Structural character of the Structural character of the terrace zone and implications for terrace zone and implications for

crater formation:crater formation:Chicxulub impact craterChicxulub impact crater

David L. GorneyDavid L. GorneySean GulickSean Gulick

Gail ChristesonGail Christeson

GSA Annual Meeting 2004GSA Annual Meeting 2004

Chicxulub impact craterChicxulub impact crater• Occurred ~65 Ma• Buried by up to 1 km of carbonates• Multi-ring basin, ~190 km diameter

Significance• K/T boundary mass extinction?• Largest well preserved impact structure on Earth

-SRTM topography

Significance of StudySignificance of Study

• Geophysical data can be used to model subsurface crater structure

• Structural data constrain numerical modeling of impact event

• The goal of this study is to reprocess the seismic reflection data, then use those data to image crater structure

1996 BIRPS seismic reflection/refraction survey with Bouger gravity anomaly overlay

Idealized Chicxulub StructureIdealized Chicxulub Structure

• Chicxulub exhibits multi-ring basin morphology

• Final crater morphology is a function of gravitational collapse

• Variation of widths of slump blocks can indicate impact angle

• narrow terrace zone indicates more intense deformation

Morgan et al., 2000

ObjectivesObjectives• Use existing seismic reflection data set to examine structural Use existing seismic reflection data set to examine structural

character: character:

– Reprocess reflection profiles to improve structural constraints Reprocess reflection profiles to improve structural constraints

– Focus processing scheme and interpretations on slump Focus processing scheme and interpretations on slump block/terrace zone features block/terrace zone features

Questions:

1) Are there signs of oblique impact?- Oblique impacts in SW-NE direction has been suggested based on gravity data- SE-NW direction based on tsunami deposits in North America

2) Can various models proposed for peak ring formation be constrained?- over-thrust model: collapsing central uplift interacts with collapsing crater rim (Morgan et al., 2000)- peak ring produced by subvertically uplifted basement (Sharpton et al., 1994)- narrow central uplift, peak ring consists of breccia (Pilkington et al., 1994)

ReprocessingReprocessingTo remove coherent noise:• Caused by reverberation, as a result of shallow water

(~20 m), hard water bottom• In response to guided waves, another noise issue in

shallow water

How?• Velocity analysis• F/K filter

Improve the images:• Post-stack time migration

Reprocessed Profile: Chicx-BReprocessed Profile: Chicx-B

before

after

Towards crater center

10 km

VE ~3x

VE ~3x

peak ring

Terrace zone

Tertiary basin

2- way time (sec)

1

2

3

3

2

1

SENW

Terrace ZoneTerrace Zone

• 40 to 80 km radial distance

• no consistent gravity signature

~15 km depth

twt (s)

1

2

3

4

5

Chicx-AChicx-ABase of Tertiary sediments

Top of slumps

• flat slump tops

• vertical offset – 4 km max, < 1 km minimum

• widths: 5 km and 10 km

• slumps underlie peak ring

peak ring

VE ~2x

Radial distance (km)

90 80 70 60 50

Chicx-BChicx-B

• inward dipping ( < 5°) tops

• widths 8 – 12 km

• slumps underlie peak ring

• individual blocks less distinct - secondary collapse?

Base of Tertiary sediments

Top of slumpsRadial distance (km)

80 70 60 50 40

twt (s) 1

2

3

4

5~15 km depth

peak ring

VE ~2x

Chicx-CChicx-CBase of Tertiary sediments

Top of slumpsRadial distance (km)

80 70 60 50 40twt (s)

1

2

3

4

5~15 km depth

• irregular dimensions – width and vertical offset

• width of blocks: 4 – 7 km

- narrower locus of deformation – oblique impact?

peak ring

VE ~2x

Radial distance (km)

80 70 60 50 40

Chicx-A1Chicx-A1Base of Tertiary sediments

Top of slumps

~15 km depth

twt (s)1

2

3

4

5

• slump tops: tilted inward 1 - 2°

• uniform widths - 9 -11 km

• vertical offsets generally increasing inward: <1 km at outer blocks, up to 2.5 km towards the peak ring

peak ring

VE ~2x

- narrow zone of deformation in NE direction

20 km

VE ~2x

A

A1

CB

A

BC

A1

Conclusions (1)Conclusions (1)Variability of terrace zone

Geometry of slump tops• Width of blocks ranges from 4 km to 12 km • Profiles A1 and B show relatively uniform

widths (within 2 km)• Profiles A and C vary (up to 5 km)• Slump tops dip inward (B), outward (A1), or

flat (A, C)

Normal Faulting• Height of scarps varies from 100’s m up to 4

km

Schrodinger - 320 km diameter lunar crater

Conclusions (2)Conclusions (2) Impact angleImpact angle – Some variations likely

reflect heterogeneities in target material, but may also suggest oblique impact

– If so, the impact direction is SW to NE

Venusian Crater – Proclus LTO Topography

- Proclus (Venusian) displays a compressed zone of deformation downrange

Direction of impact

Narrow zone of deformation to NE

15 km

From Herrick and Forsberg-Taylor, 2003

10 km

Chicx-C

Conclusions (3)Conclusions (3)Crater collapse and peak ring formation:

• Slumps blocks underlie peak ring, implying component of horizontal motion• Interaction of inwardly collapsing crater rim and outwardly collapsing central uplift

(Collins et al., 2002; Morgan et al., 2000)

From Morgan et al., 2000

[terrace zone]

Future WorkFuture Work• 2005 seismic survey2005 seismic survey

– Peak ring gridPeak ring grid– Radial profileRadial profile– Site survey for IODP drill Site survey for IODP drill

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