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Geologic Setting and Evolution of Latin America

Geologic Setting and Evolution of Latin America

Geologic Time Terminology

  • Ga (Giga Annum): Represents 1 billion years ago.

  • Ma (Mega Annum): Represents 1 million years ago.

  • Earth's Formation: Approximately 4.56 ext{ Ga}.

  • Meteorites: Most meteorites are also dated to 4.56 ext{ Ga}. (Reference: F22 class GEOS 2340 "Comets love Meteorites")

  • Oldest Rocks: The oldest known rocks on Earth are approximately 4.0 ext{ Ga} old.

Important Events in the Geologic Evolution of Latin America

  • Formation of the Continental Crust: Occurred between 3.5 and 0.5 ext{ Ga}.

  • Formation of Gondwana: Began around 600 ext{ Ma}.

  • Break-up of Pangea: Started approximately 250 ext{ Ma}.

  • Break-up of Gondwana: Occurred around 120 ext{ Ma}.

  • Formation of the Caribbean: Took place roughly 80 ext{ Ma}.

Geologic Timescale Overview

  • The geologic timescale categorizes Earth's history into Eons, Eras, Periods, and Epochs.

  • Precambrian Eon: Encompasses the Archean and Proterozoic Eras, ending at 544 ext{ Ma}.

    • Archean Era: Ends at 2.5 ext{ Ga}. Oldest rocks are approximately 4.0 ext{ Ga}. Earth forms at 4.5 ext{ Ga}.

    • Proterozoic Era: Extends from 2.5 ext{ Ga} to 544 ext{ Ma}$. Formation of Gondwanaland occurred during this time.

  • Phanerozoic Eon: Began at 544 ext{ Ma}.

    • Paleozoic Era: Ends at 250 ext{ Ma}. Formation of Pangea occurred during this era.

    • Mesozoic Era: Ends at 65 ext{ Ma}. The opening of the Atlantic Ocean began during this era.

    • Cenozoic Era: Continues from 65 ext{ Ma} to the present.

Types of Rocks

  • Igneous Rocks: Formed from the cooling of magma.

    • Volcanic (Extrusive): Cools rapidly at the Earth's surface (e.g., basalt).

    • Plutonic (Intrusive): Cools slowly at significant depths within the Earth (e.g., granite).

    • Hypabyssal: Cools at shallow depths below the surface.

  • Sedimentary Rocks: Formed by the deposition and lithification of sediments on the Earth's surface, typically by water or wind action (e.g., sandstone, limestone).

  • Metamorphic Rocks: Formed when existing sedimentary or igneous rocks are subjected to intense heat and pressure underground, causing them to transform without melting entirely.

Cratons, Shields, and Platforms

  • These terms describe the stable interior portions of continents.

  • Craton: Refers to ancient, stable continental crust that has remained relatively undeformed for a long geological period. It is primarily composed of igneous and metamorphic rocks.

  • Shield: An exposed craton, where the ancient igneous and metamorphic basement rocks are visible at the surface, uncovered by younger sediments.

  • Platform: A craton that is covered by a relatively thinner layer of younger sedimentary rocks.

South American Geologic Provinces

  • South America's geology includes Precambrian exposures, Phanerozoic sediments, and the Andean Orogen.

  • South American Shields: These consist of Archean and older Proterozoic nuclei, which are surrounded by younger Proterozoic orogenic belts.

    • They are exposed in four main locations east of the Andes:

      • Guyana Shield

      • Guaporé Shield (sometimes called "The Amazon Craton")

      • Sao Francisco Craton

      • Rio de la Plata Craton (also sometimes part of the larger Amazon Craton definition)

    • The oldest rock in Latin America (and South America) is found within these shields, dating back to approximately 3.5 ext{ Ga}.

  • The South American Platform: Characterized by thin sediments overlying the Amazon Craton.

    • The Amazonian craton boasts a strong lithospheric root that maintains its stability, preventing it from being significantly affected by Cenozoic deformation (folding and faulting).

    • Geographically, it encompasses nearly everything east of the Andes and south of the Caribbean, with Brazil being largely built upon it.

    • Topographically, the South American Platform is generally flat to hilly.

    • The deep lithospheric root of the Amazonian Craton extends to about 150 ext{ km} depth, contributing to its stability.

Orogenies, Orogenic Belts, and Orogens

  • Orogenies: Episodes of intense crustal deformation that result in the generation of new crust and mountains. They are also defined as periods of continental crust growth through the addition of material at continental margins. Cratons typically remain unaffected by orogenic processes.

  • Orogenic Belts: Elongate zones characterized by high relief and significant deformation, commonly seen as mountain chains (e.g., the Andes).

  • Orogens: Crustal tracts that underwent orogeny (mountain-building) at roughly the same geological time.

Ways to Add Crust at Continental Margins

  • 1) Magmatic Additions: The introduction of magma to the crust plays a crucial role.

    • Intra-oceanic arcs are regions where new continental crust is produced at a rate of about 1 ext{ km}^3/ ext{year}. (Based on seismic refraction profiles, e.g., Izu Arc Crust, Western Pacific).

    • The thick crust beneath the Andes is partly attributed to the addition of magma to the base of the crust above the subducting oceanic plate.

Latin American Volcanic Activity

  • Volcanoes are found in three main regions:

    1. Mexico-Central America

    2. Lesser Antilles

    3. Andes

  • There are 319 Latin American Holocene volcanoes, with 63 considered most active, showing evidence of ground deformation or thermal anomalies.

  • Andean Volcanic Arc (AVA): Earth's longest yet discontinuous continental-margin volcanic arc. It is formed by the subduction of the Nazca and Antarctic Plates beneath the South American Plate.

    • The AVA is segmented into four distinct zones:

      • Northern Volcanic Zone

      • Central Volcanic Zone

      • Southern Volcanic Zone

      • Austral Volcanic Zone

  • Notable Andean Features: While many high Andes peaks are volcanoes, not all are.

    • Aconcagua: The highest mountain in the Western Hemisphere, reaching 6962 ext{ m} (22841 ext{ feet}). Located in western Argentina, it's a "Fold-and-Thrust" Belt, not a volcano.

    • Machu Picchu: The Inca city (rediscovered in 1911) is situated on an Andean mountain top at 9060 ext{ feet} elevation. The underlying rocks are approximately 246 ext{ million year old} granite.

Supercontinent Cycle

  • This refers to the episodic process of continental crust aggregating into a single, large mass (a supercontinent) and then dispersing.

  • A supercontinent is associated with a "Superocean."

  • Supercontinents form through protracted periods of continent-continent collisions.

  • The cycle typically takes approximately 300-400 ext{ million years}.

  • Three Important Supercontinents (referenced by their approximate age of formation/existence):

    • 1000 ext{ Ma} (e.g., Rodinia)

    • 600 ext{ Ma} (e.g., Pannotia, precursor to Gondwana)

    • 200 ext{ Ma} (Pangea)

Wilson Cycle and Supercontinent Cycle

  • Wilson Cycle: Describes the opening and closing of new oceans over geological time.

  • Supercontinent Cycle: Encompasses the broader process of supercontinent formation and breakup, which includes multiple Wilson Cycles.

Pangea: The Most Recent Supercontinent

  • Formation: Pangea formed approximately 300 ext{ Ma} ago when the continents of Laurasia and Gondwana collided.

  • Components:

    • Pangea = Laurasia + Gondwana

    • Laurasia = North America and Asia

    • Gondwana = South America, Africa, Antarctica, India, and Australia

  • Gondwana Craton Connections: Within Gondwana, the cratons of South America were directly connected to cratons in Africa, for example:

    • Amazonia (S. America) connected with West Africa

    • Rio de la Plata (S. America) connected with Kalahari (Africa)

    • São Francisco (S. America) connected with West Congo (Africa)

  • Breakup of Gondwana: Began around 120 ext{ Ma}.

Passive Margins

  • Definition: These are continental shelves that are not active plate margins.

  • Significance: They are crucial areas because they facilitate thick accumulations of marine sediments.

  • Formation: The breakup of Gondwana, particularly the rifting that created the Atlantic Ocean, produced extensive passive margins on the flanks of the newly separated continents.

  • Brazilian Passive Margin: Contains significant oil reserves, making Brazil a leading offshore oil producer from new-build projects (projected lead in 2025).

  • Gulf of Mexico Passive Margin: Formed as the Gulf of Mexico (and Central Atlantic) opened along the Carboniferous Suture between Gondwana and Laurasia (around 245 ext{ Ma}).

    • The opening of the Gulf of Mexico and the evolution of the Mojave-Sonora Megashear significantly contributed to moving Mexico into its modern position.

  • Gulf of California: An example of a passive margin currently in the making, having begun opening approximately 5 ext{ Ma} ago.

Evolution of the Caribbean

  • The Caribbean region is characterized by the "Great Arc of the Caribbean," an oceanic plateau and associated volcanic islands, resulting from complex tectonic interactions, including the relative movement of the Caribbean Plate between the North and South American plates. Its formation dates back to around 80 ext{ Ma}$$.