lesson 11

Transform Plate Boundaries

• Definition and Kinematics:

  • Transform plate boundaries occur where tectonic plates slide horizontally past one another along vertical fault planes.

  • Conservation of crust: Lithosphere is neither created nor destroyed along transform boundaries.

• Geological Associations and Role in Ridge Systems:

  • The majority of transform faults are located on the ocean floor, where they connect offset segments of mid-ocean ridge spreading centers.

  • These oceanic strike-slip faults occur along linear fractures in oceanic crust known as fracture zones.

  • Active transform faults exist strictly between the offset ridge segments, whereas the extensions beyond the ridge segments are inactive fracture zones where no relative displacement occurs.

  • Transform faults are intimately connected with divergent and convergent plate boundaries, accommodating differential movement across Earth's lithospheric plates.

The San Andreas Fault System

• Continental Transform Boundary Example:

  • The San Andreas Fault is a prime continental example of a transform plate boundary, marking the strike-slip boundary between the Pacific Plate and the North American Plate in California.

• Geographic Extent and Cities Along the Fault Zone:

  • The fault trace extends across major geographic regions and cities in California, including Crescent City, Eureka, Garberville, Point Delgada, Point Arena, Ukiah, Santa Rosa, Point Reyes, Daly City, San Francisco, San Jose, Santa Cruz, Hollister, San Juan Bautista, Monterey, Parkfield, San Luis Obispo, Simmler, Carrizo Plain, Soda Lake Rd, Frazier Park, Palmdale, Santa Barbara, Los Angeles, San Bernardino, Palm Springs, Desert Hot Springs, Brawley, and San Diego.

• Surface Geomorphology and Landscape Offsets:

  • Horizontal motion along the fault causes lateral displacement of active landforms, such as river channels and stream drainages.

  • In the Carrizo Plain along Elkhorn Rd, fault slip has offset present-day stream channels from their ancestral drainages by approximately $100\,\text{yards}$.

Geological Features Associated with Plate Boundaries

• Seismic Activity Across Boundaries:

  • Earthquakes occur at all three primary types of plate boundaries: divergent, convergent, and transform boundaries.

• Volcanism Distribution:

  • Volcanic activity is restricted to divergent boundaries (mid-ocean ridges and continental rifts) and subduction-type convergent boundaries (e.g., Alaska and Japan).

  • Absence of Volcanism at Continental-Continental Collisions:

  • No volcanoes form along continental-continental convergent boundaries (e.g., the Himalayan Mountains).

  • Continental crustal material is low in density and extremely buoyant, preventing it from subduct deep into the mantle to generate magma.

  • Even if localized magma were generated deep within the crust, it cannot easily penetrate the extreme thickness of collision-thickened continental crust.

• Orogenic Processes and Environmental Impacts:

  • Continental collision zones produce large mountain ranges, substantially increasing crustal thickness (e.g., Himalayan Mountains).

  • Mountain building leads to secondary geological and environmental consequences:

  • Increased localized orographic precipitation.

  • Formation of steep topographic slopes highly vulnerable to mass wasting and landslides.

  • Increased risk of severe regional flooding.

  • Broad alterations to regional climate patterns.

Evidence for Seafloor Spreading from Ocean Drilling

• Core Sediment Deposition Patterns:

  • Direct deep-sea drilling into oceanic floor sediment provides essential physical evidence validating seafloor spreading and plate tectonics.

  • Age Distribution of Sediments:

  • Deepest sediment layer ages directly overlying basaltic crust reveal that sediment age increases systematically with distance from spreading centers.

  • The youngest sediments are found near mid-ocean ridge axes, while the oldest sediments lie closest to continental margins and trenches.

  • Sediment Thickness Accumulation:

  • Sediment thickness increases with distance from the ridge crest.

  • Sediments are virtually absent directly along active mid-ocean ridge axes because the newly erupted oceanic crust has had minimal time to accumulate biogenous or terrigenous debris.

  • Sediment reaches its thickest accumulation atop the oldest oceanic crust furthest from the spreading center.

• Deep-Sea Scientific Drilling Technology:

  • Continuous ocean sediment cores are obtained using high-capability drillships.

  • The Chikyu is a state-of-the-art Japanese scientific drilling vessel engineered to drill up to $7,000\,\text{meters}$ (more than $4\,\text{miles}$) beneath the seafloor into sediments and underlying basaltic crust.

Mantle Plumes and Hot Spots

• Fundamental Definitions:

  • Mantle Plume: A narrow, cylindrically shaped upwelling of unusually hot mantle rock rising from deep within Earth's mantle.

  • Hot Spot: An isolated area of volcanic activity located at Earth's surface directly above a mantle plume.

• Formation of Hot-Spot Tracks:

  • As an overriding tectonic plate drifts continuously over a stationary underlying mantle plume, a linear sequence of progressively older volcanoes forms, designated as a hot-spot track.

  • Measuring the age of volcanic rock along a hot-spot track defines the direction and rate of plate motion relative to the plume over geologic time.

• Hawaiian–Emperor Seamount Chain Case Study:

  • The age progression along the Hawaiian–Emperor track provides an accurate record of Pacific Plate displacement:

  • Detroit Seamount: $80\,\text{mya}$

  • Suiko Seamount: $60\,\text{mya}$

  • Koko Seamount: $48\,\text{mya}$

  • Midway Island: $27\,\text{mya}$

  • Kauai: $3.8\text{--}5.6\,\text{mya}$

  • Oahu: $2.2\text{--}3.3\,\text{mya}$

  • Molokai: $1.3\text{--}1.8\,\text{mya}$

  • Maui: $\text{less than } 1.0\,\text{mya}$

  • Hawaii (Big Island): $0.7\,\text{mya to present}$

• Global Hotspot Distribution:

  • Key global hotspots and associated volcanic chains include Bowie, Cobble, Yellowstone, Hawaii, Baja California, Caroline, Samoa, Marquesas, Pitcairn, Macdonald, Easter Island, Louisville, Juan Fernández, Bermuda, Great Meteor, Azores, Cape Verde, Fernando, Trindade, St. Helena, Tristan da Cunha, Bouvet, Iceland, Eifel, Central Massif, Hoggar, Tibesti, Darfur, Afar, Réunion, Crozet, and Kerguelen.

Measurement and Kinematics of Plate Motion

• Seafloor Spreading Rate Calculation:

  • Average plate motion rate can be calculated by dividing the distance of oceanic crust from a mid-ocean ridge axis by its radiometric age:

$\text{Average Rate of Plate Motion} = \frac{\text{Distance}}{\text{Age}}$

• Geodetic Space Measurement (GPS):

  • Space-based positioning technologies, specifically the Global Positioning System (GPS), allow direct, real-time tracking of plate displacement.

  • Ground stations positioned across multiple continents record precise coordinates continuously over many years to establish absolute vectors of plate velocity and trajectory.

• Plate Velocities and Magnitudes:

  • Tectonic plates move at average rates ranging from a few millimeters to several centimeters per year.

  • Typical plate movement rates are comparable to the growth speed of human fingernails and hair.

  • Velocity benchmarks:

  • Average global relative plate motion speed benchmark: $5.5\,\text{cm/yr}$.

  • Spreading rates along the Mid-Atlantic Ridge vary between $1.8\,\text{cm/yr}$ and $4.1\,\text{cm/yr}$.

  • Spreading rates across the East Pacific Rise reach fast velocities up to $16.0\text{--}18.3\,\text{cm/yr}$.

  • Nazca Plate convergent motion rates are approximately $10.1\,\text{cm/yr}$.

Surface Area Dynamics of Lithospheric Plates

• Conservation of Global Surface Area:

  • Earth's total surface surface area remains constant over time. The generation of new oceanic lithosphere at divergent ridges is balanced by the consumption of old lithosphere at subduction zones.

• Growth and Reduction of Plate Dimensions:

  • Tectonic plates shrink or grow depending on the nature of the boundaries enclosing them:

  • African Plate: Enclosed predominantly by spreading mid-ocean ridges (divergent boundaries), causing its overall surface area to grow.

  • Pacific Plate: Surrounded largely by subduction zones (convergent boundaries), causing its overall surface area to shrink.

Driving Mechanisms of Plate Tectonics

• Primary Driving Engine: Mantle Convection:

  • Convective flow within Earth's mantle—where hot, buoyant mantle rocks ascend and cool, dense rock slabs sink—is the primary driving engine behind plate tectonics.

  • Convection provides the physical process for transferring Earth's internal heat from the deep interior outward toward the crust.

• Forces Driving Plate Motion:

  • Slab-Pull Force:

  • Driven by density differences.

  • As cold, dense oceanic lithosphere subducts into the warmer asthenosphere, its weight pulls the rest of the trailing plate downward into the trench.

  • Slab pull acts as the single primary driver of plate velocity.

  • Ridge-Push Force:

  • Driven by gravitational sliding.

  • Topographic elevation at mid-ocean ridge axes creates a gravitational gradient, causing elevated lithosphere to slide down the flanks of the ridge toward ocean basins.

  • Mantle Drag:

  • Occurs due to shear stress along the lithosphere-asthenosphere boundary.

  • Convection currents in the underlying asthenosphere enhance plate velocity when asthenospheric motion exceeds the velocity of the overlying lithospheric plate.

• Resisting Forces:

  • Frictional resistance acting along the contact interface between the subducting slab and the overriding plate resists motion.

  • Viscous resistance (mantle drag) against subducting slabs penetrating deep into the mantle.

• Evolutionary Models of Mantle Convection:

  • Classic Historical Model:

  • Envisioned simple, rigid convection cells in which tectonic plates were viewed as passive rafts swept along by underlying convective fluid currents.

  • Modern Tomographic Model:

  • Demonstrates complex, irregular convective circulation.

  • Tectonic plates are not passive rafts but active upper thermal boundary layer components driving the convective system.

  • Seismic tomography displays complex thermal structures: warm ascending plumes (red) and cool sinking subducted slabs (blue) descending deep into the lower mantle.