Plate Motions Rifting To Spreading Transition: Oceanic Plate Kinematics And Continental Rifting

Front 1

WB provides unique opportunity to compare

Back 1

brittle extension, subsidence, and extension from long term plate motions at a rifting to spreading boundary

Front 2

Analysis of reflection seismic data near the rifting to spreading transition reveals a brittle extension of

Back 2

111km(+-23km)

Front 3

Subsidence along the same transect predicts

Back 3

115 km (+-45km)

Front 4

Long term plate motions derived from seafloor spreading and transform azimuths from the eastern portion of the WB predict _______ in the study area

Back 4

220km

Front 5

No large discrepancy btwn brittle ext and subsidence values, contrary to

Back 5

observations at ancient rifted margins

Front 6

The 2 fold discrepancy btwn long term plate motion values and brittle ext/subsidence values implies that

Back 6

seafloor spreading rates do not match continental rifting rates

Front 7

Since rifting and spreading rates do not match, extension rates in the rift can only be modeled using euler pole kinematics

Back 7

after the initiation of seafloor spreading

Front 8

The 2 different spreading rates is hypothesized to result from a

Back 8

decoupling of the lithosphere

Front 9

Decoupling of the lithosphere results in the

Back 9

upper crust moving more slowly than the mantle lithosphere

Front 10

pre-break up plate motions are primarily controlled by

Back 10

extension of mantle lithosphere due to rollback of the solomon sea plate

Front 11

After initiation of seafloor spreading, spreading rates are controlled by the mantle lithosphere, and the

Back 11

upper crust moves at the same rate as the rest of the mantle lithosphere

Front 12

Northward motion of the ______ relative to the _____ is causing rapid extension throughout the basin

Back 12

woodlark plate
australian plate

Front 13

Seafloor spreading initiated approximately ____, and has propagated _______ east to west

Back 13

6ma
500km

Front 14

Seafloor spreading helps to provide accurate constraints on plate motions via _______, derived from _________

Back 14

Euler pole solutions
magnetic isochrons and fracture zone traces

Front 15

a max of ____ km of sediments are in the deepest part of the ______

Back 15

1.5 km
North Moresby Graben

Front 16

Most of the margin is covered by less than ____ of sediment

Back 16

1 km

Front 17

Deepest ocean part of my study area is 1108 NMG

Back 17

2700m

Front 18

The northern margin has subsided virtually _____ with no visible faulting, in contrast to the southern margin, which has subsided a similar amount, but has been ________________

Back 18

2-3 km
dissected with brittle extension

Front 19

Projection of Euler pole extensions in the margins allows for predictions of ____________________ along any points of the margins

Back 19

total horizontal extension

Front 20

Therefore we can compare brittle extension from fault heaves to _____________________________, as well as extension required to produced observed ________ in the area

Back 20

long term horizontal plate motions
subsidence

Front 21

These comparisons provide insight into how the _________________ during extension

Back 21

lithosphere deforms

Front 22

At the majority of margins _______ in the upper crust is considered to be considerably less than predictions from total lithospheric extension

Back 22

brittle extension

Front 23

Basic model of simple shear rifting show an assymetric conjugate margin in which one margin deforms brittle like and the other would show ___________ deformation

Back 23

deformation

Front 24

_________________ can produce produce localized areas where subsidence does not match brittle extension, but it alone cannot explain the __________ of total extension estimates across both the rifted margins

Back 24

Lower crustal flow
mismatch

Front 25

3 hypotheses explaining mismatch

Back 25

1) depth-dependent extension, where extension is accommodated primarily by the lower lithosphere
2) polyphase faulting in which brittle extension goes unrecognized due to complex structures
3) sub-resolution faulting, to small to see

Front 26

Large amounts of subsidence with little brittle extension is _____________ at rifted margins

Back 26

commonly observed

Front 27

Depth-dependent stretching is explained by

Back 27

decoupling of the upper and lower lithosphere

Front 28

This is interpreted as the _____ and _______ accommodating a greater amount of extension than the ________

Back 28

upper mantle and lower crust
upper crust

Front 29

The discrepancy between total crustal extension vs brittle extension observed is explained by greater __________________________ of the lower lithosphere vs the upper crust

Back 29

ductile thinning

Front 30

Extension should laterally ________ at some distance away from the rift zone, i.e. and explanation of the boundaries is necesarry

Back 30

balance

Front 31

A common explanation of how the lower lithosphere is extended more than the ____________ is long wavelength flow of the lower lithosphere and subsequent __________ of the lithosphere away from the rift zone due to divergent, upwelling ___________ flow field at the onset of seafloor spreading

Back 31

upper crust
thickening
mantle

Front 32

Others suggest that greater extension occurs in the lower lithosphere when the rift ruptures the ______________, and lower lithosphere (mantle) is __________ which leads to ____________ extension

Back 32

upper crust
exhumed
ductile

Front 33

This is consistent with observations of exhumed mantle rocks on _______________ at some continental margins

Back 33

rift bounding faults

Front 34

Polyphase faulting theory suggest that only the most recent series of faulting is being accounted for in the brittle extension estimates, while _______________faults are to difficult to decipher in the seismic

Back 34

older subhorizontal

Front 35

Accounting for ___________________ via subhorizontal faults in the ________________ removes the discrepancy between the extension in the upper and lower lithosphere

Back 35

polyphase faulting
Galacia Margin

Front 36

A considerable amount of extension undoubtedly occurs in_____________________, faults to small to be resolved in seismic data

Back 36

sub-resolution faulting

Front 37

However, a _____ fold discrepancy still remains on most margins when subresolution fault estimates are applied to the amount of extension

Back 37

2-3

Front 38

Euler theorem motion of a rigid body can be described by a

Back 38

translation + a rotation

Translation = a rotation about an axis crossing the Earth
from its center to its surface.

Front 39

rigid plate motions are rotaions about an ________ passing through the ____________

Back 39

axis
center of the earth

Front 40

Euler pole is the location of the intersection of the ______________ with the __________, also can be thought of as a rotation pole

Back 40

plate rotation axis
earths surface

Front 41

Relative plate motion rate is given by seafloor

Back 41

magnetic isochron

Front 42

Relative plate motion direction can also be given by

Back 42

earthquake slip vectors

Front 43

The Papuan Plateau collided with an island arc (What island arc) through northward directed subduction in the ________________

paleogene
neogene

oligocene
miocene
pliocene
holocene

Back 43

The Late Cretaceous (100-70 ma)

65-25 ma
23-3 ma

34-23 ma
23-6 ma
5-3 ma
12000 yrs ago

Front 44

Remnants of this subduction zone now form the

Back 44

Pocklington Trough

Front 45

The Papuan Plateau was partially subducted, Obducting an______________________- the papuan ultramafic belt - on top of continental crust between _____________ and ____________

Back 45

ophiolite suite
Paleocene and Ogliocene (65-25 ma)

Front 46

obduction overthrusting of ____________ onto continental lithosphere at a convergent plate boundary where oceanic lithosphere is being subducted beneath continental lithosphere

Back 46

oceanic lithosphere

Front 47

Southward subduction initiated along the _________________ in the Early _________

Back 47

Trobriand Trough
Miocene (20 ma)

Front 48

Southward subduction produced an arc directly north of the ______________ in the eastern portion of Papua New Guinea

Back 48

what arc? D Islands
orogenic belt? Papua New Guinea

Front 49

_____________ associated with this arc was produced from the early Miocene through the ________

Back 49

Magmatism
Holocene

Front 50

Today the volcanic front extends through longitude and latitude of

Back 50

9 S 148 E
Eastern tip of Papuan peninsula

Front 51

In the Miocene the volcanic fronts west boundary was

Back 51

Egum Atoll

Front 52

East of 153 E, the ______________________________ terminates and the boundary between the Woodlark Rise and the Solomon Sea is a ________________

Back 52

Neogene Volcanic Arc
Transform Margin

Front 53

Rifting in the WB began in late Miocene (___) ~, shown by an ________ in the seismic and core data

Back 53

8.4 ma
unconformity

Front 54

Rifting is currently following a zone of weakness provided by the __________ and the ____, splitting the formerly contiguous ___________ and _____________

Back 54

orogen
arc
Woodlark Rise
Pocklington Rise

Front 55

As the orogen is off-center, the result is inherently asymmetric conjugate ____________________ (oceanic continental lithosphere), with a _______________ to the north and a _____________________ to the south

Back 55

passive margins
Neogene forearc
Paleogene collision complex

Front 56

This asymmetry has been used to explain the origin of the modern rift __________ in th WB

Back 56

asymmetry

Front 57

Based on _____________, seafloor spreading had begun about _______ in the east and propagated ~__________ west to the current rift-spread transition which extends through the ___________

Back 57

magnetic isochrons
6ma
500 km
Moresby Seamount

Front 58

To the west of the MS, ________________________ have developed along the D Islands and in the Papuan Peninsula

Back 58

metamorphic core complexes

Front 59

The met core complexes of the D islands follow the ___________ front in the latest Miocene to _______

Back 59

volcanic
Holocene

Front 60

Core complex formation accompanied __________, volcanism and westward propagation of seafloor spreading

Back 60

continental rifting

Front 61

Horizontal plate motions can be predicted based on Euler pole solutions of the motion of the Woodlark Plate relative to the ________________ derived by Taylor et al. (1999) based on _________________ and fracture zone azimuths in Woodlark Basin

Back 61

Austrailian Plate
seafloor spreading rates

Front 62

opportunity to compare an independent estimate of horizontal extension with _______________ observed in seismic data

Back 62

brittle extension

Front 63

In order to yield a minimum estimate of extension, a rift-onset age of 6Ma, the age of the ______________________________, is used rather than an age of 8.4 Ma derived from dating the ___________ seen in the odp cores

Back 63

oldest oceanic lithosphere preserved in WB
unconformity

Front 64

Taking the current width of the Papuan Peninsula in this area (odp 180) to be ∼______ from margin to margin, this yields an original width of ∼________, and a stretching factor of slightly greater than __

Back 64

320 km
110 km
3

Front 65

The geologic interpretation used in this study assumes that there have been ____________ of rifting

Back 65

two phases

Front 66

Though a single phase interpretation of brittle estimate rifting history is possible in this part of the margin, the goal is to give a ________________ of brittle extension

Back 66

maximum estimate

Front 67

Multi-channel seismic surveys with better
depth penetration and signal-noise ratio do not have good coverage over the ___________________ of Woodlark Basin where the bulk of the brittle extension has occurred

Back 67

southern margin

Front 68

Seismic line MW9304-5052, passing over Moresby
Seamount close to the active rifting-spreading transition and coincident with the ODP Leg 180 drilling transect, was chosen Fig. 5.as the primary section along which to estimate________________

Back 68

brittle extension

Front 69

Though the structural chronology and sedimentation
history is well constrained on the ____________,
sediments on the ____________ are primarily constrained to local fault-bounded depocenters (____________) and cannot be correlated regionally. The chronology of the _______________ is therefore very poorly known

Back 69

northern margin
southern margin
(deepest deposit in a sedimentary basin)
southern margin

Front 70

However, we can produce an interpretation of southern margin that is ____________ consistent with the geometries visible in seismic data and the known ___________ of the northern margin

Back 70

geologically
chronology

Front 71

the currently available seismic reflection data does not constrain the fault geometries at _____. For the purpose of the interpretation presented in Fig. 5 we have assumed a _____ geometry at depth, with the normal faults soling into a northward-dipping detachment or shear zone at the ____________ transition

Back 71

depth
listric
brittle-ductile

Front 72

As only the ____________ reaches of the faults are being used to constrain the amount of brittle extension, the interpretation of the fault geometry at depth does not influence the final result.

Back 72

planar upper

Front 73

Sediments in North Moresby Graben indicate that it began
forming quite early, around _______

Back 73

5.5 Ma

Front 74

Moresby Seamount is capped by a sediment package over _____ thick, the top of which is dated at ______, though __________ observed in cores indicated that at least 200m of younger sediments have been removed

Back 74

1 km
1.6 Ma
porosities

Front 75

The base of these sediments (MS) is interpreted to be correlative with the oldest sediments in ___________________ and slightly younger than the oldest sediments in ________________

Back 75

South Moresby Graben
NMG

Front 76

Therefore, the seamount must have previously
been near the base of _______________ and must have been subsequently uplifted due to __________ by motion on its bounding faults

Back 76

SMG
unloading

Front 77

Based on these observations, _______________, a
presently active shallow-angle normal fault, must have formed early in the basin's rifting history, but has only become the primary focus of extension since sometime after ______

Back 77

MF
1.6 Ma

Front 78

Fault blocks on the southern margin were rotated to a low angle ____________ with extension on the _______________

Back 78

simultaneously
northern margin

Front 79

SMGis interpreted to be bounded by the last __________________ and the first ______________ of this system

Back 79

South Moresby Graben

Front 80

Localization of extension on Moresby Fault
probably began after the fault blocks on the southern margin were rotated to low angle and became ______.

Back 80

locked

Front 81

southern margin faults were then crosscut by a second phase of faulting at a much ______ angle

Back 81

higher

Front 82

The long-wavelength subsidence of the northern margin, accompanied by little brittle faulting, requires southward _______________ towards the focus of ___________ throughout the rifting history of the margin

Back 82

lower crustal flow
extension

Front 83

Brittle extension is estimated by summing _____________ along seismic reflection line MW9304-5052

Back 83

fault heaves

Front 84

It should be noted that this method ____________extension, as it does not take into account ___________ of fault blocks

Back 84

overestimates
rotation

Front 85

However, correcting for this effect would require accurate ______________, which are not available for the southern margin

Back 85

depth constraints

Front 86

Summing all fault heaves interpreted from line MW9304-5052 results in _____ of extension, 31 km of which is accommodated between ______________ and the end of the seismic line

Back 86

44 km
Moresby Seamount

Front 87

Bathymetry and observations of seismic data on the ________________ record subsidence with the aid of little brittle extension

Back 87

northern margin

Front 88

Faulting beyond the northern end of line MW9304-5052 is therefore assumed to be _________

Back 88

minimal

Front 89

Considerable extension has
occurred ________ of our primary transect, as evidenced by the
previously unnamed graben visible in ___________ on Pocklington
Rise, referred to as Pocklington Graben in this work

Back 89

south
bathymetry

Front 90

bathymetry and _________ data alone clearly suggest that the amount of extension is comparable in __________ with that accommodated in the vicinity of Moresby Seamount

Back 90

gravity
magnitude

Front 91

PocklingtonGraben is assumed to accommodate 30 km(±10 km) of extension based on its similarity with the area between ______________ and the ________ end of lineMW9304-5052

Back 91

Moresby Seamount
southern

Front 92

Using the observed extension along line MW9304 and making these assumptions about the amount of extension accommodated by the rest of the margin yields ______ ext across the margin

Back 92

77 km

Front 93

This is in ___________ with previous
estimates for the region, which yielded 75 km of extension across the margin in the vicinity of _______________

Back 93

agreement
Moresby Seamount

Front 94

Additional strain is undoubtedly accommodated by faults
that are ____________ in the seismic reflection data

Back 94

not visible

Front 95

Estimates of the amount of faulting below seismic resolution predict that an additional ____ is accommodated by faults too small to be resolved

Back 95

45%

Front 96

Correcting for this amount of subseismic faulting yields an estimate of _______ (±23 km) of north–south extension accommodated across Woodlark Basin over a distance of ______ at this longitude. This yields a horizontal extension factor (β)
of ∼

Back 96

111 km
1.5
320 km

Front 97

Crustal thinning estimated from ________ subsidence of the margins provides another extension estimate to compare with our previous results. Crustal thinning can be estimated from observations of subsidence assuming _______ and correcting for uplift due to thinning and thermal expansion of the _______________ during rifting assuming instantaneous extension

Back 97

syn-rift
Airy isostasy
mantle lithosphere

Front 98

Airy Isostasy

Back 98

within a fluid in static equilibrium, the hydrostatic pressure is the same on every point at the same elevation

Front 99

Study equation 1

Back 99

crustal thinning factor

Front 100

The maximum subsidence occurs directly north of Moresby Seamount, where the bathymetry reaches ∼_____ below sea level, with an additional ∼_____ of sediments in the graben

Back 100

3.2 km
1.5 km

Front 101

However, themargin averages∼____ below sea level, with relatively ______ sedimentation. As the amount of subsidence varies widely across the margin, any point estimate of subsidence—such as that given by ______________—will not provide a representative estimate of the amount of extension across the basin

Back 101

2km
little
1D backstripping

Front 102

If _______________ is conserved during extension (i.e. no crustal flow or thermal expansion of the crust, as assumed by Eq. (1)), then the crustal thinning factor is equivalent to the horizontal stretching factor, and total horizontal extension can be estimated from observed subsidence by summing the _______________ required to produce the observed _____________ over the area undergoing extension

Back 102

crustal volume
stretching factor
subsidence

Front 103

Flow of the mid or lower crust from beneath the margins toward the ______________ is clearly occurring in Woodlark Basin

Back 103

focus of extension

Front 104

The decrease in subsidence of the deep grabens will be accompanied by an increase in the subsidence of the _____________

Back 104

margins

Front 105

Assuming Airy Isostasy will underestimate extension if the crust has a significant _______________, and assuming an instantaneous duration of rifting will _______estimate extension if the lithosphere has had time to cool and thermally ________

Back 105

flexural rigidity
over
contract

Front 106

These effects are assumed to be negligible compared to
other uncertainties and are _______ in our calculations

Back 106

ignored

Front 107

Extension estimates depend heavily on the pre-rift crustal _________, which is difficult to constrain

Back 107

thickness

Front 108

Refraction seismic profiles of the unrifted Papuan mainland west of our study area yield maximum crustal thickness estimates of _____, which is a reasonable proxy for the maximum_______________ in the study area

Back 108

50 km
pre-rift crustal thickness

Front 109

As estimating the stretching factor requires subsidence
since rifting began, it is also necessary to assume a ________________ to calculate subsidence from present-day topography

Back 109

paleo-elevation

Front 110

However, if the present-day crustal thickness at _______ is known, the paleoelevation can be estimated from the pre-rift crustal thickness

Back 110

sea level

Front 111

Assuming the crust at sea level is 35 km thick, a 50 km pre-rift crustal thickness would correspond to ∼______ paleo-elevation

Back 111

2.3 km

Front 112

Much of the margin in the vicinity of the study area was _________ prior to the initiation of rifting, and it is not unreasonable to assume a paleo-elevation of ∼2 km, as this area was part of an orogenic belt that was continuous with the modern Papuan Peninsula

Back 112

sub-aerial

Front 113

A _________ estimate of extension can be obtained by assuming a 35 km pre-rift crustal thickness, corresponding to 0 km paleo-elevation

Back 113

minimum

Front 114

Poor controls on sediment thickness across the study area require assuming __ sedimentation

Back 114

no

Front 115

_____________ clearly introduces a large degree of error.

Back 115

poor controls on initial sediment thickness

Front 116

Other parameters used in the calculation are a crustal density of_______ kg/m3, a mantle density of _____ kg/m3, _____ km depth to the ______ °C isotherm, consistent with thermal modeling of the continental lithosphere using an average geothermal gradient, and a volumetric expansion coefficient of 3×10− 5 K− 1—similar to forsterite—for the mantle

Back 116

2800
3300
160
1300

Front 117

Using Eq. (1) and these parameters, the extension
required to produce the ________________ can be calculated for every point along the margin

Back 117

observed subsidence

Front 118

Extension estimate for subsidence predicts

Back 118

115

Front 119

WB seems atypical in the regard that _________ and ______________ are the same, where as these 2 are usually different at other margins

Back 119

brittle and thinning extension estimates

Front 120

This could be due to the WB estimates accounting for

Back 120

subseismic faulting

Front 121

________________ of the crust has been shown to primarily occur after continental breakup

Back 121

depth-dependent extension

Front 122

since seafloor spreading has __________ along this
transect, it may also explain why there is no pronounced disparity between brittle extension and crustal thinning

Back 122

not initiated

Front 123

The excess of predicted plate motions as compared to observed extension and subsidence has important implications with regard to the _________________________ during continental rifting leading to seafloor spreading if all three measurements are accurate

Back 123

behavior of the lithosphere

Front 124

The results imply one of three possibilities: 1) the
methods of estimating brittle extension and predicting extension from subsidence are _______, 2) Euler pole solutions for plate motions based on seafloor spreading in the __________ portion of Woodlark Basin do not apply to the study area, or 3) __________ plate motions do not reflect syn-rift extension

Back 124

flawed
eastern
postbreakup

Front 125

______________ of faults are accounted for by our structural
interpretation

Back 125

two generations

Front 126

a third generation of faulting is difficult to justify based on _________ visible in the available seismic data.

Back 126

structures

Front 127

Even if a third phase of faulting had occurred, it alone could not resolve the discrepancy between _____________ and brittle extension

Back 127

plate motion estimates

Front 128

The southern margin, where brittle extension is concentrated, has ________ considerably less than the northern margin, despite extensive faulting in the area

Back 128

subsided

Front 129

This implicates ____________________ of the
weak and mobile mid or lower crust as a key factor in controlling the magnitude and spatial distribution of subsidence, implying a weak, mobile mid or lower crust

Back 129

ductile deformation

Front 130

Crustal flow would cause a _____ estimate of horizontal extension based on subsidence

Back 130

low

Front 131

The continuation of margin rifting after the initiation of seafloor spreading suggests that Euler pole solutions derived from magnetic isochrons may ______-predict extension in the western part of the basin, as these plate motion estimates are based solely on seafloor spreading history, and do not take into account
tectonic motions accommodated by the _________ after seafloor spreading begins

Back 131

under
margin

Front 132

The possibility exists that a third plate—the ______________, bounded on the north by the Trobriand Trough and the south by Moresby fault and the northern boundary of Egum Graben—exists in the region

Back 132

Trobriand Plate

Front 133

If the _____________ does behave as a separate tectonic plate, a triple junction would exist in the study area, and the derived Euler pole solutions may not accurately predict plate motions in this region

Back 133

Trobriand Plate

Front 134

The Euler pole solutions used in this study have been shown to accurately predict the seafloor spreading history of the basin (Taylor et al., 1999), so there seem to be few reasons why they shouldn't accurately predict extension near the ___________________

Back 134

current spreading tip

Front 135

The discrepancy between long-term (several Myr) plate
motions and observed brittle extension and subsidence must imply that ___________________ rates do not match pre-rift extension rates

Back 135

post-breakup seafloor spreading

Front 136

plate motions after continental
breakup are well described by a __________________

Back 136

rigid-body rotation

Front 137

At face value, this observation requires that the lithosphere in Woodlark Basin behave ________ after
the initiation of seafloor spreading, but not during continental rifting

Back 137

rigidly

Front 138

the best interpetation for rigid behavior after initiation of seafloor spreading but not before is that the the ________________ may behave as a rigid body, with the upper crust ____________ from the rest of the lithosphere by the weaker mid or lower crust

Back 138

mantle lithosphere
decoupled

Front 139

Extension prior to continental breakup would
then occur primarily in the _________________

Back 139

mantle lithosphere

Front 140

Initially, the ___________ in the rift zone is decoupled from overall plate motions by the mobile mid and lower crust

Back 140

upper crust

Front 141

As a result, the crust responds passively to the extension of the mantle lithosphere and extends at a much _______rate, “slipping” above the rest of the lithosphere

Back 141

lower

Front 142

Once the mantle lithosphere necks through, crustal extension localizes above the rising ______________,
leading to continental breakup and the onset of seafloor
spreading

Back 142

asthenosphere

Front 143

lithosphere
asthenosphere

Back 143

stronger rigid, brittle deformation
weaker, plastic deformation

Front 144

After the initiation of seafloor spreading, the upper
crust moves with the rest of the __________, and the conjugate margins separate at the full plate velocity

Back 144

lithosphere

Front 145

This explains the discrepancy between plate motion estimates and brittle extension measurements, as plate motions inferred from seafloor spreading are controlled by the motion of the ______________

Back 145

mantle lithosphere

Front 146

This condition is most easily met if the ________________ is the primary strength of the lithosphere, as is the case in classic “jelly sandwich” models of lithospheric strength

Back 146

mantle lithosphere

Front 147

Extension rates calculated from the seafloor spreading history of a young oceanic basin may not reflect extension rates in the upper crust during continental rifting, suggesting that _____________________ cannot be directly applied to the continental lithosphere during rifting

Back 147

rigid plate kinematics

Front 148

How did we come up with 320 km as the margin extent? Measured from woodlark rise to pocklington rise, the 2 margins that constrain the north and south of the ___________________

Back 148

Papuan Peninsula

Front 149

Indicate the direction of plate motions

Back 149

transform fault azimuths