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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Africa
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East Africa
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Sudan (1)
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North Africa
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Algeria
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Ahaggar (1)
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Atlas Mountains
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Moroccan Atlas Mountains (1)
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Morocco
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Sahara (1)
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Southern Africa
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Karoo Basin (2)
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South Africa
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Bushveld Complex (1)
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Free State South Africa
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Vredefort Dome (1)
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Limpopo South Africa (1)
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West Africa
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Guinea (1)
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Mauritania
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Richat Mountain (1)
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Niger (1)
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Nigeria (2)
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Annapolis Valley (1)
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Asia
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Far East
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Indonesia
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Australasia
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Canada
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International Ocean Discovery Program (2)
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precious metals (1)
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oxygen
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Plantae
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geochronology methods
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geologic age
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Cenozoic
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Tertiary
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Mesozoic
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Paleozoic
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lower Paleozoic (1)
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Precambrian
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upper Precambrian
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Paleoproterozoic (3)
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igneous rocks
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granodiorites (2)
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miaskite (1)
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ultramafics
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pyroxenite
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clinopyroxenite (1)
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volcanic rocks
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andesites (2)
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basalts
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tholeiite (1)
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dacites (2)
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glasses
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pyroclastics
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rhyolite tuff (1)
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tuff (1)
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rhyolites (4)
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metamorphic rocks
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gneisses (1)
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impactites
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impact breccia (2)
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metaigneous rocks (1)
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metasedimentary rocks (1)
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mylonites
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pseudotachylite (3)
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minerals
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halides
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fluorides
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oxides
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phosphates
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silicates
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chain silicates
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pyroxene group
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clinopyroxene
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framework silicates
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feldspar group
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alkali feldspar
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K-feldspar (2)
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silica minerals
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quartz (3)
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orthosilicates
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nesosilicates
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olivine group
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fayalite (1)
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topaz (1)
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zircon group
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zircon (3)
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ring silicates
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milarite group
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milarite (1)
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sheet silicates
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mica group
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biotite (1)
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phlogopite (1)
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sulfides
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molybdenite (1)
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Primary terms
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absolute age (5)
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Africa
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East Africa
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Sudan (1)
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North Africa
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Algeria
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Ahaggar (1)
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Atlas Mountains
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Moroccan Atlas Mountains (1)
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Morocco
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Moroccan Atlas Mountains (1)
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Sahara (1)
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Southern Africa
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Karoo Basin (2)
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South Africa
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Bushveld Complex (1)
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Free State South Africa
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Vredefort Dome (1)
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Limpopo South Africa (1)
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West Africa
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Guinea (1)
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Mauritania
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Richat Mountain (1)
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Niger (1)
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Nigeria (2)
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Asia
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Baikal region (1)
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Chukotka Russian Federation
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Chukchi Peninsula (1)
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Far East
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Australasia
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Canada
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Carswell Structure (2)
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Eastern Canada
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Maritime Provinces
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Nova Scotia
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Annapolis County Nova Scotia (1)
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-
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Ontario
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Sudbury igneous complex (1)
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Sudbury Structure (1)
-
-
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Western Canada
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Athabasca Basin (1)
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Saskatchewan (2)
-
-
-
carbon
-
C-13/C-12 (2)
-
-
Cenozoic
-
Quaternary
-
Pleistocene
-
Bishop Tuff (1)
-
upper Pleistocene (1)
-
-
-
Tertiary
-
Challis Volcanics (1)
-
lower Tertiary (2)
-
middle Tertiary (1)
-
Neogene
-
Miocene
-
lower Miocene (1)
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Yakima Basalt (1)
-
-
Pliocene (1)
-
-
Paleogene
-
Eocene (1)
-
Oligocene (1)
-
Paleocene
-
lower Paleocene
-
K-T boundary (2)
-
-
-
-
-
-
Central America
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Costa Rica (1)
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crust (4)
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crystal chemistry (1)
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crystal structure (1)
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deformation (5)
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diagenesis (1)
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Earth (1)
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Europe
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Azerbaijan (1)
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Baltic region (2)
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Germany
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Ries Crater (1)
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Fennoscandian Shield (4)
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Southern Europe
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Western Europe
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France
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Ireland (2)
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Scandinavia
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Finland (1)
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Sweden
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United Kingdom
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Great Britain
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Scotland
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Argyllshire Scotland (1)
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Highland region Scotland
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Ardnamurchan (1)
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Northern Ireland (1)
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faults (23)
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foliation (1)
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gems (1)
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geochemistry (5)
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geophysical methods (13)
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geothermal energy (2)
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ground water (2)
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hydrogen
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D/H (2)
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igneous rocks
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carbonatites (1)
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granophyre (2)
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phoscorites (1)
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plutonic rocks
-
diabase (2)
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diorites
-
quartz diorites (3)
-
tonalite (2)
-
-
gabbros (3)
-
granites
-
granite porphyry (1)
-
microgranite (2)
-
-
granodiorites (2)
-
pegmatite (2)
-
syenites
-
nepheline syenite
-
agpaite (1)
-
foyaite (1)
-
miaskite (1)
-
-
-
ultramafics
-
pyroxenite
-
clinopyroxenite (1)
-
-
-
-
volcanic rocks
-
andesites (2)
-
basalts
-
tholeiite (1)
-
-
dacites (2)
-
glasses
-
palagonite (1)
-
-
pyroclastics
-
ash-flow tuff (1)
-
ignimbrite (4)
-
rhyolite tuff (1)
-
tuff (1)
-
-
rhyolites (4)
-
trachytes (1)
-
-
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inclusions
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fluid inclusions (2)
-
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Indian Ocean
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Arabian Sea
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Persian Gulf (1)
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intrusions (34)
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isostasy (1)
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isotopes
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stable isotopes
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C-13/C-12 (2)
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D/H (2)
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O-18/O-16 (4)
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Sr-87/Sr-86 (1)
-
-
-
lava (3)
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magmas (13)
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mantle (2)
-
maps (1)
-
Mesozoic
-
Cretaceous
-
Graneros Shale (1)
-
Lower Cretaceous (3)
-
Upper Cretaceous
-
Carlile Shale (1)
-
Greenhorn Limestone (1)
-
K-T boundary (2)
-
Niobrara Formation (1)
-
Pierre Shale (1)
-
-
-
Jurassic (4)
-
Serra Geral Formation (1)
-
-
metal ores
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base metals (1)
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copper ores (3)
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gold ores (2)
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iron ores (2)
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lead ores (1)
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lead-zinc deposits (1)
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manganese ores (1)
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mercury ores (1)
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molybdenum ores (3)
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polymetallic ores (1)
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silver ores (1)
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uranium ores (1)
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metals
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actinides
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thorium (1)
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uranium (1)
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alkali metals
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lithium (1)
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potassium (1)
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rubidium (1)
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alkaline earth metals
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barium (1)
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strontium
-
Sr-87/Sr-86 (1)
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-
-
precious metals (1)
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rare earths (3)
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metamorphic rocks
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gneisses (1)
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impactites
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impact breccia (2)
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metaigneous rocks (1)
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metasedimentary rocks (1)
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mylonites
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pseudotachylite (3)
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metamorphism (7)
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metasomatism (7)
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Mexico
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mineral deposits, genesis (9)
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North America
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Canadian Shield (1)
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Western Interior
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Western Interior Seaway (1)
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oxygen
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O-18/O-16 (4)
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paleogeography (1)
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paleomagnetism (1)
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Paleozoic
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Cambrian (2)
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Carboniferous (1)
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Devonian
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Upper Devonian (1)
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ABSTRACT The ~11-km-wide, Paleoproterozoic Dhala impact structure in north-central India comprises voluminous exposures of impact melt breccia. These outcrops are discontinuously spread over a length of ~6 km in a semicircular pattern along the northern, inner limit of the monomict breccia ring around the central elevated area. This study of the magnetic fabrics of impact breccias and target rocks from the Dhala impact structure identified a weak preferred magnetic orientation for pre-impact crystalline target rocks. The pre- and synimpact rocks from Dhala have magnetite and ilmenite as common magnetic phases. The distributions of magnetic vectors are random for most impact melt breccia samples, but some do indicate a preferred orientation. Our anisotropy of magnetic susceptibility (AMS) data demonstrate that the shape of susceptibility ellipsoids for the target rocks varies from prolate to oblate, and most impact melt breccia samples display both shapes, with a slight bias toward the oblate geometry. The average value for the corrected degree of anisotropy of impact melt rock (P′ = 1.009) is lower than that for the target rocks (P′ = 1.091). The present study also shows that both impact melt breccia and target rock samples of the Dhala structure have undergone minor postimpact alteration, and have similar compositions in terms of magnetic phases and high viscosity. Fine-grained iron oxide or hydroxide is the main alteration phase in impact melt rocks. Impact melt rocks gave a narrow range of mean magnetic susceptibility (K m ) and P′ values, in contrast to the target rock samples, which gave K m = 0.05–12.9 × 10 −3 standard international units (SI) and P′ = 1.036–1.283. This suggests similar viscosity of the source magma, and limited difference in the degrees of recorded deformation. Between Pagra and Maniar villages, the K m value of impact melt breccias gradually decreases in a clockwise direction, with a maximum value observed near Pagra (K m = 1.67 × 10 −3 SI). The poor grouping of magnetic fabrics for most impact melt rock samples implies local turbulence in rapidly cooled impact melt at the front of the melt flow immediately after the impact. The mean K 1 for most impact melt samples suggests subhorizontal (<5°) flow in various directions. The average value of K m for the target rocks (4.41 × 10 −3 SI) is much higher compared to the value for melt breccias (1.09 × 10 −3 SI). The results of this study suggest that the melt breccias were likely part of a sheet-like body of sizeable extent. Our magnetic fabric data are also supported by earlier core drilling information from ~70 locations, with coring depths reaching to −500 m. Our extensive field observations combined with available widespread subsurface data imply that the impact melt sheet could have covered as much as 12 km 2 in the Dhala structure, with an estimated minimum melt volume of ~2.4 km 3 .
Comparison of stress orientation indicators in Chicxulub’s peak ring: Kinked biotites, basal PDFs, and feather features
ABSTRACT During hypervelocity impacts, target rocks are subjected to shock wave compression with high pressures and differential stresses. These differential stresses cause microscopic shear-induced deformation, which can be observed in the form of kinking, twinning, fracturing, and shear faulting in a range of minerals. The orientation of these shear-induced deformation features can be used to constrain the maximum shortening axis. Under the assumption of pure shear deformation, the maximum shortening axis is parallel to the maximum principal axis of stress, σ 1 , which gives the propagation direction of the shock wave that passed through a rock sample. In this study, shocked granitoids cored from the uppermost peak ring of the Chicxulub crater (International Ocean Discovery Program [IODP]/International Continental Drilling Project [ICDP] Expedition 364) were examined for structures formed by shearing. Orientations of kink planes in biotite and basal planar deformation features (PDFs) in quartz were measured with a U-stage and compared to a previous study of feather feature orientations in quartz from the same samples. In all three cases, the orientations of the shortening axis derived from these measurements were in good agreement with each other, indicating that the shear deformation features all formed in an environment with similar orientations of the maximum principal axis of stress. These structures formed by shearing are useful tools that can aid in understanding the deformational effects of the shock wave, as well as constraining shock wave propagation and postshock deformation during the cratering process.
No ring fracture in Mono Basin, California
Tracing shock-wave propagation in the Chicxulub crater: Implications for the formation of peak rings
Topaz in New Hampshire's White Mountains
Case study: methane gas in a groundwater system located in a dolerite ring structure in the Karoo Basin; South Africa
A reassessment of the proposed ‘Lairg Impact Structure’ and its potential implications for the deep structure of northern Scotland
Dynamic weakening of ring faults and catastrophic caldera collapses
Natural ring structures on the Baikal ice cover: analysis of experimental data and mathematical modeling
Discriminating between the origins of remotely sensed circular structures: carbonate mounds, diapirs or periclinal folds? Purbeck Limestone Group, Weymouth Bay, UK
The block structure and the presence of oil and gas in the Siljan impact crater
The geometry and emplacement of the Pilanesberg Complex, South Africa
The Kotuikan ring structure as possible evidence for a large impact event in the northern Siberian craton
Lateral versus vertical emplacement in shallow-level intrusions? The Slieve Gullion Ring-complex revisited
PERSPECTIVE
PARTING SHOTS
Structural controls on mud volcano vent distributions: examples from Azerbaijan and Lusi, east Java
Fairchildite K 2 Ca(CO 3 ) 2 in phoscorites from Phalaborwa, South Africa: the first occurrence in alkaline carbonatite complexes
Seismic imaging of sub-circular salt-related structures: evidence for passive diapirism in the Straits of Hormuz, Persian Gulf
Seismic images of Chicxulub impact melt sheet and comparison with the Sudbury structure
Chicxulub is the only known impact structure on Earth with a fully preserved peak ring, and it forms an important natural laboratory for the study of large impact structures and understanding of large-scale cratering on Earth and other planets. Seismic data collected in 1996 and 2005 reveal detailed images of the uppermost crater in the central basin at Chicxulub. Seismic reflection profiles show a reflective layer ~1 km beneath the apparent crater floor, topped by upwardly concave reflectors interpreted as saucer-shaped sills. The upper part of this reflective layer is coincident with a thin high-velocity layer identified by analyzing refractions on the 6 km seismic streamer data. The high-velocity layer is almost horizontal and appears to be contained within the peak ring structure. We argue that this reflective layer is the predicted coherent melt sheet formed during impact, and it may be comparable with the unit known as the Sudbury Igneous Complex at the Sudbury impact structure. The Sudbury Igneous Complex, interpreted as a differentiated impact melt sheet, appears to have a similar scale and geometry, and an uppermost lithological sequence consisting of a high velocity layer at the top and a velocity inversion beneath. This comparison suggests that the Chicxulub impact structure also contains a coherent differentiated melt sheet.