GEOL 451-2010 Geology of North America

GEOL 451-2010Geology of North America

Review of some Lithotectonic PrinciplesUpdated January 2011

University of ReginaGEOL 451-2011

R. Macdonald, Instructor

University of ReginaGEOL 451-2011

R. Macdonald, Instructor

Uniqueness and interactive nature of the Earth system

Basic Earth Structure Lithotectonic entities

Coverage in this presentation

Largely from Condie p.14 onwards

An Approach to Earth Processes

1. PETROCENTRIC Processes concerning only rocks of the earth’s crust and

mantle, e.g. sedimentation, metamorphism, even diagenesis

But rocks react with the biosphere, oceans and atmosphere Climate factor, asteroids, flood, tsunamis, etc Earth physiology - Jim Lovelock’s GAIA hypothesis The earth system maintains itself through positive

feedbacks

2. TIME-CENTRIC Geologists tend to think in very long periods of time But some earth processes can occur very rapidly A return to CATASTROPHISM?

Need to consider the entire Earth system: earth-ocean-atmosphere Earth physiology: James Lovelock’s Gaia Hypothesis Feedback loops (+) and (-) Recycling lithosphere Knowledge explosion of the past 15 or so years Tuzo Wilson (1968):

Data collecting Hypotheses (transient) New unifying theories

Uniqueness of the Earth and interaction of the Earth Elements

Thermal historyThermal history

The whole earth system

Life history

Climate

Tectonism and tectonic history

Crustal evolution

Earth’s core-mantle

Magnetic fields

ET impacts

Life

Crust

Oceans

Atmosphere

Magmatism

MetamorphismMetamorphism

Earth’s axial tiltEarth’s axial tilt

Mantle hotspots

Earth cooling

Solar radiation

Weathering

Crustal recycling

1

1. Rigid lithosphere rests on weaker asthenosphere

2. Lithosphere is fragmented into segments and plates in relative motion which continually change shape and size

Fundamental Earth Structure

What are some of the major lithotectonic features of the Earth?

Intraplate - Continental

Cratons: Shields and Platforms

Precambrian ShieldsRelatively stable older cratons, generally Precambrian and without a cover of Phanerozoic rocks.

Continental platforms Relatively stable older cratons overlain by oval shaped Phanerozoic sedimentary, shallow water, ssts, lsts, shales, deltaic and fluvial, commonly not much more than 1000 m thick


Buried Precambrian Shields, cored with older cratons

aka PlatformsRelatively stable older cratons, generally Precambrian but with a cover of Phanerozoic rocks.

(Intra)cratonic basins, aka ENSIALIC basins (2)


Deep, sometimes formed over failed rifts Other causes (see Kent) Epicontinental seas, some evaporites (e.g. Prairie

Evaporite) Examples:Williston, Hudson Bay and Michigan basins,

Amadeus and Carpentaria basins of Australia, Paris Basin, Parana Basin, Chad basin.

Sedimentary and volcanic loading produces crustal densification on cratons and continental platforms. Interior sag basins

Diverse origins, extension, thermal effects, higher density of underlying crust

Typically have the longest timeframe


(Intra)cratonic basins, aka ENSIALIC basins


Inland-sea basinsMajor I style, typically dormantOverlie continental crust, connected intermittently to open seas, or cut off with extensive saline de[positse.g. Black Sea Caspian Sea Gulf of Mexico

Regional Crustal Subsidence due to local sediment loading

Example: Gulf of Mexico and Mississippi River

Sediments delivered by major river systems eventually deposit a non-negligible load on the crust, resulting in some subsidence. This provides accommodation (space) for further sediment loading. (positive feedback).

NOTE: Some reinforcement by petroleum extraction

Basin Formation

Due to sags produced in the crust by diverse mechanisms: Magma depletion Isostatic compensation:

melting of ice caps Deep crustal/mantle

underthrusting Magma accession:

emplacement of higher temperature melts in the crust

Basement block movements by a variety of causes

Load deepening etc.

Some basin subsidence mechanisms

Intraplate - Continental Continental Rifts

Largely recognized today as formed over Mantle hotspots/plumes May be a sign of incipient plate movements, marking the beginning of

continental break-up Why do continents break-up?

Robert Macdonald:Why do Continents Break Up?The Earth's interior is hot. The heat comes from the heat of formation of the Earth that has not yet dissipated and heat generated by decay of unstable isotopes distributed through the mantle and crust. While the lithosphere cools primarily by conduction, the mantle cools by convection. Most of the convective heat from the mantle is dissipated at the midocean ridges and through cooling seafloor. Beneath large continents, however, heat builds up in the mantle. This excess heat should weaken the continental lithosphere making it easier to rift.But what forces could cause a continent to come apart?membrane stresses? The Earth's curvature is greater at the equator than at the poles and continents drifting across latitudes can experience tensional stresses.trench suction? Trench rollback at a subduction zone can generate tensional stresses within a continent.hotspots? A number of hotspots initiated along the line of breakup of Pangea. It has been proposed that they may have caused rifting or at least determined where the breakup occurred





Earth’s interior contains formational and isotope-generated heat Lithospheric crust cools by conduction, but the Mantle cools by convection dissipated at MORS and ocean floors Beneath large continents heat builds up in the Mantle, weakening

the Crust Relatively higher membrane stress in equatorial regions due to

higher amount of earth curvature Trench rollback at subduction zones Hotspots/plumes (randomly formed)

Some causes of continental rifts

The East African rift system showing the Afar Triangle as a triple-junction at the intersection of the Red Sea, Aden and East African rifts. Possibly the expression of a mantle plume. Diverging rifts starts a new round of continental drifting and ultimately “creates” new ocean floor. Dots indicate young volcanoes.


The East African rift system showing the Afar Triangle as a triple-junction at the intersection of the Red Sea, Aden and East African rifts. Possibly the expression of a mantle plume. Diverging rifts starts a new round of continental drifting and ultimately “creates” new ocean floor. Dots indicate young volcanoes.


But not so simple

1. Initial doming and normal faulting.

2. As lower crust & lithosphere thins by ductile

shear, heat flow increases and normal

faulting occurs in the brittle upper crust.

3. Increased heat flow produces bimodal

(basaltic and rhyolitic) volcanism

4. Subsiding rift basins collect infill sediments .

5. If rifting continues the crust/lithosphere thins

to zero and seafloor spreading is initiated

Sediments on continental passive margins

drape drape over normal faulted basement

6. After the initial thinning, margins continue

to subside for tens of millions of years by

continued cooling and loading subsidence


RRR Triple Junctions and AulocogensIf rifting stops before complete continental breakup, the failed rift or aulocogen infills with sediments and be buried in the subsurface, perhaps to be re-exposed by some later episode of erosion or be discovered by seismic exploration.

Aulocogens are commonly associated with continental breakup. Continental rifts seem to start as a number of rift-rift-rift triple junctions. Two of the rift arms become a new ocean basin and the third becomes a failed rift, although it may still be active as a continental rift system. The East African rift (EAR) appears to be a modern example, as ti is the failing arm from the triple junction including the Red Sea and Gulf of Aden.

See also Basin and Range Half grabens East African Rift Transcurrent rifting



Rift-related igneous activity: bimodal volcanic signature distinctive trace element geochemistry continental rift basalts are enriched in alkalis (K, Ba, Rb), and

incompatible elements, LIL. deep mantle-plume contribution mantle fluids and metasomatism. lithospheric mantle contribution

Other features: distinctive trace element geochemistry with sediment traps, accommodation space arkoses, immature sediments half grabens fault driven sedimentation: alluvial fans and debris flows Along-strike changes = segmentation and depocentres every rift basin is unique


The failed third arm (called an aulocogen) is a topographic low.

Many major rivers in the world flow down aulocogens

e.g. Amazon, Mississippi, Niger, St. Lawrence, Rhine, and parts of the Nile

Intraplate – Oceanic Crust Oceanic plateaux Ocean basins - sag basins pelagic

clays, oozes, turbidites Volcanic islands/ seamounts/guyots Produced by Mantle plume

hotspots - long-lived structures fixed within the mantle.

Lithospheric plates move over them, typically in a datable track.

e.g. Hawaii, Yellowstone, Galapagos

Intraplate Oceanic

Mantle plume hotspot tracks

Ages in million years

Intraplate Oceanic

Long lived global hotspots

Intraplate Oceanic

Divergent - Continental

Proto-oceanic troughsRed Sea <5 Ma oceanic crust in centre, thick salt

deposits due to ocean cut off

Passive marginsContinental rises and terraces (prisms/wedges,

continental crust thinned, transitory and oceanic crust, can include pelagic turbidite. May be caused by densification by metamorphism

e.g. Eastern N. America seaboard. Stable EA coast

Detailed Cross-section of a Passive Margin

Atlantic Margin

Triassic rift valley sediments

Jurassic saltCretaceous & Cenozoic sediments

What is the relative age of the basalt?

Divergent - Continental

Divergent - Oceanic

MORs (Mid-oceanic rifts)

Divergent - Oceanic

Oceanic Crustal Age revealed against passive margins

Convergent - Intraoceanic

Oceanic volcanic arcs with intra-arc basinsDeep sea trenches – arc-trench gaps (containing fore-arc basins) – active volcanic (island arc) arc – back-arc

Two oceanic slabs converge; one subducts

The subducted slab produces melting in the overlying mantle wedge

Magma Is less dense than overlying crust / lithosphere and rises as volcanoes.

If the volcanoes emerge as islands, a volcanic island arc (or archipelago) is formed

e.g. Japan, Aleutian islands, Tonga islands

Convergent - Intraoceanic

Oceanic Back-Arc Basins1. Back-arc basins (or retro-arc

basins) are submarine basins associated with island arcs and subduction zones

2. Found at some convergent plate boundaries, presently concentrated in the Western Pacific Ocean

3. Most result from tensional forces caused by oceanic trench rollback rollback and the collapse of the edge of the continent

4. Back-arc basins were not predicted by plate tectonic theory, but are consistent with the dominant model for how Earth loses heat

Ocean ic Back-Arc Basins

Continent:Continent with subduction

Convergent - Continental Common when two continents collide and the buoyant continental lithosphere does not subduct

Any original trenches are eliminated

Collision then thickens the crust, along the suture separating the original continents

Crustal thickening then responds isostatically, producing a large mass of buoyant continental crust e.g. Himalayas, Alps, Appalachians

North-south profile across the eastern Alps. Subsurface profile from seismic reflection data. After Adrian Pfiffner

Convergent - Continental

Part of Africa breaks away ca. 50 Ma ago

Travelled to the north at ca. 10 cm/annum

Is subducted under continental Asia, cause it to rise in elevation

Plate movements continue today, so Hilary had it a few centimetres easier to climb Everest than today’s climbers

Cause of the Indonesian tsunami

Continent:Continent with subductionExample from the Himalayas



Head 0n with obduction

Obduction

styles


Convergent – Continental Margin

Products: Deep sea trenches Trench slope

subduction basins Accretionary

complexes Mélange Foreland arcs Fore-arc basins Intra arc basins Back-arc basins Foreland fold-thrust

belts

Crustal melting occurs above the descending slab producing batholithic rocks surmounted by volcanic. Sediments are derived mainly from the arc and are siliclastic Sediments are subducted or scraped off into the accretionary complexese.g. Sunda, Aleutian, Peru-Chili, and Japan.


Vertical sequence:

Volcanic arc

Crust (sub-arc lithosphere TTG)

2. Upper Mantle wedge

1. Subducting slab



Convergent – Continental Margin and Oceanic

TranstensionalTranspressionalTransrotationalIntracontinental wedge basins

Transcurrent (strike –slip & transform)

Transform faults

Most transforms are prominent linear breaks associated with mid-ocean ridge segments.

Known as fracture zones these occur between offsets in the spreading ridge.

Fracture zones are a geometrical necessity due to the fact that sea-floor genesis occurs on a SPHERE.

Suspect terranes This term applies to a terranes which have been brought in from a long

distances, exotic in nature to the terranes they now abut. With accurate age-dating and other methods of establishing

provenance it may be possible where the suspect terranes come from, and how far they have travelled

Analysis of such terranes is the main basis for constructing paleo maps

Transcurrent (strike –slip & transform)

Plate Tectonic Mechanisms

No one mechanism accounts for all major facets of plate tectonics

Convective flow in the plastic 2,900 km-thick mantle is the best option

Other mechanisms generate forces that contribute to plate motion.

Slab-pull on cold plate in subduction zone

Ocean ridge-push

Gravitational sliding on oceanic ridges

The Six Major Types of Sedimentary Basin(with examples)

Six major types of sedimentary basins are shown in their plate-tectonic settings. The major physical cause or causes of subsidence for each case are shown below on above the diagram.

Michigan BasinE. AfricaNevada

Offshore Calif.

Indonesia

E. Coast NA

Seismicity related to Subduction

A scheme relating igneous rocks to plate tectonics

Documents

GEOL 451-2010 Geology of North America