REVIEW
 
 

DEFINITIONS
 
 

READINGS

 Ch. 6

LECTURE
 
 

WILSON CYCLE
 
 

It is now viewed that plates have repeatedly broken and reassembled every 400-500 Ma. Passive-to-Active breakup

 1: continental heating, doming and extension under body forces.

 2: seafloor spreading. Old rifted inactive margin - passive margin

 3: subduction

 4: continental collision and suturing
 
 

TIMOR STUDY AREA UNDER COLLISION

Foreland basins are asymmetric basins formed in front of an advancing orogen.

The advancing orogen weighs down the elastic continental lithosphere.

The lithosphere can flex and break the crust
 
 

These basins formed as a result of bending are called continental flexural extensional basins.
 
 

WILSON CYCLE

Throughout the Earth's history, continents have experienced numerous episodes of collision and breakup. As a result, flexural extensional basins can be obscured by tectonism.
 
 

It is easier to examine present-day continent-ocean foreland basins.
 
 

HEEZEN-THARP MAP

I know of at least three incipient continent-ocean collision zones:
 
 

Taiwan, Eratosthenes Seamount (couth of Cyprus) and northern Australia
 
 

RIFTING VERSUS FLEXURAL EXTENSION

If we only had the top of the crust to go by and you saw extension you might think it was caused by rifting. (1). In classical rifting the whole crust thins.
 
 

However, extension during flexure predicts that if we conserve area in this cross-section in the shallower portions of the crust may extend. Whereas below some neutral surface the crust and perhaps mantle experiences shortening and may actually thicken.

MODEL VERSUS DATA

Various elastic-viscous models have been proposed to explain how the shape the top of the subducting crust would have if it were loaded by a mountain range.

First, the observed bathymetry doesn't always match the models here.

: the models predict some region of uplift here, which we do not see.

BEACH

CROCODILES

SHARKS

We chose to study the outer NW Shelf of Australia. This is also a region whose beaches are never too crowded because of the unfriendly wildlife. There are territorial saltwater crocodiles, sharks as well as Portuguese Man-o-Wars
 
 

REGIONAL LOCATION

 The northern boundary of the Indo-Australian Plate is a collisional plate boundary of variable character. Near India it is a continent-collision. East of our study region, it's an example of ocean-ocean collision. In our area it's a zone on incipient continent-arc collision.

In this region the Indo-Australia Plate is moving to the N (~7km/yr) with respect to the SE Asian Plate.
 
 

Currently, convergence has slowed or ceased, and may be taken up locally by strike slip (a transform boundary) within the island of Timor.
 
 

This area in yellow is the study area in the Indonesian Island Arc, south of the Island of Timor. There are two key stages in the evolution of this area in yellow.
 
 

TWO-STAGE EVOLUTION

 At about 3-5 Ma (Pliocene) Australia was moving under Timor.

3-5 Ma (Pliocene) was the peak period of mountain building by thrusting and folding on Timor.

About 3 Ma volcanism north of Timor, and subduction ceased, probably in response to the continental crust entering and choking the subduction system. Earthquake studies suggest that the plate is currently undergoing rebound as a result of the underlying oceanic plate breaking off. (This is an important consideration)
 
 

BATHYMETRY MAP

 The present-day bathymetry of this area shows a marked indentation along the outer continental shelf.

3-D RENDITION OF TIMOR TROUGH

 The indentation, I believe, is a basin produced during the collision. These is a candidate flexural extensional basin.

The continental shelf has no sign of a flexural uplift.
 
 

SEISMIC DATA

 Luckily there existed a very large marine geological and geophysical data set in this area.

We have over 14,000 km and complete well log suites from about 54 commercial wells. Note the location of these lines for reference.

The white lines are the ship tracks. The dots are the well locations.
 
 

The yellow line marks the limit of the outer continental shelf (200 isobath)
 
 

BASE MIOCENE MAP

 These seismic data permitted us to map out the major stratigraphic surfaces at depth.

It sowed us that under the scalloped indentation on the continental shelf there was a Tertiary Basin

This basin is The Cartier Trough.

 This is a TWTT structure map. It's a map of the surface at the Base of the Miocene.

The blue area is the deeper and the red area is relatively high. The 200-m contour lies here.

JURASSIC STRUCTURE MAP

 Below the Cartier Trough we find a set of en echelon Jurassic rift basins. I believe that the Tertiary Basin may have been developed by reactivation of a deeper Jurassic rift basins.

If the Cartier Trough is a flexural extensional basin then major sedimentation in the Trough shouldn't have started until about the same time as the major phase of mountain-building on Timor, i.e. the Pliocene.
 
 

ISOPACH MAP OF PALEOCENE-BASE MIOCENE

 Ths orediction agrees with the following observation: The sediment thickness map for the sedimentary unit that includes the Paleocene to Base Miocene shows no evidence for a thickening in the area of the Cartier Trough before the Miocene.
 
 

ISOPACH MAP OF BASE MIOCENE-PLIOCENE SEISMIC REFLECTOR

 However, sometime as early as the Late Miocene, the Cartier Trough became reactivated and became a large depocenter (in yellow).

It seems that the Cartier Trough began to form before the main mountain-building stage on Timor. This implies that the C.T. would have been found several hundred km farther S. at the time. The plates were converging at that time.

We decided to investigate the stresses necessary to produce basin formation.
 
 

BENDING MODEL

 We decided to start by looking at present-day stresses.

 We used this model to predict stresses.

 The weight of the Island of Timor is represented by the wedge.

 The continental lithosphere is represented by this elastic beam and all are floating in the mantle.

 Note that we do not consider the effect of slab pull. We assume that today the slab has broken off.
 
 

SEAFLOOR FAULTS

 This model can explain the distribution of faults on today's sea floor.

 The boxes are proportional to the vertical throw on the fault scarps.
 
 

The lines are the ships' tracks along which the faults were identified.
 
 

There are no faults south of this line and this hatched line is the theoretical limit of faulting predicted by the previous model.
 
 

However, this model does not explain the necessary stresses to have broken the crust several hundred kilometers farther south. I hypothesize that if we include slab pull then the necessary stresses can be reached. This is ongoing research.
 
 

FLEXURAL MODEL

 We have proposed some explanations for basin formation. But, not only are basins important to explain but so is the absence of a forebulge.

In this elastic theoretical model we predict a forebulge here.

We find instead that the predicted bulge is absent. In its place there is the C.T.
 
 

3-D MODEL OF BATHYMETRY

 We have not been able to locate a clear flexural bulge either in the present day topography .
 
 

I think that a key reason is that these elastic models were developed for oceanic settings. In oceanic settings, the crust is younger, has fewer pre-existing weaknesses and is intrinsically stronger.

On continents the crust is older, weaker and fraught with old zones of weakness that can be reactivated.

The Jurassic basins under the C.T. are an example.

The crust is unable to sustain the bending and instead breaks. Faulting can subdue or destroy the bulge.
 
 

ISOSTATIC GRAVITY MODEL

 We have seen that in flexural extensional settings there are two competing features: a flexural bulge and a flexural extensional basins. Flexural extensional basins occur because of fault reactivation.

In our study area here (S. of the island of Timor) the bulge has been destroyed.