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Emily Neier

Mentor: Steve Mattox

GEO 485

28 April 2016

Comparison of the ancient Keweenaw Mid-Continent Rift System and the active East

African Rift System

Abstract

The active East African Rift System (EARS) and ancient Keweenaw Midcontinent Rift

System (KMRS) are both large areas of continental rifting that are often consider analogous to

each other. This paper compares the rift systems in order to determine if they are analogous by

looking at their size, fault type, and volume of basalt. These factors, particularly fault style,

define the functions of both rift systems. Identifying what areas the rifts are similar could help

predict how the KMRS functioned in the past as well as predict where the EARS might be

heading. The primary focus of this research is a literature review of both rift systems and using

papers of both older and emerging research to define each rift. The KMRS is a rift filled with

flood basalts that likely operated on a few listric master faults. The EARS is a rift on a

topographic high caused by the African Superplume beneath it, and contains three microplates

between the major Somalian and Nubian plates. The faulting styles of the KMRS and EARS are

not very comparable and basalt volumes of the KRMS almost double that of the EARS; however,

the possibility of a microplate in the KMRS suggests that the EARS would be helpful in

furthering this model. While both systems show a lot about intercontinental rifting, the EARS

and KMRS both have unique qualities and are ultimately not analogous on the major factors

presented in this paper.

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Introduction

Understanding active continental rift features and processes could help us hypothesize the

conditions ancient rift systems worked under; in the same sense, the frozen ancient rift structures

could help us hypothesize where active rifts are heading. A literature review of the active East

African Rift System (EARS) and the ancient Keweenaw Midcontinent Rift System (KMRS) will

be used to identify the similarities and differences of both rifts on key platforms and determine if

they are analogous. The areas of study that are an interest to this project include the overall size

of the rift, major fault types associated with both systems, basalt volumes, presence of

microplates. These areas help define the basic characteristics of how the rifts behave and are

most important in considering the system analogous.

The East African Rift System began rifting

approximately 30 million years ago in the early

Eocene (Corti 2009). The rift system is around

3500km in length north to south and approximately

1000km at its widest (Chorowicz 2005) and lies

across the countries of Ethiopia, Kenya, Somalia and

Tanzania. As shown in Figure 1, the rift is broken up

into three branches: the Eastern, Western, and

Southeastern. While the majority of faults in the rift

system are high-angle normal faults (Corti 2009),

there is a dividing transform fault between the Eastern

and Western branches of the system, and another in the middle of the Western branch

(Chorowicz, 005). These transform faults are similar to the transform faults seen in mid-ocean

Figure 1: Map view of EARS showing major faults and branches; white areas indicates lakes (Chorowicz, 2005)

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rifts (Chorowicz 2005). The Main Ethiopian Rift

(MER), located in the Eastern Branch of the

EARS, hosts a major system of normal faults on

the floor of the rift called the Wonji Fault Belt

(Corti 2009). Wonji faults are short normal faults

that occur in clusters and are different from the

long, widely spaced border faults along the edge

of the rift (Corti 2009).

The EARS makes a good laboratory for

studying continental rifting because there are very

little opposing convergent plate forces on the system (Stamps et. al. 2014). As shown in Figure 2,

the Somalian Plate moves, on average, 3mm east a year, with a clockwise rotation (Stamps et. al.

2014). Three microplates—Victoria, Rovuma, and Lwandle—lie between the Nubian and

Somalian plates and move the same 3mm east a year without rotation (Stamps et. al. 2014).

The Keweenaw Midcontinent Rift System (KMRS) is an ancient continental rift that was

active during the Mesoproterozoic approximately 1.1 billion years ago (Stein et. al. 2014). The

rift is often considered failed because seafloor spreading did not develop out of the rift, and the

tectonics associated with the rift are not apparent (Stein et. al. 2014). Several hypotheses on the

rift’s origin include that rifting began as a result from a mantle plume or as a result of the

Grenville Orogeny, but Stein et. al. (2014) has proposed that the rifting occurred between

Laurentia and Amazonia plates during the Grenville Orogeny. This hypothesis also proposes that

the rifting occurred at a plate boundary and likely included microplates as is exhibited in the

EARS (Stein et. al. 2014).

Figure 2: Major plates and plate motions from GPS data and best-fit model data in the EARS, assuming a Nubia-fixed environment (Stamps et. al. 2014)

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The KMRS is approximately 2000km

in length (Stein, et. al., 2014) and approx.

250km at its widest. The rift covers an area

across U.S. states Minnesota, Wisconsin, and

the Upper Peninsula of Michigan, Iowa, and

Kansas, with portions of the rift possibly

reaching down into Texas (Stein et. al. 2014).

Most of the KMRS is buried under younger

rock layers, but buried rift segments can be

traced by the high magnetic wander of the

mafic volcanic rocks exhibit (Ojakangas et. al.

2001). The polar wander of these rocks is due to a

period of reversed polarity when magma erupted

from the rift (Ojakangas et. al. 2001). A map view

of the rift outlined by the magnetic anomaly in the rocks is shown in Figure 3. U/Pb radiometric

dating of zircons date the volcanic activity in the rift between 1105 Ma and 1094 Ma (Ojakangas

et. al. 2001).

The St. Croix horst, marked on Figure 3, is located in the Wisconsin portion of the

Superior Zone, is an important structure in the KMRS because it shows an area where faulting

was later reactivated (Ojakangas et. al. 2001). The normal faults that originally created grabens

later reactivated as reverse faults and pushed back up into the horst (Ojakangas et. al. 2001). The

reactivation occurred around 1060 Ma, and one hypothesis is that the reactivation is related to the

Grenville Orogeny (Ojakangas, et. al., 2001).

Figure 3: The KMR outlined by the magnetic anomaly in the mafic rocks. Gravity high areas indicate volcanic rocks and gravity low areas indicate sedimentary rocks containing sediments from the volcanic rocks. (Ojakangas, et. al. 2001)

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First I will determine what aspects of rift systems must be compared to be considered

analogous, and second, determine if the KMRS and the EARS meet these requirements. If the

systems meet the criteria, then the EARS can provide insight into how the ancient KMRS

progressed, and the KMRS can provide a picture of where the EARS might be heading.

Research Strategies

The research for this project was a literature review of both the Keweenaw Mid-

Continent Rift System (KMRS) and the East African Rift System (EARS). The papers used can

be divided into two broad categories: overview and topic-specific. Overview papers covered the

basics of each rift system, including the size and age of each rift, the rock types, and the major

structural features. The reason for choosing these types of papers was to have a sense of each rift

as a system before focusing in on specific aspects.

Topic-specific papers focused on one area of interest in the rift systems like fault styles,

rifting forces, microplate formation hypothesis, and basalt volumes. As a general rule to ensure

the most accurate information, the papers used in this project were published within the last

fifteen years. Topic-specific papers older than this are foundational for the newer research. The

reason for choosing these types of papers was to focus on aspects of both rift systems and

determine if the research in a specialized area of one rift was comparable to that of the other. Not

every sub-discipline was available for both rifts, including the total magma volume of the EARS

and spreading rates for the KMRS. However, total extension of the KMRS was available. The

missing data might be due to the EARS still actively producing lava that calculating the total

volume is not possible. The missing KMRS data could be a result of compression, after rifting

stopped, that might have reset or erased any trace of earlier extension. However, the data

available for the other rift in these sub-disciplines might help hypothesis on the missing data.

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Table 1 available in Appendix A shows a compilation of the papers used to analyze the

KMRS and summarized methods within those papers. The KMRS outcrops primarily in the Lake

Superior region of the rift, but the strong reversed pole magnetic signatures of the basalts within

the rift reflects the shape and size of the rift buried beneath Phanerozoic sediments. The papers

used for the KMRS mainly used paleomagnetic data, Ur-Pb radiometric dating of zircon, and

seismic data from the GLIMPSE program. Stratigraphic data determined from drill cores and

seismic data were also used to study the KMRS.

Table 2 available in Appendix A shows a similar compilation of papers used to analyze

the EARS. Because the EARS is active, a variety of GPS and seismic data was used to measure

the direction and rate of movement of the plates involved in the EARS. Most notably, Stamps et.

al. (2014) collected vertical GPS data in order to model the vertical buoyancy forces acting on

the EARS’s spreading. Data from recent and frequent earthquakes in the EARS also show the

displacement of specific points along the rift (Stamps et. al. 2014). Maps of the EARS are more

helpful than maps of the KMRS because the rift is not buried, and topography and major

structures of the rift are visible on them.

Data Compilation

The values and information in Table 3 describe major features of both rifts in a side-by-

side comparison. The features in this table include some general geographic information as well

as major components of rifting: dominate fault type, duration of rifting, and basalt volumes.

Keweenaw Mid-Continent Rift System

The KMRS actively rifted between 1120 Ma–1086 Ma, and erupted approx. 2,000,000

km3 of flood basalt during that time (Stein et. al. 2015). While the KMRS has a comparable

volume of basalt compared to other traps, the KMRS basalts reach thicknesses of 7–20 km

because it is contained in a narrow rift basin (Stein et. al. 2015). Although most of the rift is

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buried under younger sediments, the thick layers of basalt are detectable by their negative

magnetic signature (Ojakangas et. al. 2001). This prominent magnetic feature of the KMRS is

used to map the shape of the rift under Cambrian sediments. Crustal thickening happened along

the basin as the rift filled with basalt (Stein et. al. 2015).

As shown in Figure 5, the major fault type in the KMRS is listric faulting. Stein et. al.

(2015) observed that the Douglas-Obijwa fault and Keweenaw fault were master faults

accompanying the largest extension of the rift. The listric faulting in the Lake Superior region of

the rift, observable through seismic data from the GLIPMCE program, is accompanied mainly by

rollover anticlines (shown in Figure 5) with some areas of slumping along the Keweenaw fault

(Stein et. al. 2015). Calculated extension for the Douglas-Obijwa and Keweenaw faults are 23

km and 28 km, respectively (Stein et. al. 2015). The listric normal faults were reactivated much

later by a shortening event as reverse faults, accommodating 7 km of shortening along the

Douglas-Obijwa fault and 12 km of shortening by reverse faulting along the Keweenaw fault;

this compression also attributed to crustal thickening (Stein et. al. 2015).

New evidence from Stein et. al. (2014) suggests the KMRS could have formed during the

breakup of Amazonia and Laurentia as a developing plate boundary (Figure 6). Paleomagnetic

data of the anomaly demonstrates rotation in the rift that would be consistent with microplate

formation (Merino et. al. 2013; fig 4 specifically).

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East African Rift System

The EARS began rifting the late Eocene-early Oligocene (Chorowicz, 2005). An

estimated volume of erupted basalt so far is ~845,000 km3 (Guth, 2013). The EARS is

topographically high (Behn, et. al. 2004), possibility because the source of the magma is most

likely the African Superplume (Furman, 2007). The EARS produces basalt in the triple junction

Figure 5: A cross section of the master listric Keweenaw fault from Stein et. al. (2015). The figure shows estimated extension amounts and rollover anticlines (shown as dashed gray lines indicating bent rock layers).

Figure 6: Estimated plate positions before separation between Laurentia and Amazonia showing the KMRS as part of a spreading center (Stein et. al. 2014)

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of the rift, shown in Figure 7 (Furman 2007). Sea floor spreading has been established in the Red

Sea and Gulf of Aden, but the top of the EARS, the Main Ethiopian Rift is host to the Ethiopian

Traps with an estimated volume of ~250,000 km3 (Guth 2013)

The major fault type in the EARS is high angle normal faults. In the Main Ethiopian Rift,

long, high angle, border faults are situated on each side of the rift with patches of shorter, high

angle faults in the middle of the rift, shown in Figure 8 (Corti 2009). Extension of about 3mm/yr

occurs throughout the rift (Stamps et. al. 2014). The rift experiences very little compression and

exists in a virtually divergent-only area (Stamps et. al. 2014).

The EARS contains three microplates between the Nubian and Somalian plates: Victoria,

Rovuma, and Lwandle plates, shown in Figure 2(Stamps, et. al. 2014). These plates and the

Somalian are shown to rotate clockwise as they move eastward (Stamps, et. al. 2014).

Figure 7: The triple junction of the EARS showing the area of the flood basalt (Corti 2009).

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Table 3: Comparison of KMRS and EARS

Keweenaw Midcontinent Rift East African Rift Map Note: Maps are not to scale of each other and are included in this table at this size to demonstrate their shape.

Length 2000 km 4100 km Maximum Width approx. 250km approx. 1100km Duration of Rifting 34 million year 30 million years Magma Volume 2,000,000 km3 approx. 845,000 km3 Fault Type Listric Normal Faults High Angle Normal Faults Major Rock Types Basalt Basalt Microplates 1 possible: Wisconsin Block 3 distinct: Victoria, Rovuma,

Lwandle

Figure 8: Cross section of the MER in the EARS showing border faults and swarms of short, high angle normal faults.