4.2 Continental Rifting
Continental rifting is the process by which tectonic extensional forces thin and ultimately rupture continental lithosphere, representing the earliest stage of the Wilson Cycle—the birth of a new ocean basin. The interplay between far-field plate boundary forces, mantle dynamics, and pre-existing lithospheric structure determines the style, geometry, and ultimate success or failure of a rift system.
Two end-member mechanical models describe how the lithosphere extends during rifting: the pure shear model of McKenzie (1978), which assumes uniform stretching, and the simple shear model of Wernicke (1985), which routes extension along a master detachment fault. Real rift systems typically exhibit elements of both, reflecting the three-dimensional complexity of continental breakup.
Pure Shear Model (McKenzie, 1978)
The McKenzie model treats rifting as instantaneous, uniform stretching of the entire lithosphere by a factor β, defined as the ratio of initial to final lithospheric thickness. The model is powerful because it quantitatively predicts both the initial tectonic subsidence and the subsequent thermal subsidence that follows rifting.
Stretching Factor
β = stretching factor (dimensionless, β > 1 for extension)
β = 1: no stretching
β = 2: lithosphere thinned to half its original thickness
β → ∞: complete continental breakup, onset of seafloor spreading
Initial (Syn-rift) Subsidence
The immediate isostatic response to crustal thinning produces the initial subsidence:
S_i = initial tectonic subsidence (m)
t_c = initial crustal thickness (~35 km for average continent)
ρ_m = mantle density (~3300 kg/m³)
ρ_c = crustal density (~2800 kg/m³)
For β = 2 and typical densities, Si ≈ 2.6 km. This subsidence creates the initial accommodation space filled by syn-rift sediments.
Thermal (Post-rift) Subsidence
After stretching ceases, the thinned lithosphere cools conductively and contracts, producing exponentially decaying thermal subsidence:
E_0 = \( \frac{4 \, a \, \rho_m \, \alpha \, T_m}{\pi^2 (\rho_m - \rho_w)} \) (thermal subsidence scale factor)
τ = thermal time constant = \( a^2 / \pi^2 \kappa \) ≈ 62.8 Myr
a = lithospheric thickness (~125 km)
κ = thermal diffusivity (~10&sup{-6} m²/s)
T_m = basal mantle temperature (~1333°C)
The exponential decay means that ~63% of thermal subsidence is achieved by time τ (~63 Myr), and ~95% by 3τ (~190 Myr). This broad, gentle sag creates the wide post-rift basins characteristic of passive continental margins.
Simple Shear Model (Wernicke, 1985)
Wernicke proposed that extension in many rifts is accommodated by a single, through-going, low-angle detachment fault that traverses the entire lithosphere. This produces fundamental asymmetry between the two conjugate margins:
Upper Plate Margin
- Hanging wall of the detachment fault
- Crustal thinning without corresponding lithospheric mantle thinning
- Block-faulted upper crust with tilted half-grabens
- Subdued initial subsidence, reduced thermal subsidence
- Relatively narrow continental margin
Lower Plate Margin
- Footwall of the detachment fault
- Lithospheric mantle thinned more than overlying crust
- Exhumation of lower crust and mantle rocks (metamorphic core complexes)
- Enhanced thermal subsidence, possible uplift of rift flank
- Wide transitional zone with hyperextended crust
The Iberia–Newfoundland conjugate margins represent a well-studied example of asymmetric rifting consistent with the Wernicke model, where the Iberian margin exposes exhumed serpentinized mantle while the Newfoundland margin preserves a more conventional tilted fault-block architecture.
The East African Rift System
The East African Rift System (EARS) is Earth's premier example of active continental rifting, stretching over 3,000 km from the Afar Triple Junction in the north to Mozambique in the south. It provides a natural laboratory for studying rifting at every stage of evolution.
Eastern Branch (Gregory Rift)
Volcanically active, associated with the Kenya Dome thermal uplift. Magma-rich rifting with large volcanic centers (Kilimanjaro, Kenya, Nyiragongo). Narrow rift valleys (40–60 km wide) with relatively thin crust (< 30 km). Extension rates of 2–5 mm/yr.
Western Branch (Albertine Rift)
Magma-poor rifting with deep, fault-bounded lakes (Tanganyika, Malawi, Kivu). Spectacular half-graben basins up to 7 km deep. Extension rates of 1–3 mm/yr. Thick sedimentary sequences valuable for paleoclimate studies.
Afar Triangle
Most advanced stage: continental crust thinned to < 15 km. Incipient oceanic spreading (Erta Ale volcano has a persistent lava lake). The Afar Triple Junction connects the EARS with the Red Sea and Gulf of Aden rifts—a textbook ridge-ridge-ridge triple junction.
Basin and Range Province: Wide Rift Mode
The Basin and Range Province of western North America illustrates wide rift mode extension, contrasting with the narrow rift mode of the EARS. Extension is distributed across a zone more than 800 km wide, producing regularly spaced (~30 km) north-south trending mountain ranges separated by sediment-filled valleys (basins).
The wide rift mode develops in regions with hot, weak lithosphere—here caused by Cenozoic back-arc heating and possible delamination of the Farallon slab. Total extension is estimated at 100–250%, with stretching factors β of 2–3.5 in the most extended areas.
Metamorphic Core Complexes
In regions of extreme extension (β > 3), low-angle detachment faults exhume mid-crustal metamorphic and plutonic rocks in dome-shaped uplifts called metamorphic core complexes (MCCs). Distinctive features include:
- Mylonitic shear zones recording ductile deformation at depth
- Corrugated detachment surfaces with “turtleback” geometry
- Brittle overprint in the footwall (chloritic breccia, cataclasite)
- Examples: Whipple Mountains, Snake Range, Shuswap Complex
Syn-rift vs. Post-rift Sedimentary Sequences
The sedimentary fill of rift basins records the tectonic and thermal history of extension in two distinct phases:
Syn-rift Sequence
- Deposited during active faulting and stretching
- Wedge-shaped units thickening toward border faults
- Coarse alluvial fans and fan-deltas adjacent to fault scarps
- Lacustrine sediments in basin centers (often anoxic, organic-rich)
- Growth strata with progressive onlap and angular unconformities
- Volcaniclastic interbeds in magma-rich rifts
Post-rift Sequence
- Deposited during thermal subsidence after faulting ceases
- Tabular, unfaulted strata draping the rift topography
- Regional “sag” basin much wider than the original rift
- Marine transgression as subsidence deepens the basin
- Carbonate platforms, deltaic, and deep-marine facies
- Separated from syn-rift by the breakup unconformity
Magma-Rich vs. Magma-Poor Rifted Margins
Magma-Rich (Volcanic) Margins
- Massive flood basalt eruptions during breakup
- Seaward-dipping reflector sequences (SDRs) in seismic data
- Thick underplated igneous bodies at base of crust
- Often associated with mantle plumes or hotspots
- Examples: Norway–Greenland, South Atlantic, Deccan margin
Magma-Poor (Non-volcanic) Margins
- Little to no syn-breakup volcanism
- Extreme crustal thinning and hyperextension
- Exhumed and serpentinized subcontinental mantle
- Wide ocean-continent transition zone (50–150 km)
- Examples: Iberia–Newfoundland, Alpine Tethys (Err-Platta)
Key Takeaways
- The McKenzie (pure shear) model predicts initial subsidence Si ∝ (1 - 1/β) followed by exponential thermal subsidence
- The Wernicke (simple shear) model explains asymmetric conjugate margins via a lithosphere-scale detachment
- The East African Rift System provides a modern type example spanning from incipient rifting to proto-oceanic spreading
- Wide rifts (Basin and Range) develop in hot, weak lithosphere; narrow rifts (EARS) in stronger lithosphere
- Metamorphic core complexes form under extreme extension (β > 3) on low-angle detachments
- Syn-rift sediments are fault-controlled wedges; post-rift sediments are broad thermal-sag deposits
- Volcanic margins feature SDRs and flood basalts; non-volcanic margins expose exhumed mantle