The Continental Lithosphere
Seismology Group IIT Kharagpur
Complex Continents • Age: 4200 Ma old. • Oldest material at their center flanked by younger materials: representing many events of mountain building, collision, rifting and plate convergence and subsidence.
• Direct Observations limited to exposures at or near the surface. • Depth information demands conjecture which cannot be tested directly.
Matters of Current Debate • • • •
Composition of the Lower Crust Nature of the Moho and upper mantle Strength of the Lithosphere Mountain Building, support and destruction
Geophysical Characteristics of Continents • Average Thickness: 38 km. (range 30-45 km)
Difficult to define a ‘standard’ continental crustal structure
- Generally thicker beneath younger mountain belts - Moderately thick beneath ancient Shields - Thin beneath young basins and rifts
Geophysical Characteristics of Continents • Average Seismic Wave velocities (from long seismic refraction lines, deep reflection lines and receiver function): - Pg in crystalline, continental basement: 5.9 – 6.2 km/s - P-wave in the upper crust (top 10 km): 6.0 – 6.3 km/s - middle to lower crust: > 6.5 km/s - Some regions: lower crustal layer of Vp > 7.0 km/s • Conrad Discontinuity (between upper and lower crust) is not a universal feature. • Low velocity zones at various locations at all depths.
Cross-section across an idealised continent showing the average Vp of the crust in various tectonic regions
The variability in seismic velocity reflects the bulk composition, its thermal state and metamorphic history
Laboratory measurement of P-wave velocity in various rock types
Felsic Æ Mafic ÆUltramafic
Ranges of laboratory measurement of P-wave velocity in various rock types
The composition of the continental crust The continental crust: Formed from the mantle material over the lifespan of the Earth by a series of melting, crystallization, metamorphic, erosional, depositional, subduction and endless reworking events. Compound
Continental (%)
Oceanic (%)
Si02
57.3
49.5
Ti02
0.9
1.5
Al203
15.9
16.0
FeO
9.1
10.5
MgO
5.3
7.7
CaO
7.4
11.3
Na2O
3.1
2.8
K2O
1.1
0.15
Estimated composition of the bulk continental and oceanic crust
Age of the continental crust
Only 30% of the current basement rocks younger than 450 Ma; 70% older Oldest material concentrated in the center: Cratons Flanked by the accreted terranes – continental, oceanic, island-arc origin
Growth of Continents a. Volcanism at subduction zones
The descending slab - dehydration of the crust
The decending slab - heating
The overriding mantle wedge
The base of the continental crust
Growth of Continents b. Sediments at subduction zones
Stages of development of subduction zone with thick sediments on oceanic plate
Growth of Continents c. Continent-continent collision The Himalayas
The Himalayas and Tibet
Major tectonic blocks and their sutures
ITSZ TIBET
INDIA
One attempt to explain the overall evolution of the region
Balanced restored north-south section across Nepal at ~88°E •Probable (?) sequence of thrusting events that gave rise to the Himalayas as observed today • Shortening of the crust since 16-25Ma occurred in 2 stages as location of active thrust moved progressively to the south:
Total amount of shortening ~200-250 km, with 40-70 km during stage 2 and 3
Crustal shear-wave velocity images across Southern Tibet
North
Section across Southern Tibet along Yadong-Gulu rift at 89-91째E
Deformation of the Indian plate south of the Himalayas
Earthquakes in the continental lithosphere (seismogenic thickness)
Strength of the continental lithosphere "jelly sandwich" or "crème-brûlé"? • The strength of the continental lithosphere and how it responds to long-term geological loads is a topic of much current interest in the Earth Sciences. • Flexure studies suggest a rheological model, dubbed the "jelly sandwich" model, in which the strength of the lithosphere is attributed to both the upper crust and mantle. • Studies of the thickness of the seismogenic zone, however, suggest a model, dubbed the "crème-brûlé" model, in which the strength resides in the crust, but that the mantle has no strength. This lecture will review the evidence in support of each model and will attempt to reconcile between these fundamentally different views of the mechanical behaviour of the lithosphere.
Strength envelopes Continental
Oceanic Strength 0
Strength
water crust Moho
0
Depth (km)
Crust
50
100
Moho
Mantle
50 Mantle
100
Brittle-Ductile Properties of the Lithosphere We all know that rocks near the surface of the Earth behave in a brittle manner. Crustal rocks are composed of minerals like quartz and feldspar which have high strength, particularly at low pressure and temperature. As we go deeper in the Earth the strength of these rocks initially increases. At a depth of about 15 km we reach a point called the brittle-ductile transition zone. Below this point rock strength decreases because fractures become closed and the temperature is higher, making the rocks behave in a ductile manner. At the base of the crust the rock type changes to peridotite which is rich in olivine. Olivine is stronger than the minerals that make up most crustal rocks, so the upper part of the mantle is again strong. But, just as in the crust, increasing temperature eventually predominates and at a depth of about 40 km the brittle-ductile transition zone in the mantle occurs. Below this point rocks behave in an increasingly ductile manner.
The Jelly-Sandwich
Earthquake Focal Depths and Seismogenic Strength
Figure 1. Histograms of earthquake focal depths determined by modeling of long-period teleseismic P (primary) and SH (secondary horizontal) seismograms (solid bars). White bar in North India (G) is depth determined from short period depth phases in Shillong Plateau by Chen and Molnar (1990). White bars in Tibet (C) are subcrustal earthquakes, but not necessarily in mantle of continental origin. Approximate Moho depths are indicated by dashed lines. Focal depth and Moho data are from various sources, including Nelson et al. (1987), Molnar and Lyon-Caen (1989), Foster and Jackson (1998), Mangino et al. (1999), and Maggi et al. (2000). Focal depths based on arrival times recorded at local seismic networks have also found seismicity throughout crust in several parts of North America (e.g., Wong and Chapman, 1990).