from Mexico all

the way up to Alaska

              Illustration of accreted terranes of western North America

(see Fig. )

 

V. Evolution of Continents

A. Plate Tectonics - Cause of Continental Evolution

1. Oceanic seafloor spreading and subduction processes

have caused the growth and evolution of the continents

 

             Probably started some 4 billion years ago

 

             Gradual increase of granitic crust through a multi-

step tectonic process:

 

       Partial melting of mantle peridotite at spreading

centers generates new oceanic basalt

 

       Partial melting of oceanic basalt at island arc

subduction zones = new transitional andesite

 

       Accretion of island arc terranes to form larger early

continental masses (cratons), which increase in size

and crustal thickness

 

       Partial melting of andesitic crust at base of edges of

continental masses over subduction zones generates

new granitic continental crust

 

       Rifting apart and reshuffling of older continental

crust changes the size and configuration of

continental masses and subjects continental

edges to basaltic magmatism

 

       Mixing and assimilation of older continental crust

with fresh mantle-derived basaltic and andesitic

magmas creates "new" recycled continental crust

2. The cyclic, non-ending repetition of the above tectonic

processes over four billion years have created the

complex granitic continents of today.

 

3. Illustration of Continental Evolution (Figs. )

 

VI. Vocabulary Continental Mountain Building - CH14