Continental Rifting Lecture Summary - Myweb @ CW Post

GLY 47/511 - Plate Tectonics
Continental Rifts Lecture Summary
Lecture on 2/21/12
based principally on Chapter 3 in Frisch et al. (2011), and Sections 7.1-7.4 in Kearey et
al. (2009)
normal faults
extension produces normal faults in the brittle upper crust
steep angle faults (dipping downward ~60° from the horizontal)
the hanging wall block moves down, footwall block moves up
grabens are subsided hanging wall blocks
with inward facing normal faults on either side
uplifted footwall blocks form graben shoulders
horsts are uplifted footwall blocks with normal faults on both sides
the classic view considered normal faults to be planar faults
but many normal faults that extend to depth (on the order of 10 km or more) are now
recognized (seismic data) to be listric (curving)
this is thought to be due to the transition from brittle to ductile behavior
the flat “sole” portion of the fault is commonly a ductile shear zone
at deeper depths, in the fully ductile lower crust, extension is accomplished by ductile
stretching and thinning of the crust
subsiding hanging wall blocks rotate downward along listric faults
half-grabens are the result of a hanging wall block subsiding principally along a
listric “border fault” on just one side (although there may commonly be smaller faults
throughout and on the opposite side of the basin)

foot wall blocks experience isostatic uplift because weight is being removed
as hanging wall blocks are slowly removed from them
foot wall blocks flex upwards since only one end is rising
rift geometry
active rifts: (lithosphere extension driven by upwelling asthenosphere)
driven by mantle upwelling (e.g., hotpsot mantle plume)
wide area of uplift and thinning of mantle lithosphere
! (in comparison to width of faulting and crustal thinning)
passive rifts: (asthenosphere upwelling driven by lithospheric extension)
driven by extension of lithosphere (due to plate motions, etc.)
narrow zone of lithospheric thinning and uplift
note: uplift is due to the replacement of lithosphere by asthenosphere (less dense)
symmetric rifting:
narrow graben structure with similar degree of inward facing normal faulting on both
sides
asymmetric rifting:
wide zone of faulting underlain be a regional detachment (decollement) surface
individual listric normal faults merge into the basal decollement

the decollement is a ductile shear zone which slices down through the lower crust
and lithospheric mantle
rift volcanism
magma produced by decompression melting
hot asthenosphere moves closer to surface as lithosphere thins
bimodal: basalt - rhyolite (little intermediate lava)
some tholeitic basalt (high percent melting at shallow depth)
alkaline lavas characteristic (low percent melting at greater depth)
narrow continental rifts
probably form in thick, cool (strong) lithosphere
can potentially proceed to split, forming new midocean ridge and ocean basin
Rhine Graben
part of a larger western European graben system
symmetric graben structure
thinned crust in central graben area
minor volcanism (only 2 volcanic centers)
early extension in upper portion (SW) of Upper Rhine Graben
later active extension in lower (NE) portion of Upper Rhine Graben
latest extension in the Lower Rhine Embayment resulting from a counterclockwise
! rotation of the extension direction
! causing some sinistral (left-lateral) shearing of Upper Rhine Graben
Upper Rhine Graben and Bresse Graben offset by system of strike-slip faults
! that act in aggregate like a sinistral transform fault offsetting spreading
! segments of a midocean ridge
East African Rift System
represents the third arm of a triple junction
the other 2 arms are the Gulf of Aden (midocean ridge)
and the Red Sea (newly formed ocean basin - actual ocean crust in middle)
4 parts:
Afar Triangle (Afar Depression)
Main Ethiopian Rift (East African Graben)
Western Rift (Central African Graben)
Eastern Rift (Kenya or Gregory Rift)
the Western and Eastern Rifts are like overlapping spreading centers
northern part of the rift system produces voluminous vulcanism
especially in Afar Triangle approaching triple junction
flood basalts erupt from fissures in the Afar Triangle
there may be a mantle plume beneath Ethiopia
the rift is segmented, with offsets of central volcanic zones along the rift axis, notably
in the Main Ethiopian Rift into the Afar Triangle
(not unlike offset midocean ridge spreading centers)
many of these rift segments are asymmetric with most subsidence along major border
faults on one side of the rift (half graben structure)
low seismic velocities under the Main Ethiopian/Afar indicates low density, hot, rising
asthenosphere under the rift

gravity lows across the Western and Eastern rifts also indicate very thin lithosphere
(low density asthenosphere replacing denser lithosphere)
long wavelength gravity anomalies (mathematically filtering out small scale variations)
show a circular low over the Ethiopia and an elongate gravity trough along eastern
Africa to the south, indicating thinned lithosphere (hot, low-density asthenosphere
replacing cooler, denser lithosphere)
seismic data (focal plane solutions) indicate dominant normal faulting with NE-SW
strikes, paralleling the Main Ethiopian Rift but changing to NW-SE in coastal
sections of the Afar Triangle, parallel to spreading direction in the Red Sea
some deep basins in coastal Afar Triangle may actually be composed of basaltic
(oceanic) crust
Red Sea rifting has transitioned to drifting
new ocean crust now forms in central basins in the Red Sea
these deep basins are 2 to 2.5 km deep, floored by basalt
no midocean ridge feature because ocean crust area not wide enough
most of Red sea underlain by stretched/thinned continental crust
the Arabian peninsula has broken away from and is drifting northward away from
Africa due to seaflor spreading in the Red Sea and Gulf of Aden
the Red Sea spreading center ends in a sinistral transform fault system that extends
up through the Gulf of Aqaba and up the Jordan Graben (Jordan River, Dead Sea)
along which the Arabian plate moves northward
Newark Rift Basin
the Newark Basin is one of a series of half-graben rift basins in present-day eastern
North America that were active in the Late Triassic-Early Jurassic, around 225-180
m.y. ago prior to the separation of Laurasia from Gondwana with the formation of
new ocean crust between North America and Africa beginning about 170 m.y. ago
the Newark basin subsided and tilted downward towards the Ramapo fault which
separates the basin (hanging wall block) from the uplifted footwall (Hudson
Highlands) composed of ~1 b.y. old gneiss, exposed by erosion of younger
overlying strata
the basin is filled with Triassic-Jurassic sedimentary strata which are inter-layered
toward the top with 3 major basaltic lava flows, as well as a large sill intrusion
(Palisade Sill)
wide continental rifts
probably form in thin, hot (weak) lithosphere
may produce large amounts of extension over a long period without rifting completely
through to form a new ocean basin
Basin and Range
broad area of extension & crustal thinning
! 500 - 800 km wide
! 250 - 300 km of extension during last 16 m.y.
average crustal thickness ~30 km
! adjacent Colorado Plateau is 50 km
Basin & Range originally thicker still
(was site of late Cretaceous thrusting and crustal thickening)

extend from the east side of the Sierra Nevada to the Colorado Plateau, Rocky
Mountains and as far as the Gran Teton Range in NW Wyoming
the structures include full grabens and sequences of half grabens rotating
downward on listric faults
the upper (brittle) crust is variably thinned, even though the total crustal thickness is
fairly consistently about 30 km
NW-SE shearing along the California coast continues to the boundary region
between the Sierra Nevada and the western Basin and Range
motions become westward extension on normal faults in and across the Basin
and Range
Basin & Range extension is probably caused by the relative motion between the
Pacific and North American plates (i.e., it is not perfectly transform motion)
the extended region was originally thickened during Mesozoic Era compressional
tectonics
the regular size and spacing of the basins and ranges contrasts with studies that
indicate variable total crustal thickness down to the Moho of from 15-20 km up to
40 km - so extension has not been uniform across the region
low angle normal faults (normal faults should have steep dips) act as detachment
surfaces
in places they extend from the surface to middle and lower crustal depths, perhaps
all the way through the crust (30 km)
in places middle or lower crustal rocks have been exposed as
metamorphic core complexes: brittle upper crustal rocks (hanging wall blocks)
directly overlying lower crustal mylonites (decollement footwall rocks) showing
ductile shearing, or the upper crustal rocks have been removed by faulting
entirely
the detachment fault (decollement) is a low angle normal fault
these have been quite controversial, because rock shouldnʼt be able to slide
down these low angle ramps, but there is evidence to indicate that they did
commonly the exposed mylonite/ductile shear zone is bowed up (isostatic uplift due
to local removal of overlying rocks)
ultimately, continental rifting can lead to the breakup of a continent and the formation of
a new ocean basin
the edges of the 2 new continents will bear the marks of rifting and its aftermath as an
evolving passive margin