essays for tectonics

main parts of the Himalayan cross section

Indian lithospheric mantle, Indian Crust, Asian lithospheric mantle, Greater Himalayan Crystalline Complex, Gangdese Batholith in South Tibet, North Tibet, Asian lithospheric mantle

when did the Indian plate reach the margin of Asia

~ 50 Ma in the Eocene

what happened in the Indian-Asian collision

sutures between ancient blocks exploited → renewed thrusting & sig strike-slip faulting

Gondwana detachment

Cretaceous event, Indian plate broke away, beginning its rapid N migration

magnetic-stripe data used for

evidence from Indian Ocean used to track speed and timing of India’s movement toward Asia

approximate time of the initial contact between India & Asia margin

50 Ma in Eocene

pre-collisional phase of the Himalayas

subduction of Indian oceanic crust beneath Asia, pre-thickened Asian crust before continental collision

climax of Himalayan orogeny

20 Ma in the Miocene

Himalayan Moho deepening

result of extreme crustal shortening and thickening, esp prominent in Central sector

ductile crustal flow

movement of warmer middle & lower crust laterally as it deforms under pressure

brittle upper crust

deforms thru sideways block faulting rather than flowing

topographic limit

crust is so thick it cannot support its weight → g collapse & lateral expansion

isostatic response

upward ‘rebound’ of lightened crust after denudation removes mass from peaks

ends of the Himalayan range

Western Pakistan/Karakorum, Eastern Myanmar

Himalayan modern thrusting

ongoing activity along S border causes topography to propagate S

Himalayan outline

50 Ma Eocene collision starts, 20 Ma Miocene uplift peaks, shortening/thickening Moho, vertical to lateral movement

what primary plate movements drove the Alpine orogeny during the Jurassic and Cretaceous?

Opening of the Central Atlantic moved Africa E relative to eu, opening of S Atlantic forced Africa to rotate anti-clockwise moving it N/NW towards eu

what caused the dramatic ‘Alpine Arc’ geometry?

Pliocene opening of Balearic & Tyrrhenian Seas (Western Mediterranean, rotated Italian peninsula nearly 90 deg, curved orogen into arc

describe sequence of collisions leading to ‘Main Alpine Event’

1. Subduction of Tethys oceanic crust beneath Apulia

2. Pennine terrane collided w/ Apulia

3. combined mass struck eu margin during Miocene

Describe Jura Mountains & Helvetic Zone

• Jura: outermost external zone; Mesozoic sediments in a ‘thin-skinned’ fold-thrust belt

• Helvetic: deformed eu margin; massive fold nappes cored by eu crystalline basement directed NW

what is the sig of Piemont Zone

critical suture containing ophiolites of Tethys Ocean, high-pressure metamorphism proves these rocks were subducted to great depths before exhumation

define ‘Pre-Alps’ & ‘Dent Blanche’ in structural terms

both are klippe (thrust outliers), Pre-Alps transported far from origin via gravity gliding, Dent Blanche African/Apulian basement resting on top of Pennine units

What are the S Alps & the Insubric Fault?

• Insubric fault, a major structural boundary w/ sig strike-slip movement

• S Alps, internal zone ft SE-directed thrusting creating ‘fan’ geometry w/ NW-directed Helvetic zone

contrast deformation styles of the outer vs interior Alpine zones

• Jura/S Alps brittle thin-skinned fold-thrust geometry linked to a sole thrust

• Helvetic/Pennine ductile fold nappes & shear zones formed at greater depths

what is ‘back-thrusting’ & why did it occur in the Alps?

occurred in Miocene, orogen became too thick to move NW, cont convergence forced material upward & backward (SE) toward Apulian plate

What are the modern crustal thickness (Moho) & shortening estimates for the Alps

shortening > 250 km, crustal thickness > 50 km

name the 2 primary Himalayan depocenters/basins & their sediment types

1. Northern Foredeep Basin between Helvetic & Jura, filled w/ Molasse (Oligocene-Miocene non-marine clastics/conglomerates)

2. Southern Po Basin on Apulian plate, sediment shed from S Alps

modern Alpine orogeny

dynamic, climax ~25 Ma, ongoing African convergence, eu plate rotation cause ongoing uplift & seismic activity

where do collisional orogens fit into Wilson cycle, what is the primary physical driver?

rep final stage, driven by convergence of continental lithosphere after an ocean basin completely subducted, bcuz continental crust buoyant, resists subduction → thickening & shortening

tapered orogenic wedge model

entire mtn belt acts as wedge-shaped mass, slides along basal detachment, geometry determined by balance of internal rock strength & friction at base

how is a foreland basin formed, what is its purpose?

orogenic load causes underlying lithosphere to flex down, collects sediments eroded from rising peaks

define a nappe & minimum distance requirement

nappe is a large, sheet-like body of rock moved from its og position along a thrust fault, it must have moved over 2-5 km

fold nappes vs thrust nappes

• fold - massive fold becomes so recumbent that lower limb shears out

• thrust - tectonic ‘slices’ of crust stacked on top of one another

what occurs at margins of an orogen regarding sedimentary cover?

thin-skinned deformation: cover often stripped from crystalline basement & deformed into series of parallel anticlines, synclines, & thrusts

Himalayan ‘wedge’ & ‘thickened interior’

wedge = Indian Plate, thickened interior = Tibetan Plateau

provide an example of complex nappe stacking from the Alps

Helvetic & Penninic nappes, older crystalline basement frequently thrust over younger sedimentary cover

basal detachment

‘floor’ thrust the wedge slides on

lithospheric flexure

bending of plate that creates basins

shortening

result of horizontal compression often 100s of km

recumbent fold

essentially horizontal & leads to fold nappes

orogenic memorization

wedge (the whole mtn), basin, nappe (internal slices), crust flexes to makes basin, folds to make nappes

orogenic essay outline

the big squeeze, the orogenic wedge, regional responses of basins, nappes & folds

joints vs faults

joint is fracture w/ no visible displacement, fault is fracture w/ obvious displacement

define a normal fault & its components

hanging wall moves downward relative to footwall, dominant structure in extensional settings

slickensides, slickenlines, slickenfibres

polished surfaces created by friction have lines/grooves & crystal growths, indicate direction of movement e.g. dip-slip, oblique-slip

describe process of brittle rock failure

elastic strain (micro-cracks open) until shear strength is exceeded, stored elastic e released in earthquake at point of failure

how do faults ‘grow’ or propagate?

stress concentrates at ends of new fracture driving it to spread, often creating splay faults & can eventually form massive escarpments

horsts vs grabens

horst uplifted block of crust between 2 normal faults, graben depressed block between

listric fault & rollover anticline

fault that curves from steep near surface to horizontal at depth, anticline fold in hanging wall to fill gap created as block slides along listric fault plane

evolution of crustal stretching

extension, fault blocks rotate, og horizontal strata become inclined, faults themselves rotate to lower angle becoming ‘locked’ & inefficient

West Iberian margin

deep-ocean drilling showed multiple generations of faults, oldest rotated until sub-horizontal basal crustal detachments, extreme cases mantle exposed at seafloor

final stage of extensional evolution

extreme lithospheric thinning, new ocean basin & transition from continental rift to seafloor spreading

rifting outline

1. extension → continental rifts → ocean basins

2. joints vs faults, normal fault anatomy, slickensides

3. fracture, elastic strain → brittle failure → propagation & splay faults ex East African rift

4. geometry horst & graben (planar) → listric faults & rollover anticlines (curved)

5. adv evo: fault rotation, basal detachments, mantle exposure e.g. W Iberian margin

describe the lithospheric thickness and geothermal gradient at a divergent boundary

thinnest at ridge axis, thickens as it moves away & cools, high geothermal gradient due to upwelling asthenosphere

list 4 std layers of crustal structure at a MOR

1. pelagic sediments (absent at axis)

2. basaltic lava pillows & sheet flows

3. sheeted dykes

4. gabbro intrusions (remnants of magma chambers)

explain decompression melting & its magmatic signature

as peridotite rises, pressure decreases while temp stays constant, causes rock to cross solidus w/o added heat, produces MORB

MORB

Mid-Ocean Ridge Basalt, low in silica & volatiles, leads to effusive eruptions

define Benioff Zone

plane of earthquakes marking descending cold dense slab

3 primary regions of a subduction system

• forearc region between trench & ac often containing accretionary wedge

• volcanic arc

• backarc region behind arc may experience extension

flux melting

as slab descends, hydrous minerals (chlorite, amphibole) break down & release volatiles (water) into overlying mantle wedge, lowers melting temp (shifts solidus) allowing mantle to melt at its current ambient temp

why is subduction zone volcanism typically explosive compared to divergent volcanism?

magmas rich in volatiles & often travel thru thick continental crust, become silica-rich & highly viscous → high-pressure, explosive stratovolcanoes

contrast role of solidus in P-T diagram for divergent & subduction zones

divergent: peridotite rises upward to cross normal solidus (pressure drop), subduction: solidus shifted left (to lower temp) due to chem change

what are the primary mantle rocks involved in both divergence & subduction?

peridotite from asthenosphere, method of triggering melt differs (decompression vs fluxing)

divergent & subduction essay outline

1. divergent = birth & decompression, subduction = recycling & flux

2. divergent thin lithosphere, high geotherm, layering, MORB

3. subduction asymmetric, Benioff Zone, arcs, fluids, stratovolcanoes

4. earths structure reg of mantle flow, pressure, temp, composition

what is the most critical rheological factor in folding?

viscosity contrast between folding layer & surrounding matrix

competent vs incompetent layers

• competent e.g. sandstone, limestone, high viscosity, stiff, resists internal deformation, controls fold’s wavelength

• incompetent e.g. shale/marl, low viscosity, soft, flows easily into hinges, acts as a passive matrix

how do layer thickness & viscosity ratio affect dominant wavelength

thicker, more competent layers produce larger, more widely spaced folds

describe flexural slip & its geometric result

layers slide past one another along bedding planes like deck of cards, producing parallel folds where layer thickness remains constant when measured perpendicular to bedding

flexural flow

material flow ductiley from limbs toward hinge, produces similar folds where layer thickness constant only measured parallel to axial surface (limbs thinned, hinges thickened)

buckling

high viscosity contrast, stress parallel to layering, rhythmic, periodic & typically symmetric folds

what determines if a fold hinge is rounded or angular

high viscosity contrast in thick layers is rounded, extreme viscosity contrast in thin layers creates chevron w/ sharp hinges & straight limbs to minimize e needed to bend stiff layers

how does total strain affect fold style

initial/low strain creates gentle, open folds, high strain like orogenic cores creates tight, isoclinal folds, complex strain → refolded folds

fold shape & style essay

1. fold morphology (hinge, limbs, symmetry, tightness) record of rheology

2. viscosity contrast & wavelength

3. flexural slip & flow, buckling

4. hinge shape & strain evolution

what is a lithospheric strength profile

graph mapping max stress rock can sustain before failing vs depth

avg strength profile in lithosphere

brittle to ductile regimes

how do P-T affect rock strength

pressure increases brittle strength w/ depth by increasing friction, temp decreases strength in ductile regime by weakening atomic bonds

2 main effects of fluids on rock deformation

brittle: fluid pressure counteracts confining pressure, facilitating fracturing, ductile: hydrolytic weakening enters crystal lattices, allowing easier dislocation movement & flow

continental vs oceanic mineralogical strength

quartz becomes weaker & ductile at lower temps than olivine found in the mantle

continental multi-layered profile

strong brittle upper crust, weak ductile lower crust (Quartz-rich), 2nd strength ‘spike’ at Moho w/ Olivine mantle

what controls strength of oceanic lithosphere

thermal age

how does strain rate influence deformation

higher rates promote brittle fracturing