Transcript The Mid-continent Rift

Middle Proterozoic and The
Mid-continent Rift
• Rodinia, the Proterozoic supercontinent, was finally assembled ~ 1.31.0 Ga by collision with proto-South America along the eastern edge of
present-day North America
• Development of a mantle plume beneath the modern Lake Superior
and the upper peninsula of present-day Michigan caused thermal
doming of proto North America (Laurentia), thinning of the crust, and
rifting ~1.2-1.1Ga
• The rift failed, for reasons not well understood, but not before
intrusions of mafic igneous rock (gabbro) and eruption of mafic volcanic
rock (basalt) filled much of the rift.
• Deposition of siliciclastic sediment filled the remainder of the rift, prior
to uplift and erosion at the close of the Precambrian
• Deformation at the eastern margin of Laurentia (N.America) produced
compressional forces, which reactivated rift-margin faults and exposed
occurreences of copper deposits
Proterozoic Geologic History
Reconstruction of the Rodinia
Supercontinent at time of Grenville Orogeny
Gravity Map for U.S.
• Note the Mid-continent rift, which extends from
the lower peninsula of Michigan, through
theSuperior Basin, and south into Oklahoma
Precambrian Bedrock in
Minnesota
Rifted Continent to Ocean Basin
Map of Mid-continent Rift
• ~2500 km long and 150-200 km wide
Plume Head and Mid-Continent Rift
• Note that the
northern arm of the
triple junction of the
rift did not fully
evolve.
• Continued evolution
of the rift was aborted
by collision of
Laurentia with
Amazonia to form the
Grenville Mountains
Map of Mid-continent Rift, Showing Age of
Basement Rocks and Distribution of Basalt
Lava Flows and Sedimentary Rocks
Map of Rock Units Filling Midcontinent Rift
Mid-Ocean Rift in Iceland
Red Sea is a Modern Analogue for the
Mid-continent Rift
Rifting and Magmatic Activity
• enormous volume of flood basalt erupted in rift
from1108-1094 Ma
• basalt sequences are as much as 30 km thick and
older crust was nearly completely separated during
synvolcanic extension.
• isotopic signature consistent with onset of a new
mantle plume beneath the area at about 1110 Ma.
• plume head provided a hot source for production of
basaltic magma by decompression melting.
• Deposition of ~8 km thick mostly alluvial fan, river
and lake sediments continued for a few tens of
millions of years after volcanism and extension
ceased. Some of this deposition overlapped basalts
in time
Mantle Plume
Structure of a Thick Basalt Lava Flow
• Margins cool more
rapidly, center cools
more slowly. Note
the well-developed
columnar joints at
the top and bottom
margins of the flow
Map of the Superior Syncline
Cross Section of the Superior Syncline
• Weight of pile of basalt lava flows bows the
crust downward
Duluth Complex and North Shore
Volcanic Group
Note
mineralization
along northern
margin of the
complex
Relationship of Duluth Complex to
North Shore Volcanic Group
Mid-continent Rift in the Vicinity of
the Twin Cities
Cut Face Creek Lava Flow
and Sandstone
• Lava flow overlies
reddish-colored
sandstone deposited
in the mid-continent
rift
• Sandstone contains
grains of volcanic
sand derived from
nearby volcanic
rocks at the margins
of the rift valley
Gooseberry Falls State Park, North
Shore Volcanic Group
Cascades
tumble over
edges of
successive
lava flows
Basalt Lava Flows, North
Shore Volcanic Group
Lava flows are
tilted
southward
toward center
of Superior
Syncline
Columnar Joints,
North Shore
Volcanic Group
• Form when lava
cools and shrinks
• Cracks are
polygonal (manysided) and form
perpendicular to the
cooling surface
Columnar Jointing in Basalt Flows,
North Shore Volcanic Group
Pahoehoe in Oxidized Basalt Flows, Temperance
River, North Shore Volcanic Group
Ropy
pahoehoe
structures
indicate fluid
lava
Flow Boundaries Often Marked by Oxidized
Zones, Temperance River, North Shore
Volcanic Group
Pipe Vesicles Concentrated at Top of Flow,
Sugarloaf Cove, North Shore Volcanic Group
Amygdules in Basalt Flow, North
Shore Volcanic Group
Amygdules are
mineral fillings
of gas holes
(vesicles) they are NOT
large crystals
that form early
in the
solidification of
the rock
Lake Superior Agates
• Form as silica fillings in
amygdules of basalt lava flows
• Eroded from basalt and
washed/abraded in the surf zone
of Lake Superior
• Transported southward by
glaciers and outwash
• Often found in gravel pits along
the Mississippi River in SE
Minnesota
Basalt Lava Flows at the Dalles of
the St. Croix
The lava flows
were exposed
by a
catastrophic
flood from
Glacial Lake
Duluth down
the St. Croix
River Valley
Basalt Flow in Potholes, Dalles of the
St Croix River
Note concentration of vesicles
at top of flow
Copper Deposits in the Duluth Complex
• mantle plumes rich in magnesium, chromium, copper,
nickel, platinum, and relatively low in sulfur.
• copper-bearing minerals are generally sulfides
• when mafic rocks, such as the Duluth Gabbro, are
intruded into crustal rocks, sulfur from the crustal rocks
can combine with copper in the mafic magma to be
deposited as copper sulfides
• overlying lava flows are then depleted in copper
• to expose the copper deposits, uplift of the deeper
intrusions along faults is needed
• compression during the final assembly of Rodinia
caused uplift along the faults associated with the rift,
exposing the copper deposits.
Copper and Iron Sulfides at the Northern
Margin of the Duluth Complex
Map of Copper Sulfide Deposits in the
Lake Superior District