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

ü    ü   Regional deformation and metamorphism

 

ü    ü   Magmatism

 

ü    ü   Uplift and erosion

 

·        ·      Orogenic belts are regions where major mountain-

     building activities have taken (or are taking) place

ü    ü   Characteristic of convergent plate boundaries

 

ü    ü   Compressional tectonics major driving force

 

ü    ü   Accretionary tectonics prevalent

 

·       ·      Earth's Recent Orogenic Belts (see Figure )      

 

                2. Regional Deformational activities and events

·        ·      Compressional tectonics

ü    ü   Associated mainly with convergent boundaries -

      especially cont-cont collision zones

 

ü    ü   Folding and thrust faulting common

 

ü    ü   Mountain building accompanied by major shortening,

      thickening, and uplift of crust

 

ü    ü   Examples: Himalayas, Andes & Appalachians

 

·        ·      Tensional tectonics

ü         ü      Associated mainly with divergent boundaries like

       ocean spreading ridges & continental rifts

 

ü    ü   Horst and graben normal faulting common

 

ü    ü   Mountain building accompanied by major extension,

       thinning, and uplift of crust

 

ü    ü   Best examples are Mid-Atlantic Ridge and the

East Africa Rift Valley

 

ü    ü   Regional example is the Sierra Nevada and Basin

      and Range Province

 

·        ·      Accretionary tectonics

ü    ü   Associated with convergent and transform plate

boundaries at continental margins

 

ü    ü   The accretion of an exotic or suspect terrane onto

     the edge of a continent is often the primary trigger

                                    for many orogenic episodes 

 

ü    ü   A suture zone marks the boundary where a terrane

       is accreted to the edge of a continental plate

 

·        ·      Regional metamorphism

ü    ü   Accompanies folding and faulting activities

 

ü    ü   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

ü    ü   Sustained regional compressional forces

 

ü    ü   Sustained upward-directed forces

 

ü    ü   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

ü    ü   Juvenile volcanic-plutonic arc complex made up

       mainly of new andesitic crust

 

ü    ü   Uplifted subduction accretionary prism

 

ü    ü   Little to no compressional deformation

 

·        ·      Illustration of this orogenic setting (see Fig. )

 

        2. Oceanic-Continental Plate Boundary

·        ·      Pair set of parallel orogenic belts

ü    ü   Mature volcanic-plutonic arc complex made up of

      mainly new/reworked granitic crust

 

ü    ü   Uplifted, subduction accretionary prism that may

        include blocks of ophiolite

 

ü    ü   Accreted terranes may be sutured into the paired

       volcanic arc-trench deposit belts

 

ü    ü   Moderate compressional deformation

 

·       ·      Illustration of this orogenic setting (see Fig. )

 

 

        3. Continental-Continental Plate Boundaries

·        ·      Broad, massive, and complex orogenic belt

ü    ü   Number of Fold and Thrust mountain systems

 

ü    ü   Massive uplift of mountain systems and formation

     of deep crustal root

 

ü    ü   Numerous accreted terranes may be sutured into

     the collision zone

 

ü    ü   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)

 

ü    ü   Carried via the ocean crust conveyor belt to

a continental margin subduction zone

 

ü    ü   Carried laterally along a continental margin

                   via regional-scale transform faulting

 

·        ·     Terranes can have great variation in age and geology

 

ü    ü   Old slivers of exotic continental crust

 

ü    ü   Island arcs from near and far

 

ü    ü   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:

 

ü    ü   Partial melting of mantle peridotite at spreading

      centers generates new oceanic basalt

 

ü    ü   Partial melting of oceanic basalt at island arc

        subduction zones = new transitional andesite

 

ü    ü   Accretion of island arc terranes to form larger early

     continental masses (cratons), which increase in size

                                and crustal thickness

 

ü    ü   Partial melting of andesitic crust at base of edges of

    continental masses over subduction zones generates

     new granitic continental crust

 

ü    ü   Rifting apart and reshuffling of older continental

     crust changes the size and configuration of

     continental masses and subjects continental

     edges to basaltic magmatism

 

ü    ü   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